Effects of green macroalgal blooms on the meiofauna community structure in the Bay of Cádiz

Marine Pollution Bulletin xxx (2013) xxx–xxx

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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul

Effects of green macroalgal blooms on the meiofauna community structure in the Bay of Cádiz J. Bohórquez a,⇑, S. Papaspyrou a,b, M. Yúfera b, S.A. van Bergeijk a,1, E. García-Robledo a, J.L. Jiménez-Arias a, M. Bright c, A. Corzo a a b c

Department of Biology, Faculty of Marine and Environmental Science, University of Cádiz, Pol. Río San Pedro s/n, 11510 Puerto Real, Spain Instituto de Ciencias Marinas de Andalucía (ICMAN-CSIC), Pol. Río San Pedro s/n, 11510 Puerto Real, Spain Department of Marine Biology, University of Vienna. Althanstr. 14, 1090 Vienna, Austria

a r t i c l e

i n f o

a b s t r a c t

Key words: Meiofauna Macroalgal bloom Intertidal sediment Bay of Cádiz

The effect of macroalgal blooms on the abundance and community structure of intertidal sediment meiofauna was studied using an in situ enclosure experiments (Bay of Cádiz, Spain). Meiofaunal abundance (3500–41,000 ind 10 cm2) was three to sevenfold higher in the presence of macroalgae. Nematoda were the dominant taxon both in Control (52–82%) and Macroalgae plots (92–96%), followed by Harpacticoida Copepoda and Ostracoda. Non-metric Multi-Dimensional Scaling (MDS) analysis clearly separated the meiofaunal community from Control and Macroalgae plots. Organic matter, organic carbon, total nitrogen, chlorophyll a and freeze-lysable inorganic nutrients were higher in Macroalgae plots, and were highly correlated with the horizontal MDS axis separating Control and Macroalgae meiofaunal communities. Meiofaunal abundance and taxonomic composition in the Bay of Cádiz seem to be bottom–up controlled either through a grazer system based on microphytobenthos in bare sediments or through a decomposer system in macroalgae affected sediments. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Giere, 1993; Knox, 2000), at a very small vertical scale, which in turn affects meiofaunal diversity and abundance (Coull, 1999; Findlay and Tenore, 1982). Macroalgae have a significant impact on a number of key variables in intertidal sediments (Corzo et al., 2009; García-Robledo and Corzo, 2011; García-Robledo et al., 2008). On the one hand, macroalgae reduce the light quantity reaching the sediment surface up to 95%, thus reducing photosynthetic activity of benthic microalgae. As a consequence, microphytobenthic biomass, a food resource for many meiobenthic species, is reduced and hypoxic/anoxic conditions develop near to the sediment surface (Corzo et al., 2009). On the other hand, macroalgal biomass increases the organic matter content of the sediment favouring meiobenthic detritivores. The balance between these processes determines ultimately the abundance and vertical distribution of meiofauna and its community structure in shallowwater ecosystems (Gee et al., 1985; Sandulli and De Nicola Giudici, 1989; Vopel et al., 1998). Aim of our study was to study the effects of green macroalgal blooms of Ulva spp. on the intertidal meiofaunal community structure. To achieve this we used an enclosure experimental setup in situ. We wanted to determine whether the presence of macroalgae (1) has an overall negative or positive effect on meiobenthic abundance compared with bare sediments, (2) changes the vertical distribution of meiofauna, promoting higher abundances near the sediment surface and (3) produces a shift of the meiofaunal community structure. For this purpose we compared two experimental

