Antoine Grémare

Assessing coastal benthic macrofauna community condition using best professional judgement - Developing consensus across North America and Europe

Benthic indices are typically developed independently by habitat, making their incorporation into large geographic scale assessments potentially problematic because of scaling inequities. A potential solution is to establish common scaling using expert best professional judgment (BPJ). To test if experts from different geographies agree on condition assessment, sixteen experts from four regions in USA and Europe were provided species-abundance data for twelve sites per region. They ranked samples from best to worst condition and classified samples into four condition (quality) categories. Site rankings were highly correlated among experts, regardless of whether they were assessing samples from their home region. There was also good agreement on condition category, though agreement was better for samples at extremes of the disturbance gradient. The absence of regional bias suggests that expert judgment is a viable means for establishing a uniform scale to calibrate indices consistently across geographic regions.

Long-term comparison of macrobenthos within the soft bottoms of the Bay of Banyuls-sur-mer (northwestern Mediterranean Sea)

The macrofauna present at seven of the stations within the Bay of Banyuls-sur-mer (northwestern Mediterranean) initially sampled by Guille during the late 1960s was reassessed in 1994, using the same gear and techniques as those used during the reference study. Results showed an increase both in the number of species and of individuals (all species pooled) per unit of surface area at nearly all stations. This trend was not significant for total biomass. The most important changes were linked to the increase of the polychaete Ditrupa arietina (both within the Spisula subtruncata and the Nephtys hombergii community) and the decrease of the polychaetes Scoloplos armiger and Notomastus latericeus within the Scoloplos armiger community. The possible causes underlying these changes are discussed. The most probable cause is considered to be a slight modification of sediment composition (decrease of fine material in the sediment) within the Spisula subtruncata, the Nephtys hombergii and the Scoloplos armiger communities. Temporal changes in (1) Rhone river input, (2) small coastal river inputs, and (3) frequency of easterly storms may all have contributed to such a decrease.

Bathymetric changes in the distributions of particulate organic matter and associated fauna along a deep-sea transect down the catalan sea slope (Northwestern Mediterranean)

Abstract In March 1998 (BBD1) and September 1998 (BBD2) two oceanographic cruises were undertaken in the Catalan Sea (North-western Mediterranean) during which a transect was sampled down a bathymetric gradient encompassing depths of ~189 m to ~1645 m. There were five stations along the transect nominally at depths of ~200 m (Station 1), 400 m (Station 2), 600 m (Station 3), 1200 m (Station 4) and 1600 m (Station 5). At each station the main physical parameters of the water column were measured and the sediments were sampled to assess their biochemical characteristics and the abundances and demographic structure of the macro- and meiobenthos. During March, the whole water column was homogeneous for both temperature and salinity, whereas in September it was thermally stratified, as is typical of summer conditions. Fluorescence was measured throughout the water column while profiling with a CTD, and the primary production (PPeu) was deduced from satellite imagery. Both fluorescence and primary production were higher in March (average PPeu: 181 gC m−2) than in September 1998 (average PPeu: 102.8 gC m−2). Also during March, the organic carbon and enzymatically hydrolizable amino acids (EHAA) contents of the sediments were found to increase with depth, and a peak in lipid concentrations was observed at one of the deeper stations (Station 4: 0.254 μg mgDW−1). During September, organic carbon, lipid and amino acid contents and the C/N ratio, all decreased with depth, except at St. 5. Overall concentrations of most sedimentary organic compounds were higher in March than in September, indicating a greater availability of food to the bathyal communities in spring. Total abundances of meiobenthos and of copepod nauplii were positively correlated with lipid concentrations, but not with any of the other organic parameters measured. The abundances of near-bottom zooplankton and suprabenthos (vagile macrobenthos) showed no significant correlations with any of the properties of the sedimentary organic content. However, at the level of population structure, during March abundances of Furcilia and Calyptopis larval stages of euphausiids were higher, and recruitment peaked in several of the deposit-feeding species (e.g. the cumaceans Cyclaspis longicaudata and Platysympus typicus, and the amphipod Scopelocheirus hopei). In contrast, no recruitment peaks were detected for species potentially feeding on meiofauna (i.e. the isopod Munnopsurus atlanticus and the amphipods Bathymedon spp. and Rhachotropis caeca). Hence the only responses detected in the population structure of the benthic/benthopelagic fauna over the short time scales of this study were in recruitment patterns, and these appeared to be dependent on autoecological features such as the trophic guild of the individual species.

