Feeding ecology of mesopelagic zooplankton of the subtropical and subarctic North Pacific Ocean determined with fatty acid biomarkers

Deep-Sea Research I 57 (2010) 1278–1294

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Feeding ecology of mesopelagic zooplankton of the subtropical and subarctic North Pacific Ocean determined with fatty acid biomarkers S.E. Wilson a,n, D.K. Steinberg b, F.-L.E. Chu b, J.K.B. Bishop c,d a

Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, USA Virginia Institute of Marine Science, College of William and Mary, Route 1208 Greate Rd., Gloucester Point, VA 23062, USA c Department of Earth and Planetary Science, University of California Berkeley, Berkeley, CA 94720, USA d EO Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA b

a r t i c l e in f o

a b s t r a c t

Article history: Received 17 March 2009 Received in revised form 22 June 2010 Accepted 6 July 2010 Available online 18 July 2010

Mesopelagic zooplankton may meet their nutritional and metabolic requirements in a number of ways including consumption of sinking particles, carnivory, and vertical migration. How these feeding modes change with depth or location, however, is poorly known. We analyzed fatty acid (FA) profiles to characterize zooplankton diet and large particle ( 451 mm) composition in the mesopelagic zone (base of euphotic zone  1000 m) at two contrasting time-series sites in the subarctic (station K2) and subtropical (station ALOHA) Pacific Ocean. Total FA concentration was 15.5 times higher in zooplankton tissue at K2, largely due to FA storage by seasonal vertical migrators such as Neocalanus and Eucalanus. FA biomarkers specific to herbivory implied a higher plant-derived food source at mesotrophic K2 than at oligotrophic ALOHA. Zooplankton FA biomarkers specific to dinoflagellates and diatoms indicated that diatoms, and to a lesser extent, dinoflagellates were important food sources at K2. At ALOHA, dinoflagellate FAs were more prominent. Bacteria-specific FA biomarkers in zooplankton tissue were used as an indicator of particle feeding, and peaks were recorded at depths where known particle feeders were present at ALOHA (e.g., ostracods at 100–300 m). In contrast, depth profiles of bacterial FA were relatively constant with depth at K2. Diatom, dinoflagellate, and bacterial biomarkers were found in similar proportions in both zooplankton and particles with depth at both locations, providing additional evidence that mesopelagic zooplankton consume sinking particles. Carnivory indices were higher and increased significantly with depth at ALOHA, and exhibited distinct peaks at K2, representing an increase in dependence on other zooplankton for food in deep waters. Our results indicate that feeding ecology changes with depth as well as by location. These changes in zooplankton feeding ecology from the surface through the mesopelagic zone, and between contrasting environments, have important consequences for the quality and quantity of organic material available to deeper pelagic and benthic food webs, and for organic matter sequestration. & 2010 Elsevier Ltd. All rights reserved.

Keywords: Biomarkers Fatty acids Feeding ecology Mesopelagic zone North Pacific Ocean Zooplankton

1. Introduction Zooplankton residing within the mesopelagic zone (base of the euphotic zone to  1000 m) include a diverse assemblage of organisms that play a key role in the biological pump (Angel, 1989; Steinberg et al., 2008a, 2008b). The community structure and feeding activities of mesopelagic zooplankton are biotic factors that can affect the efficiency by which sinking POM is transported to the deep sea (Fowler and Knauer, 1986; Bishop et al., 1987; Sasaki et al., 1988; Angel, 1989; Noji, 1991; Lampitt, 1992; Buesseler et al., 2007, 2008). Mesopelagic zooplankton must obtain their food via carnivory and particle feeding (on

n

Corresponding author. E-mail address: [email protected] (S.E. Wilson).

0967-0637/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2010.07.005

aggregates of sinking or suspended detritus), or migrate to the epipelagic zone (0–150 m) to feed (Steinberg et al., 2008b). However, there is limited information on the feeding ecology and nutrition of mesopelagic zooplankton. Approaches to studying the feeding ecology of mesopelagic zooplankton include microscopic analysis of gut contents or fecal pellets, and observations of changes in types of zooplankton fecal pellets with depth. Gut content and fecal pellet analysis is useful for detecting remains of food items such as phytoplankton tests, crustacean carapaces, and detritus in deep-sea zooplankton (e.g., Silver and Bruland, 1981; Gowing and Wishner, 1986, 1992; Lampitt et al., 1993a, 1993b; Steinberg, 1995; Uttal and Buck, 1996; Kosobokova et al., 2002; Schnetzer and Steinberg, 2002). Carnivory and particle feeding in the upper mesopelagic is evidenced by the emergence in lower mesopelagic sediment traps of new, distinctive fecal pellet types derived from crustacean

S.E. Wilson et al. / Deep-Sea Research I 57 (2010) 1278–1294

or gelatinous carnivores and filter feeding larvaceans, respectively (Wilson et al., 2008). These changes in fecal pellet type with depth are generally consistent with observations of distinct peaks in both particle feeders (e.g., salps, ostracods, and poecilostomatoid copepods) and carnivores (e.g., chaetognaths, medusae, mysids, and ctenophores) within the mesopelagic zone in the subarctic and subtropical North Pacific Ocean (Steinberg et al., 2008a). Gut content and fecal pellet analyses, however, provide a snapshot of the diet- only what was most recently consumed. Fatty acid lipid biomarkers provide a useful tool for investigating zooplankton nutrition and food-web interactions over longer time periods as they are indicative of the feeding history of an ¨ organism (e.g. Hakanson, 1984; Boer et al., 2005; DeMott and ¨ Muller-Navarra, 1997). Fatty acids are described here as x:y(n  z); where x ¼number of carbon atoms, y¼number of double bonds/ degree of unsaturation, and if unsaturated, n  z is used where z¼ the location of first double bond on the carbon chain after CH3. They are the main constituents of lipids and are vital for normal growth and development of zooplankton (Ackman et al., 1970; Sargent and Henderson, 1976; Sargent and Falk-Petersen, 1981). These compounds (or biochemicals) can be linked to certain taxa of zooplankton, algae, or bacteria based on structural features including number of double bonds, methyl group structure, and carbon chain length, and most polyunsaturated fatty acids are incorporated into the animal without much modification (Lee et al., 1971b; Graeve et al., 1994). As a result, fatty acids can be used to investigate diet history, nutrient upgrading, trophic transfer, and prey assimilation (reviewed by Dalsgaard et al., 2003). Fatty acid indicators for different taxa of plankton as identified from the literature, with their respective biochemical structural features, are shown in Table 1. For example, a comparatively high ratio of (n  3) to (n 6) polyunsaturated fatty acids (PUFA) (or low (n  6)/(n  3)) is a general indicator of herbivory in zooplankton and bivalve tissues. Fatty acids such as docosahexanoic acid (DHA, 22:6(n  3)), arachidonic acid (AA, 20:4(n 6)), and 22 carbon-chain (C22) PUFA are biomarkers for dinoflagellates, whereas eicosapentanoic acid (EPA, 20:5(n 3)), 16:1(n  7), 16:1/16:0, and 20 carbon-chain (C20) PUFA are biomarkers for diatoms. Zooplankon are high in oleic acid (OA, 18:1(n  9)) and therefore carnivores tend to accumulate this fatty acid in their tissue. Carnivorous zooplankton can also have a high ratio of 18:1(n  9) to18:1(n 7). Herbivorous

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calanoid copepods are uniquely able to biosynthesize the long chain monounsaturated fatty acids/alcohols 20:1(n  9) and 22:1(n 11), which if measured in other non-calanoid zooplankton, could be an indication of carnivory as well (reviewed in Dalsgaard et al., 2003). Sinking particles are microbial hotspots and a food source for mesopelagic zooplankton, but detrital particles can be difficult to recognize in guts (Gowing and Wishner, 1986; Alldredge and Silver, 1988; Gowing and Wishner, 1992; Azam and Long, 2001). As many larger, non filter-feeding zooplankton cannot directly feed on free-living microorganisms such as bacteria and picophytoplankton, zooplankton feeding on marine snow is a food web ‘shortcut’ (Lampitt et al., 1993a; Dilling et al., 1998). Fatty acids within zooplankton tissue that are specific to bacteria would provide evidence of feeding on items such as marine snow or microzooplankton. Experiments in which fatty acid biomarkers have been utilized specifically for indicating microbial/particle feeding are rare and have varied results based on location and interpretation (e.g., Desvilettes et al., 1997; Stevens et al., 2004c). Fatty acids with odd-numbers of carbon atoms such as 13:0, 15:0, and 17:0 or branched chain have been used as biomarkers for heterotrophic bacteria (see Table 1). Increasing values of MUFA and combinations of particular MUFAs have also been used as indicators for bacterial association/feeding (Pranal et al., 1996; Desvilettes et al., 1997). Previous studies show that the proportion of bacterial fatty acids in zooplankton is too high to derive from symbiotic bacteria alone and thus bacterial FAs must also originate from either ingestion of bacteria via microzooplankton or marine snow (Desvilettes et al., 1997). In the present study, the concentration and composition of FA associated with zooplankton and particles (a combination of both sinking and suspended, 451 mm) throughout the mesopelagic zone were measured to investigate diets of the zooplankton community in the subarctic and subtropical North Pacific Ocean. The study was part of the VERtical Transport In the Global Ocean (VERTIGO) project, designed to investigate the controls on the efficiency of particle export to the deep sea (Buesseler et al., 2007, 2008). We examined the relative importance of carnivory, herbivory, and particle feeding by mesopelagic zooplankton with depth in these two contrasting regions using specific fatty acid biomarkers and fatty acid ratios previously established as indicators of zooplankton diet. We also compared the fatty acids in bulk zooplankton to those of zooplankton with known dietary

Table 1 Listing of common biomarkers for zooplankton diet used in this study. n/a ¼ no common name for ratio or group of compounds, FA ¼fatty acids, PUFA¼ polyunsaturated fatty acids. Food source

Biomarker

Common name

Example references

General herbivory

(n  3)/(n 6)

n/a

Viso and Marty (1993), Desvilettes et al. (1997), Stevens et al. (2004a)

Diatoms

16:1(n  7) 20:5(n 3) C20 PUFA 16:1/16:0

Palmitoleic acid Eicosapentanoic acid (EPA) n/a n/a

Ackman et al. (1968), Viso and Marty (1993), Budge and Parrish (1998) Reuss and Poulson (2002)

Dinoflagellates

DHA/EPA 20:4(n 6) 22:6(n  3) C22 PUFA C18 PUFA

n/a Arachidonic acid (AA) Docosahexanoic acid (DHA) n/a n/a

Same as above

Heterotrophic bacteria

OBFA 18:1(n  7)

Odd or branched FA Vaccenic acid

Kaneda (1991), Wakeham (1995), Pranal et al. (1996), Graeve et al. (1997) Budge and Parrish (1998), Meziane and Tsuchiya (2000)

General carnivory

18:1(n  9) 18:1(n  9)/18:1(n 7)

Oleic acid (OA) n/a

Phleger et al. (1998), Cripps and Atkinson (2000), Nelson et al. (2001) Auel et al. (2002); Stevens et al. (2004b, c)

Calanoid copepods

20:1(n 9) 22:1(n  11)

Eicosenoic acid Docosenoic acid

Sargent and Henderson (1976), Kattner et al. (2003)

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habits to check the accuracy and utility of the selected biomarkers. Ultimately this information can be used to increase our understanding of how depth-related changes in zooplankton feeding ecology and particle composition can affect the biological pump.

