Μm ESD Research Articles

Implications of Anthropic Activities in the Catchment Area of a Temporary Mediterranean Wetland Complex in the South of Spain

The Lagunas de Campillos Natural Reserve and adjacent ponds are fundamentally surrounded by regularly fertilized crop fields and livestock industry, producing leachates which can be found in the ponds. The interest in this Site of European Importance and the RAMSAR wetland complex lies in the habitats within it, which are included in the Directive on Habitats of Community Interest. It is essential to determine the trophic status of the ponds and the quality of these habitats, as well as whether corrective measures need to be established in order to maintain a good environmental status. To characterize and compare the ponds, different parameters were measured, such as conductivity, pH, nutrient concentration, Chl-a concentration, phytoplankton composition, phytoplankton abundance (<20 µm), and the quantification of heterotrophic microorganisms indicating contamination of the aquifers. The obtained results showed that all ponds, except a mesotrophic pond, are eutrophic or even hypertrophic, with high levels of total nitrogen (>8 mg L−1), total phosphorous (>165 μg L−1), and chlorophyll-a concentration. These findings explain the high densities of phytoplankton observed, with the predominant presence of small cells (<3.6 μm ESD). In addition, concentrations of heterotrophs and coliforms are, in some ponds, higher than expected. Eutrophication hinders ecological functions and ecosystem services, which finally affects biodiversity and human wellbeing. Five of the six analyzed ponds are within various protection figures for their essential importance to local and migrating avifauna. Therefore, ponds’ status analysis and the implementation of measures for maintaining ecosystem services and trophic state are fundamental for the sustainable management of the studied area.

Mechanistic understanding of diazotroph aggregation and sinking: “A rolling tank approach”

AbstractDiazotrophs are ubiquitous in the surface (sub)tropical ocean, where they sustain most new primary production. Several recent studies also report the presence of diverse cyanobacterial diazotrophs in the mesopelagic and bathypelagic ocean, suggesting that they gravitationally sink, potentially supporting organic matter export and the biological carbon pump. Yet, the mechanisms leading to their export are not elucidated. Here, we simulated the sinking of diazotrophs in the water column by using rolling tanks, and measured the aggregation capacity and sinking velocities of four globally distributed strains having different sizes, shapes, and abilities to produce transparent exopolymer particles (TEP): two filamentous diazotrophs (Trichodesmium erythraeum, Calothrix sp.) and two unicellular cyanobacterial diazotrophs (UCYN‐B, Crocosphaera watsonii) and (UCYN‐C, Cyanothece sp.). All diazotrophs tested, regardless their size and shape, were capable of forming aggregates and sunk, albeit at different velocities depending on the aggregation capacity. Overall, UCYN formed aggregates as large as those formed by the filamentous diazotrophs (7000–32,014 μm ESD, equivalent spherical diameter), and sunk at 100–400 m d−1, i.e., at the same velocity as filamentous diazotrophs (92–400 m d−1). Although TEP are generally considered as enhancers of aggregation, TEP did not clearly influence aggregation rates nor sinking velocities during our study. We conclude that diazotrophs may be important contributors to carbon export in the ocean and need to be considered in future studies to improve the accuracy of current regional and global estimates of export.

Spatio-Temporal Variations in Community Size Structure of Arctic Protist Plankton in the Fram Strait

