The relative contribution of marine snow of different origins to biological processes in coastal waters

ContinentalShelfResearch,Vol. 10, No. 1, pp.41-58, 1990.

0278-434N90$3.00 + 0.00 ~) 1990PergamonPressplc

Printedin GreatBritain.

T h e relative c o n t r i b u t i o n o f m a r i n e s n o w o f d i f f e r e n t o r i g i n s to biological p r o c e s s e s in c o a s t a l w a t e r s ALICE L. ALLDREDGE* and CHRIS C. GOTSCHALK* (Received 3 January 1989; in revised form 7 May 1989; accepted 14 July 1989) Abstract--While a disproportionate quantity of photosynthetic and hetrotrophic microbial activity can occur on marine snow in surface waters, the significance of these macroscopic aggregates as sites for the transformation of matter in the euphoric zone appears to be highly variable. We investigated the hypothesis that this variability results from differences in aggregate origin by comparing the properties of marine snow at 13 stations in the Southern California Bight and California Current. Four types of marine snow were encountered including larvacean houses, diatom floes, fecal aggregates and aggregates composed primarily of miscellaneous debris and detritus. More than 95% of the aggregates observed at any one station were of the same type. Bacteria grew >3 fold faster and phaeopigments were significantly concentrated on marine snow of all types. However, an insignificant fraction ( < 1 % ) of total bacterial abundance, bacterial production, primary production, Chl a and nutrients occurred in association with marine snow of larvacean, fecal or miscellaneous origin. Marine snow formed from the coagulation of living diatom cells contributed more than 5% to these parameters, but only when the aggregates were large, abundant and recently formed. Our results indicate that only marine snow of direct phytoplankton origin contributes significantly to primary production in surface waters, and that a significant fraction of total photosynthetic and heterotrophic microbial activity is likely to occur on marine snow only episodically when aggregates are newly formed.

INTRODUCTION

ABUNDArCr amorphous marine aggregates 0.5 mm or larger in diameter, known as marine snow, harbor rich assemblages of phytoplankton, flagellates, bacteria and protozoans at concentrations 2-5 orders of magnitude higher than those occurring in the surrounding seawater (SILVERe t al., 1978; BEERS et al., 1986; CAROr~ et al., 1986; ALLDREDGEand SILVER,1988). Descriptions of these dense microbial communities have led to the suggestion that marine snow may be disproportionately important as a site for the biologically mediated transformation of matter in the pelagic zone (KARL, 1982a). However, the actual contribution of marine snow to microbial and photosynthetic processes in the euphotic zone appears to be highly variable. For example, while KNAUER et al. (1982) found that up to 58% of primary production occurred on marine snow at one station off California, most other studies report values of less than 20% and usually less than 2% (ALLDREDOE and Cox, 1982; PREZELIN and ALLDREDGE, 1983). Likewise, heterotrophic bacterial production on marine snow has been reported to be as high as 26% of total bacterial production but it is usually less than 10% (ALLDREDGEand YOUNGBLLrrH,1985; ALLDREDGEet al., 1986). * Department of Biological Sciences and Marine Science Institute, University of California, Santa Barbara, CA 93106, U.S.A. 41

42

A.L. ALLDREDGEand C. C. GOTSCHALK

This variability in the relative significance of marine snow as a site of photosynthetic and microbial activity in the water column results primarily from properties of the aggregates themselves, especially variations in aggregate size, abundance, composition and age, which are related directly to aggregate origin. Marine snow originates by two major pathways in the ocean (see ALLDREDGEand SILVER, 1988, for review). In the first pathway, zooplankton, especially larvaceans, pteropods and doliolids, produce aggregates de novo as discarded mucus feeding webs or flocculent fecal pellets. In the second pathway, aggregates in the marine snow size range (>0.5 ram) are formed primarily via the collision and subsequent attachment of smaller particles present in the water column, particularly those in the size range of tens to hundreds of microns (MCCAVE, 1984). The rate at which these particles aggregate is a function of the concentration of component particles, the probability of attachment on collision, and the intensity of the physical processes, especially turbulent shear and differential settlement, bringing about collision of particles (STuMM and MORGAN, 1981). Moreover, the composition of marine snow formed via this second pathway largely reflects the types and relative abundances of the micro-aggregates and large particles available in the water column for coagulation. Thus, if senescent diatoms are abundant, diatom flocs are produced; if fecal pellets dominate then the aggregates formed are largely composed of fecal matter, etc. The importance of aggregate origin has not been previously considered in evaluating the significance of marine snow as a site of biological activity in the pelagic zone. We hypothesized that the magnitude of the contribution of marine snow to photosynthetic and heterotrophic processes would be dependent on aggregate origin. In the following study we investigated this hypothesis by characterizing the contribution of marine snow of various origins to production processes in surface waters of the Southern California Bight and California Current. Herein we report variations in the properties of the four types of marine snow encountered and discuss the effect of aggregate origin on the significance of marine snow in surface waters. MATERIALS AND METHODS

Field collection o f marine snow

We observed and collected marine snow using SCUBA at 13 stations in the Southern California Bight and California Current during two cruises of the R.V. Point Sur in April and September 1987 (Fig. 1; Table 1). All samples were collected between 0900 and 1030 h local time from depths of 15-20 m. At each station, 250 aggregates were collected individually for determination of primary production, chlorophyll, bacterial production, taxonomic composition, particle size, and dry weight in 6 ml open-ended polypropylene syringe barrels. Additional samples for nutrient analysis were collected by drawing eight aggregates into each of five, 20 ml syringes and adjusting the volume with surrounding seawater to 15 ml. Identical surrounding seawater samples for nutrients were collected without aggregates. Whole seawater samples containing marine snow at ambient concentrations were also collected by divers in 4 liter polyethylene bottles. Aggregate densities were measured visually at each station by a diver who swam three replicate horizontal transects while counting the number of aggregates 2 mm or larger passing through a 7 cm hoop attached to a General Oceanics flowmeter (Model 2100) equipped with a low speed rotor.

Origins of marine snow

43

35

0 @ 34 °03 .J

! ~

Lnrvaceansnow Fe¢ar =now

oo.i

MIIcellaneou, snow

tomb Diatom snow

z

\

33 ° Pacific

[] 32 ° 123 °

•

Ocean

,

,

121 °

119 °

17 °

W e s t Longitude

Fig. 1.

Location and types of marine snow found at 13 stations off Southern California. Stations

represented by squares were sampled in April 1987; circles were sampled in September 1987.

Table 1.

Type, abundance and mean dry weight (+1 S.D.) of marine snow sampled at 13 stations off coastal Southern California

Date and snow type

Aggregate

Dry

weight (~tg agg. -1)

Station position

abundance (no. 1-1)

Miscellaneous 3 April 24 Sept.