2. Methods 2.1. Sample collection Zooplankton were collected between 0 and 1000 m using a 1 m2, 335 mm mesh, 10 net multiple opening/closing net and environmental sensing system (MOCNESS, Wiebe et al., 1985) and 7 net intelligent operative net sampling system (IONESS, similar to MOCNESS) at two contrasting sites in the North Pacific Ocean (Steinberg et al., 2008a). Vertical plankton tows were made at nine discrete depth intervals: 0–50, 50–100, 100–150, 150–200, 200–300, 300–400, 400–500, 500–750, and 750–1000 m. Paired day/night tows were conducted at the Hawaii Ocean Time seriesHOT station ALOHA in the oligotrophic N. Pacific subtropical gyre (27.751N, 1581W) June 22–July 9, 2004 aboard the R/V Kilo Moana, and at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) time-series site K2, in a high nutrient, variable chlorophyll region of the N. Pacific subarctic gyre (471N, 1601E) July 22–August 11, 2005, aboard the R/V Roger Revelle. At ALOHA, the base of the euphotic zone was 125 m (0.1% surface irradiance), water column-integrated net primary production was 180–220 mg C m  2 d  1, new production was 18–38 mg C m  2 d  1, mixed layer nutrients were at nanomolar concentrations, and the phytoplankton assemblage consisted of small diatoms, coccolithophorids, dinoflagellates, picoplankton, and cyanobacteria (Buesseler et al., 2007; Boyd et al., 2008; Buesseler et al., 2008; Elskens et al., 2008; Lamborg et al., 2008). At K2, the base of the euphotic zone (50 m) was shallower than at ALOHA; net primary production (302– 603 mg C m  2 d  1), new production (70–150 mg C m  2 d  1), and nutrients (12 mM mixed layer DIN) were all higher than at ALOHA, and the K2 phytoplankton assemblage consisted of picoplankton and large diatoms (Buesseler et al., 2007; Boyd et al., 2008; Buesseler et al., 2008; Lamborg et al., 2008). Copepods were the most abundant taxa at both sites followed by ostracods and chaetognaths (Steinberg et al., 2008a). Zooplankton biomass was an order of magnitude higher at K2 and dominated by the calanoid copepods Neocalanus spp. and Eucalanus bungi (Steinberg et al., 2008a). A total of 4 day/night MOCNESS/IONESS tows were conducted at each location. Upon net recovery, the 9 cod-ends were removed, stored in buckets, and processed within 2 h. The two pairs of day/night tows at each site were used for bulk zooplankton lipid analyses, and the data presented herein are an average of day and night samples. These samples were divided using a Folsom plankton splitter and 1/8, 1/16, or 1/32 of the total biomass and was rinsed with Milli-Q water to remove salts and stored at  80 1C until analysis. Additional net tows were made at ALOHA using a 1-m diameter opening and closing conical net with a 335-mm mesh and non-filtering cod end. Common mesopelagic zooplankton from both K2 and ALOHA were sorted from net tows. Species were selected based on their known or hypothesized dietary preferences and ecological importance within the region and included: chaetognaths (carnivores, at both sites), euphausiids (omnivores, at both sites), and assorted calanoid copepods (omnivores). Three of the calanoid copepod species at K2, Eucalanus bungii, Neocalanus cristatus, and Neocalanus plumchrus, are large-sized seasonal vertical migrators which feed upon phytoplankton and detrital aggregates, accumulating large stores of lipids in their bodies (e.g. Kobari et al., 2008b). The calanoid Paraeuchaeta sp. (carnivore,

both sites) was also analyzed. One set of multiple individuals was processed for each depth the taxa were present (n¼ 1 set per depth). Individuals were collected from 2 to 7 different depth intervals. The remaining sample was used for analysis of zooplankton gut contents, size-fractionated biomass, and enumeration of taxa (Kobari et al., 2008b; Steinberg et al., 2008a; Wilson et al., 2010). At both K2 and ALOHA, a combination of large (451 mm) suspended and sinking particles were collected simultaneously from 3 to 12 depths within the upper 1000 m (ca. 10, 35, 60, 85, 135, 185, 235, 315, 465, 565, and 765 m) using the multiple unit large volume filtration system (MULVFS), which consists of 12 specialized particulate matter pumps tethered to a 1000 m electrochemical cable and powered by ship’s electricity (Bishop et al., 1985; Bishop and Wood, 2008). The MULVFS collected three size fractions of particles, o1 mm, 1–51 mm, and 451 mm. MULVFS protocols for VERTIGO are described in detail by Bishop and Wood (2008). Particle samples acquired for this study were collected during two day-night deployments at each location, from approximately 1/8 of the large-fraction, 51 mm-polyester filter on the MULVFS. Volumes of water filtered represented by these subsamples range from 500 L in the euphotic zone to  1500 L in deeper waters. The 451 mm fraction was chosen for analysis as it includes large phytoplankton, aggregates, and suspended and sinking particles available to mesozooplankton. Below the euphotic zone, most of this material is sinking. Visible zooplankton were removed from filters, and particles were then rinsed onto pre-weighed GF/F filters, lightly rinsed again with Milli-Q water to remove salts, and frozen at 80 1C until further analysis. The data presented from MULVFS particles also represent are the averages of day and night samples. The larger particle feeders examined in the present study generally prefer food 420 mm, and to avoid sampling free-living bacteria from the water column, only the 451 mm sample data are presented in this paper.

2.2. Sample analysis Lipid analyses were conducted on zooplankton and sinking/ suspended particles to determine the relative importance of particle/detritus feeding, herbivory, and carnivory between different mesopelagic taxa and between the two sites. Bulk zooplankton splits from tows were thawed and homogenized using a Ultraturrax T-25 homogenizer. Two 1.5 ml aliquots were placed in test tubes, one was used for lipid extraction and the second was placed in a drying oven overnight at 50 1C to determine dry-weight for each aliquot. The sorted zooplankton species and the MULVFS particles on pre-weighed and combusted GF/F filters were freeze dried using a Virtis Alcatel 2008A freeze dryer. Dry weight of bulk zooplankton, sorted zooplankton species, and MULVFS particle samples was determined using a Sartorious BP211D microbalance. Lipids were extracted from both the second 1.5 ml bulk zooplankton aliquot and the freeze-dried samples using chloroform:methanol:water in the ratio 2:2:1 v/v/v (modified from Folch et al., 1957; Bligh and Dyer, 1959). Extracted samples were dried completely under nitrogen gas and weighed on the same microbalance to obtain mg of total lipid, then resuspended in 1 ml chloroform:methanol (1:1 v/v) and stored in a  20 1C freezer. A separate aliquot of the extracted lipid samples was utilized for fatty acid analysis. 20 mg of 23:0 FA internal standard was added to the lipid aliquot and subsequently derivatized to form fatty acid methyl esters (FAMEs) using boron trifluoride (BF3, Metcalfe and Schmitz, 1961) in a 100 1C circulating water bath. The FAMEs were then extracted with carbon disulfide (CS2, Marty

S.E. Wilson et al. / Deep-Sea Research I 57 (2010) 1278–1294

et al., 1992) and suspended in hexane for gas chromatographyflame ionization detection (GC-FID) analysis. FAMEs were analyzed on a Varian model 3800 GC-FID using a DB-WAX capillary column (25 m  0.32 mm; 0.2 mm film thickness; J&W Scientific, Folsom, CA). Identification of FAMEs was based on the comparison of their retention times with FA authenticated laboratory standards, and on FAMEs derived from phytoplankton, microzooplankton, and parasitic protist samples that were analyzed in both GC and GC/MS employing the same column. Each fatty acid peak was quantified based on the 23:0 internal standard and expressed as a percentage of the total fatty acid as well as the concentrations mg FA per unit carbon, (mg FA mg C  1) and total FA per unit volume (mg FA m  3 seawater). To determine total organic carbon content, duplicates of dried, pulverized bulk zooplankton samples were weighed (1–10 mg) in acetone-rinsed tin capsules and acidified using 10% HCl to remove inorganic carbon (Hedges and Stern, 1984). Total organic carbon (TOC) was measured using a Fisions CHN analyzer (Model EA 1108). Particulate organic carbon (POC) from MULVFS particle samples were measured using a Carlo Erba elemental analyzer (Bishop and Wood, 2008) using standard procedures described in Bishop et al. (1985). Along with the gravimetric measurements of total lipid, this information was used to calculate mg lipid mg C  1. 2.3. Data analysis Results are presented as depth profiles using the median depth for each MOCNESS net sampling interval. Average water column depths were also computed: since the sampling depth interval of the MOCNESS nets varied, depth-weighted averages were calculated for the ten most abundant fatty acids (expressed as percent total fatty acid) as well as the ratios and the sum of all saturated fatty acids (SFA), monounsaturated fatty acids (MUFA), and polyunsaturated fatty acids (PUFA). This was done using the equation Zooplankton depth-weighted average ð%, ratio, or mg lipid=mg CÞ X ¼ ðz2 z1 Þ x=ztot where z2 z1 is the depth interval of each net (m), x is the fatty acid value (%, ratio, or mg FA mg C  1), and ztot is the total depth interval (m) of interest. In order to compare the net tow zooplankton – collected in depth intervals, with the MULVFS particles – collected at single depths, a calculation was used to transform the single depth values into a depth interval for a depth-weighted average using trapezoidal integration in the equation: Particle depth-weighted average ð%, ratio,or mg lipid=mg CÞ X  ¼ ðx1 x2 Þ=2 ðz2 z1 Þ=ztot where x1  x2 is the difference in particle fatty acid values (%, ratio, or mg FA mg C  1) between depth intervals z2 z1 (m) and ztot is the total depth interval (m) of interest.

3. Results 3.1. Comparison of bulk zooplankton and particle fatty acids with depth and between sites Differences in total FA can reflect differences in zooplankton biomass, individual body sizes, and amount of lipid storage. Total FA concentration per unit body carbon (mg FA mg C  1) was considerably higher at K2 than ALOHA. On average, zooplankton FA was 7  (epipelagic zone; 0–150 m) to 19  (upper mesopelagic zone; 4150–500 m) higher at K2, and 15.5  larger at K2 for

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the entire 0–1000 m water column (Table 2). Peaks in total FA/body carbon were present in both the epipelagic and mesopelagic zones at K2 and ALOHA (Fig. 1A and B). For particle samples collected from 10 to 850 m depth, total FA/carbon was on average 7  (K2) and 2  (ALOHA) lower than for zooplankton (Table 2). Total FA/carbon generally decreased with depth for particles at ALOHA with some variability above 350 m and lower average concentrations at depth (Fig. 1A). Total FA/carbon for particles tracked the concentrations for zooplankton at K2 (although with much lower concentrations; Fig. 1B). Particles from 10 to 850 m contained on average 5  more mg FA mg C  1at K2 than at ALOHA (Table 2). Total FA/volume (mg FA m  3 seawater) in the epipelagic was 33  higher for zooplankton collected at K2 than at ALOHA, and 342–115  higher at K2 in the upper and lower mesopelagic, respectively (Table 2). At ALOHA, FA/volume for particles were 2  (epipelagic) to 4  (upper mesopelagic) higher than for zooplankton. In contrast, FA/volume for particles from K2 were up to 12  lower than for zooplankton (Table 2). At both locations, FA/volume for both particles and zooplankton were generally highest in the surface waters and decreased with depth, with the exception of a large peak between 200 and 400 m at K2 (Fig. 1D). Since concentrations of all individual major fatty acids were much higher at K2 than ALOHA, the remaining results are expressed as a proportion (%) of the total FA at each station. This presentation allows a better comparison of the differences in FA composition between the two sites. The depth-weighted average concentrations of the major FA for both zooplankton and particles are also presented in Table 2. For all depth zones, there were significant differences (paired ttest, p o0.05) in the contribution of a number of these fatty acids to the % total FA between sites and between zooplankton and particles at each site. For both the epipelagic and the upper+ lower mesopelagic zones combined, zooplankton at K2 had significantly higher proportions of 14:0, 20:1(n  9), 22:1(n  11), EPA, MUFA, C20 PUFA, 16:1/16:0, and (n 3)/(n  6) than ALOHA. Zooplankton at ALOHA had significantly higher 16:0, 18:0, OA, AA, DHA, SFA, PUFA (similar in epipelagic), OBFA, C22 PUFA, and the ratios DHA/ EPA, and 18:1(n  9)/18:1(n  7). These between-station trends were generally similar for particle samples collected over the entire depth range 0–1000 m. Particles collected from K2 had higher 16:0, 18:0, 16:1(n 7), 18:1(n  9), 18:1(n  7), SFA, OBFA, and DHA/EPA than K2 zooplankton while particles collected from ALOHA had higher 14:0, 18:0, and SFA than zooplankton.