Coastal zones represent only 7–8% of marine areas but are some of the most productive marine environments, exhibiting in addition a high organismal diversity (Alongi, 1998). However, coastal areas are largely affected by increasing human pressure, resulting in increased nutrients’ load and frequency of eutrophication events (GEO5, 2012). In estuaries and other shallow-water ecosystems, eutrophication often manifests itself in the form of green macroalgal blooms, with macroalgae accumulating on the sediment surface in large quantities (Valiela et al., 1997). These blooms have negative effects on the abundance and diversity of the benthic community such as microbenthos (Sundbäck et al., 1996), meiofauna (Neira and Rackemann, 1996), macrofauna (Norkko and Bonsdorff, 1996), and seagrasses (den Hartog, 1994). In addition, macroalgal blooms affect the ecosystem function of shallow-water systems by altering rates of carbon and nitrogen biogeochemical cycles (Corzo et al., 2009; García-Robledo and Corzo, 2011; García-Robledo et al., 2008). Intertidal zones are highly dynamic environments characterised by large fluctuations of abiotic and biotic variables (Coull, 1999; ⇑ Corresponding author. Tel.: +34 956 016177; fax: +34 956 016019. E-mail address: [email protected] (J. Bohórquez). Present address: IFAPA Centro El Toruño, Junta de Andalucía, Camino Tiro del Pichón s/n, 11500 El Puerto de Santa María, Spain. 1

0025-326X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpolbul.2013.02.002

Please cite this article in press as: Bohórquez, J., et al. Effects of green macroalgal blooms on the meiofauna community structure in the Bay of Cádiz. Mar. Pollut. Bull. (2013), http://dx.doi.org/10.1016/j.marpolbul.2013.02.002

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J. Bohórquez et al. / Marine Pollution Bulletin xxx (2013) xxx–xxx

in situ enclosures with and without macroalgae in the Sancti Petri Channel of the Bay of Cádiz, Spain between July 2002 and May 2003. 2. Materials and methods 2.1. Study site and experimental design of in situ experiment The experiments were performed at an intertidal mud flat of the Sancti Petri Channel (36°250 23.500 N, 6°120 53.800 W, Bay of Cádiz, Spain) from July 2002 to May 2003 (Fig. 1). Sancti Petri Channel is a permanent channel connecting the inner Bay of Cadiz with the Atlantic Ocean. This channel is influenced by semi-diurnal tides with a mean amplitude of 2.2 m. Sediment at the sampling site consisted of >90% silt and clay particles (<63 lm). This channel is affected by a permanent green macroalgal bloom (mainly tubular Ulva spp.) throughout the year. Macroalgal biomass changed seasonally with a minimum in winter and a maximum in summer (Table 2) (Corzo et al., 2009). Once per season (July and November 2002 and February and May 2003), two plots (1.5  1.5 m2) were selected; one without macroalgae (Control) and one with macroalgae naturally present on top of the sediment (+Macroalgae). The typical separation between plots was about 15 m. Around each plot, an enclosure was installed to avoid movement of macroalgae out of the plot (+Macroalgae) or into the plot (Control). Enclosures (2 m height) were constructed by a frame of PVC tubes and a plastic net (2 cm mesh size) and placed around the plots for a minimum of two weeks before each sampling. The upper edge of the enclosures was always higher than spring tides height. 2.2. Meiofauna sampling Three sediment cores (10 cm length, 2.4 cm internal diameter) were collected in each plot once per season. The cores were sliced in 0.5 cm layers down to 1.5 cm depth. Samples were fixed with 4% formalin in filtered seawater and stored at 4 °C until analysis. For the Macroalgae plots, the macroalgae covering the sediment (Canopy) were separated from the sediment and fixed.

36° 35' 44'' N 6° 16' 25'' W

Temporary and permanent metazoan meiofaunal specimens passing through a 500 lm mesh size and retained on a 42 lm mesh size were counted and identified to higher taxa level using a dissecting microscope. Foraminifera, part of the permanent meiofauna, were abundant but not quantified as we could not determine if they were alive at the time of sampling. Total abundances of meiofauna in the 0–1.5 cm depth layer were standardized to 10 cm2 surface area. 2.3. Environmental variables 2.3.1. Freeze-lysable inorganic nutrients Nutrients (nitrate, ammonium, phosphate and silicate) were determined from cores (1.2 cm i.d.; n = 6) randomly collected from both enclosure types. After freezing (80 °C), sediment cores were sliced at a 1 mm resolution. Slices corresponding to the same depth were pooled and centrifuged to extract pore water as described in Corzo et al. (2009). Inorganic nutrients concentrations were measured using a TRAACS-800 autoanalyzer. Data from the three first millimetres were averaged. 2.3.2. Organic matter, total organic carbon and total nitrogen contents Organic matter content was determined in the first centimetre of the sediment (1.2 cm i.d.; n = 3) by loss on ignition (Heiri et al., 2001). Carbon and nitrogen content was measured on an elemental CHNS analyzer (LECO CHNS 932) (Corzo et al., 2009). 2.3.3. Microbenthic net production and respiration rates Oxygen concentration was measured with oxygen microelectrodes (Unisense A/S, Denmark) (Revsbech, 1989) in the light (800 lmol photons m2 s1) and in darkness as described in Corzo et al. (2009). Net production (Pn) and dark respiration (Rd) rates were calculated according to Corzo et al. (2009). 2.3.4. Chlorophyll a Chlorophyll a in the first 5 mm was extracted in 2 ml 100% methanol at 4 °C overnight in darkness (Thompson et al., 1999). Extracts were centrifuged at 4500 rpm for 10 min and absorption