32°13'N, 121°14'W 33°12'N, 119°21'W

0.25 + 0.01 0.85 + 0.07

21 + 12 9 + 10

Fecal 2 April 30 Sept.

33°15'N, 119°49'W 34°37'N, 121°43'W

0.65 + 0.05 0.20 + 0.01

65 + 52 10 ± 7

tarvacean 25 Sept. 26 Sept. 27 Sept.

33029'N, 119°34'W 34° 2'N, 120037'W 34°19'N, 121°ll'W

0.50 ± 0.02 0.59 + 0.09 0.41 ± 0.03

10 ___ 9 7 _+ 6 8 -+ 7

Diatom 5 April 6 April 7 April 8 April 9 April 29 Sept.

32°53'N, 32°36'N, 33°11'N, 33°43'N, 33°44'N, 34°25'N,

0.89 1.00 1.00 1.20 1.10 1.65

118°18'W l18°10'W l19010'W 119°32'W 119°31'W 121 ° I ' W

Shipboard m e a s u r e m e n t s Phytoplankton-related m e a s u r e m e n t s .

+ + ± + + ±

0.07 0.06 0.15 0.08 0.42 0.10

25 49 41 150 71 35

_+ 15 ± 61 + 25 _+ 80 ± 44 _+ 81

At each station, we measured the primary production, dry weight, Chl a, and phaeopigment content of 30 individual aggregates and of surrounding seawater at mid-day, within 90 min of sample collection. All glassware used for production measurements was washed according to FrrZWATER et al. (1982), except that vessels were soaked in Micro for 24 h and then soaked in acid for 48 h prior to ample rinsing with Milli-Q water. Each aggregate was pipetted in 2 ml seawater from its collecting syringe into a glass tissue grinder and the volume increased to 4 ml by adding

44

A.L. ALLDREDGEand C. C. GOTSCHALK

surrounding seawater. Multiple measurements of each aggregate were then achieved by gently disrupting the aggregate in the tissue grinder with 10, cavitation-free strokes and splitting it into two, 2 ml aliquots. Comparisons of carbon fixation rates per unit dry weight of disrupted and intact marine snow aggregates, and SEM examination of cell integrity indicated that gentle disruption did not result in significant reduction of photosynthetic activity or cell breakage (GoTsCrIALK, 1988). 14C-bicarbonate was added to one, 2 ml aliquot of each aggregate in a 10 ml test tube and to five, 2 ml surrounding seawater samples at a final 14C concentration of 0.4 ~tCi m1-1. The samples were incubated at ambient seawater temperature (11-15°C) for 2 h at 150 ~tEin m -2 s-1. Following incubations, samples were filtered onto 25 mm diameter, 0.4 Ixm Nuclepore filters which had been pre-weighed to the nearest 0.1 Ixg on a Cahn Electrobalance (Model 4600), rinsed twice with filtered seawater and once, very quickly, with Milli-Q water to remove salts. The filters were then dried in a desiccator, reweighed for determination of particle dry weight, placed in a liquid scintillation vial and counted with an LKB Liquid Scintillation Counter, Model 1217. Carbon fixation was corrected for dark fixation and adsorption by subtracting the mean value of five additional controls each of seawater and aggregates incubated as above with 0.5 ~tM D C M U (3-(3,4-dichlorophenyl)-l, 1, dimethylurea), a photosynthetic inhibitor (LEGENDRE et al., 1983). The remaining 2 ml aliquot of each sample was filtered onto a 25 mm Whatman GF/F glass fiber filter for determination of Chl a and phaeopigments using standard fluorometric methods (PARSONS et al., 1984). Background seawater blanks of 14C uptake, pigments, or dry weight were subtracted from all aggregate samples to obtain the values for the aggregates alone. Nutrients. Duplicate nutrient samples of both snow and surrounding seawater were filtered through 0.45 I~m syringe filters directly into clean 25 ml polyethylene vials and stored frozen at -30°C until analysed by flow injection for nitrate, nitrite and phosphate (JOHNSONet al., 1985). Ammonia concentrations of 0.45 lam filtrates from three snow and three surrounding seawater samples were measured within 30 min of sample collection by the phenol-hypochlorite method (SOLORZANO,1969). Approximate mean concentrations of nutrients dissolved within the interstitial fluid of aggregates was calculated by dividing the mean Ixg-at. of each nutrient per aggregate by the mean aggregate volume at each station. Mean aggregate volumes were approximated from aggregate dry weight using the equations for the conversion of marine snow dry weight to volume in ALLDREDGE and GOrSCaALK (1988).

Bacteria-related measurements Thymidine methodology. Prior to the cruises we determined the incubation times appropriate for studies of [3H]thymidine incorporation of marine snow by measuring the incorporation of labeled thymidine into RNA, D N A and protein as a function of incubation time. A 450 ml sample of seawater from the Santa Barbara Channel containing 1 aggregate ml-~ was obtained by hand-collecting aggregates in 18, 25 ml syringes. This aggregate slurry was divided into 42, 10 ml sub-samples in cleaned, sterile test tubes. All glassware was soaked in a solution of 6 N HCI in 30% EtOH, rinsed with sterile water treated with 0.1% diethylpyrocarbonate to remove any RNAase contamination, and then autoclaved. Each tube was inoculated with [3H]thymidine for a final concentration of 4 nM. Six tubes containing 0.2% formalin served as adsorption controls.

Origins of marine snow

45

Incubations in the dark at ambient seawater temperature were terminated for subsets of six tubes each after time intervals of 5, 15, 30 45, 60 and 120 min. Samples were filtered onto GF/F filters and stored frozen at -30°C for no longer than one week before being analysed for relative incorporation of thymidine into D N A , RNA and protein as described in KARL (1982b). Results of these measurements indicate that the percent of labeled thymidine incorporated into D N A by bacteria attached to marine snow remained fairly constant at 86-89% over the first 45 min of incubation. However, at incubation times longer than 45 min, the distribution of label into R N A and protein increased dramatically (Fig. 2a). Thus 45 rain was selected as the longest incubation time which would allow maximum incorporation of [3H]thymidine into bacterial DNA. We also conducted an isotope dilution study to determine the concentration of natural thymidine available within aggregates in order to select an appropriate quantity of labeled thymidine for experimental addition. Although the concentration of free thymidine present within the interstitial water of marine snow probably varies considerably with location and among individual particles, this study was conducted to provide an 100 90

A

80 2

70 60

oo 50= _c

._==

4030-

..¢: 2 0 I10 0 0

20

40

60

80

100

Incubation Time (min)

2.0 1.8]

B

1.6

+

1.4 1.2.

e, GI 1.0 \ 0.8 '¢c, 0.6-

+

+.