3.2. Major fatty acids in individual zooplankton taxa The percent composition of the major FAs was also calculated for individual zooplankton taxa with known dietary preferences and was highly variable between species (Table 3). Statistically significant differences between taxa were observed for many FAs (one-way ANOVA, p40.05). Of the taxa analyzed, N. cristatus, N. plumchrus, and E. bungii are considered to be omnivorous, with diets changing seasonally from phytoplankton to microzooplankton plus sinking detritus (Kobari et al., 2003 and references therein). Both Neocalanus spp. were high in (n 3)/(n 6), EPA, and C20 PUFA, which are consistent with diatom sources. However, N. cristatus had significantly higher 20:1(n 9) and 22:1(n 11) than any of the other zooplankton groups analyzed. E. bungii also had similarly high C20 PUFA and EPA; however, ratios of (n3)/(n 6) were lower and 16:0 and 16:1(n 7) were higher than Neocalanus spp., suggesting some ingestion of diatoms. Omnivorous euphausiids were collected from both stations but %OBFA, DHA, DHA/EPA, and C22 PUFA were generally higher at ALOHA, suggesting higher consumption of bacteria (from sinking and suspended particles) and dinoflagellates

Zooplankton

ALOHA

K2 150–500 m

500–1000 m

150–1000 m

0–1000 m

Fatty acids (%) 14:0 16:0 18:0 16:1(n  7) 18:1(n  9) 18:1(n  7) 20:1(n  9) 20:4(n  6) 20:5(n  3) 22:1(n  11) 22:6(n  3) SFA MUFA PUFA OBFA C20 PUFA C22 PUFA

5.1 70.3 22.4 71.3 7.1 70.8 4.1 70.3 8.2 70.7 1.8 70.2 0.57 70.06 1.6 70.06 7.9 70.6 0.5 70.1 21.7 71.7 38.9 72.5 19.0 70.8 40.5 72.3 3.4 70.2 11.0 70.4 24.3 71.7

3.2 7 0.3 20.2 7 0.9 5.2 7 0.5 5.5 7 0.3 16.2 7 2.0 2.4 7 0.1 1.2 7 0.3 1.8 7 0.07 6.9 7 0.2 1.0 7 0.2 19.9 7 0.6 31.8 7 2.0 30.5 7 2.3 36.7 7 0.9 2.8 7 0.3 9.8 7 0.2 23.0 7 0.6

2.77 0.3 18.7 7 1.9 4.87 0.3 5.87 0.5 22.1 7 2.5 2.37 0.2 1.87 0.4 1.57 0.2 5.27 0.4 1.27 0.4 15.9 7 0.9 28.8 7 2.6 37.2 7 3.8 33.3 7 1.2 2.17 0.3 8.07 0.3 21.4 7 1.0

3.3 70.2 21.9 71.5 5.6 70.4 6.4 70.4 22.3 71.6 2.6 70.08 1.8 70.4 1.8 70.13 6.7 70.2 1.2 70.3 19.8 70.4 34.0 72.1 39.0 72.8 39.4 70.9 2.7 70.2 9.9 70.2 25.0 70.8

3.27 0.2 19.87 1.1 5.37 0.4 5.47 0.3 17.97 1.2 2.27 0.07 1.47 0.3 1.67 0.09 6.27 0.15 1.07 0.3 18.17 0.5 31.47 1.8 32.17 2.2 35.67 0.8 2.57 0.2 9.17 0.2 22.47 0.6

7.9 70.6 15.0 70.9 2.5 70.3 6.1 70.6 6.0 70.7 3.1 70.4 2.2 70.2 0.79 70.07 17.4 71.1 3.6 70.2 12.0 71.0 26.3 70.8 32.4 70.5 38.7 71.2 1.1 70.1 19.4 70.9 13.5 70.9

Fatty acid ratios DHA/EPA 18:1(n  9)/18:1(n  7) 16:1/16:0 (n  3)/(n  6)

2.7 70.02 5.1 70.7 0.25 70.02 6.0 70.4

2.9 7 0.06 6.9 7 1.1 0.35 7 0.02 5.3 7 0.2

3.17 0.05 10.07 1.0 0.467 0.11 3.97 0.5

3.4 70.02 9.9 70.6 0.47 70.08 5.1 70.4

3.07 0.02 8.27 0.3 0.397 0.06 4.77 0.3

Total fatty acid (mg/mgC) (mg/m3)

29.1 75.3 43.7 713.0

17.8 7 3.2 6.4 7 1.7

17.2 7 4.1 3.87 0.9

19.8 74.0 5.5 71.0

19.27 3.5 10.77 2.3

Fatty acids (%) 14:0 16:0 18:0 16:1(n  7) 18:1(n  9) 18:1(n  7) 20:1(n  9) 20:4(n  6) 20:5(n  3) 22:1(n  11) 22:6(n  3) SFA MUFA PUFA OBFA C20 PUFA

ALOHA

0–150 m

150–500 m

500–1000 m

150–1000 m

0–1000 m

9.77 0.4 11.9 7 0.9 2.17 0.2 6.07 0.5 7.07 0.1 2.47 0.3 1.97 0.2 0.67 0.08 14.1 7 1.2 6.37 1.4 8.87 0.7 24.6 7 1.1 38.2 7 1.1 33.1 7 0.7 0.87 0.06 16.3 7 1.1 10.37 0.6

8.7 7 0.3 7.0 7 0.3 1.6 7 0.1 4.7 7 0.2 7.3 7 0.4 2.0 7 0.2 3.3 7 0.3 0.4 7 0.03 10.4 7 0.9 10.1 7 1.5 9.5 7 0.5 18.2 7 0.7 48.0 7 2.5 30.9 7 1.4 0.9 7 0.02 12.8 7 1.2 10.5 7 0.4

10.4 70.10 10.2 70.5 2.0 70.2 5.9 70.13 8.1 70.2 2.4 70.3 3.1 70.3 0.53 70.06 13.5 71.1 9.7 71.5 10.4 70.6 23.6 70.8 49.8 71.8 36.1 70.8 0.9 70.03 16.1 71.3 11.8 70.5

9.07 0.10 9.97 0.5 1.97 0.2 5.37 0.2 7.07 0.2 2.37 0.3 2.77 0.2 0.527 0.04 12.8 7 0.9 7.87 1.2 9.67 0.4 21.6 7 0.7 42.2 7 1.3 32.9 7 0.6 0.97 0.03 15.07 1.0 10.97 0.4

0.7 70.04 2.0 70.2 0.49 70.05 14.6 71.3

0.77 0.11 4.77 1.5 0.607 0.04 13.1 7 0.7

0.9 7 0.12 4.0 7 0.6 0.77 7 0.05 13.5 7 1.0

0.94 70.13 4.8 71.1 0.80 70.03 15.1 70.9

0.817 0.10 3.97 0.8 0.677 0.03 13.5 7 0.7

203.9 748.4 1461 7291

333.6 7 36.7 2177 7 528.3

301.9 7 35.1 504.1 7 53.5

356.9 734.7 1352 7267

298.3 7 21.3 1233 7 174

K2

10–150 m

150–475 m

475–850 m

150–850 m

10–850 m

9.97 0.3 25.17 1.2 6.97 0.8 5.97 0.2 7.47 0.4 2.37 0.04 0.147 0.10 1.87 0.06 6.97 0.3 0.277 0.19 16.87 1.1 45.57 0.3 17.67 0.3 36.57 0.02 3.37 0.2 9.57 0.2

6.2 7 1.0 23.3 7 1.6 8.8 7 1.0 5.4 7 0.5 12.2 7 1.9 2.8 7 0.4 0.6 7 0.2 1.1 7 0.2 5.9 7 0.4 2.4 7 2.3 14.4 7 1.1 41.9 7 2.5 26.3 7 4.0 28.9 7 2.3 2.9 7 0.3 8.0 7 0.5

3.57 0.3 18.6 7 1.0 11.8 7 0.7 4.17 0.3 13.8 7 1.2 2.87 0.3 1.87 0.8 0.97 0.2 3.47 0.3 0.57 0.2 7.97 1.4 37.5 7 0.8 29.07 3.6 24.3 7 2.8 2.87 0.4 8.07 0.3

4.47 0.2 21.27 1.6 10.87 0.3 4.77 0.5 13.97 1.2 2.97 0.3 1.27 0.5 0.947 0.14 4.37 0.3 1.47 1.1 10.57 0.8 39.97 1.7 28.67 2.9 25.77 1.3 2.87 0.3 8.07 0.3

4.7 70.3 21.5 71.6 10.5 70.3 4.9 70.4 13.6 71.4 2.9 70.3 1.1 70.4 1.0 70.2 4.5 70.2 1.4 71.1 11.0 70.8 40.3 71.7 28.1 73.0 26.3 71.6 2.8 70.3 8.1 70.4

10–150 m

8.67 1.0 17.9 7 0.7 3.17 0.4 8.97 0.3 5.67 0.3 2.37 0.4 0.787 0.1 2.37 0.4 16.2 7 1.1 0.507 0.1 11.9 7 1.1 31.7 7 0.7 24.3 7 1.2 41.6 7 1.9 1.37 0.05 18.9 7 1.2

150–475 m

5.1 70.9 15.1 70.6 4.9 70.9 6.6 70.2 13.0 70.8 2.8 70.1 2.4 70.2 2.8 70.1 9.2 70.3 3.4 70.4 10.1 70.6 26.5 71.5 39.8 71.5 30.9 70.5 1.5 70.04 12.1 70.3

475–850 m

150–850 m

10–850 m

4.4 7 0.3 14.2 7 1.2 5.2 7 1.0 7.9 7 0.6 14.9 7 1.5 3.3 7 0.3 3.1 7 0.3 3.3 7 0.3 5.6 7 0.3 6.5 7 0.5 7.6 7 0.5 25.07 2.0 49.4 7 3.0 23.6 7 1.3 1.4 7 0.2 8.4 7 0.4

4.77 0.5 14.67 0.9 5.07 0.9 7.17 0.4 14.17 1.0 3.07 0.2 2.77 0.2 3.07 0.2 7.77 0.1 4.77 0.3 9.07 0.3 25.67 1.7 44.17 2.2 27.87 0.6 1.47 0.11 10.67 0.4

5.3 70.6 15.1 70.8 4.7 70.8 7.4 70.4 12.7 70.9 2.9 70.2 2.4 70.2 2.9 70.2 9.1 70.2 4.0 70.3 9.4 70.3 26.6 71.5 40.9 71.9 30.0 70.7 1.4 70.10 11.9 70.5

S.E. Wilson et al. / Deep-Sea Research I 57 (2010) 1278–1294

0–150 m

451 lm particles

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Table 2 Major individual fatty acids (FA) as a proportion of total FA, FA ratios, and the total FA concentration for both zooplankton and 4 51 mm particles at ALOHA and K2. Data are presented for the epipelagic zone (0–150 m, which spans the different euphotic zone depths at K2 and ALOHA), upper mesopelagic (150–500 m), lower mesopelagic (500–1000 m), total mesopelagic (150–1000 m), and the 0–1000 m water column. For zooplankton, depthweighted averages (7 standard error) are presented. Values for particles are trapezoidal integrated averages ( 7 standard error). SFA¼ saturated fatty acids, MUFA¼ monounsaturated fatty acids, PUFA ¼polyunsaturated fatty acids, OBFA¼ iso/anteiso 13:0, 15:0, and 17:0 SFA. C20,22¼20,22 carbons in fatty acid. n¼4 for both K2 and ALOHA zooplankton and particles. All fatty acid data will be available on the VERTIGO project data section of the Biological and Chemical Oceanography Data Management Office website (BCO-DMO)—http://bcodmo.org/home.

