Phenological changes of blooming diatoms promoted by compound bottom-up and top-down controls

Abstract

Understanding phytoplankton species-specific responses to multiple biotic and abioticstressors is fundamental to assess phenological and structural shifts at the communitylevel. Here we present the case of Thalassiosira curviseriata, a winter blooming diatomin the Bahía Blanca Estuary, Argentina, which displayed a noticeable decrease in thepast decade along with conspicuous changes in phenology. We compiled interannualfield data to assess compound effects of environmental variations and grazing by theinvasive copepod Eurytemora americana. The two species displayed opposite trendsover the period examined. The diatom decreased towards the last years, mainly duringthe winters, and remained relatively constant over the other seasons, while thecopepod increased towards the last years, with an occurrence restricted to winter andearly spring. A quantitative assessment by structural equation modeling unveiled thatthe observed long term trend of T. curviseriata resulted from synergistic effects ofenvironmental changes driven by water temperature, salinity and grazing. Theseresults suggest that the shift in the abundance distribution of T. curviseriata towardshigher annual ranges of temperature and salinity -as displayed by habitat associationcurves- constitutes a functional response to avoid seasonal overlapping with itspredator in late winters. The observed changes in timing and abundance of theblooming species resulted in conspicuous shifts in primary production pulses. Ourresults provide insights on mechanistic processes shaping the phenology and structureof phytoplankton blooms.