+

0.4

•

Y = 0.014 X + 0.04

0.2 0 -10

0

10

20 30

40 50

60 70

80 90 100

Total Thymidine Added (nm)

Fig. 2. [3H]thymidine incorporation and isotope dilution of marine snow. (A) Percent of labeled thymidine incorporated into bacterial DNA, RNA, and protein with increasing incubation time. (B) Isotope dilution of marine snow estimated via incorporation of labeled thymidine at increasing thymidine concentrations. The x-intercept indicates the concentration of natural thymidine present within the interstices of the marine snow. Vertical bars are 95% confidence

limits.

46

A . L . ALLDREDGEand C. C. GOTSCHALK

estimate of the order of magnitude of natural concentrations. We divided a 300 ml slurry of seawater containing 1 aggregate m1-1 obtained by collecting 300 aggregates in 12, 25 ml syringes in the Santa Barbara Channel, into 30 sterile, acid-washed test tubes. Three tubes containing 0.2% formalin served as adsorption controls. [3H]thymidine yielding a final concentration of 20 nM plus unlabeled thymidine was added to nine subsets of three tubes each to yield final total thymidine concentrations of 20, 30, 40, 50, 60, 70, 80, 90 and 100 nM. All tubes were incubated for 45 rain in the dark at ambient seawater temperature and thymidine incorporation determined as in FUHRMAN and AZAM (1982). This study yielded levels of natural thymidine within marine snow on the order of 3 nM (Fig. 2b) Thus, we selected 20 nM concentrations of [3H]thymidine for addition as adequate to dilute natural concentrations and inhibit thymidine synthesis by bacterial cells (POLLARD and MORIARTY, 1984). Field methods. At each station we measured bacterial production, dry weight, and bacterial abundance on each of 30 particles and five surrounding seawater samples. The volume of each sample was adjusted to 4 ml. Multiple measurements of each aggregate were then obtained by disrupting and splitting each sample into three aliquots. Dry weight was measured on one, 2 ml aliquot as described above. A second, 1 ml aliquot was preserved in 2% formalin for later determination of bacterial abundance using the acridine orange technique (HOBBIE et al., 1977). The length and width of 150-200 bacteria from each station were also measured from enlarged photomicrographs. [3H]thymidine incorporation was measured on the third, 1 ml aliquot of each sample. Samples were incubated with 20 nM [3H]thymidine for 45 rain in the dark at ambient seawater temperature, extracted with cold TCA, and filtered onto 25 mm, 0.2 Bin Nuclepore filters as described in FUHRMAN and AZAM (1982). The means of five adsorption blanks each of snow or seawater containing 0.2% HgC12 were subtracted from each sample. Community composition. Ten aggregate samples and 25 ml of surrounding seawater at each station were preserved in 2% buffered formalin for later microscopic enumeration of associated organisms. Slides were prepared by the filter-transfer-freeze (FTF) technique (HEwES and HOLM--HANSEN,1983) and 10 fields covering 3.6% of the available filterable area on each slide were counted. Differences in the mean characteristics among aggregate types were tested statistically using single classification Analysis of Variance and the Student-Newman-Keuls test for comparisons among means of samples with unequal size. Data was transformed with a log transformation (SOKAL and ROHLF, 1969). Comparisons of mean bacteria size of attached and free-living bacteria were made with a Student's t-test. RESULTS

Origin of aggregates Marine snow occurred at all 13 stations in the Southern California Bight and California Current at densities ranging from 0.2 to 1.65 aggregates 1-1. Mean aggregate size, measured as dry weight, ranged from 7 to 150 gg agg-1. (Table 1). We categorized aggregates as to origin by their physical appearance, community composition, and the identity of the material forming the bulk of the matter in the particle. We encountered four types of marine snow; larvacean, fecal, diatom and miscellaneous. Discarded larvacean houses were easily identified by their spherical shape, small, uniform size

Origins of marine snow

47

(Table 1) and the presence of two particle-laden incurrent filters on the house exterior and one larger feeding filter within the interior of the house. Most of the houses observed were formed by Oikopleura longicauda or O. dioica (ALLDREDGE,1977). Marine snow categorized as "fecal" consisted largely of numerous macrocrustacean and copepod fecal pellets embedded in a mucus matrix laden with bacteria and naked flagellates. Marine snow of fecal origin was variable in size (Table 1) and contained very few diatoms (Table 2). Aggregates composed primarily of diatoms by volume and containing at least 30% diatoms and frustules by particle number (excluding bacteria) were categorized as diatom flocs (Table 2). These flocs were generally large (Table 1), up to 2-3 cm in length, cometshaped, brown-green, and porous in appearance. The final category, "miscellaneous", consisted of small aggregates composed of a variety of component particles, especially unidentifiable organic matter and debris, mucus, and a few fecal pellets. Identifiable cells of any type, except bacteria, were a minor component of these particles (Table 2). More than 95% of the aggregates present at any one station were of the same origin. Of the 13 stations investigated, six contained snow almost exclusively of diatom origin, three contained marine snow of larvacean origin, two were dominated by fecal snow, and two stations had aggregates composed of miscellaneous, unidentifiable debris. The 1-5% dissimilar aggregates at each station were usually larvacean houses or the feeding webs of planktonic pteropod molluscs.

Primary production Only marine snow of diatom origin showed substantial photosynthetic activity. The mean carbon fixation of all aggregates of miscellaneous or larvacean origin was less than 5 ng C agg. -1 h -1, while fecal snow averaged 21.8 ng C agg. -1 h-1. The mean production of each of these aggregate types was significantly lower (P < 0.01, ANOVA) than the mean of 132 ng C agg. -1 h -1 observed for diatom flocs (Table 2). This difference also persisted when primary production was normalized for aggregate size. The mean carbon fixation of diatom aggregates was 2.2 ng C lag agg. -1 h-1 (Table 2), significantly higher than the mean of 0.5 ng C lag agg. -1 h-1 for non-diatom aggregate types (P < 0.01). Primary production increased significantly with increased aggregate size for aggregates of diatom and fecal origin (Fig. 3a), but no significant relationship was found for marine snow of larvacean or miscellaneous origin. Chl a was also low in aggregates of larvacean, fecal and miscellaneous origin, averaging 1.5, 9.6 and 1.7 ng Chl a agg. -1, respectively. Diatom flocs averaged 78 ng Chl a agg. -1 and contained a mean of 1.3 _+ 1.6 ng Chl a lag dry wt-1, significantly higher than the mean of 0.2 + 0.1 ng Chl a lag-1 of all non-diatom snow types (P < 0.01; Table 2). Chl a content also increased significantly with increased aggregate size for diatom and fecal aggregates but showed no significant relationship for larvacean or miscellaneous snow (Fig. 3b). The proportion of total Chl (Chl a + phaeopigments) existing as Chl a was highly variable among particle types. While fecal aggregates had a uniformly low mean proportion of Chl a of 27%, Chl a averaged from 68 to 96% of total chlorophyll for aggregates of other types. Nearly 100% of the total chlorophyll in the surrounding seawater was Chl a at most stations, with the exception of stations containing diatom snow where Chl a in the surrounding seawater averaged 76 + 14% of total chlorophyll. Mean chlorophyll-specific primary production of aggregates across the 13 stations ranged from 0.8 to 3.8 ng Chl a-1 h -1 (Table 2) and was significantly lower for

