Organic nitrogen fuels phytoflagellate expansion in browning lakes: Evidence from paleolimnological records

The increasing influx of terrestrially derived dissolved organic matter (DOM) into lakes, commonly referred to as browning, is a widespread phenomenon with significant ecological and societal consequences. Browning is frequently associated with the proliferation of mixotrophic phytoflagellates (PFs), yet the underlying mechanisms remain debated. In this paleolimnological study, sediment cores from seven lakes in southeastern Norway were analyzed to assess the role of potential drivers of PF success. The study focused on two ecologically distinct PFs, Gonyostomum semen and cryptophytes, using pigment biomarkers to reconstruct their historical abundance alongside proxies for DOM, nutrients, and climate. Piecewise regression analysis revealed increases in both PFs with a relatively high degree of synchrony. Generalized additive models identified nitrogen as the most consistent predictor of PF-specific pigments, while DOM was the primary predictor of nitrogen concentrations. These findings support the hypothesis that browning promotes PF expansion primarily by increasing the input of organic nitrogen that enters lakes as a component of DOM. Other proposed mechanisms, such as enhanced stratification, reduced transparency, or iron enrichment, were not consistently supported across lakes. The temporal patterns of browning and increasing PF success suggest a strong influence from historical land-use changes, particularly afforestation and wetland drainage. Results of the present study highlight the importance of land-use legacies in shaping phytoplankton community structure and provide new insights into the ecological consequences of ongoing DOM enrichment in inland waters.