Seasonality of pico-phytoplankton abundance and cell death in a Mediterranean Bay (Bay of Palma, Majorca Island)

A pico-phytoplankton community, dominated by Synechoccoccus sp., was present through the year in the Bay of Palma, showing high dynamics, with changes across the Bay observed at a scale of a week. The populations of pico-phytoplankton showed high seasonality. Synechococcus sp. abundance was maximal during summer, with an important bloom observed at mid August. Prochlorococcus sp. showed three moderate blooms at spring, summer, and a larger one in winter. Pico-eukaryotes abundance varied seasonally, showing the highest cell concentration by mid April. Despite these differences in cell abundance, the three pico-phytoplanktonic groups showed similar patterns in the percentage of living cells, with peaks in late winter and in early summer, corresponding to the major peaks in nutrient concentration in the Bay. Similarly, the lowest % living cells was observed at late spring, indicative of high pico-phytoplankton cell death when nutrient concentration decreased markedly. The high pico-phytoplankton cell viability of late winter, was followed by moderate increase in cell abundances indicating that factors other than nutrients were controlling pico-phytoplankton populations. Despite the reduced pico-phytoplankton cell death at early summer, only Synechococcus sp. developed a sizeable bloom, indicating that summer conditions were more favourable for Synechococcus sp. as compared to other groups. Cell viability helped to elucidate the role of losses vs. stresses in controlling the population dynamics of pico-phytoplankton communities in the Bay of Palma.