Guy Boucher

Contribution of soft-bottoms to the community metabolism (primary production and calcification) of a barrier reef flat (Moorea, French Polynesia)

Abstract The relative contribution of soft bottoms to the community metabolism (primary production, respiration and net calcification) of a barrier reef flat has been investigated at Moorea (French Polynesia). Community metabolism of the sedimentary area was estimated using in situ incubations in perspex chambers, and compared with estimates of community metabolism of the whole reef flat obtained using a Lagrangian technique (Gattuso et al., 1996. Carbon flux in coral reefs. 1. Lagrangian measurement of community metabolism and resulting air–sea CO 2 disequilibrium. Mar. Ecol. Prog. Ser. 145, 109–121). Net organic carbon production ( E ), respiration ( R ) and net calcification ( G ) of sediments were measured by seven incubations performed in triplicate at different irradiance. Respiration and environmental parameters were also measured at four randomly selected additional stations. A model of Photosynthesis–irradiance allowed to calculate oxygen (O 2 ), organic carbon (CO 2 ) and calcium carbonate (CaCO 3 ) evolution from surface irradiance during a diel cycle. As chlorophyll a content of the sediment was not significantly different between stations, primary production of the sediment was considered as homogeneous for the whole lagoon. Thus, carbon production at the test station can be modelled from surface light irradiance. The modelled respiration was two times higher at the test station than the mean respiration of the barrier reef, and thus underestimated sediment contribution to excess production. Sediments cover 40–60% of the surface and accounted for 2.8–4.1% of organic carbon excess production estimated with the modelled R and 21–32% when mean R value was considered. The sedimentary CaCO 3 budget was a very minor component of the whole reef budget.

Spatio-temporal structure of the nematode assemblages of the Brouage mudflat (Marennes Oléron, France)

The density and species composition of the nematode assemblage was investigated in the Brouage mudflat (Marennes-Oleron, France) at three stations over four seasons in relation to environmental parameters (organic matter percentage, chlorophyll a and heavy metal content). From the 43 species recorded, six dominant genera represented 75% of the assemblage. Fluctuations of these species were mainly in response to the observed temporal patterns. Over the course of a year, epigrowth feeders were the most important trophic group (43%) with three dominant species Metachromadoroides remanei, Chromadora macrolaima and Ptycholaimellus jacobi but some non-selective deposit feeders, notably Daptonema oxycerca, also contributed to the utilisation of primary production. The feeding strategies of the dominant species particularly for epistrate feeders are discussed. An assessment of the influence of nematodes grazing on microphytobenthos biomass and production was then performed. Comparison of nematode species grazing rates based on nematode abundance and ingestion rates and microphytobenthic production are discussed. Using the trophic structure of nematode populations to extend those results, grazing rates represent a significant part of microphytobenthic production in springtime.

Bacteria and meiofauna in an experimental sand ecosystem. I. Material and preliminary results

Abstract A marine-sand, closed-circuit, ecosystem was successfully maintained for 16 months by the regular addition of soluble amino acids (205 g C/m 2 /yr). The nitrogen was rapidly and totally mineralized, no organic matter accumulating within the sand. The changes in density of the nematode and bacterial populations were similar with stabilization, at an in situ -like level, after 7 months. About one half of the 130 offshore species of nematodes developed in the experimental sand, some being dominant in both biotopes. The copepod population, after a 12 months exponential growth, started to fluctuate dramatically around a mean level much higher than that in nature.

Spatial variability in Sediment Oxygen Consumption under winter conditions in a lagoonal system in New Caledonia (South Pacific)

Sediment Oxygen Consumption (SOC) was investigated during a winter (Southern Hemisphere) cruise in the southwest lagoon of New Caledonia. Oxygen fluxes were measured at 11 sampling stations distributed along two coast to reef transects. Three different methods of flux measurements were used: diver-operated benthic chambers, laboratory incubation of sediment cores and oxygen microprofiles determinations. SOC values varied between 450 and 2250 μmol O2 m−2 h−1. The level of agreement between the three techniques strongly varied as a function of sediment type. Most of the SOC values from the grey sand zone in the middle part of the lagoon and the muddy bottoms of the bays did not show significant differences. A central station presenting a dense seagrass bed gave lower SOC determined by oxygen microprofiles compared to the two other methods. In coarse carbonated sands from the back reef area, SOC measured by in situ benthic chambers were higher than SOC measured by incubation techniques. This discrepancy could be explained by physical disturbance of the sediments, macroscale variability in benthic communities or technical efficiency of the sediment sampling device and probably by a combination of all three processes. Nevertheless, for the other sediment types that represented 85% of the lagoon bottoms, the results from the three techniques used for SOC determination were strongly convergent. Based on this assumption, it could be stated that the oxygen fluxes were essentially driven by microbial activity compared to biologically mediated vertical transport of solutes. The SOC values determined during this study were in agreement with budgets previously calculated for the lagoon. Regardless of the back reef area, spatial variability in SOC can be explained by the organic matter content of sediments which clearly showed a coast to reef gradient with higher organic carbon and nitrogen contents in the coastal sediments. The C/N ratios demonstrate the higher rate of freshly deposited organic matter near the coast compared to more central stations in the lagoon.