Mean

NA NA

2.9 + 3.0 0.2 + 0.2 25.4 0.2 21.7 + 21.7 0.2 + 0.1 1.8 ___ 1.8 0.6 + 0.5 0.04 + 0.04 0.04 + 0.04

Nutrients (~M) NH + aggregates SW NO~ aggregates SW NO 2- aggregates SW PO4a- aggregates SW

NA NA 0-43.5 0.1-0.2 0-3.6 0-0.1 0-0.07 0-0.07

NA NA

1.7-3.6 0.1-1.0 1.3-1.6 0.1-0.5 1.1-1.2 91-100

1.9 5 + 6 8.9 + 9.3

2.4 0.2 1.3 0.1 0.1 5

11-21 0--6 62-75 0-14 0--1 3-4

4-20 1.0-1.6 0.11-0.30

+ + + + + +

5 3 7 7 1 1

19 + 22 1.2 + 1.4 0.21 + 0.10

2.9 0.2 1.5 0.1 1.2 96

Primary production ng C fixed agg. -1 h -1 ng C fixed Ixg agg. -1 h ~ ng Chl a agg. -1 ng Clad a lag agg. -~ ng C ng Chl a -1 h-1 Percent Chl a (Chl a/total Chl)

+ + + + + +

9-21

Range

Bacteria 105 ceils agg. -1 105 cells lag agg. -x Cell volume (pro3) Factor cell volume enlarged over free-living bacteria 10-16 mol thy lag agg. -1 h -1 10-21 mol thy cell-lh -1 Factor cellular thy uptake enriched over free-living cells Bacterial production (ng C agg-1 h -1)

16 3 69 7 1 4

15 + 6

Composition (% by number) Living diatoms Frustules and spines Naked flagellates Coccolithophorids Fecal pellets Miscellaneous

Dry weight (pg agg. -1)

Miscellaneous (n = 2)

+ + + + + +

+ + + + + + 31.8 0.3 8.2 0.3 1.4 12

2 1 17 10 7* 1

N 0.3 UD 2.8 + 2.4 UD 0.2 + 0.2 2.80 + 2.80 0.02 + 0.02

0.5 + 0.6

34.3 + 63.5

1.4 5 + 7 27.4 + 50.8

47 + 35 1.2 + 0.8 0.15 + 0.04

21.8 0.6 9.6 0.2 2.5 26

2 1 67 19 9 2

38 + 28

Mean

N NA UD 0.4-5.2 UD 0-0.3 0-5.60 0-0.40

NA

NA

NA NA

1.8--6.7 1.1-1.6 0.11-0.19

3.4-37.8 0.3--0.9 1.4-15.6 0.1-0.8 1.1-3.8 14-37

1--4 0-2 50-83 8-29 2-15" 1-3

10--65

+ + + + + +

4.0 0.6 0.5 0.2 1.2 16

13 12 4 1

3

10.9 0.20 46.2 0.6 1.2 0.3 10.80 0.06

+ + + + + + + +

10.7 0.01 30.3 0.4 1.7 0.3 8.90 0.03

0.1 + 0.1

3.9 + 4.2

1.1 6 + 7 7.0 + 7.5

22 + 22 2.2 + 2.2 0.12 + 0.01

5.0 0.7 1.7 0.2 2.2 76

9 + 0 63 + 18 + 6 + 4 +

8 + 2

Mean

0--25.4 0.17-0.20 3.9-73.3 0.2-1.2 0-3.6 0.1-0.7 0-21.70 0-0.10

0.06--0.19

1.0-7.0

2-9 0.9-17.6

12-31 1.1-3.3 0.11-0.12

2.3--6.3 0.4-1.6 0.6--2.1 0.1-0.7 0.8-3.8 55-92

5-12 0 42-98 1-29 1-11 3-5

7-10

Range

Larvacean (n = 3)

Aggregate type

Range

Fecal (n = 2)

23 4 17 3 2 2

5.7 0.2 2.8 1.0 0.2 0.2 1.80 0.04

+ + + + + + + +

8.2 0.1 3.4 1.6 0.3 0.1 3.60 0.05

1.4 + 1.4

2.4 + 1.1

1.3 11 + 12 16.3 + 15.2

0.2-23.9 0.1-0.4 0-9.9 0--4.4 0-0.9 0--0.3 0-9.90 0--0.13

0.4-2.3

1.8-28.0

3-17 5.6--20.8

0.14--0.21

0.16 + 0.04

15.9--463.7 0.3-5.1 4.6--343.0 0.1-3.1 1.8-3.4 33-91

24--92 3-14 3--48 0-9 0-8 0-3

20-149

+ 154.2 ___ 3.2 + 120.0 + 1.6 + 0.6 + 24

+ + + + + +

25-150

Range

58 + 64

132.3 2.2 77.9 1.3 2.6 68

57 7 29 3 2 2

62 + 42

Mean

Diatom (n = 6)

Table 2. Mean characteristics (___1 S.D.) of the four types of marine snow observed at 13 stations off of Southern Californm. Thirty aggregates were analysed at each station, n, Number of stations averaged; range, the range of station means. Miscellaneous particles composing aggregates included molts, clay-mineral particles, armoured dinoflagellates, protozoans, silicoflagellates and other rare algae. Nutrients presented as interstitial concentrations within aggregates. NA, not applicable because data available for only one station; UD, nutrient concentrations below detection limit; N, no data available; SW, seawater. *Dominated by large macrocrustacean pellets

4

103/

i lot

"g0"

1 0 1/ .... ?x

xo ~~"12-' q~ ~ °

"

-

101~

0~

io21o

A

10

10

10

10

3

-I

Dry Weight (ug agg ) 4

103f

~g 1 0 t

el~

A

e

1021

=O~ 101_c co

o

10°~ x 1() 1,

x~,u...,,~_ []

x

o_

:~E]

B

-2

10

100

101

10 2

10 3

-1

Dry Weight (ug agg )

^

"T

10 t

•;-ram 103~

10 z-

~

101_

"0 o

10

•i

10

a.

