Seasonal dynamics of phytoplankton community assembly at the Blanes Bay Microbial Observatory (BBMO), NW Mediterranean Sea

Abstract

The dynamics of phytoplankton biomass and community composition is important for the functioning of marine ecosystems and ocean biogeochemical cycles. However, there is a shortage of studies addressing the interannual seasonal patterns of phytoplankton community assembly due to sampling limitations. Here we study the seasonal dynamics of eight major phytoplankton groups over a 12 year period (2006 to 2018) using a time-series of taxonomic composition from the Blanes Bay Microbial Observatory (BBMO) in the North Western Mediterranean Sea: dinoflagellates, diatoms, coccolithophores, Prochlorococcus, Synechococcus, picoeukaryotes, nanoeukaryotes, and photosynthetic nanoflagellates. We combine the analysis of biotic factors (primary production, phytoplankton taxa, cell abundance, cell size, chlorophyll-a concentration, and phytoplankton biomass) and abiotic factors (nutrients, temperature, and irradiance) to provide a coherent picture of the observed seasonal patterns of phytoplankton community assembly. The BBMO ecosystem is seasonally heterogeneous in community composition, displaying large fluctuating alternations in phytoplankton group dominance throughout the year. The seasonal succession of phytoplankton groups tends to repeat itself every year in a regular fashion, being the seasonal variability of the phytoplankton groups larger than their interannual variability. We compute α-diversity, a measure of the effective richness of phytoplankton groups. The seasonality of α-diversity shows that it is lowest during winter and highest during summer. We compute temporal β-diversity, a measure of compositional heterogeneity of the phytoplankton community. The data show a sinusoidal behavior of β-diversity as a function of the temporal distance between samples, with a period of one year. We use the mirror index (1 - β-diversity) at a temporal distance of one month to compute the phytoplankton group turnover. The seasonality of turnover shows that it is highest during spring and autumn. To evaluate the validity of niche and neutral theories in predicting the interannual sinusoidal behavior of β-diversity, we performed numerical simulations using a mechanistic model. The results provide support to the niche theory for marine phytoplankton ecology and community assembly. The phytoplankton groups appear to follow their specific ecological niches, tracking the seasonal changes in environmental conditions. The ecological implications of these findings are that marine phytoplankton groups appear to fill distinct environmental niches and thus may have different functional roles in the ecosystem. Furthermore, they may be predictable using both mechanistic and species distribution modeling approaches. The climatological time-series presented here can be an excellent testing ground for evaluating the performance of these marine ecosystem models having an explicit representation of different phytoplankton groups.