Change of Phaeocystis globosa ecology displayed by a 50-year time series (Dutch Wadden Sea)

Phaeocystis globosa is a colony-forming microalgal species with a complex multiple morphotype life cycle that can produce Harmful algal bloom (HAB) inducing ecological and societal issues. In the Dutch Wadden Sea the P.globosa colony and solitary cell densities have been monitored since 1974 as a component of a long-term phytoplankton series. During these years this ecosystem has endured anthropic effects through eutrophication and climate change modifying the drivers of the phytoplankton communities. Along the phytoplankton series, numerous variables (e.g. temperature, chl a, nutrients) are measured to better describe the changes and the impacts of those changes. Generalized Additives Modelling (GAM) is used to describe the change of ecology of P.globosa along the time series and pinpoint which drivers. In the longest term, the analysis highlights a shift from a period of eutrophication with high densities of P.globosa followed by the treatment of water inducing a reduction of densities. Whereas in recent years marked by the rise of temperature, the densities seem to decrease slightly. Throughout the year insolation has increased and temperature while after the eutrophic period, phosphorus has been reduced. These three parameters are pinpointed as the main drivers controlling the densities of P.globosa. Temperature and phosphorus also control the occurrence of bloom. As the temperature pattern has changed the phenology of bloom has also been modified with earlier bloom. Since 1994 the proportion of the colony morphotype has also increased steadily modifying the ecology of the species. The time series allows demonstrating how extremes in condition are affecting and driving densities ecology and phenology of P.globosa with potential consequences on human activities and higher trophic levels.