10

-1

C

-3 ,

-2

10

,

-1

10

r

0

i

1

10 10 10 -1 Chla (ng agg )

r

2

10

3

104

Fig. 3. Primary production and Chl a content of marine snow. Sofid circles, solid line - - diatom aggregates; squares, broken line - - marine snow of fecal origin; X - - larvacean houses and miscellaneous aggregates. (A) Primary production as a function of aggregate dry weight. Diatom and fecal aggregates were significantly correlated (P < 0.01). Diatom flocs: Y = 2.24 X °'s4, correlation coefficient ( c c ) = 0.54; fecal aggregates: Y = 0.54 X °s9, cc = 0.59. (B) CbJ a content as a function of aggregate dry weight. Diatom and fecal aggregates were significantly correlated ( P < 0.01); diatom flocs: Y = 1.17 X °s2, cc = 0.55; fecal aggregates: Y -- 0.15 X ° ~ , cc = 0.64. (C) Primary production per aggregate as a function of Chl a content. Regression for all aggregate types is significant ( P < 0.01), cc = 0.87.

50

A.L. ALLDREDGEand C. C. GOTSCHALK

miscellaneous aggregates (P < 0.05). Primary production increased significantly with increasing Chl a content of all aggregates, but the correlation was best for larger particles containing more than 2 ng Chl a agg. -1 (Fig. 3c) due to scatter introduced by working near the detection limit of the fluorometer.

Heterotrophic bacterial production Bacteria abundance. The mean abundance of bacteria on aggregates was highly variable across the stations, ranging from 1.2 to 14.9 million cells per aggregate and from 0.08 to 0.33 million cells Ixg agg. -1 (Table 2). The number of bacteria per ~tg of aggregate was significantly higher on larvacean aggregates than for aggregates of other origins. Many of these bacteria were filtered onto the house filters by the larvacean prior to house abandonment. Although the number of bacteria per aggregate increased significantly with increasing aggregate dry weight (Fig. 4a), large aggregates, regardless of origin, had significantly fewer bacteria per unit dry weight than did smaller ones (Fig. 4b). Heterotrophic production. Diatom aggregates had significantly higher thymidine incorporation per aggregate (P < 0.001), averaging 78 + 41 x 10-15 mol agg. -1 h-1, than did non-diatom aggregate types, which averaged only 8 + 14 x 10-15 mol agg. -1 h-1 collectively. This difference resulted, in part, from a significant increase in thymidine incorporation with increasing aggregate size (Fig. 4c) since most larger aggregates were diatom flocs. However, diatom aggregates also had significantly higher size-specific thymidine incorporation as well (Table 2). Bacteria on aggregates, regardless of aggregate type, were consistently more metabolically active than free-living bacteria. Mean cell-specific thymidine incorporation was significantly higher by 3-34 times for bacteria attached to aggregates than for free-living bacteria in the surrounding seawater. Bacteria on fecal snow were particularly active (Table 2), although their high enrichment of activity over free-living bacteria resulted, in part, from low activity by free-living forms. Thymidine incorporation per cell decreased significantly with increasing bacteria abundance per gg dry weight on aggregates (Fig. 4d). Bacteria on marine snow were also significantly larger by about 50% than free-living bacteria (P < 0.01), averaging 0.16 I.tm3 in volume vs 0.11 gm 3 for free-living forms, with the exception of bacteria on larvacean houses, which were the same size as free-living bacteria (Table 2). We calculated production rates of bacterial carbon by multiplying the mean cell volume by the carbon content per unit volume (assumed to be 0.12 pg C gm-3; FUnRUANand AZAM, 1980) by 1.4 X 1018 cells produced per mole of thymidine incorporated (HoBBIE and COLE, 1984). Mean bacterial production on marine snow ranged from 0.06 to 2.26 ng C agg. -1 h-1. Heterotrophic bacterial production per aggregate was significantly higher on diatom aggregates than on non-diatom aggregates (P < 0.001).

Nutrients Table 2 summarizes the mean nutrient concentrations within the interstitial fluid of aggregates and in the surrounding seawater for each snow type. Ammonia concentration within marine snow was enriched above that of an equal volume of surrounding seawater by up to 127 times at 80% of the 10 stations where ammonium was analysed. Nitrate and phosphate were enriched within aggregates at 54 and 46% of the stations, respectively. Nitrite was enriched within aggregates at only 23% of the stations. Nutrient enrichment within aggregates was not significantly correlated with aggregate type.

51

Origins of marine snow

10 ¸

,==

- lOsl ,°i/

A

==

107

xx

x

x rID

O

x•

•

x-x

i

10s

ee

~

x

< -~ 105

•

xO ° •

ee

•

D

•

101

i ,oi

Y = 501.187 X

102

-1

103

104

a o ~ ~ eeY o501 187 X °46

lo

m 102 10 o

101

Dry Weight (u9 a g g )

A

"6 ~

• •

D []

102t

[]-

~

+, •

~

~e •

10

10 I"

x

-

Y = 0.74 X

:

o83

1

-O+6a

Y = 9772 X

~ •

,•

.

ec 10] ~5

10 2

.

..c 10 +

1(~

104

D

~'o E 10101

® 10 0

I-

103 )

~= lO~

10 x

-1

,°i/

C

7~ 103

×

102

Dry Weight (ug agg

10

10 ~

[]

=~ lo 1

3 10'~ 100

x

lO+ 10 0

101

10 2

Dry Weight (ug

10 a

10

~-

10 a

-1

agg )

10 a

10 4

10 s

10 6 -1

Bacterial Abundance (No. ug agg

10 7

)

Fig. 4. Bacterial abundance and thymidine incorporation of marine snow from 13 stations in the Southern California Bight. Circles, diatom flocs; squares, aggregates of fecal origin; X, larvacean houses and miscelleous aggregates. Regression lines significant at P < 0.01 for all aggregate types combined. (A) Abundance of bacteria per aggregate as a function of aggregate dry weight, cc = 0.63. (B) Dry-weight specific bacterial abundance as a function of aggregate dry weight, cc = -0.56. (C) [3H]thymidine incorporation as a function of aggregate dry weight, cc = 0.54. (D) [3H]thymidine incorporation per bacteria cell vs bacterial density, cc = -0.43.