36° 36' 24'' N 6° 08' 09'' W

(a) 36° 34' 16'' N 6° 06' 45'' W

(b)

N 36° 25' 10'' N 6° 06' 10'' W 2 km

Fig. 1. Map showing location of the study site (a) and drawing of the experimental set-up (b).

Please cite this article in press as: Bohórquez, J., et al. Effects of green macroalgal blooms on the meiofauna community structure in the Bay of Cádiz. Mar. Pollut. Bull. (2013), http://dx.doi.org/10.1016/j.marpolbul.2013.02.002

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J. Bohórquez et al. / Marine Pollution Bulletin xxx (2013) xxx–xxx

Table 1 Seasonal variations in abundance of major meiofaunal taxonomic groups in bare sediment (Control), in sediment below macroalgal mats (+Macroalgae) and within the macroalgal canopy. Data shown are mean number of individuals per 10 cm2 ± SE. Combined group ‘‘Other’’ includes polychaeta, oligochaeta, gastropoda (Hydrobia spp.), gastropoda larvae, amphipoda, bivalvia larvae, cyclopoid copepods and unknown arthropoda. Total Mean ± SE

Nematoda Mean ± SE

Harpacticoida Copepoda Mean ± SE

Ostracoda Mean ± SE

Other Mean ± SE

Control July November February May

3519 ± 1010 4686 ± 1020 6720 ± 375 14,220 ± 800

1540 ± 430 3825 ± 1140 5080 ± 960 11,650 ± 1200

2±2 60 ± 31 260 ± 200 270 ± 56

27 ± 19 78 ± 39 82 ± 55 130 ± 41

1949 ± 766 722 ± 217 1299 ± 1080 2167 ± 1157

+Macroalgae July November February May

20,900 ± 5640 32,026 ± 10,710 37,620 ± 5530 38,565 ± 13,824

20,260 ± 5640 31,470 ± 10,670 36,790 ± 5380 37,170 ± 13,640

42 ± 33 177 ± 92 224 ± 63 191 ± 48

170 ± 20 230 ± 70 235 ± 130 360 ± 179

430 ± 150 152 ± 69 375 ± 140 846 ± 248

3846 ± 1530 4820 ± 3820 1225 ± 710 2690 ± 1380

3590 ± 1520 3460 ± 2830 640 ± 200 1980 ± 1050

159 ± 59 985 ± 740 484 ± 413 530 ± 320

50 ± 24 310 ± 194 89 ± 89 94 ± 30

43 ± 43 62 ± 56 9±5 80 ± 26

Canopy July November February May

spectra (400–800 nm) were measured using a Unicam UV/VIS UV2Ò spectrophotometer. 2.4. Statistical analysis Differences in meiofaunal abundance (ln(x + 1) transformed) and differences in higher taxa percentages (arcsin transformed) over time due to the presence/absence of macroalgae were tested by analysis of variance (ANOVA). Statistical analyses were performed using the software Statgraphics Plus 5.1. Community structure was analyzed by non-metric Multi-Dimensional Scaling (MDS) using the Bray-Curtis similarity index on square root transformed data. Projection biplots were drawn onto the MDS axes to examine their relationship with measured environmental variables. Significant differences between communities were tested using PERMANOVA (Anderson, 2001). Multivariate analyses were performed with the PRIMER 6.0 software package (Plymouth Marine Laboratory, UK). 3. Results 3.1. Abundance and taxonomic composition of meiofauna Total meiofaunal abundance in the sediment of the intertidal mudflat of the Sancti Petri Channel, Bay of Cádiz, ranged from 3500 to 41,000 ind 10 cm2. The macroalgal canopy, an additional habitat for meiofauna, had a total meiofauna abundance varying from 1200 to 4800 ind 10 cm2. In the presence of macroalgae, meiofaunal abundances in the sediment (32,200 ± 8100 ind 10 cm2) were similar throughout the experiment. In control bare sediment, abundance was significantly higher in May 2003 (12,100–15,900 ind 10 cm2) compared to July 2002 and February 2003 (3500–6700 ind 10 cm2). Meiofaunal abundance in the sediment was three to seven times higher in the presence of macroalgae compared to the control bare sediment (p < 0.001, two-way ANOVA), except for May 2003 (Fig. 2) (Table 1). This was due to significant differences only in the upper 0.5 cm when compared to control sediment (p < 0.05, two-way ANOVA). A total of twelve higher metazoan taxa were identified in Sancti Petri Channel during the sampling period. Nine taxa were found in bare sediment and 8 taxa in sediment covered with macroalgae. The macroalgae canopy contained 9 higher taxa. Nematoda were the dominant taxon both in bare sediment (52–82% of total abundance) and in sediments covered with macroalgae (92–96%) (Table 1). Nematoda abundance increased six times in the presence

of macroalgae compared to bare sediment (Fig. 3). Harpacticoida Copepoda and Ostracoda represented less than 3% of the total meiofaunal abundance in both treatments, with their abundance being four times higher in the presence of macroalgae (p < 0.05, one-way ANOVA). Temporary meiofauna, including gastropod and bivalve larvae as well as some unidentified larvae, were more abundant in bare sediment (15–41%) than in sediment covered with macroalgae (1–2%) (Fig. 3). Another eight higher taxonomic categories (polychaetes, oligochaetes, Hydrobia spp. gastropod, amphipods, cyclopod copepods, arthropods) contributed less than 1% of the total abundance (Table 1). Meiofaunal abundance from other taxonomic groups (temporary meiofauna, polychaetes, amphipods) decreased significantly in the presence of macroalgae (p < 0.05, ANOVA one-way) (Fig. 3). 3.2. Vertical distribution of meiofauna Meiofaunal abundance showed maxima values in the upper 0.5 cm layer and a decrease with depth in both control and macroalgae-covered sediment (Fig. 4). Nematoda and Ostracoda showed the typical vertical distribution with higher abundance in the upper layer decreasing with depth (Fig. 5). Harpacticoida Copepoda was restricted to the upper 0.5 cm layer. Similarly to the macroalgae covered sediment, Nematoda were the dominant taxon in the macroalgal canopy. However, Harpacticoida Copepoda represented a higher fraction (10–50%) of the total community compared to the sediment covered by macroalgae (less than 1%) (p < 0.05, ANOVA one-way) (Fig. 5). 3.3. Environmental variables The presence of macroalgae over the sediment induced important changes in the environmental variables measured (Table 2). With the exception of May, chlorophyll content was two to fourfold higher in the macroalgae plot compared to the control. Similarly, organic matter content, total organic carbon and total nitrogen were generally higher in the presence of macroalgae, showing a maximum in November. The differences between macroalgal and control plots were most pronounced for freeze lysable nutrients, being up to two order of magnitudes higher in the for4  3 mer (NHþ 4 2–4, NO3 10–100, SiO4 2–14, PO4 3–9 times). Net Production and Respiration rates were higher in control plot in all seasons, with highest rates being observed in February (see Corzo et al. (2009) for extended description of biogeochemical changes induced by macroalgae).

