Long-term 50-year changes in phenology and phenotypic state of Phaeocystis globosa in Dutch coastal waters

Phaeocystis globosa is a widespread phytoplankton species characterized by a complex polymorphic life cycle including a colonial phase. Due to its high ecological importance and capacity to produce Harmful Algal Blooms (HABs), understanding its population dynamics under increasing anthropogenic influences is essential. This study examined the seasonal dynamics of colonies of Phaeocystis globosa in the Marsdiep, the westernmost tidal inlet of the southern North Sea to the coastal Wadden Sea. Between 1994 and 2021, densities of different morphotypes of P. globosa and environmental conditions were measured ca. 40x per year. Between 1994 and 2009, the timing of the peak in densities of colony cells was highly variable (ranging from mid-March to mid-July), and the peak in colony cells was followed by one in flagellates (the sum of diploids and haploids). Between 1994 and 2021, variations in colony-cell densities were significantly and non-linearly correlated with those in phosphate concentrations, water temperature and light conditions. These three factors most likely co-limited the growth of this species. Although long-term increases or decreases were observed in these three factors, no phenological shift in Phaeocystis blooms was observed. The power of our analysis, however, was decreased due to a change in the assignment of cell types within 'solitary' and 'colony' categories in 2009/2010, and a longer time series may be needed to reveal causal factors influencing Phaeocystis blooms. As warming and de-eutrophication will progress, continuation of these time series is of utmost importance for monitoring the species, and for understanding impacts on coastal food webs.