Mediterranean copepods' functional traits

Information on the functional traits was gathered for the most commonly-sampled copepod species of the Mediterranean Sea. Our database includes 191 species described by 7 traits encompassing diverse ecological functions: minimal and maximal body length (mm), trophic group (Omnivore/Carnivore/Herbivore/Detritivore), feeding type (Cruise-feeding/Filter-feeding/Ambush-feeding), spawning strategy (Sac-spawner/Free-spawner), diel vertical migration (Non-migrant/Weak-migrant/Strong-migrant) and vertical habitat (prefered depth layer). Using cluster analysis in the functional trait space revealed that Mediterranean copepods can be gathered into groups that have different ecological roles.

Carbon cycling in the northern Humboldt Current off Chile

The structure of the zooplankton foodweb and their dominant carbon fluxes were studied in the upwelling system off northern Chile (Mejillones Bay; 23°S) between October 2000 and December 2002. High primary production (PP) rates (18 gC/m**2 d) were mostly due to the net-phytoplankton size fraction (>23 µm). High PP has been traditionally associated with the wind-driven upwelling fertilizing effect of equatorial subsurface waters, which favour development of a short food chain dominated by a few small clupeiform fish species. The objective of the present work was to study the trophic carbon flow through the first step of this 'classical chain' (from phytoplankton to primary consumers such as copepods and euphausiids) and the carbon flow towards the gelatinous web composed of both filter-feeding and carnivorous zooplankton. To accomplish this objective, feeding experiments with copepods, appendicularians, ctenophores, and chaetognaths were conducted using naturally occurring plankton prey assemblages. Throughout the study, the total carbon ingestion rates showed that the dominant appendicularian species and small copepods consumed an average of 7 and 5 µgC/ind d, respectively. In addition, copepods ingested particles mainly in the size range of nano- and microplankton, whereas appendicularians ingested in the range of pico- and nanoplankton. Small copepods and appendicularians removed a small fraction of total daily PP (range 6-11%). However, when the pico- + nanoplankton fractions were the major contributors to total PP (oligotrophic conditions), grazing by small copepods increased markedly to 86% of total PP. Under these more oligotrophic conditions, the euphausiids grazing increased as well, but only reached values lower than 5% of total PP. During this study, chaetognaths and ctenophores ingested an average of 1 and 14 copepods/ind d, respectively. In terms of biomass consumed, the potential impact of carnivorous gelatinous zooplankton on the small-size copepod community (preferred prey) was important (2-12% of biomass removed daily). However, their impact produced more significant results on copepod abundance (up to 33%), which suggests that carnivorous gelatinous zooplankton may even modulate (control) the abundance of some species as well as the size structure of the copepod community.

Experiment: Food availability outweighs ocean acidification effects in juvenile Mytilus edulis

Ocean acidification is expected to decrease calcification rates of bivalves. Nevertheless in many coastal areas high pCO2 variability is encountered already today. Kiel Fjord (Western Baltic Sea) is a brackish (12-20 g kg-1) and CO2 enriched habitat, but the blue mussel Mytilus edulis dominates the benthic community. In a coupled field and laboratory study we examined the annual pCO2 variability in this habitat and the combined effects of elevated pCO2 and food availability on juvenile M. edulis growth and calcification. In the laboratory experiment, mussel growth and calcification were found to chiefly depend on food supply, with only minor impacts of pCO2 up to 3350 µatm. Kiel Fjord was characterized by strong seasonal pCO2 variability. During summer, maximal pCO2 values of 2500 µatm were observed at the surface and >3000 µatm at the bottom. However, the field growth experiment revealed seven times higher growth and calcification rates of M. edulis at a high pCO2 inner fjord field station (mean pCO2 ca. 1000 µatm) in comparison to a low pCO2 outer fjord station (ca. 600 µatm). In addition, mussels were able to outcompete the barnacle Amphibalanus improvisus at the high pCO2 site. High mussel productivity at the inner fjord site was enabled by higher particulate organic carbon concentrations. Kiel Fjord is highly impacted by eutrophication, which causes bottom water hypoxia and consequently high seawater pCO2. At the same time, elevated nutrient concentrations increase the energy availability for filter feeding organisms such as mussels. Thus M. edulis can dominate over a seemingly more acidification resistant species such as A. improvisus. We conclude that benthic stages of M. edulis tolerate high ambient pCO2 when food supply is abundant and that important habitat characteristics such as species interactions and energy availability need to be considered to predict species vulnerability to ocean acidification.

Biological and environmental time series obtained from Point B, Bay of Villefranche

Ecological succession provides a widely accepted description of seasonal changes in phytoplankton and mesozooplankton assemblages in the natural environment, but concurrent changes in smaller (i.e. microbes) and larger (i.e. macroplankton) organisms are not included in the model because plankton ranging from bacteria to jellies are seldom sampled and analyzed simultaneously. Here we studied, for the first time in the aquatic literature, the succession of marine plankton in the whole-plankton assemblage that spanned 5 orders of magnitude in size from microbes to macroplankton predators (not including fish or fish larvae, for which no consistent data were available). Samples were collected in the northwestern Mediterranean Sea (Bay of Villefranche) weekly during 10 months. Simultaneously collected samples were analyzed by flow cytometry, inverse microscopy, FlowCam, and ZooScan. The whole-plankton assemblage underwent sharp reorganizations that corresponded to bottom-up events of vertical mixing in the water-column, and its development was top-down controlled by large gelatinous filter feeders and predators. Based on the results provided by our novel whole-plankton assemblage approach, we propose a new comprehensive conceptual model of the annual plankton succession (i.e. whole plankton model) characterized by both stepwise stacking of four broad trophic communities from early spring through summer, which is a new concept, and progressive replacement of ecological plankton categories within the different trophic communities, as recognised traditionally.