The Arctic Ocean is subject to severe environmental changes, including the massive decline in sea ice due to continuous warming in many regions. Along with these changes, the Arctic Ocean's ecosystem is affected on various scales. The pelagic microbial food web of the Arctic is of particular interest, because it determines mass transfer to higher trophic levels. In this regard, variations in the size structure of the microbial community reflect changes in size-dependent bottom-up and top-down processes. Here we present analyses of microscopic data that resolve details on composition and cell size of unicellular plankton, based on samples collected between 2016 and 2018 in the Fram Strait. Using the Kernel Density Estimation method, we derived continuous size spectra (from 1 μm to ≈ 200 μm Equivalent Spherical Diameter, ESD) of cell abundance and biovolume. Specific size intervals (3–4, 8–10, 25–40, and 70–100 μm ESD) indicate size-selective predation as well as omnivory. In-between size ranges include loopholes with elevated cell abundance. By considering remote sensing data we could discriminate between polar Arctic- and Atlantic water within the Fram Strait and could relate our size spectra to the seasonal change in chlorophyll-a concentration. Our size spectra disclose the decline in total biovolume from summer to autumn. In October the phytoplankton biovolume size-spectra reveal a clear relative shift toward larger cell sizes (> 30 μm). Our analysis highlights details in size spectra that may help refining allometric relationships and predator-prey dependencies for size-based plankton ecosystem model applications.

The ZooCAM, a new in-flow imaging system for fast onboard counting, sizing and classification of fish eggs and metazooplankton

In this paper we present the ZooCAM, a system designed to digitize and analyse on board large volume samples of preserved and living metazooplankton (i.e. multicellular zooplankton) and fish eggs >300 µm ESD. The ZooCAM has been specifically designed to overcome the difficulties to analyse zooplankton and fish eggs in the framework of the PELGAS survey, and provide high frequency data. The ZooCAM fish eggs counts were comparable to those done with a dissecting microscope. The ZooCAM enabled the accurate prediction and fast on board validation of staged anchovy and sardine eggs in almost real time after collection. A comparison with the ZooScan, on a more complex zooplanktonic community, provided encouraging results on the agreement between the 2 instruments. ZooCAM and ZooScan enabled the identification of similar communities and produced similar total zooplankton abundances, size distributions, and size spectra slopes, when tested on the same samples. However these results need to be further refined due to the small number of samples used to compare the two instruments. The main ZooCAM drawback resides in a slight but sensible underestimation of abundances and sizes, and therefore individual and community biovolumes. The ZooCAM have been successfully deployed over 4 years, on numerous surveys without suffering any major failure. When used in line with the CUFES it provided high resolution maps of staged fish eggs and zooplanktonic functional groups. Hence the ZooCAM is an appropriate tool for the development of on board, high frequency, high spatial coverage zooplanktonic and ecosystemic studies.

Feeding traits of the European flat oyster, Ostrea edulis, and the invasive Pacific oyster, Crassostrea gigas

Two oysters, the native flat oyster, Ostrea edulis, and the non-indigenous Pacific oyster, Crassostrea gigas, have partially overlapping distributions in European waters. Relatively little is known about particle selection by O. edulis, and the goal of the present study was to establish baselines for particle selection by both oyster species under controlled conditions in the laboratory. The study was carried out with adult oysters of similar shell size collected in the Limfjord estuary, Denmark (56°47′N, 08°51′E), in November 2011. The feeding traits of both species [clearance rate (CR), retention efficiency (RE) and lower threshold for clearance (LTC)] were compared using five algal species with different cell sizes (5−32 µm ESD) (Isochrysis galbana, Rhodomonas salina, Thalassiosira weissflogii, Prorocentrum micans and Akashiwo sanguinea). Oysters were acclimated to an experimental temperature of 22 °C and a salinity of 25. The CR of O. edulis was one to three times lower than that of C. gigas for the three smaller algal species (5−15 µm) but not different for the two larger algae. Algae in the size range 7−32 µm were retained with 100% RE by both oysters, but the smallest alga was retained with reduced RE (~35%). There were no differences in LTC between the two oysters. A closer examination of the relationship between LTC and different measures of the algae present (cell size, cell volume, chl a, C and N) suggests that the LTC, and also the CR, may depend in part on the internal energy status of the oysters.

