Effect of CO2, nutrients and light on coastal plankton. III. Trophic cascade, size structure and composition

We investigated the impacts of climate change-associated abiotic factors on the species composition and size structure of coastal phytoplankton communities. Surface coastal water collected off the coast of Málaga (Spain) was incubated outdoors during a 7 d microcosm experiment. The natural phytoplankton communities were exposed to high and low conditions of CO2, nutrients and light. During the first 2 d, a positive response to increased CO2 and nutrient concentration was observed in terms of abundance and chlorophyll in all size fractions (<2, 2 to 20, and >20 µm). After 2 d, a trophic cascade effect was observed within the phytoplankton communities for all treatments. The absence of mesozooplankton led to an increase in microzooplankton abundance, which coincided with a decrease in the abundance of phytoplankton <6 µm equivalent spherical diameter (ESD). At the same time, an increased concentration of larger phytoplankton was observed. Consequently, a diatom bloom dominated by Leptocylindrus danicus and Chaetoceros sp. developed, peaking on Day 5 in the high-light treatment and on Day 6 in the low-light treatment. The cascade effect was evident in both the smaller and the larger ranges of the size-abundance spectra (SAS). Although this trophic interaction occurred in all treatments in a similar way, there were still significant differences among treatments. Diatoms with cell sizes >20 µm ESD showed a positive response to the effects of increasing CO2 and nutrient concentration. These results highlight the importance of trophic interactions other than abiotic factors such as CO2 and nutrient availability in shaping the size structure of Mediterranean phytoplankton. More specifically, this work shows the importance of trophic cascade effects in scaling the plankton SAS and should be considered in both enclosure experiments and field measurements that deal with size distribution.

Abstract licence: CC BY

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