Copepod Foraging on the Basis of Food Nutritional Quality: Can Copepods Really Choose?

Copepods have been considered capable of selective feeding based on several factors (i.e., prey size, toxicity, and motility). However, their selective feeding behaviour as a function of food quality remains poorly understood, despite the potential impact of such a process on copepod fitness and trophodynamics. In this study, we aimed to evaluate the ability of copepods to feed selectively according to the nutritional value of the prey. We investigated the feeding performance of the calanoid copepod Acartia grani under nutritionally distinct diets of the dinoflagellate Heterocapsa sp. (nutrient-replete, N-depleted and P-depleted) using unialgal suspensions and mixtures of prey (nutrient-replete vs. nutrient-depleted). Despite the distinct cell elemental composition among algal treatments (e.g., C:N:P molar ratios) and the clear dietary impact on egg production rates (generally higher number of eggs under a nutrient-replete diet), no impact on copepod feeding rates was observed. All unialgal suspensions were cleared at similar rates, and this pattern was independent of food concentration. When the prey were offered as mixtures, we did not detect selective behaviour in either the N-limitation (nutrient-replete vs. N-depleted Heterocapsa cells) or P-limitation (nutrient-replete vs. P-depleted Heterocapsa cells) experiments. The lack of selectivity observed in the current study contrasts with previous observations, in which stronger nutritional differences were tested. Under normal natural circumstances, nutritional differences in natural prey assemblages might not be sufficiently strong to trigger a selective response in copepods based on that factor alone. In addition, our results suggest that nutritional quality might depend not only on the growing conditions but also on the inherent taxonomical properties of the prey.     

Future Climate Scenarios for a Coastal Productive Planktonic Food Web Resulting in Microplankton Phenology Changes and Decreased Trophic Transfer Efficiency

We studied the effects of future climate change scenarios on plankton communities of a Norwegian fjord using a mesocosm approach. After the spring bloom, natural plankton were enclosed and treated in duplicates with inorganic nutrients elevated to pre-bloom conditions (N, P, Si; eutrophication), lowering of 0.4 pH units (acidification), and rising 3°C temperature (warming). All nutrient-amended treatments resulted in phytoplankton blooms dominated by chain-forming diatoms, and reached 13–16 μg chlorophyll (chl) a l⁻¹. In the control mesocosms, chl a remained below 1 μg l⁻¹. Acidification and warming had contrasting effects on the phenology and bloom-dynamics of autotrophic and heterotrophic microplankton. Bacillariophyceae, prymnesiophyceae, cryptophyta, and Protoperidinium spp. peaked earlier at higher temperature and lower pH. Chlorophyta showed lower peak abundances with acidification, but higher peak abundances with increased temperature. The peak magnitude of autotrophic dinophyceae and ciliates was, on the other hand, lowered with combined warming and acidification. Over time, the plankton communities shifted from autotrophic phytoplankton blooms to a more heterotrophic system in all mesocosms, especially in the control unaltered mesocosms. The development of mass balance and proportion of heterotrophic/autotrophic biomass predict a shift towards a more autotrophic community and less-efficient food web transfer when temperature, nutrients and acidification are combined in a future climate-change scenario. We suggest that this result may be related to a lower food quality for microzooplankton under acidification and warming scenarios and to an increase of catabolic processes compared to anabolic ones at higher temperatures.     

Mesozooplankton distribution in relation to hydrology of the Northeastern Aegean Sea, Eastern Mediterranean

The Northeastern Aegean Sea (NEA) is the area where Black Seawaters (BSW) outflow in the Mediterranean enhancing local productivityand inducing high hydrographic complexity. We describe the structureand distribution (both vertical and horizontal) of mesozooplanktonassemblages in the NEA during the thermal stratification periodin July and September 2003, in an effort to identify BSW effects.The overall standing stocks of mesozooplankton in the NEA weremuch higher than those typically reported for other pelagicwaters of the eastern Mediterranean. Higher abundance and biomassvalues and a distinctive copepod and cladoceran species assemblagewere recorded in the surface layer (directly influenced by theBSW). Variability in the supply and subsequent advection ofBSW in the NEA seemed to be the major factor affecting the structureand distribution of mesozooplankton assemblages. In July, lowersurface salinity reflected the increased supply of BSW (richin organic carbon) and, consequently, the abundance of filterfeeding taxa (cladocerans, appendicularians and doliolids) washigher than in September. During both surveys, the copepod/cladoceranassemblages of the surface (0–50 m) layer were sensitivetracers of horizontal hydrological and biological variability(e.g. location of fronts) associated with the advection of theBSW. This paper was presented at Plankton Symposium III, held atFiguera da Foz, Portugal between 17 and 20 March 2005, underthe auspices of the University of Coimbra and the Universityof Aveiro, and coordinated by Mário Jorge Pereira andUlisses M. Azeiteiro.