Springtime zooplankton size structure over the continental shelf of the Bay of Biscay

Abstract. Linking lower and higher trophic levels requires special focus on the essential role played by mid-trophic levels, i.e., the zooplankton. One of the most relevant pieces of information regarding zooplankton in terms of flux of energy lies in its size structure. In this study, an extensive data set of size measurements is presented, covering parts of the western European continental shelf and slope, from the Galician coast to the Ushant front, during the springs from 2005 to 2012. Zooplankton size spectra were estimated using measurements carried out in situ with the Laser Optical Plankton Counter (LOPC) and with an image analysis of WP2 net samples (200 μm mesh size) performed following the ZooScan methodology. The LOPC counts and sizes particles within 100–2000 μm of spherical equivalent diameter (ESD), whereas the WP2/ZooScan allows for counting, sizing and identification of zooplankton from ~ 400 μm ESD. The difference between the LOPC (all particles) and the WP2/ZooScan (zooplankton only) was assumed to provide the size distribution of non-living particles, whose descriptors were related to a set of explanatory variables (including physical, biological and geographic descriptors). A statistical correction based on these explanatory variables was further applied to the LOPC size distribution in order to remove the non-living particles part, and therefore estimate the size distribution of zooplankton. This extensive data set provides relevant information about the zooplankton size distribution variability, productivity and trophic transfer efficiency in the pelagic ecosystem of the Bay of Biscay at a regional and interannual scale.

Coupling between upper ocean layer variability and size-fractionated phytoplankton in a non-nutrient-limited environment

We describe the coupling between upper ocean layer variability and size-fraction- ated phytoplankton distribution in the non-nutrient-limited Bransfield Strait region (BS) of Antarc- tica. For this purpose we use hydrographic and size-fractionated chlorophyll a data from a transect that crossed 2 fronts and an eddy, together with data from 3 stations located in a deeply mixed region, the Antarctic Sound (AS). In the BS transect, small phytoplankton ( 20 µm ESD) accounted for 80% of total chl a. The proportion of large phytoplankton increases as the depth of the upper mixed layer (ZUML), and the corresponding rate of vertical mixing, increases. We hypothesize that this change in phytoplankton composition with varying ZUML is related to the competition for light, and results from modification of the light regime caused by vertical mixing.

Mechanistic insights into the effects of climate change on larval cod

Understanding the biophysical mechanisms that shape variability in fisheries recruitment is critical for estimating the effects of climate change on fisheries. In this study, we used an Earth System Model (ESM) and a mechanistic individual-based model (IBM) for larval fish to analyze how climate change may impact the growth and survival of larval cod in the North Atlantic. We focused our analysis on five regions that span the current geographical range of cod and are known to contain important spawning populations. Under the SRES A2 (high emissions) scenario, the ESM-projected surface ocean temperatures are expected to increase by >1°C for 3 of the 5 regions, and stratification is expected to increase at all sites between 1950-1999 and 2050-2099. This enhanced stratification is projected to decrease large (>5μm ESD) phytoplankton productivity and mesozooplankton biomass at all 5 sites. Higher temperatures are projected to increase larval metabolic costs, which combined with decreased food resources will reduce larval weight, increase the probability of larvae dying from starvation and increase larval exposure to visual and invertebrate predators at most sites. If current concentrations of piscivore and invertebrate predators are maintained, larval survival is projected to decrease at all five sites by 2050-2099. In contrast to past observed responses to climate variability in which warm anomalies led to better recruitment in cold-water stocks, our simulations indicated that reduced prey availability under climate change may cause a reduction in larval survival despite higher temperatures in these regions. In the lower prey environment projected under climate change, higher metabolic costs due to higher temperatures outweigh the advantages of higher growth potential, leading to negative effects on northern cod stocks. Our results provide an important first large-scale assessment of the impacts of climate change on larval cod in the North Atlantic.

Reproduction, grazing, and development of the large subarctic calanoid Eucalanus bungii: is the spring diatom bloom the key to controlling their recruitment?