Please cite this article in press as: Bohórquez, J., et al. Effects of green macroalgal blooms on the meiofauna community structure in the Bay of Cádiz. Mar. Pollut. Bull. (2013), http://dx.doi.org/10.1016/j.marpolbul.2013.02.002

34.7 56.3 25 21.9 372.6 229.1 64.3 29.7 6.2 10.5 4.1 5.8 335.4 4180.8 573.8 213 1082.5 1735.8 1074.8 463.1 1.5 ± 1 0.9 ± 0.4 1.2 ± 0.3 0.8 ± 0.1 4.5 ± 0.7 2.4 ± 0.2 7.3 ± 2.7 2.5 ± 0.8 0.41 ± 0.03 0.36 ± 0.01 0.36 ± 0.01 0.25 ± 0.02 July November February May +Macroalgae

281.1 ± 22.9 123.3 ± 25.2 208.9 ± 8.5 255.8 ± 133

10.5 ± 6.28 9.0 ± 0.49 23.8 ± 8.31 5.1 ± 2.55

13.5 9.9 13.1 10.9

2.45 ± 0.13 2.15 ± 0.06 1.86 ± 0.08 1.55 ± 0.05

6.95 ± 0.12 6.90 ± 0.06 6.09 ± 0.09 7.19 ± 0.40

15.5 3.8 5 6.4 68.3 25.5 20.9 23.4 8.2 2.2 2.6 3.9 34 42 10.3 18.1 555.4 386.4 165.9 191.7 2.3 ± 0.4 2.3 ± 0.1 3 ± 0.8 2.2 ± 0.2 9.2 ± 1.5 7.7 ± 0.1 9.3 ± 1.3 16 ± 2.2 7.87 ± 0.73 7.93 ± 0.37 7.13 ± 0.06 7.13 ± 0.23 0.27 ± 0.03 0.25 ± 0.001 0.24 ± 0.01 0.26 ± 0.01 1.82 ± 0.32 1.72 ± 0.09 1.49 ± 0.07 1.61 ± 0.09 July November February May Control

– – – –

2.5 ± 1.24 4.0 ± 0.41 5.5 ± 3.16 8.6 ± 5.84

10.6 8.8 10.1 14.2

TN (% DW) Time (months) Treatment

Macroalgae biomass (g DW m2)

Chl. a (lm/ sediment (DW))

Organic matter (%)

TOC (% DW)

C:N (mol/ mol)

Net production (mmol O2 m2 h1)

Respiration (mmol O2 m2 h1)

NHþ 4 (lmol L1)

NO 3 (lmol L1)

NO 2 (lmol L1)

PO3 4 (lmol L1)

SiO4 4 (lmol L1)

J. Bohórquez et al. / Marine Pollution Bulletin xxx (2013) xxx–xxx Table 2 Chemical and biological characteristics of the sediment in bare sediment (Control) and in sediment below macroalgal mats (+Macroalgae). Data represent mean (n = 3) and SE when available. TOC and TN correspond to total organic carbon and total nitrogen. C:N is the ratio between carbon and nitrogen.

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Fig. 2. Seasonal changes in meiofaunal abundance in the presence and absence of macroalgae. Canopy meiofauna has been considered together with the sediment macroalgal abundance data. Data represent mean ± SE (n = 3).

3.4. Relationship between the community structure and environmental variables Non-metric Multi-Dimensional Scaling (MDS) analysis on square root transformed community data clearly distinguished the bare sediment meiofaunal community from that of sediment covered by macroalgae (Fig. 6). In addition, the community in sediments affected by macroalgae showed a higher within similarity, than that of the bare sediment (Fig. 6). PERMANOVA test verified that differences between both communities were significant (pairwise comparisons, p < 0.024), except in May (p > 0.05). Several environmental variables showed a high correlation with the MDS axes (Fig. 6). Macroalgal biomass, organic matter, organic carbon and nitrogen, and freeze-lysable inorganic nutrients correlated with the horizontal axis separating the Control and +Macroalgae plots indicating that these may exert some role in determining community changes. Respiration in the dark (Rd) and net production (Pn) showed a high correlation with the vertical axis and seemed to be related with differences occurring in July and May in bare sediment.

