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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. [ABSTRACT FROM AUTHOR]

1. Planktonic bacterial community dynamics over short timescales can be of great importance for food webs and ecosystem functioning but are rarely described when microbial community and composition are assessed. To study the significance of such dynamics we sampled the surface water at the deepest point of a mesotrophic lake (Lake Erken, Sweden) every third hour over two days. 2. By combining 454 pyrosequencing of 16S rRNA genes with bromodeoxyuridine immunocapturing of DNA, replicating populations were identified and compared to the community retrieved from total DNA samples. This comparison revealed a significant difference between the actively replicating and total community. 3. The high-frequency diurnal sampling was compared to a year-long survey conducted in the same lake in order to compare the diurnal and seasonal variation in bacterioplankton community composition. At the diurnal-scale, the variation was significantly higher in the replicating than in the total community. However, variation in both active and total diurnal community was significantly lower than the variation in the seasonal total community. 4. Our analysis revealed pronounced short-term dynamics of individual bacterial populations uncoupled from the diurnal light cycle. For example, the proliferating fraction of the most abundant bacterial tribe (LD12) followed a cyclic pattern that covaried with viral abundance. This implies that environmental factors other than light may act as important drivers of microbial community composition, at least in mesotrophic Lake Erken. [ABSTRACT FROM AUTHOR]

Compared to the well-studied open water of the "growing" season, under-ice conditions in lakes are characterized by low and rather constant temperature, slow water movements, limited light availability, and reduced exchange with the surrounding landscape. These conditions interact with ice-cover duration to shape microbial processes in temperate lakes and ultimately influence the phenology of community and ecosystem processes. We review the current knowledge on microorganisms in seasonally frozen lakes. Specifically, we highlight how under-ice conditions alter lake physics and the ways that this can affect the distribution and metabolism of auto- and heterotrophic microorganisms. We identify functional traits that we hypothesize are important for understanding under-ice dynamics and discuss how these traits influence species interactions. As ice coverage duration has already been seen to reduce as air temperatures have warmed, the dynamics of the under-ice microbiome are important for understanding and predicting the dynamics and functioning of seasonally frozen lakes in the near future. [ABSTRACT FROM AUTHOR]