Internal phosphorus loading drives long-term eutrophication trends and short-term deviations in a shallow hypertrophic lake

Enäjärvi is one of the numerous lakes that have experienced increasing blooms of cyanobacteria. We hypothesized that the cause includes an increasing role of internal phosphorus (P) loading. Using a long-term data series (1997–2024), we quantified the internal P load as IL_{in situ} (based on an increase in TP mass in the water column over summer), and analysed its annual variation and potential causal mechanisms. For mechanistic insights, we measured the redox potential and sediment characteristics, and computed sediment P release rates (by diffusion) at three sampling sites in June, August and October 2024. Finally, we conducted a hydroacoustic survey to support the spatial resolution of the studies on sediment P dynamics. The internal P load increased over the years from 1997 to 2024 and correlated with the chlorophyll a concentration, water transparency (Secchi), and the cyanobacterial biomass and its proportion of the phytoplankton biomass. The increase in the internal P load was primarily attributed to an increase in the anoxic area factor (a measure of anoxia), and additionally to a decrease in the water level. Our short-term studies confirmed a governing role of redox-related P release through the observation of 1) (anoxic) P-releasing sediment surfaces during summer and 2) a significant positive relationship between the sediment P release rate and the concentration of iron-bound P. In general, the sediment organic matter content has increased in the long term together with an increase in the internal P load and eutrophication. In addition, the pool of sediment P has remained high over the long term, suggesting that previous restoration measures have been ineffective in reducing the internal P load. The hydroacoustic survey revealed a good representativeness (approximately 82%) of the lake sediment characteristics by the three sites sampled in the current study. Consequently, measures to reduce the legacy P input are needed to combat the eutrophication trend with ongoing climate change in Enäjärvi, as in other similar shallow lakes.