Plankton Taxonomic and Size Diversity of Mediterranean Brackish Ponds in Spring: Influence of Abiotic and Biotic Factors

In this study, performed in Mediterranean brackish ponds during spring season, we assessed the effects of biotic interactions and abiotic factors on the size and taxonomic structure of the phytoplankton and zooplankton. We used a taxonomic and a size diversity index as a descriptor of the community structure. We predicted that the size diversity of each trophic level would be mainly related to biotic interactions, such as size-based fish predation (in the case of zooplankton) and food resource availability (in the case of phytoplankton), whereas taxonomic diversity would be more affected by abiotic variables (e.g., conductivity, pond morphology). Our results showed a negative relationship between phytoplankton size diversity and food resource availability leading to low size diversities under food scarcity due to dominance of small species. Conductivity also negatively affected the phytoplankton size diversity, although slightly. Regarding zooplankton size diversity, none of predictors tested seemed to influence this index. Similar fish size diversities among ponds may prevent a significant effect of fish predation on size diversity of zooplankton. As expected, taxonomic diversity of phytoplankton and zooplankton was related to abiotic variables (specifically pond morphometry) rather than biotic interactions, which are usually body size dependent, especially in these species-poor brackish environments.

Biomass changes and trophic amplification of plankton in a warmer ocean

Ocean warming can modify the ecophysiology and distribution of marine organisms, and relationships between species, with nonlinear interactions between ecosystem components potentially resulting in trophic amplification. Trophic amplification (or attenuation) describe the propagation of a hydroclimatic signal up the food web, causing magnification (or depression) of biomass values along one or more trophic pathways. We have employed 3-D coupled physical-biogeochemical models to explore ecosystem responses to climate change with a focus on trophic amplification. The response of phytoplankton and zooplankton to global climate-change projections, carried out with the IPSL Earth System Model by the end of the century, is analysed at global and regional basis, including European seas (NE Atlantic, Barents Sea, Baltic Sea, Black Sea, Bay of Biscay, Adriatic Sea, Aegean Sea) and the Eastern Boundary Upwelling System (Benguela). Results indicate that globally and in Atlantic Margin and North Sea, increased ocean stratification causes primary production and zooplankton biomass to decrease in response to a warming climate, whilst in the Barents, Baltic and Black Seas, primary production and zooplankton biomass increase. Projected warming characterized by an increase in sea surface temperature of 2.29 ± 0.05 °C leads to a reduction in zooplankton and phytoplankton biomasses of 11% and 6%, respectively. This suggests negative amplification of climate driven modifications of trophic level biomass through bottom-up control, leading to a reduced capacity of oceans to regulate climate through the biological carbon pump. Simulations suggest negative amplification is the dominant response across 47% of the ocean surface and prevails in the tropical oceans; whilst positive trophic amplification prevails in the Arctic and Antarctic oceans. Trophic attenuation is projected in temperate seas. Uncertainties in ocean plankton projections, associated to the use of single global and regional models, imply the need for caution when extending these considerations into higher trophic levels.