4. Discussions 4.1. Abundance and taxonomic composition of meiofauna Abundance of meiofauna in the bare sediment of our sampling site in the Bay of Cadiz (Fig. 2) was within the range reported for comparable intertidal environments (Coull, 1989; Phillips and Fleeger, 1985). The taxonomic composition was similar to that found in other intertidal areas (McArthur et al., 2000; McGwynne et al., 1988). Nematoda was the dominant taxon, representing between 52% and 82% of the total community in bare sediment. Harpacticoida Copepoda and Ostracoda, the second and third most abundant taxa in muddy sediments, accounted for 2% and 1% of the total community, respectively. The dominance of Nematoda and the presence of Harpacticoida Copepoda and Ostracoda is a typical characteristic of muddy sediments (Fenchel, 1978). The accumulation of macroalgae on top of the sediment produced significant modifications of the sediment environment (Table 2) that probably also affected the meiofauna community. Macroalgal accumulation reduced oxygen availability and oxygen penetration depth in the dark, favouring an increase in sulfate reduction activity and release of hydrogen sulfide near the sediment surface (Corzo et al., 2009; García-Robledo et al., 2008). One would expect that the decrease of oxygen availability and increase in hydrogen sulfide concentration would reduce meiofaunal

Please cite this article in press as: Bohórquez, J., et al. Effects of green macroalgal blooms on the meiofauna community structure in the Bay of Cádiz. Mar. Pollut. Bull. (2013), http://dx.doi.org/10.1016/j.marpolbul.2013.02.002

J. Bohórquez et al. / Marine Pollution Bulletin xxx (2013) xxx–xxx

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Fig. 3. Percentage contribution of the major meiofauna taxonomic groups in the presence and absence of macroalgae. Canopy meiofauna has been considered together with the sediment macroalgal abundance data. Notice break in vertical axes.

Fig. 4. Vertical distribution of meiofaunal abundance in the presence and absence of macroalgae. Data are presented as mean ± SE (n = 3).

abundance (Coull, 1989; Neira and Rackemann, 1996). However, other studies have shown that meiofauna, and in particular Nematoda, can tolerate the presence of hydrogen sulfide for long periods of time (Schratzberger et al., 2004; Wetzel et al., 2002). Indeed, our results showed a three to sevenfold increase of meiofaunal abundance in the presence of macroalgae compared to bare sediment, primarily attributed to a dramatic increase in both the absolute and relative abundance of Nematoda (Figs. 2 and 3). This increase in meiofaunal abundance and the increase in Nematoda dominance could be the result of an increase of organic matter input to the sediment from the macroalgae (Knox, 2000). Macroalgae increased the organic matter supply to the sediment (Table 2) (Corzo et al., 2009; García-Robledo et al., 2008), which probably stimulated microbial degradation and detritic trophic webs within the sediment (Montagna, 1984; Montagna et al., 1983). Therefore, bottom–up interactions could explain the increase in meiofauna abundance and the major changes found in the community composition. Alternatively, meiofauna abundance might be controlled by top–down interactions. Meiofauna is a food source for sediment macrofauna and other secondary consumers, such as pelagic fish and birds (Knox, 2000). The physical presence of the macroalgae canopy might have protected meiofauna from predation (Frame et al., 2007) thus resulting in an increase in abundance. However,

in our experimental conditions, birds and large pelagic marine organisms (>2 cm) were excluded from both the Control and +Macroalgae enclosures and therefore a possible increase of meiofauna abundance due to a decrease of top–down pressure just in the +Macroalgae is unlikely. 4.2. Vertical distribution of meiofauna Meiofaunal abundance in the Bay of Cádiz showed the typical vertical profile observed in intertidal sediments, with higher abundance in the upper sediment layer decreasing with depth (Giere, 1993; Knox, 2000). The decrease in meiofaunal abundance with depth is due to (1) the concurrent decrease of labile fresh organic matter, either sedimented from the water column or produced in situ by microphytobenthos (MPB), and (2) the reduction of oxygen availability with depth, frequently coupled with an increase in sulfide. The presence of macroalgae increased the abundance of meiofauna at all depths. However, differences were statistically significant only in the upper 0.5 cm layer, where macroalgae increased meiofaunal abundance up to 7 times compared to the bare sediment (Fig. 4). Therefore, the input of fresh organic detritus from macroalgae seems to affect mainly the upper sediment layer. In addition, macroalgae can act as foundation species (Bruno and

