Responses of a Mediterranean coastal lagoon plankton community to experimental warming

Coastal lagoons are extremely sensitive to increasing temperature, especially in the Mediterranean basin, which has been identified as a hotspot for global warming. The warming effects on the abundance and size structure of a Mediterranean coastal lagoon plankton community were investigated in this study. Water from Cabras Lagoon (Italy) was incubated in laboratory for 16 days in winter, excluding mesozooplankton. Three temperature treatments were applied: (i) the in situ winter mean water temperature; (ii) + 3 °C, as forecasted for the Mediterranean region by the next century; (iii) + 6 °C, as forecasted for the Mediterranean region by the next two centuries. A direct and positive effect of warming on ciliate density was observed in absence of their predators, as well as a taxonomic composition shift from Oligotrichs to Scuticociliatida. In turn, a decrease in heterotrophic nanoflagellate density was detected under warming. Phytoplankton (autotrophic cell size > 2 µm) density increased significantly, while their mean cell size decreased strongly throughout the experiment at the highest temperature. A significant change in phytoplankton class composition, consisting of the increase of smaller Chlorophyceae which replaced larger Bacillariophyceae, was observed under heating. Considering picoplankton (cell size < 2 µm), a decrease in larger autotrophic cell density was contemporary to an increase in smaller heterotrophic cell density, especially at the highest temperature. This work adds novel information to the predictions about plankton community responses on warming considering several trophic levels, which has been little studied in shallow coastal lagoons and in the Mediterranean basin.