The response of large calanoid, Eucalanus bungii, to environmental fluctuation, particularly in relation to the spring diatom bloom in the Oyashio region, western subarctic Pacific Ocean, was examined by investigating egg production, grazing, development and starvation tolerance. Mean in situ egg production rate increased with ambient chlorophyll-a concentration, ranging from 0 to 47 eggs female−1 d−1, while no diurnal synchronous spawning behavior was observed. Under the spring bloom condition, E. bungii showed prey preference for less mobile and larger-sized prey (≥30 μm ESD) and bloom-forming diatom Thalassiosira spp. accounted for >80% of ingested carbon. In the laboratory, E. bungii was successfully reared from newly hatched nauplii to adult with the diatom, Thalassiosira nordenskioldi, as a food resource. Nauplii newly hatched from eggs reached the adult stage in ca. 150 days (5°C) with a sigmoidal developmental pattern and no sexual difference in development pattern. Starvation experiments indicated that the starved copepodids (C1–C4) became more vulnerable to high temperature with the progression of developmental stage, suggesting that the post-bloom condition with low food availability and increased temperature is harsh for their copepodids. The results of this study in conjunction with previous findings suggest that E. bungii is well adapted to utilize large-sized phytoplankton, such as a bloom-forming diatoms and, therefore, their recruitment processes, including egg production, development and mortality would be strongly affected by the duration and intensity of the spring bloom.

Controls on the ratio of mesozooplankton production to primary production in marine ecosystems

An ecosystem model was used to (1) determine the extent to which global trends in the ratio of mesozooplankton production to primary production (referred to herein as the “ z-ratio”) can be explained by nutrient enrichment, temperature, and euphotic zone depth, and (2) quantitatively diagnose the mechanisms driving these trends. Equilibrium model solutions were calibrated to observed and empirically derived patterns in phytoplankton biomass and growth rates, mesozooplankton biomass and growth rates, and the fraction of phytoplankton that are large (>5 μm ESD). This constrained several otherwise highly uncertain model parameters. Most notably, half-saturation constants for zooplankton feeding were constrained by the biomass and growth rates of their prey populations, and low zooplankton basal metabolic rates were required to match observations from oligotrophic ecosystems. Calibrated model solutions had no major biases and produced median z-ratios and ranges consistent with estimates. However, much of the variability around the median values in the calibration dataset (72 points) could not be explained. Model results were then compared with an extended global compilation of z-ratio estimates (>10 000 points). This revealed a modest yet significant ( r=0.40) increasing trend in z-ratios from values ∼0.01–0.04 to ∼0.1–0.2 with increasing primary productivity, with the transition from low to high z-ratios occurring at lower primary productivity in cold-water ecosystems. Two mechanisms, both linked to increasing phytoplankton biomass, were responsible: (1) zooplankton gross growth efficiencies increased as their ingestion rates became much greater than basal metabolic rates and (2) the trophic distance between primary producers and mesozooplankton shortened as primary production shifted toward large phytoplankton. Mechanism (1) was most important during the transition from low to moderate productivity ecosystems and mechanism (2) was responsible for a relatively abrupt transition to values >0.1 in high productivity ecosystems. Substantial z-ratio variations overlying these mean trends remained unexplained by these mechanisms. Potential sources of this variability include zooplankton patchiness, unresolved effects of advection and unsteady dynamics, unresolved shifts in mesozooplankton sizes and species, and unresolved aspects of zooplankton bioenergetics. Comparison of the modeled z-ratio patterns and mechanisms diagnosed herein with those obtained using models with expanded biological dynamics embedded in global circulation models will help further elucidate the causes of this variation.

