Gaston Desrosiers

Predation and sediment disturbance effects of the intertidal polychaete Nereis virens (Sars) on associated meiofaunal assemblages

A microcosm experiment was carried out to determine the effects of the activity of the burrowing polychaete Nereis virens (Sars) on the associated meiofauna. The sediment basin (76×41 cm) was filled with 10 cm of sandy sediment previously sieved with a 1-mm mesh to remove any undesired macrofauna and macrodetritus. Fifteen 13-cm long polyvinyl-chloride (PVC) tubes (I.D.=10 cm) were pushed into the sediment to partition treatments. Nereis were added to the tubes at two densities, low (N=1) and high (N=3). Five tubes were used as controls (no Nereis), while two sets of five tubes were used for the low (L) and high (H) density treatments, respectively. After 14 days, meiofauna was sampled by coring. Cores were cut into three slices: surface (0–1 cm), subsurface (1–5 cm), and deep (5–10). High densities of Nereis (H) significantly affected nematodes, harpacticoid copepods, and nauplii abundance. However, lower abundances were found only in the top cm of the sediment. Moreover, a significant number of dead nematodes found in this sediment layer of treatment H allowed a distinction between sediment disturbance effects and predation effects. Sediment disturbance caused by Nereis may be related to an intensive “ploughing” of surface sediment during food-searching activity. Diversity indices were affected only in the top cm of the sediment with generally lower values in treatment H. Differences in the relative survival of the different feeding groups were found in treatment H, where microvores and deposit feeders respectively showed greater and lower survival. Multivariate analysis (multidimensional scaling) revealed significant differences in nematode species composition among treatments in all sediment layers. It is concluded that N. virens significantly affects meiofauna mostly by disturbance of the top cm of the sediment where its predation represents an influent force as well. The structure of nematode assemblages in subsurface and deeper sediment layers is also affected, most likely by changes in redox conditions caused by the bioirrigating effects of Nereis burrows.

Size spectra, body width and morphotypes of intertidal nematodes: an ecological interpretation

Nematode species from three intertidal assemblages (St Lawrence Estuary, Quebec, Canada) were studied in order to form an ecological interpretation of three allometric attributes: body width, size spectra, and morphotypes. The three assemblages were characterized by a very similar sediment grain median (Md) but different silt–clay proportions: A1 (upper-tidal level; Md=122 μm; silt=34.8%), A3 (mid-tidal level; Md=182 μm; silt=12.8%), and A5 (lower-tidal level; Md=122 μm; silt=6.8%). Silt–clay proportions were an influential factor in determining the mean nematode body width, used as a morphological discriminant between burrowing and interstitial organisms. A plot of the number of species vs the body width-classes showed two peaks: between 19.3 and 22.6 μm (interstitial), and between 32.0 and 45.5 μm (burrowers). As for the size spectra, in sandy sediments the mean nematode individual biomass was smaller than in muddy sediments. As a consequence, the estimated mean individual respiration rate was greater in muddy (A1=2.26 nl O 2  h −1 ) than sandy sediments (A3=1.25 nl O 2  h −1 ; A5=1.12 nl O 2  h −1 ). In contrast, estimated metabolic ratios were lower in A1 (2.78 nl O 2  h −1  μg −1 dry weight, DW) than in A3 (2.95 nl O 2  h −1  μg −1 DW) and A5 (3.01 nl O 2  h −1  μg −1  DW) suggesting different productivity and/or physiological adaptations to different lifestyles (burrowing vs interstitial) between species inhabiting muddy or sandy sediments. Morphotypes (body width/body length ratio=w/l ratio) were found to be associated with feeding groups. Small w/l ratios were typical of microvores, while greater ratios were typical of epigrowth feeders and predators. Ciliate-feeders, deposit-feeders and facultative predators had intermediate ratios. A morphotype food-related hypothesis is proposed: the species morphotype reflects the quality of exploited food; a small w/l ratio (i.e. long gut) would favour digestive efficiency and would be an adaptation to low quality food (microvores); inversely, a greater w/l ratio (i.e. short gut) would be an adaptation to high quality food (epigrowth-feeders and predators).

Assessment of the spatial variability of intertidal benthic communities by axial tomodensitometry: importance of fine-scale heterogeneity

At the water–sediment interface of aquatic ecosystems, the presence of biogenic structures produced by benthic invertebrates strongly affects biogeochemical processes. The quantification of these structures and the assessment of the vertical distribution of fauna are essential for determining the impact of communities in sediments. In the present study, computer axial tomodensitometry (CAT-scan) was used to measure the space occupied by an intertidal community of the St. Lawrence estuary. Three cores were sampled at a site that was considered homogeneous according to surface sediments. The vertical distribution of biogenic structures and gravel were measured in the three cores using CAT-scan; the vertical distribution of fauna was also analysed for each core. The biogenic structures were highest at the water–sediment interface and decreased with depth in the three cores. The number of invertebrates also decreased with depth. We observed similar distributions of biogenic structures in cores 1 and 2. However, fewer biogenic structures were observed below 90 mm in core 3. This result was correlated with a high quantity of gravel from 90 to 140 mm in core 3 whereas the other cores had lower quantities of coarse material. We found relationships among the distributions of biogenic structures, fauna, and sediment characteristics (gravel quantity) that can affect species distribution. The vertical distributions of Macoma balthica, Mya arenaria, Nereis virens, and small-sized gallery-producing species (nematodes and oligochaetes) could also be recorded with the CAT-scan method. Thus, CAT-scan is an excellent tool to determine the fine-scale heterogeneity in the space occupied by benthic invertebrates in sediments.