Biochemical characteristics of settling particulate organic matter at two north-western Mediterranean sites : a seasonal comparison

Abstract Gross sedimentation rates (GSR) and the main biochemical characteristics of settling particulate matter were monitored at two NW Mediterranean sites (the Bay of Banyuls-sur-Mer, France, and the Medes Islands, Spain) throughout a year to assess possible differences in particulate organic matter (POM) availability to the benthic community. A similar seasonal pattern was observed at both sites with higher organic contents and lower GSR during the spring–summer than during the autumn–winter period. At both sites, there was a negative correlation between GSR and sediment trap organic contents, which is indicative of the importance of resuspension in driving GSR. Along the same line, GSR in Medes correlated positively with wave height. The principal component analysis based on GSR and biochemical characteristics of sediment trap material always segregated samples from the two studied sites mostly due to the occurrence of higher lipid and lower carbohydrate contents in Medes. These differences were indicative of the presence of more labile settling POM in the Medes Islands. They were more pronounced during the spring–summer than during the autumn–winter period. Such a pattern may reflect differences in the sensitivity of the two studied sites to resuspension. According to this interpretation, between-sites differences would be high when resuspension is low (i.e. during the spring–summer period when resuspension would mainly affect Banyuls) and become low when resuspension is high (autumn–winter period when resuspension would affect both sites).

Relationship between sedimentary organic matter and benthic fauna within the Gulf of Lion: synthesis on the identification of new biochemical descriptors of sedimentary organic nutritional value

The abilities of several biochemical characteristics of sedimentary organic matter to describe quantitative changes in benthic fauna were assessed based on several surveys carried out within the Gulf of Lion during the last decade. Two sets of high frequency samplings were carried out both inshore and offshore, together with an assessment of seasonal variability along a depth gradient, and a synoptic assessment of mesoscale spatial variability. The considered biochemical parameters were: total organic matter, organic carbon, nitrogen, total proteins, available proteins, carbohydrates, lipids, total and available amino acids. The considered faunal parameters were the abundance and the biomass of both meio- and macrofauna. Our results contribute to unravel the relative importance of food digestibility and composition in controlling particulate organic matter nutritional value. Spectra of both total and available amino acids were almost constant irrespective of the seasons and/or the environments, which were studied during these surveys. This underlines the role of particulate organic matter digestibility relative to its ability to meet specific nutritional requirements in controlling benthic fauna. Multivariate analysis used to relate biochemical and faunal parameters suggest that available amino acids and lipids are the best descriptors of food nutritional value. In contrast with carbohydrates, these two parameters are both associated with the most labile fraction of particulate organic matter. This result suggests that the use of the sum of proteins, carbohydrates and lipids as a proxy for labile organic matter is precluded at least in areas similar to the Gulf of Lion.

Growth rates and RNA: DNA ratios in Paracentrotus lividus (Echinodermata: Echinoidea) fed on benthic macrophytes

We measured growth rates and RNA: DNA ratios of young sea urchins Paracentrotus lividus cultured for 6 months in the laboratory where they were fed on 12 macrophytes (Rissoella verruculosa, Cystoseira mediterranea, Corallina elongata, Stypocaulon scoparium, Padina pavonica, Ulva rigida, Cystoseira compressa, Colpomenia sinuosa, Posidonia oceanica, Codium vermilara, Asparagopsis armata, and Dilophus spiralis). Average increases in test diamter ranged from 0.3 mm · month dash1 for urchins fed on Dilophus spiralis to 1.9 mm · month 1 for urchins fed on Rissoella verruculosa. Average increases in wet mass ranged from 0.10 g wet mass · month 1 for urchins fed on Dilophus spiralis and Asparagopsis armata to 1.20 g wet mass · month 1 for urchins fed on Rissoella verruculosa. In terms of proteins, average growth rates ranged from 0.23 mg protein · month−1 for urchins fed on Codium vermilara to 3.08 mg protein · month−1 for urchins fed on Cystoseira mediterranea. In all three cases, growth rates were significantly (p = 0.0001) affected by the nature of the offered macrophytes. There was a high correlation between increase in test diameter and (1) increase in wet mass and (2) increase in proteins (r = 0.98 and 0.83, respectively). RNA: DNA ratios of Paracentrotus lividus (0.47 < r < 2.02) were within the range of what has previously been reported for larval fishes, bivalves, cephalopods, and crustaceans. Average RNA: DNA ratios were significantly affected by the food regime. However, results of a posteriori LSD tests were different for growth rates and RNA: DNA ratios. There were significantly (p = 0.0001, 0.0001, and 0.0004, respectively) positive correlations between growth rate (in terms of diameter, wet mass, and proteins) and RNA : DNA ratio. However these relationships accounted for only 23.8, 23.6, and 19.4%, respectively, of the variance of RNA : DNA ratios. These results suggest that the use of RNA : DNA ratios would be inefficient in predicting growth rates of field collected Paracentrotus lividus.