Structure and biodiversity of nematode assemblages in the SW lagoon of New Caledonia

Abstract. Meiofaunal density and the taxonomic composition of nematodes were investigated in three bottom types of the SW lagoon of New Caledonia (SW Pacific). Meiofaunal density was significantly higher in white-sand bottoms than in other biotopes (grey-sand and muddy bottoms). Thirty-three nematode families and 172 genera were identified. Species diversity indices were significantly lower in back-reef white-sand than in seagrass grey-sand and coastal mud. The familial, generic and species diversity were analysed in relation to ecological and biotic parameters by multivariate factorial correspondence analysis and hierarchical clustering. The analysis at familial level was unable to discriminate between stations and biotopes. The analysis at generic level detected three main nematode associations related to microphytobenthic primary productivity, macrophyte biomass and silt content. Taxonomic composition of one white-sand station (Mbéré reef) differed from all others in relation to an increased chlorophyll/pheophytin ratio and a lower silt content. Grey-sand stations were split in two distinct groups. The first combined two of the three white-sand stations and the grey-sand stations, all of which were characterised by a medium silt content and a medium productivity. The second group combined the muddy bottom stations and the siltiest grey-sand stations which were characterised by a high silt content, a lower macroflora biomass and low productivity.

Biodiversity of littoral nematodes from two sites in the Gulf of California

The Gulf of California, Mexico, consists of unique environmental conditions resulting in a rich array of biological diversity. Nematodes are present in most marine sediments and are probably the most abundant metazoans. This research analyses and compares nematode biodiversity in two shallow, littoral locations of the Gulf and compares the results with other Pacific and Atlantic faunae. Samples collected in Punta Estrella and Santa Clara were processed for nematode extraction by standard methods. A total of eighty genera were identified, sixty-seven occurring in coarser sediments at Punta Estrella and fifty-five in more silty sediments at Santa Clara. Nematode abundance was higher at the latter location, on average. The region was not especially high or low in diversity compared to other littoral sites, and was most similar to locations in Europe with a corresponding ecology but at temperate latitudes.

Estimation of bottom ammonium affinity in the New Caledonia lagoon

Ammonium affinity of New Caledonia lagoon benthic communities was measured during the course of 33 in situ enrichment experiments, in order to estimate the contribution of benthos to ammonium fluxes. Ammonium chloride was injected into enclosures pushed into the sediment, in order to obtain a concentration of 20–22 μmol l-1 in the enclosed water which approximated the interstitial water content. Ammonium kinetic uptake was then followed for two hours. Grey-sand bottom displayed the highest affinity for ammonium, but white-sand and muddy bottom affinity was of the same order of magnitude. Macrophytes, and microphytes (when macrophytes are absent), account for the bulk of ammonium bottom uptake. As a result, grey-sand bottoms with their dense macrophyte cover represent a sink for water column nitrogen and play a key role in nutrient cycling of the lagoon.

The use of photosynthesis inhibitor (DCMU) for in situ metabolic and primary production studies on soft bottom benthos

A selective chemical photosynthesis inhibitor, DCMU (Dichorophenyl-dimethylurea), dissolved in DMSO (Dimethyl sulfoxide) was substituted for the dark incubation method commonly used to measure the oxygen consumption in metabolic and primary production studies. We compared oxygen fluxes during light incubations with DCMU and dark incubations procedure, on soft bottom benthos. For this purpose, we studied the effects of different DCMU concentrations. A concentration of 5 · 10−5 mol l−1 inside a clear incubation enclosure completely inhibits photosynthesis without affecting the metabolism of soft bottom benthos.

In situ study of the effect of oyster biomass on benthic metabolic exchange rates

Oxygen consumption and dissolved nitrogen fluxes at the water-sediment interface of an oyster-bed were measured in situ using transparent enclosures inserted on undisturbed sediment. Experiments were performed in summer, under dark and light conditions, with various densities of the oyster Crassostrea gigas (0–150 animals m − 2). The influence of oyster biomass on oxygen and ammonia exchange rates was similar in both lighting conditions. Oxygen consumption increased with increasing biomass, though not at the level of prediction for the highest biomasses, suggesting a depressed respiration rate. Ammonia release never matched the rates predicted by adding sediment efflux to oyster excretion, when biomass exceeded 100 g DW m − 2. The coupling between oxygen consumption and ammonia release (O : N ratio) was thus influenced by oyster biomass. Stabilization of nitrogen release was related to enhanced nitrification in the presence of oyster and/or sediment uptake of ammonia against the molecular gradient. Urea release was erratic but appeared uninfluenced by oyster biomass. Fluxes of oxygen and of each nitrogen compound displayed thus a specific response to modifications of the oyster biomass. Both the organisms and the sediment are involved in regulation processes of metabolic exchange rates at the water-sediment interface.

Fluctuations des nutriments au cours de la marée sur les parcs Ostreicoles de la rivière Penze (Nord-Finistère)

The importance of short-term fluctuations in nutrients and primary amines over oyster beds during spring and neap tidal cycles was analysed. The range of the variations recorded over 24 h is of the same order of magnitude as that observed over an annual cycle.Multivariate analysis, taking into account all available parameters concerning the tidal cycles, showed a short-term evolution of primary amines and ammonia nitrogen, independent from that of other parameters. The latter fluctuate in synchronisation with the down-and-up-stream flow of the river Penzé. The mean ammonia Nitrogen concentration is higher during spring tide than during neap tide, and ammonia is more concentrated at the bottom than in the surface water. This higher concentration of ammonia in bottom water is related neither to current speed, nor to interstitial water dilution by sediment resuspension. Nutrient exchanges between the sediment and the water column did not seem to be an important source of ammonia, while ammonia excretion by oysters might be the main source of this nutrient at the bottom level, particularly during spring tide.