Responses of benthic macrofauna and biogeochemical fluxes to various levels of mussel biodeposition: An in situ benthocosm experiment

An in situ experiment was done to evaluate the dose-dependent response of mussel biodeposition on benthic communities and biogeochemical fluxes. Natural benthic communities were exposed to 7 different levels of mussel biodeposition (equivalent to that produced by 0-764 mussels m(-2)) over 50 days. Benthic communities responded as predicted from the Pearson, T.H., Rosenberg, R., 1978. Macrobenthic succession in relation to organic enrichment and pollution of the marine environment. Oceanogr. Mar. Biol. Annu. Rev. 16, 229-311 model of organic enrichment. Total abundance and species richness decreased with increasing biodeposition. The abundance and biomass of opportunistic species (Capitella spp.) increased in the mesocosms subject to the greatest biodeposition. Sensitive species Tellina agilis and Pherusa plumosa tended to decrease in abundance and biomass with increasing biodeposition. The biotic index M-AMBI responded clearly to increased biodeposition and may be a useful tool for assessing the effect of mussel biodeposition on the benthic environment. These results are important for the construction of predictive models for determining environmental carrying capacity for bivalve aquaculture.

Intertidal meiofauna of the St Lawrence estuary (Quebec, Canada): diversity, biomass and feeding structure of nematode assemblages

The meiofauna of the St Lawrence estuary was investigated in the intertidal zone of the Parc du Bic (Quebec, Canada). Five nematode assemblages were distinguished by a cluster analysis: A1 and A2 (uppertide level); A3 (mid-tide level); A4 and A5 (low-tide level). Discriminant function analysis showed that exposure time during low tide was the most important environmental factor in determining diierences between assemblages. Chlorophyll-a, phaeopigments, sediment water content, and per cent of silt followed in the same order. Nematode densities (400^1500 ind 10 cm 72 ) were found to be lower than those generally reported for other estuarine intertidal zones of the eastern Atlantic coast. Mean nematode biomass in the ¢ve assemblages ranged between 96 � 14 and 248 � 86m gC org 10 cm 72 . Deposit feeders were generally the dominant nematode feeding group in terms of abundance and biomass. Correlation of epigrowth-feeders with chlorophyll-a and phaeopigments, respectively, suggested that in the upper-tide level, old or partially degraded phytodetritus contribute more to the diet of this nematode feeding group; and in the low-tide level epigrowth-feeders may rely more on ‘fresher’ phytodetritus.

Juvenile growth of Nereis diversicolor (O.F. Müller) feeding on a range of marine vascular and macroalgal plant sources under experimental conditions

Abstract A laboratory experiment was conducted in 1992 (January–March) to evaluate variation of juvenile growth (% · day −1 ) of the polychaete Nereis diversicolor (O.F. Muller) in relation to tidal flat plant species as food sources. Experimental design used plant materials (alga and vascular plants) found abundantly in the intertidal ecosystem along the southeastern coast of the English Channel in which juveniles were sampled for the 3-month experiment. Juveniles ( ≤ 1-yr-old) markedly increased in wet weight with the alga Enteromorpha intestinalis (weight-specific growth rate μ: 1.9 ± 0.01% · day −1 ) as food source. A higher value of assimilation efficiency was observed for algal species ( E. intestinalis : 86.0%) than for marine vascular plants ( Salicornia europae 61.7 ± 9.5%; Halimione portulacoides 48.3 ± 6.6%; Spartina anglica 37.0 ± 2.7%). The digestion of marine vascular species took longer (17 to 33 h) than that of algae. All these characteristics (growth, assimilation efficiency and duration of digestive process) seem to be correlated positively with lignin and cellulose concentrations in the plant tissue.