39.37 7.6 1417 26 17.7 7 3.5 42.8 7 8.0

40.0 76.9 199 727

1.27 0.04 4.77 0.4 0.617 0.08 6.87 0.4 1.4 7 0.08 4.7 7 0.7 0.67 7 0.11 6.07 0.6

1.1 70.03 4.4 70.4 0.61 70.07 7.0 70.3

10.57 0.3 8.8 7 0.6

11.0 70.3

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than at K2. Euphausiids at K2 had significantly higher EPA and 22:1(n 11) than ALOHA (indicating both diatoms and calanoid copepods in their diet). Euphausiids at both sites had moderate (n 3)/(n 6), 18:1(n9), and C20 and C22 PUFA, providing additional evidence of their omnivorous diet. Carnivorous chaetognath FAs were generally similar to the FA content of their presumed prey (Neocalanus spp.). However, chaetognaths had higher %18:1(n9) than Neocalanus spp., as is typical of carnivores. Paraeuchaeta sp. are generally considered carnivorous copepods. They had a high %MUFA, 18:1(n9), 20:1(n 9), 22:1(n 11), 18:1(n 9)/18:1(n 7), and low ratio of (n 3)/(n 6), all indicative of carnivorous feeding. The miscellaneous large copepods collected at ALOHA had high %18:1(n 9) and lower ratios of (n 3)/(n 6), which may reflect an omnivorous diet or consumption of other copepods ranging from herbivorous to carnivorous.

58.4 715.2 227.3 754.8 43.5 7 11.7 499.8 7 95.4 8.4 71.3 24.1 73.7 6.67 1.6 11.07 2.8 9.2 7 3.0 26.2 7 6.7 11.17 2.5 95.97 16.1

8.17 1.2 18.37 2.6

1.1 70.04 4.7 70.2 0.54 70.04 7.4 70.3 0.757 0.02 2.77 0.4 0.637 0.04 7.77 0.6 2.3 70.13 4.8 70.5 0.31 70.02 4.1 70.9 2.37 0.13 4.97 0.5 0.317 0.03 4.17 0.9 2.27 0.2 5.17 0.8 0.347 0.05 4.67 0.7 2.4 7 0.06 4.4 7 0.5 0.28 7 0.01 4.1 7 1.2 2.47 0.07 3.37 0.1 0.287 0.02 4.47 0.6

A high ratio of (n 3) to (n6) PUFA is a general indicator for herbivory. This ratio was higher at K2 than ALOHA for bulk/mixed zooplankton (Fig. 2A). At ALOHA, the ratio was relatively constant throughout the water column and decreased slightly in the deepest sample. In contrast the (n 3)/(n 6) ratio was constant in the euphotic zone at K2 and decreased in the mid and lower mesopelagic (Fig. 2A). An increase in the (n3)/(n 6) ratio from 11.5 at 200–300 m to 14.6 at 750–1000 m depth at K2 may be an indication of elevated feeding on sinking/suspended particles which can include aggregates of senescent phytoplankton cells. The (n 3)/ (n 6) ratio for individual calanoid copepods at both sites is shown in Fig. 2B for comparison. Neocalanus species at K2 had the highest ratios and did not change considerably in the mesopelagic zone, suggesting these species may also be feeding on sinking/suspended particles. The (n 3)/(n 6) ratio also decreased with depth for Paraeuchaeta sp. at ALOHA, possibly indicating a switch in feeding mode from omnivory to carnivory with depth. E. bungii at K2 was intermediate between Neocalanus spp. and Paraeuchaeta sp. Miscellaneous large calanoid copepods from ALOHA had low (n 3)/(n 6) ratios suggesting carnivory as well. Although there was a diverse assemblage of phytoplankton groups at ALOHA and K2 (Boyd et al., 2008), we used several diatom and dinoflagellate biomarkers as examples to explore zooplankton herbivory in more detail. The %C20 PUFA and 16:1/ 16:0 ratio were used as an index for diatom sources (Fig. 3A and B) and %C22 PUFA and DHA/EPA ratio were used as an index for dinoflagellates (Fig. 3C and D). Conversely, a low DHA/EPA ratio can also be used as an index for diatoms. Both %C20 PUFA and 16:1/16:0 were higher at K2 (ANOVA, p o0.05), consistent with the greater importance of diatoms at K2 vs. ALOHA (Fig. 3A and B). The DHA/EPA ratio was close to 1 and the 16:1/16:0 was below 1 at K2, suggesting there is a dinoflagellate source of nutrition in the zooplankton at K2 as well (Fig. 3B and D). In contrast, %C18 (which also occurs in haptophytes), C22 PUFA, and DHA/EPA were higher at ALOHA than at K2 (ANOVA, p o0.05), consistent with a higher dinoflagellate source for the zooplankton residing in both the epipelagic and mesopelagic zones at ALOHA compared to K2 (Table 2, Fig. 3). Values for both dinoflagellate indices were relatively constant with depth (Fig. 3C and D). 3.4. Depth profiles of fatty acid indicators of carnivory/omnivory

Total fatty acid (mg/mgC) (mg/m3)

16.6 7 1.6

10.27 2.3 20.37 0.05 C22 PUFA

Fatty acid ratios DHA/EPA 18:1(n  9)/18:1(n  7) 16:1/16:0 (n  3)/(n  6)

12.77 1.0

13.1 71.2

13.2 7 1.0

11.9 70.6

3.3. Depth profiles of fatty acid indicators of herbivory

OA (18:1(n 9)), which is found in larger proportions in carnivorous zooplankton, increased with depth in mixed bulk zooplankton at ALOHA, and peaked at both 100–200 m and 400–500 m at K2 (Fig. 4A), which corresponds to above and below the midwater peak in K2 zooplankton biomass (Steinberg et al.,

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ALOHA Total FA (µg/mgC) 10

20

30

0

40

0

0

100

100

200

200

300

300

400

400 Depth (m)

Depth (m)

0

K2 Total FA (µg/mgC)

500

600

700

700 Zooplankton

400

500

4000

5000

600

Particles

Particles 900

1000

1000 ALOHA Total FA (µg/m sw)

K2 Total FA (µg/m3 sw)

3

0

50

100

150

200

250

0

300

0

0

100

100

200

200

300

300

1000

2000

3000

400 Zooplankton Particles

Depth (m)

400

Depth (m)

300

Zooplankton

800

900

500

200

500

600

800

100

500 Zooplankton

600

600

700

700

800

800

900

900

1000

1000

Particles

Fig. 1. Depth profiles of total fatty acid (FA) concentration in zooplankton and 451 mm particles (mg/mg C) at (A) ALOHA and (B) K2, and (mg/m3) at (C) ALOHA and (D) K2. For zooplankton, values are plotted at the median depth of each net sampling interval. For particles, values are plotted at actual depth of sampling of the MULVFS. sw¼ seawater, error bars ¼ 1 standard error, n¼4 for both zooplankton and particles.

2008a). The carnivorous copepods (e.g., Paraeuchaeta sp. at K2, large copepods at ALOHA) were significantly higher in OA than the Neocalanus spp. at K2 (ANOVA, p o0.05, Fig. 4B). In the middle range were E. bungii at K2 (Fig. 4B). The relative percentages of OA in Paraeuchaeta sp. also increased with depth. The miscellaneous large copepods collected from ALOHA contained similar proportions of OA as Paraeuchaeta sp. below 300 m. Like OA, the omnivory ratio 18:1(n  9)/18:1(n  7) was also higher at ALOHA than at K2 (Table 2). The long chain MUFAs, 20:1(n 9) and 22:1(n 11), indicative of calanoid copepods, were both higher at K2 than ALOHA in the bulk zooplankton and increased through the mesopelagic (Fig. 5A). Chaetognaths, as well as the calanoids N. cristatus, N. plumchrus, and Paraeuchaeta

sp., were also elevated in these MUFAs (Fig. 5B). OA and 20:1(n  9)+ 22:1(n  11) in particle samples at both locations increased with depth throughout the mesopelagic, indicating an increase in zooplankton-derived material (e.g., fecal pellets, Figs. 4C and 5C).

3.5. Depth profiles of fatty acid indicators of particle feeding Although a minor component of the total fatty acid concentration, bacterial fatty acids in zooplankton provide a useful index for particle feeding. Bacterial fatty acid biomarkers examined in zooplankton and particles include OBFA, 18:1(n  7), and %MUFA

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Table 3 Major individual fatty acids (FA) as a proportion of total FA, as FA ratios, and the total FA concentration for individually sorted zooplankton taxa from K2 and ALOHA. Values are depth-weighted averages (7 standard error) for zooplankton collected between 0 and 1000 m. n¼ number of replicates for each taxa, SFA¼ saturated fatty acids, MUFA¼ monounsaturated fatty acids, PUFA¼polyunsaturated fatty acids, OBFA¼ iso/anteiso 13:0, 15:0, and 17:0 SFA. C20,22¼ 20,22 carbons in fatty acid. K2

ALOHA

Chaetognaths (n¼ 7) Fatty acids (%) 14:0 16:0 18:0 16:1(n  7) 18:1(n  9) 18:1(n  7) 20:1(n  9) 20:4(n  6) 20:5(n  3) 22:1(n  11) 22:6(n  3) SFA MUFA PUFA OBFA C20 PUFA C22 PUFA Fatty acid ratios DHA/EPA 18:1(n  9)/ 18:1(n 7) 16:1/16:0 (n  3)/(n  6)

Paraeuchaeta sp. (n¼ 5)

Euphausiids (n¼ 2)

N. plumchrus (n¼ 7)

lg. copepods (n¼ 4)

Euphausiids (n¼ 2)

1.27 0.3 14.07 1.1 9.37 2.2 13.4 7 1.4 23.6 7 1.1 1.47 0.1 1.97 0.2 0.237 0.01 4.27 1.2 5.17 1.3 5.97 1.5 25.2 7 2.9 54.3 7 5.2 15.9 7 3.0 0.697 0.1 5.07 1.3 6.27 1.6

4.2 7 1.0 22.5 7 2.2 1.6 7 0.09 4.8 7 0.9 9.9 7 0.8 5.7 7 0.1 1.2 7 0.4 1.3 7 0.4 20.6 7 1.6 1.3 7 0.3 11.4 7 4.6 29.2 7 3.3 26.8 7 3.7 41.3 7 7.9 0.69 7 0.1 23.1 7 2.1 12.4 7 4.7

13.6 7 0.7 9.07 0.6 4.6 7 0.15 2.7 7 0.2 1.2 7 0.1 1.1 7 0.1 0.937 0.11 0.527 0.03 13.3 7 0.9 4.2 7 0.5 6.5 7 1.5 28.2 7 0.5 16.07 0.5 34.7 7 2.4 1.1 7 0.1 16.9 7 0.9 7.3 7 1.5

7.8 70.2 6.8 70.14 3.7 70.4 2.2 70.09 1.1 70.04 1.1 70.03 3.4 70.2 0.60 70.06 16.5 70.8 10.4 70.9 6.2 70.3 19.4 70.6 25.0 71.1 37.9 71.3 1.2 70.1 20.1 70.9 7.6 70.3

6.5 70.2 24.7 70.5 2.0 70.2 16.6 70.4 6.2 70.05 6.5 70.3 0.47 70.02 0.95 70.04 21.4 70.3 0.3 70.01 4.7 71.0 33.6 70.9 32.9 70.1 32.7 71.0 0.25 70.04 23.5 70.3 5.6 70.9

4.1 7 1.3 19.2 7 5.2 9.2 7 4.1 4.7 7 0.6 18.7 7 4.0 2.1 7 0.13 1.0 7 0.4 1.5 7 0.14 7.4 7 0.9 0.7 7 0.2 15.1 7 3.3 35.2 7 3.1 30.9 7 5.1 31.5 7 3.5 2.7 7 0.5 9.7 7 1.3 16.7 7 3.4

5.0 7 0.5 20.7 7 0.3 3.3 7 0.4 3.9 7 0.6 9.5 7 0.2 3.5 7 0.12 0.807 0.08 4.0 7 0.15 13.3 7 0.2 0.0 7 0.0 23.4 7 0.6 32.0 7 0.1 18.5 7 0.8 48.6 7 0.5 3.4 7 0.1 17.8 7 0.03 25.6 7 0.6