Contribution of aggregates to water column processes We calculated the mean percent contribution of marine snow larger than 2 mm in diameter to various processes and components of the water column at each station by multiplying the mean aggregate abundance per liter by the mean value of each variable per aggregate divided by the total value of that variable per liter of whole seawater. The results are summarized in Table 3. Marine snow of larvacean, fecal or miscellaneous origin made insignificant contributions, less than 1% in all cases, to total primary production, Chl a, total bacteria abundance, thymidine incorporation, and ammonium concentrations of the bulk seawater. A significant proportion of nitrate, nitrite and phosphate occurred on marine snow at a few stations, but only when these nutrients were undetectable in the surrounding seawater. With one exception, a maximum of 5% and generally less than 1% of total primary production, Chl a, bacterial abundance, thymidine incorporation and ammonium occurred on marine snow of diatom origin as well. The exception was 8 April when 17% of primary production, 30% of Chl a and 11% of thymidine incorporation occurred in

~

Zo ~

V

ooooo~

0

Z

VV

) 0

~z~ e ~

~.~ E

V

.r, 0

;II

o~

e~

e~

0

~==~'zz~

Origins of marinesnow

53

association with aggregates. The uniformly porous and homogeneous appearance of aggregates at this station and their exceptionally high proportion of healthy diatom cells (91%) indicated that they had been recently formed, probably within 24-48 h of sampling. Older diatom flocs appear more compacted and heterogeneous in composition, with internal clumps of higher density matter. The only consistent contribution made by all aggregate types was to total phaeopigments. Ten to 100% of the phaeopigments present in the bulk seawater occurred on aggregates at 62% of the stations. DISCUSSION Three of the four types of marine snow that were observed, diatom, fecal and miscellaneous aggregates, originated via physical coagulation of smaller component particles. Of these, only diatom flocs are formed by the aggregation of predominantly living cells. Diatom aggregates form via rapid mass flocculation of chain-forming diatoms at the termination of diatom blooms, possibly in response to nutrient stress (SMETACE~:, 1985; ALLDREDGEand GOTSCHALK, 1989). AS these flocs age in the water column they also accumulate non-living particles including fecal pellets and miscellaneous debris. Since the composition of aggregates formed by physical coagulation reflects the composition of smaller particles in the surrounding seawater present for aggregation, it is not surprising that almost all of the aggregates at any one station were of the same type. At stations where larvacean houses were abundant, we also encountered very few aggregates of other types. However, larvacean houses and other types of marine snow formed de novo by zooplankton can co-occur in abundance with other types of aggregates (ALLDREDGE, personal observation). The four types of marine snow that were observed do not make an exhaustive list of possible types. Additional de novo sources include pteropod webs (CARONet al., 1986), doliolid fecal pellets (POMEROYand DEIBEL, 1980) and phytoplankton with gelatinous sheaths (see ALLDREDGEand SILVER, 1988, for review). Additional types of marine snow could also form when other types of component particles are abundant for coagulation. For example, although particles composed of clay minerals were not observed, resuspended sediments are likely to be highly significant components of marine snow formed nearshore and in the nephaloid layer. Aggregates composed predominantly of coccolithophorids and the cells and spines of phaeodarians entangled in mucus and debris have also been described ( R I E ~ N , 1989). Diatom flocs were more abundant than other types of marine snow. This is consistent with coagulation theory, which predicts higher rates of aggregate formation with increasing abundance of component particles available for aggregation. Diatom blooms generate high abundances of chains, which, when physiologically stressed, develop the ability to stick on collision. Aggregate size, on the other hand, is a function of the strength of the polymers binding the component particles and the intensity of turbulent shear present to break aggregates apart (PARKERet al., 1972). Fecal aggregates attained large size via high strength. Considerable agitation was required to disrupt them. Diatom flocs, however, were both large and fragile suggesting that they were formed under conditions of relatively low turbulence. Primary production

Several previous studies have investigated the primary production of marine snow. The range of station means of 16-463 ng C agg. -1 h -1 for diatom aggregates was very similar to

54

A.L. ALLDREDGEand C. C. GOTSCHALK

the means of 20-591 ng C agg. -1 h -1 reported previously for diatom aggregates (ALLDREDGEand GOTSCHALK,1989). This range is also similar to the productivities of marine snow reported by KNAUERet al. (1982; 666 and 1078 ng C agg. -1 h-l). Neither of these later studies identified the origin or composition of their aggregates. However, because of the location of the Knauer study (Monterey Bay), the large size, high aggregate abundance (10 agg. 1-1), and "loose" appearance of their aggregates, it is our opinion that they were most certainly studying diatom flocs. The large aggregate volumes, location (Southern California Bight), and season (spring) of the study of ALLDREDGEand Cox (1982) suggest that their most productive snow may also have been of diatom origin. The carbon fixation of larvacean houses and miscellaneous aggregates reported here was lower than any previously reported values for marine snow. Both types of aggregates were very small. While we had expected larvacean houses to be photosynthetically active due to phytoplankton cells captured on their filters, the houses we encountered were old and predominately contained high populations of bacteria and small fagellates, many presumably filtered into the house while still occupied by the animal. Larger houses and those more recently discarded by their animals would be expected to be more productive. The results indicate that only marine snow of direct phytoplankton origin is a significant site (>5%) of photosynthesis in surface waters and then only episodically when the aggregates are recently formed. PREZELINand ALLDREDGE (1983) report one exception to this generalization. They sampled a station where 20% of primary production occurred on larvacean houses. However, the houses were recently formed and occurred at densities of 80 1-1. This is among the highest abundances reported in the literature (TAGUCHI,1982) and is unlikely to be encountered often. Comparison of recently formed flocs from 8 April, when 17% of primary production occurred in association with snow, with flocs from all other stations suggests that three conditions must be fulfilled for marine snow to be a significant site of primary production. First, aggregates must have a high Chl a content per unit dry weight with correspondingly high primary production. Only aggregates composed primarily of living phytoplankton contained more than 1 ng Chl a I.tg-1 and contributed more than 1% to total primary production. Second, aggregates must be abundant. Diatom flocs were 2- to 4-fold more abundant than were aggregates of other origins at their respective stations due to increased abundance of component particles available for aggregation. Finally, aggregates must be relatively large in size. For example, although diatom flocs at several of the stations were as photosynthetically active per unit dry weight as those from 8 April, they were 2- to 6-fold smaller. Thus, their total contribution to primary production was also correspondingly smaller. Removal of larger aggregates via settlement and possible particle erosion by turbulent mixing or grazing are the major processes reducing both aggregate size and abundance in the mixed layer over time. Thus, diatom flocs would be expected to make the largest contribution to primary production processes in surface waters directly after they are formed while they were still large, photosynthetically active and abundant. Although only one type of marine snow composed primarily of living phytoplankton was studied, other types of phytoplankton aggregates are known. The coccolithophorids Umbellicosphaera sibogae and Emiliani huxleyi and prymnesiophyte species, Phaeocystis pouchetii and Corymbellus aureus produce large colonies that dominate particulate flux on some occasions (Homo, 1981; CADEE, 1985, 1986; WASSMANN, 1987). A significant