Spatial interactions in the Macoma balthica community control biogeochemical fluxes at the sediment-water interface and microbial abundances

We examined how interactions among the three dominant species of the Macoma balthica community in the St-Lawrence estuary influence net biogeochemical fluxes and the composition of the sedimentary bacterial community. We manipulated the biodiversity of the community via combinations of Macoma balthica, Mya arenaria, and Nereis virens in sediment microcosms containing sieved and homogenized tidal-flat sediment. Each combination was adjusted to similar total biovolumes and in situ animal densities. Compared to treatments with single species, which were also adjusted to similar biovolumes and densities, combinations of multiple species changed the fluxes of oxygen, phosphate, ammonium, and nitrate across the sediment-water interface and altered the composition of the microbial community beyond the level predicted by linear addition of the single species effects. Most combinations involved positive interactions that increased net fluxes. Although N. virens dominated system behavior, we also observed positive interactions between the two bivalves, and between N. virens and M. balthica. Weaker interactions, sometimes negative, were observed between N. virens and M. arenaria. Most effects could be directly linked to total oxic sediment volumes and burrow volumes generated by the different species. N. virens had the maximum species-specific burrow volume/biovolume ratio, and its presence maximally impacted fluxes. The principal interactions between species that affect net solute fluxes reflect space occupation, life habit (position, location), and irrigation of burrows. These factors interact with reactive substrate distributions and remineralization patterns to produce particular flux balances. Future studies should include the relationships of biodiversity to spatial scales over which the functional groups are distributed, the volumes actively occupied and irrigated, production rates of burrows, and the depths to which the organisms burrow. 1. Institut des Sciences de la Mer de Rimouski (ISMER), 310 allee des Ursulines, Rimouski (Quebec), Canada, G5L 3A1. 2. Laboratoire de Microbiologie, de Geochimie et d’Ecologie Marines (LMGEM), Centre d’Oceanologie de Marseille (COM), Campus de Luminy, Avenue de Luminy, case 901, 13288 Marseille Cedex 09, France. 3. Present address: Laboratoire des Sciences de l’Environnement Marin (LEMAR), UMR6539 UBO/CNRS/ IRD, Institut Universitaire Europeen de la Mer (IUEM), Technopole Brest Iroise, 29280 Plouzane, France. email: emma.michaud@univ-brest.fr 4. School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY 11794-5000, U.S.A. 5. UMR-CNRS 5023, Laboratoire d’Ecologie des Hydrosystemes Fluviaux (LEHF), Universite Claude Bernard Lyon 1, 69622 Villeurbanne Cedex, France. 6. Earth & Planetary Sciences, McGill University, 3450 University Street, Montreal (Quebec), Canada, H3A 2A7. Journal of Marine Research, 67, 43–70, 2009

Juvenile growth of the polychaete Nereis virens feeding on a range of marine vascular and macroalgal plant sources

A laboratory experiment was conducted to examine variation of juvenile growth (% d-1) of the polychaete Nereis virens (Sars) in relation to tidal flat plant species as food sources. We used vegetable materials (algae and vascular plants) which are carried along by tidal currents and are found abundantly at the upper tidal level. Juveniles (≤2-yr-old) markedly increased in wet weight with the algae Laminaria longicruris (weight-specific growth rate μ: 1.7% d-1) and Enteromorpha intestinalis (μ: 1.6% d-1) as food sources. A higher value of assimilation efficiency was observed for algal species (L. longicruris 55.1±7.9%; E. intestinalis 54.8±0.5%; Fucus vesiculosus 40.6%) than for marine vascular plants (Spartina alterniflora 26.8±10.9%; Zostera marina 1.4%). The digestion of marine vascular species lasts longer (19 to 38 h) than that of algal species. All these characteristics (growth, assimilation efficiency and duration of the digestive process) seem to correlate positively with lignin and cellulose concentrations in the plant structure.

Orientation of the infaunal bivalve Mya arenaria L. in relation to local current direction on a tidal flat

Abstract The deep-burrowing and sedentary bivalve Mya arenaria L. has a preferential orientation on a tidal flat. Circular distribution of individuals is axial, the mean bearing of an organisms sagittal plane is perpendicular to the principal component of current direction, and variability in orientation is related to the variability of current direction. This mean orientation can be explained through optimization of energy acquisition during an entire tidal cycle. This position prevents recirculation during flood and ebb because exhalant and inhalant apertures are always in a plane normal to the tidal bidirectional current. No effects of individual orientation on shell growth, on somatic or reproductive tissue weights, and on gonadosomatic index could be demonstrated for individuals 6–19 yr old. This could result from a strong selective pressure of current direction on youngest individuals.

Périodicité d’alimentation du polychète Nereis diversicolor (O.F. Müller) selon les changements de la marée

Abstract:The feeding rhythm of the polychaete Nereis diversicolor (O.F. Muller) was studied during tidal cycles (spring and neap tide) at three sites in baie du Mont St-Michel and the Rance estuary in summer 1990. For most of the sites, the polychaetes were mainly feeding during spring tides when immersion occurred. Dietary activity decreased at emersion sites depending on low tide duration. During this period, the physical and chemical conditions in the mud flat sediments are not favourable to polychaetes’ activities (high temperature, high desiccation, low oxygen content) and can cause mortality or migration of the population. Repletion indices from polychaetes of the Hirel sector (baie du Mont-St-Michel) showed a decrease of their dietary’s activity during the neap tide period even if the site was permanently immersed.