1.0 70.1 4.0 70.9

1.47 0.1 16.8 7 1.0

0.5 70.2 1.8 7 0.2

0.477 0.07 1.1 7 0.07

0.38 70.02 1.0 70.05

0.22 70.04 1.0 70.03

2.1 7 0.3 9.3 7 2.3

1.8 7 0.07 2.7 7 0.04

0.27 7 0.01 8.8 7 0.6

0.477 0.02 15.9 7 1.5

0.48 70.02 16.5 70.4

0.72 70.01 9.0 70.3

0.5 7 0.3 5.5 7 0.6

0.21 7 0.03 5.0 7 0.1

174.9 7 28.4 158.6 7 51.1

159.4 76.0 630.5 7103

142.1 749.6 71.3 726.5

85.4 7 25.2 94.6 7 62.3

24.9 7 1.2 29.3 7 0.8

1.17 0.1 8.47 2.3

Total fatty acid mg/mg dry weight 75.2 7 9.1 mg/mg individual n/d

159.4 7 17.7 278.1 7 60.1

107.9 7 45.6 n/d

Bulk Zooplankton (n-3)/(n-6) 5

10

15

Calanoid Copepods (n-3)/(n-6) 0

20

0

0

100

100

200

200

300

300

400

400 Depth (m)

Depth (m)

E. bungii (n¼3)

6.7 7 2.4 12.8 7 0.9 3.6 7 0.9 7.8 7 0.7 7.5 7 1.5 1.9 7 0.2 2.2 7 0.3 0.57 7 0.05 12.7 7 1.6 4.5 7 1.1 12.7 7 1.9 24.0 7 2.9 33.1 7 3.3 32.8 7 3.2 1.2 7 0.1 14.1 7 1.7 13.9 7 1.8

0.78 7 0.08 15.4 7 1.3

0

N. cristatus (n ¼6)

500

500

600

600

700

700

5

10

15

20

25

N. plumchrus N. cristatus E. bungii. Paraeuchaeta spp. lg. calanoids

ALOHA 800

K2

800

900

900

1000

1000

Fig. 2. Herbivory biomarkers vs. depth. Depth profiles of (n  6)/(n  3) polyunsaturated fatty acids (PUFA) herbivory index for (A) bulk zooplankton (error bars ¼ 1 standard error, n ¼4), and (B) calanoid copepods at ALOHA and K2 (n ¼1). Values are plotted at the median depth of each net sampling interval. For (B), dashed lines indicate carnivorous taxa, solid lines indicate omnivorous taxa. N, Neocalanus. ALOHA ¼ lg. calanoids. K2 ¼N. plumchrus, N. cristatus, Eucalanus sp., and Paraeuchaeta sp.

(although MUFA are not exclusive to bacteria). OBFA and 18:1(n  7) made-up a small percentage of the total FA in all samples (Table 2), and there was no difference in the 18:1(n  7)

between stations in both bulk zooplankton and particles. However, OBFA was significantly higher (t-test, p o0.05) in zooplankton tissue throughout the mesopelagic and epipelagic zone at

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0

% C20 PUFA 10 15

5

20

0.0 0

25

100

100

200

200

300

300

400

400

Depth (m)

Depth (m)

0

500 ALOHA 600

16:1/16:0 0.4 0.6

0.2

1.0

500 ALOHA

600

K2

0.8

K2 700

700

800

800

900

900

1000

1000 DHA/ EPA

% C22 PUFA 5

10

15

20

25

0.0

30

0

0

100

100

200

200

300

300

400

400

Depth (m)

Depth (m)

0

500 600

ALOHA

0.5

1.0

1.5

2.0

2.5

3.0

3.5

500 600

K2 700

700

800

800

900

900

1000

1000

ALOHA K2

Fig. 3. Diatom and dinoflagellate biomarkers vs. depth. (A) Bulk zooplankton mean %C20 polyunsaturated fatty acid (PUFA) (diatom biomarker) of total fatty acid, (B) bulk zooplankton 16:1/16:0 ratio (diatom biomarker), (C) bulk zooplankton mean %C22 PUFA (dinoflagellate biomarker) of total fatty acid, and (D) docosahexanoic acid/ eicosapentanoid acid (DHA/EPA) ratio (dinoflagellate biomarker). Values are plotted at the median depth of each net sampling interval. Error bars ¼ 1 standard error, n¼ 4.

ALOHA vs. K2, and similar at ALOHA between the particles and zooplankton (Table 2). For the individual taxa, E. bungii at K2 and copepods at ALOHA were enriched in OBFA relative to the other species sampled (ANOVA, p o0.05). E. bungii at K2 and euphausiids at both sites were also higher in 18:1(n 7) than other species measured (Table 3). %MUFA in zooplankton was higher at K2 than ALOHA through 1000 m (Table 2) and increased significantly (ANOVA, p o0.05) with depth at both sites (Fig. 6A and B). %MUFA in both particles and zooplankton were similar with depth at ALOHA until 750–1000 m, where MUFA was slightly higher in zooplankton than in particles (Fig. 6A). %MUFA was similar in zooplankton and particles throughout the water column at K2 with the exception of the epipelagic zone where %MUFA was

higher in zooplankton (Table 2, Fig. 5B). MUFA was similar in individual taxa analyzed at both sites with the exception of Paraeuchaeta spp, due to higher OA, which is also a MUFA.

4. Discussion 4.1. Bulk zooplankton total FA The occurrence of higher fatty acid concentrations in bulk zooplankton at K2 is a reflection of their higher biomass, body sizes, and lipid storage at this site. Changes in zooplankton FA concentration, expressed as mg per unit volume (mg m  3

S.E. Wilson et al. / Deep-Sea Research I 57 (2010) 1278–1294

Bulk Zooplankton % 18:1(n-9) 5

10

15

20

25

30

0

35

0

0

100

100

200

200

300

300

400

400 Depth (m)

Depth (m)

0

500

5

Calanoid Copepods % 18:1(n-9) 10 15 20 25 30

600

700

700

N. plumchrus N. cristatus E. bungii Paraeuchaeta spp. lg. calanoids

ALOHA 800

K2

35

500

600

800

1287

900

900

1000

1000

Particles % 18:1(n-9) 0

5

10

15

20

25

30

35

0 100 200 300

Depth (m)

400 500 600 700

ALOHA K2

800 900 1000 Fig. 4. Carnivory/omnivory biomarkers vs. depth. (A) Bulk zooplankton, (B) calanoid copepod, and (C) particle mean carnivory/omnivory biomarker % oleic acid 18:1(n  9) of total fatty acid vs. depth. Values are plotted at the median depth of each net sampling interval (error bars ¼ 1 standard error, n¼4). For (B), dashed lines indicate carnivorous taxa, solid lines indicate omnivorous taxa. N, Neocalanus. ALOHA ¼ lg. calanoids. K2 ¼N. plumchrus, N. cristatus, Eucalanus sp., and Paraeuchaeta sp., n ¼1.

seawater), with depth also reflected the considerably higher zooplankton biomass at K2 (Steinberg et al., 2008a). The majority of the zooplankton biomass was o2 mm in size at station ALOHA and 42 mm at station K2, due to the high numbers of large Neocalanus spp. copepods at K2 present at all depths (Kobari et al., 2008b; Steinberg et al., 2008a). Neocalanus spp. and E. bungii at K2 were beginning their seasonal ontogenetic vertical migrations to

depth during our study (Tsuda et al., 1999; Kobari et al., 2008b; Steinberg et al., 2008a) and are well known to store lipids for their winter diapause and spawning (Tsuda et al., 2001). Large lipid globules obtained by summer feeding were clearly visible in these species at K2 and contributed to their higher total lipid and total FA content, and thus the high lipid and FA content of bulk zooplankton. Lipid globules in copepods were not clearly visible at

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ALOHA, however particle FA concentration (mg FA m  3 seawater) was much higher at ALOHA vs. K2. Higher FA concentration in particles at ALOHA suggest that mesopelagic zooplankton can derive a steady supply of essential nutrition from a food source that is less seasonal than at K2, as suggested by Conte et al. (1998) for sinking particles in the Sargasso Sea. Zooplankton FA concentration expressed as per unit carbon (mg FA mg C  1) was variable with depth at both locations, peaking at two distinct depths at K2 (epipelagic and 200– 400 m) and ALOHA (euphotic/upper epipelagic and 400–750 m).

The double peaks in zooplankton FA concentration at K2 mirrored the abundance of several individual zooplankton taxa; calanoid copepods (mostly Neocalanus spp.), ostracods, chaetognaths, and hydrozoan medusae had bimodal distributions at K2 at similar depths (Steinberg et al., 2008a). Other taxa at K2 such as radiolaria, pteropods, salps, and doliolids also peaked in abundance at 200–500 m. Similar to zooplankton FA trends in concentration at ALOHA, most individual species abundances at ALOHA decreased with depth and a few (including cyclopoid and harpacticoid copepods, gammarid amphipods, and mysids)

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increased slightly in abundance at 500–750 m (Steinberg et al., 2008a). At K2, these double FA peaks likely come from calanoid copepods with large oil sacs such as Neocalanus and Eucalanus spp. that were abundant at these depths and beginning their ontogenetic vertical migrations to depth (Dagg, 1993; Kobari and Ikeda, 1999, 2001; Shoden et al., 2005). At ALOHA, although these peaks are small, they could be indicative of vertical migrators coming from both the mesopelagic and from 41000 m (Steinberg et al., 2008a). These deeper-residing taxa would likely have a greater stored lipid concentration (in the form of wax esters) than shallower-residing zooplankton (Lee et al., 1971a; Lee and Hirota, 1973). Total FA concentrations from the bulk zooplankton samples collected at night above 400 m at ALOHA were indeed slightly higher than those collected during the day, but in general day-night differences in zooplankton FA at both stations were not statistically significant (data not shown).

4.2. Modes of feeding—herbivory The contrasting community composition of the primary producers between the two sites was reflected in the diets of the mesozooplankton. At the times of our study, ALOHA was dominated by cyanobacteria (Prochlorococcus spp. and Synechococcus spp.) and dinoflagellates, and K2 was dominated by both cyanobacteria (Synechococcus spp.) and larger diatoms (Chaetoceros spp. and Coscinodiscus spp.), and to a lesser extent dinoflagellates (Boyd et al., 2008; Buesseler et al., 2008; Zhang et al., 2008). There was a higher herbivory ratio [(n  3)/(n  6) PUFA] at K2 vs. ALOHA, likely due to higher phytoplankton productivity and biomass in the form of large diatoms at K2 (Boyd et al., 2008). Higher herbivory ratios in calanoid copepods have been reported in areas of high diatom production in the Arctic (Stevens et al., 2004a). A comparatively large proportion of (n  6) PUFA in grazers can be due to the consumption of both bacteria

and ciliates (Desvilettes et al., 1997). Therefore the lower (n  3)/ (n 6) ratio at ALOHA may be due to a greater influence of either bacteria or ciliates in zooplankton diets there. Many studies do not use the general (n  3)/(n  6) ratio but instead relate the more specific biomarkers such as EPA, DHA, and C18PUFA–C22PUFA to infer dinoflagellate vs. diatom diets. However Viso and Marty (1993) proposed the use of (n 6)/(n  3) as an index of nutritional value in bivalves in conjunction with the other more specific biomarkers such as EPA and DHA. Group-specific phytoplankton trophic biomarkers were utilized to determine if dinoflagellate- or diatom-based nutrition dominated at each location. This approach is supported by Graeve et al. (1994) who used feeding experiments to show that herbivorous calanoid copepods in the Arctic fed either diatoms or dinoflagellates were capable of completely replacing dinoflagellate biomarkers with diatom biomarkers, and vice versa, within 24–42 days. EPA and DHA made-up the highest proportions of individual fatty acids in bulk zooplankton tissue at K2 and ALOHA indicating diatoms and dinoflagellates were an important component of zooplankton nutrition at both sites and at all depths. Although the ratio DHA/EPA in zooplankton tissue indicated dinoflagellate feeding was important at ALOHA and diatom feeding was important at K2, dinoflagellates were also a substantial dietary component at K2. Diatom-derived C20 PUFA and 16:1/16:0 ratio, both higher in the bulk zooplankton tissue at K2 than ALOHA, also provide evidence of a diatom-origin diet at K2. At depth, these PUFA were likely obtained via vertical migration or feeding on sinking aggregates (see below). Although cyanobacteria were abundant in the water column (Zhang et al., 2008), and consistently found in zooplankton guts at both locations (Wilson et al., 2010), they may be a poor food ¨ ¨ source for zooplankton (Muller-Navarra et al., 2000; MullerNavarra et al., 2004). Cyanobacteria contain less PUFA and essential fatty acids and therefore are less nutritious than diatoms or dinoflagellates (Viso and Marty, 1993; Desvilettes et al., 1997).