Please cite this article in press as: Bohórquez, J., et al. Effects of green macroalgal blooms on the meiofauna community structure in the Bay of Cádiz. Mar. Pollut. Bull. (2013), http://dx.doi.org/10.1016/j.marpolbul.2013.02.002

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herbivorous meiofauna to migrate to the canopy, where it can graze on the macroalgae and their epiphytes. Harpacticoida Copepoda and Ostracoda are epibenthic, detritus feeders, surface feeders and microphagous, showing different degrees of trophic specialization. Ostracoda are not known to consume macroalgae directly but they graze on bacteria and microalgae that grow on macroalgal fronds (Frame et al., 2007). In addition these taxa are generally more sensitive to decreased oxygen availability in the sediment than Nematoda (Giere, 1993), which could explain their increase in the macroalgae canopy. Nonetheless, a part of their populations remained in the sediment surface showing tolerance to more hypoxic/anoxic conditions (Vopel et al., 1998). 4.3. Relationship between meiofauna and environmental changes induced by green macroalgal blooms

Fig. 5. Vertical distribution of major taxa abundance in the presence and absence of macroalgae. Data are annual mean ± SE (n = 4).

Bertness, 2001) creating a new ecological niche for meiofauna. The structure of the meiofaunal community inhabiting the macroalgal canopy was significantly different from that inhabiting the sediment below it. The complexity of the macroalgal canopy presumably increases the meiofaunal abundance due to the provision of a high number of surfaces on which they can feed and live. Furthermore, the canopy offers an effective refuge against predation and desiccation (Frame et al., 2007). Harpacticoida Copepoda and Ostracoda, in particular, were relatively more abundant in the macroalgae canopy than in both bare and macroalgae covered sediment (Fig. 5 and Table 1). The presence of macroalgae reduces the amount and quality of light reaching the sediment, resulting in a decrease or even complete inhibition of MPB primary production and a reduction in the abundance of MPB (Corzo et al., 2009; García-Robledo et al., 2008, 2012). This reduction might induce

Meiofaunal community showed a higher seasonal variability in bare sediment than in the presence of macroalgae (Fig. 6). Most likely the macroalgae canopy created more stable microenvironmental conditions throughout the year, protected meiofauna against tides, erosion and desiccation (Frame et al., 2007), and thus favoured the same assemblage of taxa. Mineralization of organic matter released a huge amount of organic and inorganic nutrients inside the sediment and to the water column (Corzo et al., 2009; García-Robledo et al., 2008). This input of macroalgal detritus to the sediment probably augments the importance of the detritic pathways, the so-called decomposer system. Organic matter and inorganic nutrient content showed a clear positive correlation with the horizontal axis of MDS separating the meiofaunal community in sediments affected by macroalgae from those of bare sediment (Fig. 6). Therefore, macroalgal detritus was a direct or indirect (through bacterial growth) source of matter and energy for meiofauna, increasing its abundance and shifting the community structure to higher Nematoda dominance. Nematoda are generally deposit feeders consuming detritus and the complex microalgal and bacterial community associated with the decaying organic matter (Giere, 1993). On the other hand, Pn and Rd were higher in the Control plot than in the Macroalgae plot indicating a higher importance of MPB primary production for meiofauna in bare sediment than in macroalgae covered sediment. This is not surprising since macroalgae inhibit MPB primary production (Corzo et al., 2009; García-Robledo et al., 2008, 2012). Consequently, the microbial and meiofaunal trophic web based on MPB production (grazer system) (Blanchard, 1991; Middelburg et al., 2000; Montagna et al., 1983; Nilsson et al., 1991) will lose importance with important effects on the meiofaunal community composition. However, our results show a higher chlorophyll content in the Macroalgae plot than in Control plot (Table 2). Chlorophyll measured in Control plot comes exclusively from MPB primary production; while in the Macroalgae plot, decomposition of macroalgae might released a large amount of chlorophyll to the sediment (Corzo et al., 2009). Overall, our results shown that bottom–up interactions, by way of either a grazer system based on MPB primary production or a decomposer system based on macroalgal detritus, seem to control meiofaunal abundance and taxonomic composition in our in situ experiment. The importance of bottom–up mechanisms to control meiofaunal abundance has been shown in different coastal systems (Pinckney et al., 2003), although in some cases a top–down control by macrofauna and other organisms has been reported (Ólafsson, 2003). Our enclosures excluded animals larger than 2 cm, but we do not have direct information on how our experimental manipulation could have affected the grazing pressure on meiofauna within the enclosures. However, since both enclosures were identical the differential increase in meiofauna in the presence of macroalgae clearly supports an important role for