Feeding of tropical cladocerans (Moina micrura, Diaphanosoma excisum) and rotifer (Brachionus calyciflorus) on natural phytoplankton: effect of phytoplankton size-structure

The proliferation of large phytoplankton in tropical shallow freshwater ecosystems may be attributable to inefficient feeding by the dominant zooplankton (small cladocerans and rotifers) on large particles, but more information on the feeding behavior of tropical organisms is required to explore this hypothesis. In this study, food size selectivity and functional feeding responses of three major tropical freshwater zooplankton species (Moina micrura, Diaphanosoma excisum and Brachionus calyciflorus) were studied to test their ability to control phytoplankton. Eleven grazing experiments were performed, using natural phytoplankton assemblages as a food source. Moina micrura fed efficiently on a wide range of sizes of phytoplankton particles, from unicellular picoplankton Chlorella sp. (2–4 μm equivalent spherical diameter, ESD) to large Coelastrum reticulatum coenobia (20–40 μm ESD), but the selectivity depended on the nature and size distribution of the phytoplankton. Diaphanosoma excisum ingested only very small particles (Monoraphidium, Chlorella). Brachionus calyciflorus fed on a wide size range but showed a clear preference for the largest algae (Cyclotella sp, Scenedesmus opoliensis). These three species increased their ingestion rate linearly with the food concentration and the saturation point was reached for M. micrura within the range of experimental conditions. The results suggest a strong food partitioning between these three species and showed that B. calyciflorus and M. micrura were better able to exploit and control algal blooms than D. excisum, which was a more selective feeder controlled by the availability of small food particles.

Naupliar development times and survival of the copepods Calanus helgolandicus and Calanus finmarchicus in relation to food and temperature

We measured egg hatching times and stage specific naupliar development times of the key calanoid copepods Calanus helgolandicus and C. finmarchicus in response to temperature, food quantity and food type. Calanus helgolandicus development times decreased with increasing temperature and when fed Isochrysis galbana (4.4 μm ESD) rather than Prorocentrum micans (29.5 μm ESD). Nauplii needed higher food carbon concentration to develop past the first feeding stage (N3) when fed I. galbana compared with P. micans. At low food carbon concentrations nauplii developed more slowly past N3 than at more saturated levels. The survival of nauplii fed P. micans increased with temperature, but starved nauplii survival decreased at higher temperatures. We parameterized a temperature-dependent model of development for both species which fitted the observed stage durations under non-limiting food extremely well and demonstrated that C. finmarchicus develops faster than C. helgolandicus. Further data are needed to clarify the effect of food-temperature interactions on development rates.

Effects of frontal processes on marine aggregate dynamics and fluxes: An interannual study in a permanent geostrophic front (NW Mediterranean)

This study provides and discusses the spatial distributions of abundances and sizes of marine-snow aggregates across the Ligurian Sea frontal system. A cross-front transect was sampled 34 times between 1992 and 1996, using the Underwater Video Profiler (UVP). Atlantic Water flows parallel to the Ligurian coast in the NW Mediterranean Sea, where that current creates a quasi-permanent front that separates the central and coastal waters. The horizontal distribution of aggregates (> 150 μm ESD, Equivalent Spherical Diameter) in the upper 1000 m shows two main features. First, the smaller aggregates (150 μm < ESD < 1 mm) are more abundant in coastal waters, as a result of continental input, cross-slope export, and re-suspension along the slope. The layers that contain very high concentrations of small aggregates are observed from surface down to 1000 m, and extend from the continental slope to the front. Second, the concentrations of large aggregates (ESD > 1 mm) are highest in and under the frontal zone, probably as a result of physical coagulation, and/or biological transformations. The seasonal intensity of large aggregate accumulations in and under the frontal structure seems to be more related to the autumn–winter increase in sub-mesoscale and mesoscale activity of the current flow than to the surface phytoplankton biomass. Interestingly, the horizontal distribution of aggregates is affected not only in the frontal zone (0–300 m depth), but also deeper down to 1000 m, probably as a consequence of rapid sinking or vertical transport. Results suggest that the settling of large aggregates under the frontal zone may limit the cross-slope transport of fine-grained particles by coagulation due to differential settling between the small particles suspended in the continental nepheloid layer and the large aggregates. This process, which takes place in sub-mesoscale zones (5–10 km wide), was also observed in one other front in the Western Mediterranean Sea. This led us to hypothesize that the impact of frontal processes on particle and aggregate dynamics might be generalized. Since fronts exist in many other coastal regions, the vertical fluxes at sub-mesoscale may have consequences for the transport of continental particles to the ocean's interior.