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In feeding experiments, the freshwater cladoceran Daphnia was fed a diet of Synechococcus and other autotrophic picoplankton to determine if these crustaceans could get their essential fatty acids from these small cells (Desvilettes et al., 1997; Bec et al., 2006). The consumption of these species resulted in poor growth and fecundity without the addition of other phytoplankton (DeMott ¨ and Muller-Navarra, 1997; Desvilettes et al., 1997) or via trophic upgrading through an intermediate grazer (Bec et al., 2003; Bec et al., 2006). There is also speculation that cyanobacteria are resistant to digestion by zooplankton and pass through their guts still viable (Silver and Alldredge, 1981; Lampitt et al., 1993b). Of the specific taxa analyzed, N. cristatus, N. plumchrus, and E. bungii are considered omnivorous with diets changing seasonally from phytoplankton to microzooplankton and sinking particles (Dagg, 1993; Kobari and Ikeda, 1999, 2001; Kobari et al., 2003; Shoden et al., 2005; Kobari et al., 2008a). The high (n  3)/(n 6) ratios in Neocalanus spp. are consistent with a more herbivorous diet in the late summer at K2 that included diatoms (e.g., Kobari and Ikeda, 1999, 2001), as indicated by their high EPA and C20 PUFA content. A high %EPA and C20 PUFA in Neocalanus spp. was generally consistent throughout the 1000 m depth profile. Mesopelagic Neocalanus spp. are likely obtaining these diatom biomarkers through particle feeding as mentioned above. E. bungii, however, contained lower (n  3)/(n  6) ratios, higher oleic acid, as well as higher 16:1(n  7), EPA, and C20 PUFAs, indicating an omnivorous diet with a large diatom influence, consistent with feeding on sinking particles (e.g., Shoden et al., 2005).

4.3. Modes of feeding—carnivory/omnivory Carnivores are an abundant component of the mesopelagic food web and carnivory is essential (along with diel vertical migration) for satisfying the metabolic demands of deeper fauna (Steinberg et al., 2008b). Carnivores and omnivores tend to accumulate particular MUFA in their tissue, therefore the high relative proportions of OA 18:1(n  9), 18:1(n  9)/18:1(n  7), as well as 20:1(n  9) and 22:1(n  11) (in non-calanoid zooplankton) is consistent with the increasing importance of carnivory. Mayzaud et al. (1999) determined that the arctic euphausiid Meganyctiphanes norvegica was a carnivore from its comparatively high percentages of OA, 20:1(n 9), and 22:1(n  11). Mayzaud et al. (2007) further demonstrated carnivory in select zooplankton species analyzed in a subtropical station compared to a subantarctic one using these same fatty acid biomarkers. In the subtropical station, an increase in the importance of carnivory was related to a lack of algal food (Mayzaud et al., 2007). Similarly, OA and 18:1(n  9)/18:1(n  7) indices were both higher at ALOHA than at K2, likely also due to a greater reliance on carnivory due to lower primary productivity at our subtropical site. The increase in OA and/or 20:1(n  9) +22:1(n  11) biomarkers with depth at ALOHA also signifies the increasing importance of carnivory with depth (Figs. 4a and 5a) and is consistent with an increase with depth in the proportion of ALOHA zooplankton 45 mm, the fraction containing many of the large carnivores (Fig. 8; Steinberg et al., 2008a). This increasing dependence on carnivory with depth has also been shown in the Arctic, where the amphipod Themisto abyssicorum had a higher 18:1(n  9)/ 18:1(n  7) than its congener Themisto libellula. T. abyssicorum is distributed deeper and preys upon other carnivorous zooplankton whereas the surface-dwelling T. libellula were preying upon diatom-feeding copepods (Auel et al., 2002). The lower proportions of 20:1(n  9) and 22:1(n  11) at ALOHA likely result from a lack of exclusively herbivorous feeding calanoid copepods in this region; however, like K2, the proportion of these FAs do increase

by  40% from 25 to 1000 m, supporting the hypothesis of an increase in carnivory with depth. At K2, there were distinct peaks in the abundance of large carnivores (e.g., chaetognaths at 0–50 and 150–200 m, medusae at 200–300 m, and ctenophores at 50–150 m) within the mesopelagic zone (Steinberg et al., 2008a, b). These peaks, and peaks in the proportion of zooplankton in the 45 mm size fraction, correspond with peaks observed in OA (Fig. 8) and 20:1(n  9)+ 22:1(n  11) in this study. Within the tissue of the carnivorous chaetognaths and copepods analyzed at K2, 20:1(n  9) and 22:1(n 11) were in similar proportions to their prey, N. plumchrus. Similarly, Auel et al. (2002) found higher amounts of 20:1(n  9) and 22:1(n  11) in carnivorous Arctic amphipods as did Graeve et al. (2008) in Arctic ctenophores, both suspected of consuming Calanus spp. Mesopelagic carnivores are also important for supplying ‘repackaged’ carbon at depth by the production of fecal pellets (Wilson et al., 2008). Mayzaud et al. (2007) showed that the fecal pellets of carnivores can be important vectors of essential fatty acids to deeper waters, with PUFA, DHA+ EPA, and 20:1(n  9)+ 22:1(n  11) content all highest in the fecal pellets of the most carnivorous copepods and euphausiids sampled. The increase in OA and 20:1(n 9) + 22:1(n 11) in particles with depth at both K2 and ALOHA supports this assertion.

4.4. Modes of feeding—sinking/suspended particles Fatty acids associated with particulate organic matter generally reflect their origins which could be from a combination of phytoplankton, zooplankton, and bacterial sources—which makes finding appropriate biomarkers for detrital feeding difficult (Wakeham, 1995). We used the proportions of OBFA and 18:1(n  7) in zooplankton tissue as indicators of feeding on bacteria. Relative abundances of these FA tend to be small, causing their importance to be overshadowed by the larger contribution of other fatty acids from zooplankton and phytoplankton (Wakeham and Canuel, 1988). In our study, %OBFA was useful for indicating particle feeding between sites and with depth. Stevens et al. (2004c) also used an OBFA index to determine the importance of microbial feeding in Arctic zooplankton around a polynya in Baffin Bay and concluded that particle feeding by copepods occurred but varied by region. A strong linear correlation between OBFA in marine seston and the calanoid copepod, Calanus glacialis was also found (Stevens et al., 2004a). At both ALOHA and K2, there were distinct peaks in abundance of known particle feeders (e.g., salps above 150 m at ALOHA and between 200 and 400 m at K2, ostracods 100–300 m at ALOHA and 150–400 m at K2, and poecilostomatoid copepods above 200 m at ALOHA and below 200 m at K2, Steinberg et al., 2008a). Similar to salps, poecilostomatoid copepod, and ostracod distribution at ALOHA, the % OBFA was highest between 0 and 300 m. The %OBFA at K2 however, remained constant throughout the mesopelagic zone. OBFA in particles and zooplankton was significantly higher at ALOHA than K2 although the %18:1(n  7) was similar at both sites. %18:1(n  7) can also be present in copepod tissue in more productive regions due to the elongation of 16:1(n 7) into 18:1(n  7) (Stevens et al., 2004c). As a result, this biomarker may not be useful as an index for bacteria at K2, as illustrated by the somewhat higher %18:1(n  7) in the epipelagic zone where diatoms dominated, than the mesopelagic (Table 2). In contrast, %18:1(n 7) was higher in bulk zooplankton collected from the mesopelagic vs. those collected from the epipelagic zone at ALOHA (Table 2). However 18:1(n  7) is also produced in cyanobacteria, coccolithophores, and flagellates; all present at ALOHA (Boyd et al., 2008). Thus the 18:1(n 7) measured in

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zooplankton tissue at ALOHA was likely obtained via feeding on sinking particles which can aggregate phytoplankton taxa (see below). EPA and DHA, as well as other (n  3) and (n  6) PUFAs can be indicators of particle feeding at depths below the epipelagic. These compounds are considered essential fatty acids (EFAs). Zooplankton cannot synthesize EFAs de novo so these FA must be obtained from food sources for normal growth and development (Dalsgaard et al., 2003). In order to obtain EFAs, mesopelagic zooplankton would have to vertically migrate to surface waters to feed, consume sinking particles, or consume other zooplankton. Vertical migration was significant at both K2 and ALOHA with both the strength and amplitude varying between regions (Steinberg et al., 2008a, 2008b). Sinking particles however, include fecal pellets, viable or dead/ dying phytoplankton cells, microzooplanton and bacteria which would also carry nutritious PUFAs and EFAs to the deep sea (e.g., Fowler and Knauer, 1986; Wakeham and Canuel, 1988; FalkPeterson et al., 1999). EPA and DHA however, are also assumed to be an indicator of ‘‘freshness’’ of sinking particles as the half lives of these and other highly labile PUFAs are on the order of days (Conte et al., 1995; Canuel and Martens, 1996; Conte et al., 2003). The percentage of total PUFA in particles at ALOHA and K2 decreases slightly from the epipelagic to the mesopelagic but remained at high levels in total FA throughout. The high PUFA could also be due to the presence of microzooplankton, but their contribution is unknown. The availability of PUFA in the mesopelagic zone is undoubtedly an important nutritional option for zooplankton residing there. The correlation between compounds found in both zooplankton and particles may also be a general indicator of particle feeding. Alternatively, this relationship could mean that zooplankton ingest particles in constant proportion. Many of the FA depth profiles of sinking particles mirrored the FA depth profiles of the zooplankton at both ALOHA and K2. For example, the proportion of MUFA in bulk zooplankton and particles at ALOHA and K2 both increase and are nearly identical with depth (Fig. 6). Increasing values of total MUFA (or combinations of specific MUFA) have also been used as indicators of bacterial association/ feeding (e.g., Pranal et al., 1996; Desvilettes et al., 1997). The percentages of (n  3) EFA (including EPA and DHA), OBFA (at ALOHA, not shown due to scale), PUFA (at K2 only), as well as

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MUFA in bulk zooplankton all were significantly correlated with the percentages measured in the particles (Fig. 7, Pearson’s r, po0.05). These linear correlations provide additional evidence that zooplankton can obtain essential FA from sinking and suspended particles in the mesopelagic zone. Although a poor nutritional supplement, the presence of cyanobacteria in zooplankton tissue can also demonstrate particle feeding. Like bacteria, cyanobacteria are too small to be eaten by non-filter feeding zooplankton. The increase in 16:1/16:0 in zooplankton tissue at ALOHA and K2 with depth may be due to the fact that 16:1(n  7) is found in bacteria and cyanobacteria as well as diatoms (e.g., Wakeham, 1995; Desvilettes et al., 1997). Wakeham (1995) found increasing 16:1(n  7) in POC of the deeper 1000 m waters in the central north pacific was well as in the anoxic region of the Black Sea, indicative of bacterial decomposition of POC. Zooplankton fecal pellets sinking through the mesopelagic can also provide essential nutrition to zooplankton residing there. The increase with depth in both 18:1(n  9) and 20:1(n  9)+ 22:1(n  11) MUFA in the particles is indicative of an increase in zooplankton-derived material, likely fecal pellets, within the mesopelagic zone. As mentioned above, FA in subantarctic and subtropical zooplankton fecal pellets contained significant amounts of EFA, including DHA and EPA (Mayzaud et al., 2007). Sheridan et al. (2002) and Conte et al. (1998) both conclude that the production and consumption of fecal pellets produced by mesopelagic zooplankton contributes to or has an effect on the nutritional composition of particles reaching the deep sea. At ALOHA and K2 the percentage of the total carbon flux that consisted of intact fecal pellets was as much as 35% and 29%, respectively (Wilson et al., 2008). A larger proportion of sinking material may also be of fecal origin but in the form of broken-up ‘‘fecal fluff’’ which was present in large amounts at K2 but not quantifiable (Wilson et al., 2008).