Effects of demersal trawling on marine sedimentary habitats analysed by sediment profile imagery

Abstract Demersal trawling causes one of the most widespread physical and biological changes in marine shallow and shelf sedimentary habitats: trawl otter boards may create furrows in the sediment surface, while trawl nets and attached weights scrape the sediment surface. As a consequence, benthic animals are disturbed or killed, and resuspension of particles increase. The impact of trawling on benthic animals has traditionally been analysed by changes in species composition and abundance, whereas frequency and distribution of trawl tracks are frequently analysed by side-scan sonar. We have used sediment profile images (SPIs) (30×22 cm) and observed furrows and other physical disturbances on the sediment surface that we attribute to trawling. In a manipulative experimental trawl study in Sweden (BACI design), significant impacts were found in trawled benthic habitats (73–93 m deep) compared with pre-trawling conditions and with reference areas. In particular, furrows from trawl boards had a severe ecological impact. In the Gulf of Lions (northwest Mediterranean), similar patterns were observed in the vast majority of 76 images taken at random at depths between 35 and 88 m in four different areas. Epifauna and polychaete tubes were generally either rare or not observed at all on trawled sediment surfaces. Burrows and feeding voids were, however, frequently present in some trawled areas and seemed to be comparatively less affected. Such biogenic structures in the sediment were generally associated with rather deep (3–4 cm) mean apparent redox profile discontinuities (aRPDs), which were measured digitally as the visible division zone between oxidised (sub-oxic) and reduced sediments. Increased roughness caused by the trawl boards acting on the sediment surface, e.g. depressions and protrusions, could have effects on sediment solute fluxes.

Experimental study of suspension-feeding activity in the serpulid polychaete Ditrupa arietina (O.F. Müller)

As time spent feeding can be an index of the feeding intensity, we attempted to describe the filtering activity in the suspension-feeding polychaete Ditrupa arietina. This description was based on the detection of the opening of the gill-crown by an automated image analysis system. The common pattern of activity could be described as the succession of filtering events and resting periods of which the number and the mean duration differed greatly from one worm to another. Filtering activity in D. arietina was neither a continuous process nor even a process having a particular rhythm. Within a same batch of worms, total filtration durations could represent between 12.5 and 87.5% of the total experimental time. Despite a strong inter-individual variability, our results showed the existence of pronounced seasonal variations in the activity of the gill-crown. In May, worms spent less than 25% of time feeding compared to more than 50% during the rest of year. These temporal changes appeared to result from the physiological state of the worms (reproductive period and ageing) at the time of the experiment.

A bioassay approach to temporal variation in the nutritional value of sediment trap material

Gross sedimentation rates (GSR) are determined weekly over a 1 yr cycle at a shallow station (18 m) of the bay of Banyuls-sur-Mer. GSR range between 0.6 and 107.8 g · m−2 · d−1. Spring and summer are characterized by relatively low and constant GSR whereas fall and winter are characterized by relatively high and highly variable GSR. The shape of the inverse relationship between GSR and organic content suggests the importance of resuspension in controlling GSR. At the studied station resuspension appears to be mainly cued by the wind. Ingestion rates and absorption efficiencies of the deposit-feeding bivalve Abra ovata fed on 14C-labelled sediment trap material collected on eight sampling dates characteristic of the main meteorological regimes are measured. When expressed in terms of organic matter, ingestion rates are similar (3.2 and 4.9 10−3 mg OM · mg OM−1 · h−1 for materials collected on May 11, 1993 and January 19, 1993, respectively). The relative consistancy of ingestion rates supports the compensatory intake theory. Absorption efficiencies (ranging from 6.4 to 21.0%) are highly affected by sampling dates. During fall and winter, absorption efficiencies are minimal when resuspension is maximal. During spring and summer, absorption efficiencies depend on the quality of materials sedimenting directly from the water column. Temporal differences in the quality of particulate organic matter available to benthic deposit-feeders should be included in the existing models of continental shelf food webs.

Food sources, ingestion and absorption in the suspension-feeding polychaete, Ditrupa arietina (O.F. Müller)

Food sources, food intake and absorption were studied for Ditrupa arietina at different periods of the year. The description of the diet was based on seasonal monitoring of fecal-pellet content produced by freshly collected worms. The size spectrum of prey was rather large, between 1 and almost 50 μm. Throughout the year, the origin of the food was planktonic as well as benthic. Food utilization rates were determined by radioactive pulse–chase design feeding experiments on four potential food sources: Skeletonema costatum, Isochrysis galbana, Pseudoalteromonas holoplanktis and Synechococcus sp. Food intake increased linearly with food availability, except in the case of S. costatum. However, the threshold cell concentrations (i.e., 1×104 and 4×104 cells ml−1) above which the ingestion rate of diatoms became constant was far above the maximum concentration observed in the field. This suggests that in the bay of Banyuls-sur-Mer, the filtering capacity of D. arietina is probably not saturated by the natural concentrations of phytoplanktonic cells. Absorption efficiencies measured for D. arietina are the first reported for a suspension-feeding polychaete. These efficiencies were in the range of values generally observed for other suspension feeders fed on the same kind of food sources. Worms absorbed diatoms, haptophytes, bacteria and cyanobacteria with respective efficiencies of 84.7%, 70.9%, 72.3% and 63.7%.