| 19 Feb 2026
Long-term changes in phytoplankton phenology, biomass, and community structure in the northern Baltic Sea
Abstract. Phytoplankton seasonality integrates signals of climate forcing, anthropogenic pressure, and natural physical variability, making it a sensitive indicator of ecosystem change in coastal seas. We analysed a 52-year phytoplankton monitoring time series (1966–2018) from twenty-six stations in the Helsinki Archipelago, Gulf of Finland, to examine long-term changes in phytoplankton phenology, biomass, and community composition.
Generalized additive mixed models revealed a pronounced seasonal cycle dominated by a short spring bloom, whose timing advanced steadily at an average rate of approximately 2.7 days per decade. In contrast, total phytoplankton biomass exhibited stepwise changes rather than gradual trends, with a marked decline in both spring and summer biomass in the late 1980s and early 1990s. This shift coincided temporally with a period of major restructuring of municipal wastewater treatment in the Helsinki metropolitan area.
Community composition responded differently from biomass. Distance-based ordination showed that long-term temporal trends and seasonal variation acted largely independently on community structure, with compositional change more strongly associated with decadal-scale trends than with seasonality. Time-series decomposition identified changepoints in community seasonality around 1972 and 2000. Following the latter, summer biomass increased while spring bloom intensity remained comparatively unchanged, consistent with a reorganisation of seasonal community structure rather than a reversal of long-term recovery.
Together, these results demonstrate that phytoplankton phenology, biomass, and community composition capture complementary aspects of ecosystem change. Long-term observations allow disentangling gradual climate-driven shifts, targeted management effects, and episodic physical disturbances, highlighting the value of phytoplankton seasonality as an integrative indicator in the northern Baltic Sea.