5. Summary and conclusions Our results illustrate clear differences in the feeding ecology of zooplankton between the surface and the mesopelagic zone, and between locations, both of which have implications for the transport of organic material to the deep sea. For example, the

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larger zooplankton at K2 produce larger, faster sinking fecal pellets, and along with the higher fatty acid concentrations at K2, suggest a more rapid transport of essential nutrients to the seafloor compared to ALOHA. Biomarkers for herbivory remained more pronounced in zooplankton at K2 during late summer where, although at the end of a diatom bloom, primary production was still higher than at ALOHA. On an annual basis however, K2 is more seasonal than ALOHA, making lipid storage in seasonal migrators at K2 essential. Zooplankton FA composition at K2 was consistent with zooplankton feeding on diatom and diatom-derived aggregate material throughout the mesopelagic. In contrast, dinoflagellate biomarkers were proportionately more abundant in zooplankton at ALOHA, which like diatoms at K2, can also supply essential fatty acids to zooplankton at depth. At ALOHA, where production is lower and particulate organic carbon attenuated more rapidly than at K2, carnivory was more prominent at depth. Carnivory may help mesopelagic zooplankton compensate for the lower export efficiency of POC at ALOHA vs. K2. Particle feeding by zooplankton was evident at both locations, and fecal pellets are likely to be a major source of essential fatty acids for mesopelagic zooplankton. Food-web transfer and alteration of fatty acids through the mesopelagic zone thus affects the efficiency of the biological pump as differences in zooplankton feeding ecology can alter the quality and quantity of organic material reaching the deep sea.

Acknowledgements Our thanks to the captains and crews of the R/V Kilo Moana and R/V Roger Revelle for their assistance during the VERTIGO cruises. Thanks to Ken Buesseler for his leadership in VERTIGO and for his help at sea. We are very grateful to Elizabeth Canuel and Eric Lund

for their excellent advice on all things lipid. Joe Cope, Vincent Encomio, Toru Kobari, Carl Lamborg, Elizabeth Lerberg, Jennifer Podbesek, and Todd Wood also provided help with sample collections or processing. The comments of four anonymous reviewers improved the manuscript. This study was supported by grants from the US National Science Foundation NSF OCE0324402 (Biological Oceanography) to D.K.S, and by the Department of Energy, Office of Science, Biological and Environmental Research Program to J.K.B.B. This paper is Contribution no. 3099 from the Virginia Institute of Marine Science. References Ackman, R.G., Eaton, C.A., Sipos, J.C., Hooper, S.N., Castell, J.D., 1970. Lipids and fatty acids of two species of North Atlantic krill (Meganyctiphanes norvegia and Thysanoessa inermis) and their role in the aquatic food web. Journal of the Fisheries Research Board of Canada 27 (3), 513–533. Ackman, R.G., Tocher, C.S., McLachlan, J., 1968. Marine phytoplankter fatty acids. Journal of the Fisheries Research Board of Canada 25, 1603–1620. Alldredge, A.L., Silver, M.W., 1988. Characteristics, dynamics and significance of marine snow. Progress in Oceanography 20, 41–82. Angel, M.V., 1989. Does mesopelagic biology affect the vertical flux?. In: Berger W.H., Smetacek, V., Wefer, G. (Eds.), Productivity of the Ocean: Present and Past. John Wiley and Sons, Ltd., New York, pp. 155–173. Auel, H., Harjes, M., da Rocha, R., Stubing, D., Hagen, W., 2002. Lipid biomarkers indicate different ecological niches and trophic relationships of the arctic hyperiid amphipods Themisto abyssorum and T. libellula. Polar Biology 25, 374–383. Azam, F., Long, R.A., 2001. Sea snow microcosms. Nature 414, 495–498. Bec, A., Desvilettes, C., Ve´ra, A., LeMarchand, C., Fontvielle, D., Bordier, G., 2003. Nutritional quality of a freshwater heterotrophic flagellate: trophic upgrading of its microalgal diet for Daphnia hyaline. Aquatic Microbial Ecology 32, 203–207. Bec, A., Martin-Creuzburg, D., von Elert, E., 2006. Trophic upgrading of autotrophic picoplankton by the heterotrophic nanoflagellate Paraphysomonas sp. Limnology and Oceanography 51, 1699–1707. Bishop, J.K.B., Schupack, D., Sherrell, R.M., Conte, M.H., 1985. A multiple unit large volume in situ filtration system (MULVFS) for sampling oceanic particulate matter in mesoscale environments. In: Zirino, A. (Ed.), Mapping Strategies

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in Chemical Oceanography. American Chemical Society, Washington, DC, pp. 155–175. Bishop, J.K.B., Stepien, J.C., Wiebe, P.H., 1987. Particulate matter distributions, chemistry and flux in the Panama Basin: response to environmental forcing. Progress in Oceanography 17, 1–59. Bishop, J.K.B., Wood, T.J., 2008. Particulate matter chemistry and dynamics in the twilight zone at VERTIGO ALOHA and K2. Deep-Sea Research I 55, 1684–1706. Bligh, E.G., Dyer, W.J., 1959. A rapid method of total lipid extraction and purification. Canadian Journal of Biochemistry and Physiology 37, 911–917. ¨ Boer, M., Gannefors, C., Kattner, G., Graeve, M., Hop, H., Falk-Petersen, S., 2005. The Arctic pteropod. Marine Biology 147, 707–717. Boyd, P.W., Gall, M.P., Silver, M.W., Coale, S.L., Bidigare, R.R., Bishop, J.K., 2008. Quantifying the surface-subsurface biogeochemical coupling during the VERTIGO ALOHA and K2 studies. Deep-Sea Research II 55 (14–15), 1578–1593. Budge, S.M., Parrish, C.C., 1998. Lipid biogeochemistry of plankton, settling matter and sediments in Trinity Bay, Newfoundland. II. Fatty Acids. Organic Geochemistry 29 (5–7), 1547–1559. Buesseler, K.O., Bidigare, R.R., Bishop, J.K., Boyd, P.W., Honda, M.C., Karl, D.M., Lam, P.J., Lamborg, C.H., Siegel, D., Silver, M.W., Steinberg, D.K., Trull, T.W., 2008. What we know from VERTIGO (VERtical Transport In the Global Ocean). Deep-Sea Research II 55 (14–15), 1522–1539. Buesseler, K.O., Lamborg, C.H., Boyd, P.W., Lam, P.J., Trull, T.W., Bidigare, R.R., Bishop, J.K.B., Casciotti, K.L., DeHairs, F., Karl, D.M., Siegel, D., Silver, M.W., Steinberg, D.K., Valdez, J., Van Mooy, B., Wilson, S.E., 2007. Revisiting carbon flux through the ocean’s twilight zone. Science 316, 567–570. Canuel, E.A., Martens, C.S., 1996. Reactivity of recently-deposited organic matter: degradation of lipid compounds near the sediment-water interface. Geochimica et Cosmochimica Acta 60, 1793–1806. Conte, M.H., Dickey, T.D., Weber, J.C., Johnson, R.J., Knap, A.H., 2003. Transient physical forcing of pulsed export of bioreactive material to the deep Sargasso Sea. Deep-Sea Research I 50, 1157–1187. Conte, M.H., Eglinton, G., Madureira, L.A.S., Rabouille, C., Labeyrie, L., Mudge, S., 1995. Origin and fate of organic biomarker compounds in the water column and sediments of the eastern North Atlantic. Philosophical Transactions: Biological Sciences 348, 169–178. Conte, M.H., Weber, J.C., Ralph, N., 1998. Episodic particle flux in the deep Sargasso Sea: an organic geochemical assessment. Deep-Sea Research I 45 (11), 1819–1841. Cripps, G.C., Atkinson, A., 2000. Fatty acid composition as an indicator of carnivory in Antarctic krill, Euphausia superba. Canadian Journal of Fisheries and Aquatic Science 57 (Suppl. 3), 31–37. Dagg, M.J., 1993. Sinking particles as a possible source of nutrition for the large calanoid copepod Neocalanus cristatus in the subarctic Pacific Ocean. Deep-Sea Research I 40 (7), 1431–1445. ¨ Dalsgaard St., J., John, M.A., Kattner, G., Muller-Navarra, D.C., Hagen, W., 2003. Fatty acid trophic markers in the pelagic marine environment. Advances in Marine Biology 46, 225–340. ¨ DeMott, W.R., Muller-Navarra, D.C., 1997. The importance of highly unsaturated fatty acids in zooplankton nutrition: evidence from experiments with Daphnia, a cyanobacterium and lipid emulsions. Freshwater Biology 38, 649–664. Desvilettes, C., Bourdier, G., Amblard, C., Barth, B., 1997. Use of fatty acids for the assessment of zooplankton grazing on bacteria, protozoans and microalgae. Freshwater Biology 28, 629–637. Dilling, L., Wilson, J., Steinberg, D.K., Alldredge, A.L., 1998. Feeding by the euphausiid Euphausia pacifica and the copepod Calanus pacificus on marine snow. Marine Ecology Progress Series 170, 189–201. Elskens, M., Brion, N., Buesseler, K.O., Van Mooy, B.A.S., Boyd, P.W., Dehairs, F., Savoye, N., Baeyens, W., 2008. Primary, new and export production in the NW Pacific subarctic gyre during the VERTIGO K2 experiments. Deep-Sea Research II 55 (14–15), 1594–1604. Falk-Peterson, S., Sargent, J.R., Lønne, O.J., Timofeev, S., 1999. Functional biodiversity of lipids in Antarctic zooplankton: Calanoides acutus, Calanus propinquus, Thysanoessa macrura, and Euphausia crystallorophias. Polar Biology 21, 37–47. Folch, J., Lees, M., Sloane-Stanley, G.H., 1957. A simple method for the isolation and purification of total lipids from animal tissues. Journal of Biological Chemistry 226, 497–509. Fowler, S.W., Knauer, G.A., 1986. Role of large particles in the transport of elements and organic compounds through the oceanic water column. Progress in Oceanography 16, 147–194. Gowing, M.M., Wishner, K.F., 1986. Trophic relationships of deep-sea calanoid copepods from the benthic boundary layer of the Santa Catalina Basin, California. Deep-Sea Research 33 (7), 939–961. Gowing, M.M., Wishner, K.F., 1992. Feeding ecology of benthopelagic zooplankton on an eastern tropical Pacific seamount. Marine Biology 112, 451–467. Graeve, M., Kattner, G., Hagen, W., 1994. Diet-induced changes in the fatty acid composition of Arctic herbivorous copepods: experimental evidence of trophic markers. Journal of Experimental Marine Biology and Ecology 182, 97–110. Graeve, M., Kattner, G., Piepenburg, D., 1997. Lipids in Arctic benthos: does the fatty acid and alcohol composition reflect feeding and trophic interactions? Polar Biology 18, 53–61. ¨ Graeve, M., Lundberg, M., Boer, M., Kattner, G., Hop, H., Falk-Petersen, S., 2008. The fate of dietary lipids in the Arctic ctenophore Mertensia ovum (Fabricius 1780). Marine Biology 153, 643–651.