Phytoplankton co-existence: Results from an individual-based simulation model

We have investigated phytoplankton competition in a 1D, coupled physical-biological, individual-based model, designed to simulate a size-structured phytoplankton community, whose members belong to the same functional group but differ in size (20, 40 and 60 μm ESD), and compete for two resources (light and nutrient-nitrogen) in the frame of a food-chain plankton ecosystem, forced by astronomical and climatological conditions of a subtropical site. Allometric relationships established ranked performance: small-sized individuals have higher mass-specific metabolic rates (photosynthesis, nutrient uptake and respiration) and sink more slowly than do individuals in the larger size-classes. No a priori form of niche diversification was considered. The simulation reproduced the seasonal pattern of the environmental variables and phytoplankton biomass, displayed seasonality in relative demography and sustained multi-year co-existence. Phytoplankton biomass rose during the spring bloom until nutrient depletion, decreasing afterwards due to zooplankton grazing. In the light-controlled phase of the spring bloom, the dominance ranking in the mixed layer was consistent with the allometric ranking of energetics; small, middle and large-sized phytoplankton accounted for 77.2%, 22.4% and 0.4% of total biomass (ca. 5 gC m −2). Vernal subduction into the seasonal thermocline shaped a summer nutricline at ca. 30 m depth, below which reproduction generated a deep chlorophyll maximum. During summer, zooplankton diel vertical migration, foraging and excretion, and microbial remineralisation of detritus produced a feeble and declining source of ammonium in the oligotrophic layer. Differential subduction into the seasonal thermocline and nutrient stress promoted the dominance of small phytoplankton in this layer. By the end of summer, the survivors of the three size-classes lay at different depths, which provides a mechanism to relax competition: small cells survived in the mixed layer, the middle-sized in the seasonal thermocline, and the largest in both the seasonal and permanent thermoclines. Large phytoplankton survived longer in the eutrophic but poorly lit environment due to their lower mass-specific respiration. Oligotrophy lasted until the mixed layer reached the nutricline in autumn. Those cells in the seasonal and permanent thermoclines were entrained into the mixed layer as it deepened, seeding the growing season next year. The numbers of plankton in the three seed populations depended critically on their reproduction during summer. In winter, growth was accelerated by the re-establishment of the diurnal thermocline. From year-to-year, the relative demographic success (the annual competitive advantage, ACA) of the competing populations depends critically of their relative energetics and the biomasses in the seed populations. Taken together, these two factors yielded negligible ranking among the size-classes, and thus co-existence was achieved over three simulated years despite substantial seasonal variation in competitive advantage.

Environmental conditions and phytoplankton assemblages in two shallow reservoirs of Ivory Coast (West Africa)

Phytoplankton biomass, productivity and assemblages were studied during the main hydrological seasons in two shallow reservoirs in Ivory Coast (West Africa). In this area, the hydrological pattern begins with the flood from July to October, that is the main change in environmental condition. The flood is followed by a dry period from November to March; then the rainy season resumes from April with numerous and intense storms. Chlorophyll-α concentrations ranged from 5.0 to 27.4 μg l -1 in mesotrophic Brobo reservoir and from 15.1 to 123.0 μg l -1 in eutrophic Nambengue reservoir. Despite their different trophic states phytoplankton production varied between 0.2 and 1.8 g C m -2 d -1 in both reservoirs. Both Chlorophyceae and Cyanobacteria largely dominated the phytoplankton communities. Opportunist species (mainly Chlorophyceae), with medium sized cells (10-15 μm equivalent spherical diameter, ESD), were observed during the flood. Slow growing/slow sinking species, with relatively large cells (> 15 μm ESD, mainly Cyanobacteria and Chlorophyceae), were present during the dry season. Small species (< 10 μm ESD), dominated by Chlorophyceae and Cyanobacteria were abundant during the rainy season. At this late stage of the hydrological year, N 2 fixing Cyanobacteria like Anabaena and Anabaenopsis were also present. Eight functional associations (H I, J, K, L M , N, P, S 1, X 1) from the thirty-one proposed by REYNOLDS et al. (2002) were outlined in the two systems surveyed; these functional groups were representative of warm, shallow, turbid, nutrient-enriched and frequently mixed habitats. A large part of the biomass was composed of cells hardly matching the needs of superior consumers, in size as well as in shape. These conditions could explain the inefficient transfer of biomass within the pelagic food web. resulting in trophic dead end and contributing to the eutrophication of the sites.