Please cite this article in press as: Bohórquez, J., et al. Effects of green macroalgal blooms on the meiofauna community structure in the Bay of Cádiz. Mar. Pollut. Bull. (2013), http://dx.doi.org/10.1016/j.marpolbul.2013.02.002

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Fig. 6. Non-metric Multi-Dimensional Scaling ordination plot based on Bray Curtis dissimilarities of square-root transformed meiofaunal abundance data (stress: 0.01). Points represent centroids of replicate data (n = 3). Groupings based on a group average cluster analysis are also drawn (inner circle: 25 distance, outer circle: 35 distance). Vector overlay of the environmental variables with correlation >0.5. Arrows indicate direction and relative magnitude of influence.

bottom–up mechanisms. Therefore, macroalgal detritus favored detritic pathways within the sediment and likely was responsible for the observed increase in meiofauna abundance and changes in the meiofaunal community. The study of how these changes in the meiofaunal community might affect macrofauna and other higher trophic levels was beyond our aim. This is a very critical gap in our knowledge, since there is very little information available on how the increase in frequency and extension of seasonal and permanent macroalgal blooms induced by eutrophication affect the trophic interactions between meiofauna and higher trophic levels. Acknowledgments The research was funded by Grants CTM 2009-10736 (Ministry of Education and Science, Spain), P06-RNM-01787 (Andalusian Regional Government). J. Bohórquez was funded by a FPI Grant (BES2010-035711) from the Ministry of Education and Science, Spain. S. Papaspyrou was funded by a JAE-Doc fellowship (Programa JAE, JAE-Doc109, Spanish National Research Council) and a Marie Curie ERG action (NITRICOS, 235005, European Union). E. García-Robledo was funded by Ramon Areces Foundation (Spain). J.L. Jiménez-Arias was funded by a FPI Grant (2012-063) from the University of Cádiz, Spain. References Alongi, D.M., 1998. Coastal Ecosystem Processes. CRC Press, Boca Raton, Florida, p. 419. Anderson, M.J., 2001. A new method for non-parametric multivariate analysis of variance. Austral Ecol. 26, 32–46. Blanchard, G.F., 1991. Measurement of meiofauna grazing rates on microphytobenthos: is primary production a limiting factor. J. Exp. Mar. Biol. Ecol. 147, 37–46. Bruno, J.F., Bertness, M.D., 2001. Habitat modification and facilitation in benthic marine communities. In: Bertness, M.D., Hay, M.E., Gaines, S.D. (Eds.), Marine Community Ecology. Sinauer Associates, Massachusetts, pp. 201–218. Corzo, A., van Bergeijk, S.A., Garcia-Robledo, E., 2009. Effects of green macroalgal blooms on intertidal sediments: net metabolism and carbon and nitrogen contents. Mar. Ecol.–Prog. Ser. 380, 81–93. Coull, B.C., 1989. A long-term variability of estuarine meiobenthos: an 11 years of study. Mar. Ecol. Prog. Ser. 24, 205–218. Coull, B.C., 1999. Role of meiofauna in estuarine soft-bottom habitats. Aust. J. Ecol. 24, 327–343.

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Please cite this article in press as: Bohórquez, J., et al. Effects of green macroalgal blooms on the meiofauna community structure in the Bay of Cádiz. Mar. Pollut. Bull. (2013), http://dx.doi.org/10.1016/j.marpolbul.2013.02.002