Origins of marine snow

55

proportion of photosynthesis may also occur in association with aggregates when these phytoplankton are the main sources of marine snow in the water column as well. Heterotrophic bacterial processes Marine snow supports a diverse microbial community dominated by abundant bacteria and microflagellates. Aggregates are sites of active remineralization as indicated by their high ammonium content (this study; SHANKSand TRENT, 1979), abundant microflagellates, and high bacterial growth rates. The heterotrophic bacteria inhabiting all aggregate types in this study were growing at least three times faster than unattached forms in the surrounding seawater. This result is consistent with results for attached bacteria from midwater depths (ALLDREDGEand YOUNGBLUTH,1985) but deviates from ALLDREDGEet al. (1986) who found highly variable thymidine incorporation per cell on aggregates collected in both Pacific and Atlantic surface waters. Despite their active, microbial communities, a significant percentage of bulk heterotrophic bacterial production occurred in association with aggregates only when aggregates were abundant, large and newly formed; e.g. on recently formed diatom flocs. Newly formed particles are high in labile organic matter providing a rich substrate for heterotrophic bacteria. The most favorable conditions for bacterial growth on aggregates were on particles with low densities of bacteria per unit size (Fig. 4d). Bacteria under low density conditions may compete less for available resources and thus exhibit higher growth rates. Our results indicate that, although marine snow may provide significant microhabitats rich in nutrients and organic matter, attached bacteria are so rare relative to free-living forms that aggregates are unlikely to be significant sites of bacterial production except episodically on newly formed snow. This is consistent with previous studies of the significance of attached microbes on marine snow (ALLDREDGEand YOUNGBLUTH,1985; ALLDREDGE et al., 1986) and on aquatic particles in general (AzAMand HODSON, 1977; HANSON and WEIBE, 1977; DUCKLOW and KIRCHMAN, 1983) which indicate that all attached forms generally contribute less than 10% to total heterotrophic bacterial production. Bacterial populations on aggregates increase and decline very rapidly (under 72 h, POMEROY et al., 1984) and labile organic matter also becomes solubilized very quickly (1-2 days, SIMON, personal communication) on newly formed aggregates. Aggregate age and abundance, rather than specific origin, may be most important in determining the contribution of marine snow to heterotrophic microbial production. Static sampling, such as that used here, probably misses the relatively short time window during which bacterial activity is highest on most aggregates. Thus we may underestimate their significance. Aggregate age While the bulk of the visible matrix comprising marine snow results from collision of microaggregates, fecal pellets, and other large particles of relatively high mass, the abundance of microscopic organisms inhabiting aggregates, including bacteria, naked flagellates, and protozoans may reflect the age, successional stage, and availability of labile organic matter on the aggregate. Microbial successional changes are well documented on several types of marine snow and planktonic detritus. A rapid burst of bacterial growth after initial colonization is followed by increased populations of flagellates and protozoans. These graze the bacteria producing rapid decline of the

56

A.L. ALLDREDGEand C. C. GOTSCHALK

bacterial populations after 2-3 days (POMEROY and DEIBEL, 1980; POMEROYet al., 1984; DAVOLL and SILVER, 1986; GOTSCHALK, 1988). These successional changes suggest that aggregates with high bacterial and flagellate populations may be midway through the successional process and on the order of a few days old. Determination of aggregate age, especially by the relative proportions of components of the microbial community, is highly problematic. Many microbes present within larvacean houses, for example, are filtered into the house by the larvacena rather than originating from population growth of microbial colonizers. Moreover, marine snow formed by coagulation most likely consists of component particles of many disparate ages including both young and old fecal pellets, aged detrital debris, living diatom chains etc., each with its own complement of detrital microbes at various stages of successional development. The concept of age may have little meaning when applied to marine snow formed by coagulation of many variably aged component particles. Aggregates whose bulk matrices are formed at one time are exceptions. Larvacean houses, doliolid fecal pellets, and pteropod webs are produced and discarded at identifiable times. Recently formed larvacean houses have distinct and relatively particlefree outer perimeters, while the surfaces of aged houses become ragged and speckled with particles. Newly formed diatom flocs, such as those we observed on 8 April, can be identified by the homogeneous distribution of the diatom chains throughout the flocs, giving them a highly fluffy, uniformly porous appearance. However, once formed, all these particles begin to collide with surrounding particles. All begin to accumulate fecal pellets, organic debris, clay-minerals and micro-organisms of disparate ages as their own original complement of organic matter also undergoes degradation. Thus, it may be possible to approximate the age of only very recently formed marine snow. CONCLUSIONS

The results indicate that there is high variability in the characteristics of different types of marine snow. However, only aggregates formed predominantly of living phytoplankton are likely to be significant sites of photosynthetic activity in the euphotic zone. Moreover, while bacterial production is relatively high on all types of aggregates, regardless of origin, a significant proportion of both primary production and heterotrophic microbial production occurs on marine snow only episodically soon after mass formation of aggregates. This is a consequence, not of low biological activity of organisms inhabiting aggregates, but of the low abundance of attached algae and bacteria relative to free-living forms in the water column most of the time. Moreover, we caution that our results apply only to photosynthetic and heterotrophic microbial processes. The high interstitial ammonium concentrations and potentially low oxygen concentrations (ALLDREDGE and COHEt~, 1987) occurring within marine snow suggest that these aggregates sustain unique chemical microhabitats where many other biological processes important in the recycling of organic matter may be active. Acknowledgements--We thank K. Crocker, D. Campbell, N. Larsen, D. Steller, E. Schnitzler, D. Martin, J. Alstatt, D. Steinberg, B. Harrison and F. Fabry for diving and technical assistance, the captain and crew of the R.V. Point Sur for field support, and especially E. Triplett for advice on the RNA-DNA-protein analysis. This study was funded by NSF grants OCE85-10826 and OCE88-00396.