Efficiency of particle retention and clearance rate in the polychaete Sabellaria alveolata L.

The development of Sabellaria alveolata, a gregarious reef-building polychaete species, is maximal in Mont-Saint-Michel Bay (France), where trophic capacity is now threatened by increasing shellfish farming. As no data are available concerning the ecophysiological response of this species, the purpose of the present study was to obtain clearance rate and retention efficiency values to provide a first order of magnitude for the trophic role of this species. Data were obtained using a flow-through system with novel troughs suitable for 225 cm 2 reef blocks containing a mean number of 940±102 (S.E.) individuals. The experimental diet used consisted of a mixture of two live microalgae, Skeletonema costatum (3800 cell ml −1) and Isochrysis galbana (23 700 cell ml −1), chosen to cover a broad size range (2 to 16 μm equivalent spherical diameter, ESD), as determined by a particle counter. On the basis of a mean clearance rate of 0.7 l h −1 obtained with reef blocks, the mean rate for an individual was estimated at 7.5×10 −4 L h −1. Particles larger than 6 μm ESD were cleared with 100% efficiency, but S. alveolata was unable to retain particles smaller than 2 μm ESD. The results are compared with data obtained for other polychaete species, and clearance rate values are extrapolated to an entire reef. To cite this article: S. Dubois et al., C. R. Biologies 326 (2003).

Feeding of Acartia clausi and Pseudodiaptomus hessei (Copepoda: Calanoida) on natural particles in a tropical lagoon (Ebrié, Côte d'Ivoire)

Grazing of the copepods Acartia clausi and Pseudodiaptomus hessei on natural particles was studied from on board experiments during several 24 h time series performed between 1993 and 1997 in four sites of the Ebrié Lagoon (Côte d'Ivoire). Ingestion rates of both species increased linearly with food concentration until a concentration threshold (5.5×10 9 μm 3 l −1 for A. clausi and 5.2×10 9 mm 3 l −1 for P. hessei) beyond which the relation presented a plateau. Both species poorly selected the peak of available particles (range 3–6 mm equivalent spherical diameter, ESD) but A. clausi seek preferentially smaller particles (6–21 μm ESD) than P. hessei (9–33 μm ESD). When the proportion of the preferred particles in the food offered decreased, A. clausi extended its selectivity towards both smaller and larger particles whereas P. hessei extended its selectivity towards larger particles only. As a consequence of these patterns, the useful particle concentration (UPC) was higher for A. clausi than for P. hessei. In addition, the ratio of the UPC for the two species showed a positive relationship with the ratio of their respective biomass. The significance of these results for the adaptation capacities of the two species and for the ecosystem functioning are discussed. A. clausi which is more suited than P. hessei to exploit smaller particles (3–6 μm) which dominate the seston, has a food competitive advantage. The inadequacy between the seston food-size composition and the selective patterns of the two main zooplankton species of the Ebrié Lagoon explains that they could be food limited despite the high trophic level of the lagoon. It could also partly explain the low transfer efficiency between phytoplankton and zooplankton in this ecosystem.