1293

Hakanson, J.L., 1984. The long and short term feeding condition in field-caught Calanus pacificus, as determined from the lipid content. Limnology and Oceanography 29 (4), 794–804. Hedges, J.I., Stern, J.H., 1984. Carbon and nitrogen determinations of carbonatecontaining solids. Limnology and Oceanography 29 (3), 657–663. Kaneda, T., 1991. Iso- and Anteiso-fatty acids in bacteria: biosynthesis, function, and taxonomic significance. Microbiological Reviews 55 (2), 288–302. Kattner, G., Albers, C., Graeve, M., Schnack-Schiel, S.B., 2003. Fatty acid and alcohol composition of the small polar copepods, Oithona and Oncaea: indication on feeding modes. Polar Biology 26, 666–671. Kobari, T., Ikeda, T., 1999. Vertical distribution, population structure and life cycle of Neocalanus cristatus (Crustacea: Copepoda) in the Oyashio region, with notes on its regional variations. Marine Biology 134, 683–696. Kobari, T., Ikeda, T., 2001. Ontogenetic vertical migration and life cycle of Neocalanus plumchrus (Crustacea: Copepoda) in the Oyashio region, with notes on regional variations in body sizes. Journal of Plankton Research 23 (3), 287–302. Kobari, T., Moku, M., Takahashi, K., 2008a. Seasonal appearance of expatriated boreal copepods in the Oyashio–Kuroshio mixed region. ICES Journal of Marine Science 65, 469–476. Kobari, T., Shinada, A., Tsuda, A., 2003. Functional roles of interzonal migrating mesozooplankton in the western subarctic Pacific. Progress in Oceanography 57, 279–298. Kobari, T., Steinberg, D.K., Ueda, A., Tsuda, A., Silver, M.W., Kitamurae, M., 2008b. Impacts of ontogenetically migrating copepods on downward carbon flux in the western subarctic Pacific Ocean. Deep-Sea Research II 55 (14–15), 1648–1660. Kosobokova, K.N., Hirche, H.-J., Scherzinger, T., 2002. Feeding ecology of Spinocalanus antarcticus, a mesopelagic copepod with a looped gut. Marine Biology 141, 503–511. Lamborg, C.H., Buesseler, K.O., Valdez, J., Bertrand, C.H., Bidigare, R.R., Manganini, S.J., Pike, S., Steinberg, D.K., Trull, T.W., Wilson, S.E., 2008. The flux of bio- and lithogenic material associated with sinking particles in the mesopelagic ‘‘twilight zone’’ of the northwest and North Central Pacific Ocean. Deep-Sea Research II 55 (14–15), 1540–1563. Lampitt, R.S., 1992. The contribution of deep-sea macroplankton to organic remineralization: results from sediment trap and zooplankton studies over the Madeira Abyssal Plain. Deep-Sea Research 39 (2), 221–233. Lampitt, R.S., Hillier, W.R., Challenor, P.G., 1993a. Seasonal and diel variation in the open ocean concentration of marine snow aggregates. Nature 362, 737–739. Lampitt, R.S., Wishner, K.F., Turley, C.M., Angel, M.V., 1993b. Marine snow studies in the Northeast Atlantic Ocean: distribution, composition and role as a food source for migrating plankton. Marine Biology 116, 689–702. Lee, R.F., Hirota, J., 1973. Wax esters in tropical zooplankton and nekton and the geographical distribution of wax esters in marine copepods. Limnology and Oceanography 18 (2), 227–239. Lee, R.F., Hirota, J., Barnett, A.M., 1971a. Distribution and importance of wax esters in marine copepods and other zooplankton. Deep-Sea Research 18, 1147–1165. Lee, R.F., Nevenzel, J.C., Paffenhofer, G.-A., 1971b. Importance of wax esters and other lipids in the marine food chain: phytoplankton and copepods. Marine Biology 9, 99–108. Marty, Y., Delaunay, F., Moal, J., Samain, J.-F., 1992. Changes in the fatty acid composition of Pecten maximus (L.) during larval development. Journal of Experimental Marine Biology and Ecology 163, 221–234. Mayzaud, P., Laureillard, J., Merien, D., Brinis, A., Godard, C., Razouls, S., Labat, J.-P., 2007. Zooplankton nutrition, storage and fecal lipid composition in different water masses associated with the Agulhas and subtropical fronts. Marine Chemistry 107, 202–213. Mayzaud, P., Virtue, P., Albessard, E., 1999. Seasonal variations in the lipid and fatty acid composition of the euphausiid Meganyctiphanes norvegica from the Ligurian Sea. Marine Ecology Progress Series 186, 199–210. Metcalfe, L.D., Schmitz, A.A., 1961. The rapid preparation of fatty acid esters for gas chromatographic analysis. Analytical Chemistry 33, 363–364. Meziane, T., Tsuchiya, M., 2000. Fatty acids as tracers of organic matter in the sediment and food web of a mangrove/intertidal flat ecosysterm, Okinawa, Japan. Marine Ecology Progress Series 200, 49–57. ¨ Muller-Navarra, D.C., Brett, M.T., Liston, A.M., Goldman, C.R., 2000. A highly unsaturated fatty acid predicts carbon transfer between primary producers and consumers. Nature 403, 74–77. ¨ Muller-Navarra, D.C., Brett, M.T., Park, S., Chandra, S., Ballantyne, A.P., Zorita, E., Goldman, C.R., 2004. Unsaturated fatty acid content in seston and trophodynamic coupling in lakes. Nature 427, 69–72. Nelson, M.M., Mooney, B.D., Nichols, P.D., Phleger, C.F., 2001. Lipids of Antarctic Ocean amphipods: food chain interactions and the occurence of novel biomarkers. Marine Chemistry 73, 53–64. Noji, T.T., 1991. Macrozooplankton influence on the fate of particles in the ocean. In: Wassmann, P., Heiskanen, A.S., Lindahl, O. (Eds.), Sediment Trap Studies in the Nordic Countries, Symposium Proceedings. Phleger, C.F., Nichols, P.D., Virtue, P., 1998. Lipids and trophodynamics of Antarctic zooplankton. Comparative Biochemistry and Physiology. Part B 120, 311–323. Pranal, V., Fiala-Me´dioni, A., Guezennec, J., 1996. Fatty acid characteristics in two symbiotic gastropods from a deep hydrothermal vent of the West Pacific. Marine Ecology Progress Series 142, 175–184.

1294

S.E. Wilson et al. / Deep-Sea Research I 57 (2010) 1278–1294

Reuss, N., Poulsen, L.K., 2002. Evaluation of fatty acids as biomarkers for a natural plankton community. A field study of a spring bloom and post-bloom period off West Greenland. Marine Biology 141, 423–434. Sargent, J.R., Falk-Petersen, S., 1981. Ecological investigation of the zooplankton community in Balsfjorden northern Norway: lipids and fatty acids in Meganyctiphanes norvegica, Thysanoessa raschi, and T. inermis during midwinter. Marine Biology 62, 131–137. Sargent, J.R., Henderson, R.J., 1976. The structure, metabolism and function of lipids in marine organisms. In: Malins, D.C., Sargent, J.R. (Eds.), Biochemical and Biophysical Perspectives in Marine Biology. Academic Press, London, pp. 149–212. Sasaki, H., Hattori, H., Nishizawa, S., 1988. Downward flux of particulate organic matter and vertical distribution of calanoid copepods in the Oyashio Water in summer. Deep-Sea Research 35 (4), 505–515. Schnetzer, A., Steinberg, D.K., 2002. Natural diets of vertically migrating zooplankton in the Sargasso Sea. Marine Biology 141, 89–99. Sheridan, C.C., Lee, C., Wakeham, S.G., Bishop, J.K.B., 2002. Suspended particle organic composition and cycling in surface and midwaters of the equatorial Pacific Ocean. Deep-Sea Research I 49, 1983–2008. Shoden, S., Ikeda, T., Yamaguchi, A., 2005. Vertical distribution, population structure and life cycle of Eucalanus bungii (Copepoda: Calanoida) in the Oyashio region, with notes on regional variations. Marine Biology 146, 497–511. Silver, M.W., Alldredge, A.L., 1981. Bathypelagic marine snow: deep-sea algal and detrital community. Journal of Marine Research 39, 501–530. Silver, M.W., Bruland, K.W., 1981. Differential feeding and fecal pellet composition of salps and pteropods, and the possible origin of the deep-water flora and olive-green ‘‘cells’’. Marine Biology 62, 263–273. Steinberg, D.K., 1995. Diet of copepods (Scopalatum vorax) associated with mesopelagic detritus (giant larvacean houses) in Monterey Bay, California. Marine Biology 122, 571–584. Steinberg, D.K., Cope, J.S., Wilson, S.E., Kobari, T., 2008a. A comparison of mesopelagic mesozooplankton community structure in the subtropical and subarctic North Pacifc Ocean. Deep-Sea Research II 55 (14-15), 1615–1635. Steinberg, D.K., Van Mooy, B., Buesseler, K.O., Boyd, P.W., Kobari, T., Karl, D.M., 2008b. Bacterial vs. zooplankton control of sinking particle flux in the ocean’s twilight zone. Limnology and Oceanography 53 (4), 1327–1338. Stevens, C.J., Deibel, D., Parrish, C.C., 2004a. Copepod omnivory in the North Water Polynya (Baffin Bay) during autumn: spatial patterns in lipid composition. Deep-Sea Research I 51, 1637–1658.

Stevens, C.J., Deibel, D., Parrish, C.C., 2004b. Incorporation of bacterial fatty acids and changes in a wax ester-based omnivory index during a long-term incubation experiment with Calanus glacialis Jaschnov. Journal of Experimental Marine Biology and Ecology 303, 135–156. Stevens, C.J., Deibel, D., Parrish, C.C., 2004c. Species-specific differences in lipid composition and omnivory indices in Arctic copepods collected in deep water during autumn (North Water Polynya). Marine Biology 144, 905–915. Tsuda, A., Saito, H., Kasai, H., 1999. Life histories of Neocalanus flemingeri and Neocalanus plumchrus (Calanoida: Copepoda) in the western subarctic Pacific. Marine Biology 135, 533–544. Tsuda, A., Saito, H., Kasai, H., 2001. Life history strategies of subarctic copepods Neocalanus flemingeri and N. plumchrus, especially concerning lipid accumulation patterns. Plankton Biology and Ecology 48, 52–58. Uttal, L., Buck, K.R., 1996. Dietary study of the midwater polychaete Poebius meseres in Monterey Bay. Calinfornia. Marine Biology 125, 333–343. Viso, A.-C., Marty, J.-C., 1993. Fatty acids from 28 marine microalgae. Phytochemistry 34 (6), 1521–1533. Wakeham, S.G., 1995. Lipid biomarkers for heterotrophic alteration of suspended particulate organic matter in oxygenated and anoxic water columns of the ocean. Deep-Sea Research I 42 (10), 1749–1771. Wakeham, S.G., Canuel, E.A., 1988. Organic geochemistry of particulate matter in the eastern tropical North Pacific Ocean: implications for particle dynamics. Journal of Marine Research 46, 183–213. Wiebe, P.H., Morton, A.W., Bradley, A.M., Backus, R.H., Craddock, J.E., Barber, V., Cowles, T.J., Flierl, G.R., 1985. New developments in the MOCNESS, an apparatus for sampling zooplankton and micronekton. Marine Biology 87, 313–323. Wilson, S.E., Steinberg, D.K., Buesseler, K.O., 2008. Changes in fecal pellet characteristics with depth as indicators of zooplankton repackaging of particles in the mesopelagic zone of the subtropical and subarctic North Pacific Ocean. Deep-Sea Research II 55 (14–15), 1636–1647. Wilson, S.E., Steinberg, D.K., 2010. Autotrophic picoplankton in mesozooplankton guts: evidence of aggregate feeding in the mesopelagic zone and export of small phytoplankton. Marine Ecology Progress Series, doi:10.3354/ meps08648. Zhang, Y., Jiao, N.Z., Hong, N., 2008. Comparative study of picoplankton biomass and community structure in different provinces from subarctic to subtropical oceans. Deep-Sea Research II 55 (14–15), 1605–1614.