Origins of marine snow

57

REFERENCES ALLDREDGE A. L. (1977) House morphology and mechanisms of feeding in the Oikopleuridae. Journal of Zoology, 181, 175-188. ALLI)REDGE A. L. and J. L. Cox (1982) Primary production and chemical composition of marine snow in surface waters of the Southern California Bight. Journal of Marine Research, 40, 517-527. ALLDREDGEA. L. and M. J. YOUNOBLUTH(1985) The significance of macroscopic aggregates (marine snow) as sites for heterotrophic bacterial production in the mesopelagic zone of the subtropical Atlantic. Deep-Sea Research, 32, 1445-1456. ALLDREDGEA. L. and Y. COHEN (1987) Can microscale chemical patches persist in the sea? Microelectrode study of marine snow, fecal pellets. Science, 235, 689-691. ALLDREDGE A. L. and C. C. GOTSCHALK(1988) In situ settling behaviour of marine snow. Limnology and Oceanography, 33, 339--351. ALLDREDGE A. L. and M. W. SILVER (1988) Characteristics, dynamics and significance of marine snow. Progress in Oceanography, 20, 41-82. ALLDREDGEA. L. and C. C. GOTSCHALK(1989) Direct observations of the mass flocculation of diatom blooms: characteristics, settling velocities and formation of diatom aggregates. Deep-Sea Research, 36, 159-171. ALLDREDGE A. L., J. J. COLE and D. A. CARON (1986) Production of heterotrophic bacteria inhabiting macroscopic organic aggregates (marine snow) from surface waters. Limnology and Oceanography, 31, 68-78. AZAM F. and R. E. HODSON(1977) Size distribution and activity of marine microheterotrophs. Limnology and Oceanography, 22, 492-501. BEERS J. R., J. D. TRENT, F. M. H. REID and A. L. SHANKS(1986) Macroaggregates and their phytoplanktonic components in the Southern California Bight. Journal of Plankton Research, 8, 475-487. CADEE G. C. (1985) Macroaggregates of Emiliana huxleyi in sediment traps. Marine Ecology Progress Series, 24, 193-196. CADEE G. C. (1986) Organic carbon in the water column and its sedimentation, Fladen Ground (North Sea), May 1983. Netherlands Journal of Sea Research, 20, 347-358. CARON D. A., P. G. DAVIS, L. P. MADr~ and J. MeN. SIEBURTH(1986) Enrichment of microbial populations in macroaggregates (marine snow) from surface waters of the North Atlantic. Journal of Marine Research, 44, 543-565. DAVOLL P. J. and M. W. SILVER (1986) Marine snow aggregates: life history sequence and microbial community of abandoned larvacean houses from Monterey Bay, California. Marine Ecology Progress Series, 33, 111-120. DUCKLOWH. W. and D. L. KIRCHMAN(1983) Bacterial dynamics and distribution during a diatom bloom in the Hudson River Plume, USA. Journal of Plankton Research, 5,333-355. FITZWATER S. E., G. A. KNAUERand J. H. MARTtN (1982) Metal contamination and its effect on primary production measurements. Limnology and Oceanography, 27, 544-551. FUHRMANJ. A. and F. AZAM (1980) Bacterioplankton secondary production estimates for coastal waters of British Columbia, Antarctica and California. Applied and Environmental Microbiology, 39, 1085-1095. FUH~ J. A. and F. AZAM (1982) Thymidine incorporation as a measure of heterotrophic bacterioplankton production in marine surface waters: Evaluation and field results. Marine Biology, 66, 109-120. GOTSCHALKC. C. (1988) Enhanced primary production and nutrient regeneration within aggregated marine diatoms: Implications for the mass flocculation of diatom blooms. Masters Thesis, University of California, Santa Barbara, 49 pp. HA~qSONR. B. and W. J. WmBE (1977) Heterotrophic activity associated with particulate size fractions in a Sparfina alterniflora saltmarsh estuary, Sapelo. Is., Georgia, USA and the Continental shelf waters. Marine Biology, 42, 321-330. HEWES C. D. and O. HOLM-HANSEN (1983) A method for recovering nanoplankton from filters for identification with the microscope: The filter-transfer-freeze (FrF) technique. Limnology and Oceanography, 28, 389-394. HOBBIE J. E. and J. COLE (1984) Response of the detrital food web to coastal eutrophication. Bulletin of Marine Science, 35, 357-363. HOBBIE J. E., R. J. D~a~EVand S. JASPER(1977) Use of Nuclepore filters for counting bacteria by fluorescence microscopy. Applied and Environmental Microbiology, 33, 1225-1228. HONJO S. (1982) Seasonality and interaction of biogenic and lithogenic particulate flux at the Panama Basin. Science, 218, 883-884. JorIrqSON K. S., R. L. PETTY and J. THOMSEN (1985) Flow-injection analysis of seawater micronutrients. Advances in Chemistry Series, 209, 7-30. D. M. (1982a) Microbial transformations of organic matter at oceanic interfaces: A review and perspectus. EOS, 63, 138-140.

58

A . L . ALLDREDGEand C. C. GOTSCHALK

KARL D. M. (1982b) Selected nucleic acid precursors in studies of aquatic microbial ecology. Applied and Environmental Microbiology, 44, 891-902. I~AtrER G., D. HEBEL and F. Cn'RIANO (1982) Marine snow: Major site of primary production in coastal waters. Nature, 300, 630--631. LEGENDREL., S. DEMERS,C. M. YENTSCHand C. S. YENTSCH(1983) The 14C method: Patterns of dark CO2 fixation and DCMU correction to replace the dark bottle. Limnology and Oceanography, 2,8, 996--1003. MCCAVE I. N. (1984) Size spectra and aggregation of suspended particles in the deep ocean. Deep-Sea Research, 31,329-352. PARKER D. S., W. J. KAOFMA~ and D. JENKINS (1972) Floc break-up in turbulent flocculation processes. Journal of the Sanitary Engineering Division of the American Society of Ovil Engineers, 98, 79-99. PARSONST. R., Y. MALTAand C. M. LALLI (1984) A manual of chemical and biological methods for seawater analysis. Pergamon Press, New York, 173 pp. POLLARD P. C. and D. J. W. MORIARTV (1984) Validity of the tritiated thymidine method for estimating bacterial growth rates: Measurement of isotope dilution during DNA synthesis. Applied and Environmental Microbiology, 48, 1076--1083. POMEROY L. R. and D. DEmEL (1980) Aggregation of organic matter by pelagic tunicates. Limnology and Oceanography, 25, 643-652. POMEROV L. R., R. B. HANSON, P. A. McGtLLIVARY, B. F. SHERR, D. KmCHMAN and D. DEraEL (1984) Microbiology and chemistry of fecal products of pelagic tunicates: Rates and fates. Bulletin of Marine Science, 35, 426--439. PREZELIN B. B. and A. L. ALLDREDGE(1983) Primary production of marine snow during and after an upwelling event. Limnology and Oceanography, 28, 1156-167. RIEMANNF. (1989) Gelatinous phytodetritus on the Atlantic dead-sea bed. Marine Biology, 100, 533-539. SHANKSA. L. and J. D. TRENT (1979) Marine snow: microscale nutrient patches. Limnology and Oceanography, 24, 850-854. SILVERM. W., A. L. SHANKSand J. D. TREYr (1987) Marine snow: microplankton habitat and source of smallscale patchiness in pelagic populations. Science, 201,371-373. SOKAL R. R. and F. J. ROHLF (1969) Biometry, Freeman, San Francisco, 776 pp. SOLORZANO L. (1969) Determination of ammonium in natural waters by the phenolhypochlorite method. Limnology and Oceanography, 14, 799-801. SMETACEKV. S. (1985) Role of sinking in diatom life-history cycles: ecological, evolutionary and geological significance. Marine Biology, 84, 239-251. STUMMW. and J. P. MORGAN(1981) Aquatic chemistry. Wiley-Interscience, 731 pp. TAGUCHIS. (1982) Seasonal study of fecal pellets and discarded houses of appendicularia in a subtropical inlet, Kaneohe Bay, Hawaii. Estuarine, Coastal and Shelf Science, 14, 545-555. WASSMANNP. (1987) Sedimentation of organic matter and silicate out of the euphotic zone of the Barents Sea. EOS, 68, 1728.