Planktonic ciliates in the Baltic Sea in summer: distribution, species association and estimated grazing impact

Samples for studying the ciliate communities of the open Baltic Sea were taken on 2 transects from the Kattegat to the entrance of the Gulf of Finland during 1988 and 1990. The abun- dance of HCIL (heterotrophic ciliates) was highest close to the surface, the maximum values ranging from ca. 7000 (entrance of the Gulf of Finland in 1988) to ca. 20 000 cells l -1 (Arkona Basin in 1990). The dominating HCIL groups were small strobilidiids, strombidiids, or prostomatiids. The photo- autotrophic ciliate Mesodinium rubrum was most abundant in the surface water of Arkona Basin in 1990 (ca. 26 000 cells l -1 ) but was also found at a high concentration in deeper layers during both cruises in daytime (down to 80 m in 1988). Large ciliates dwelling in the oxic/anoxic boundary layer at ca. 100 m depth were found in both years, the highest numbers being ca. 1000 cells l -1 (1988). HCIL community grazing was estimated by using a biovolume-dependent, mostly experimentally derived exponential function. We estimated that in 1988 the ciliate community cleared close to 50% of the water volume per day, whereas in 1990 the highest values reached up to 125% cleared d -1 . In both years, small (≤ 30 µm ESD) ciliates dominated the communities and were also responsible for most of the estimated grazing. Three distinct ciliate associations were revealed from the data of both years by correlation analysis. A deep-water association characterized by large ciliates was found at the oxic/anoxic water interface at the 2 deepest stations, Bornholm and Gotland Basins, in both years, whereas the other associations found were found closer to the surface. Some of these groups may rep- resent true feeding guilds bound together by utilization of same resources, while others could be united by abiotic factors or internal dynamics (e.g. predator-prey relationships) of the association.

Ingestion, faecal pellet and egg production rates of Calanus helgolandicus feeding coccolithophorid versus non-coccolithophorid diets.

Ingestion rates, faecal pellet and egg production were obtained in laboratory experiments with females of the copepod Calanus helgolandicus collected from the English Channel in November 1994. Five different algal monocultures were used as food: Prorocentrum micans (30 µm ESD), Thalassiosira weissflogii (13 µm ESD), Dunaliella tertiolecta (7 µm ESD), Emiliania huxleyi (5 µm ESD) and Coccolithus pelagicus (14 µm ESD). Results obtained suggest the low ingestion efficiency of the copepod when feeding on coccolithophorids during late autumn-early winter. From the five species offered, only the largest non coccolithophorid Prorocentrum micans and Thalassiosira weissflogii supported efficient feeding and calculated respiratory demand for C. helgolandicus. Both coccolithophorids, irrespective of cell size, were ingested at very low rates even when offered at high concentrations (233-468 µg C l(-1)). Besides low ingestion, no egg production was found in the copepods fed with Emiliania huxleyi, although unusual high gross efficiency (reaching 72%) was obtained in experiments performed with Coccolithus pelagicus. The late seasonal timing of the experiments (November) could explain the low ingestion and egg production rates.

Zooplankton food limitation and grazing impact in a eutrophic brackish-water tropical pond (Côte d'Ivoire, West Africa)

Gut fluorescence, feeding and egg production rates of zooplankton assemblages were measured in a shallow, eutrophic brackish-water pond for 24 days. Brachionus plicatilis, Hexarthra intermedia and Apocyclops panamensis successively developed and exhibited differences in food selectivity. Rotifers selected small particles but also had a preference for larger particles (15–21 μm, Equivalent Spherical Diameter, ESD). B. plicatilis appeared less selective than H. intermedia, which fed mostly on particles <6 μm. A. panamensis adults showed a selectivity for 6–21 μm ESD particles. Laboratory experiments suggested that A. panamensis adults were able to shift from seston to carnivorous feeding, depending on the availability of these food resources. Measurements of gut fluorescence and grazing gave comparable ingestion rates. Rotifers displayed the highest ingestion rates (up to 486% body C d-1). Despite high total phytoplankton and seston biomasses, the high ingestion rates and selective grazing of rotifers induced auto food-limitation phenomena and caused major changes in seston abundance and size structure. Grazing impact was less important when A. panamensis dominated.