Wet and Dry Hydrological Conditions Reduce Chlorophyll-a at River-Lake Interfaces

Shahrokh Shahbazi , Lars Ribbe, Kerstin Stahl, Carolin Winter, Benjamin Kraemer

Hydrological variation is intensifying under climate change, yet its net effects on lake water quality remain uncertain. Wet and dry hydrological conditions are commonly assumed to exacerbate eutrophication by increasing nutrient inputs during wet conditions or concentrating nutrients during dry conditions. However, these events can also activate counteracting processes—such as dilution, or enhanced mixing— that shorten residence time or redistribute nutrients in ways that suppress eutrophication. Here, we evaluate how hydrological variability shapes chlorophyll-a (Chl-a) dynamics across 53 large lakes spanning broad gradients in morphometry, watershed extent, and nutrient availability. Focusing on inflow-dominated nearshore “impact zones”, we used boosted regression trees to relate satellite-derived Chl-a (1998–2019) to inflow discharge, climatic forcing, and watershed characteristics. Inflow discharge, indexed using the Standardized Streamflow Index (SSI), emerged as the most influential predictor of Chl-a anomalies, exceeding the effects of lake surface temperature and direct precipitation. However, overall explanatory power was low (5% variance explained in cross-validation), indicating that hydrological forcing represents only one component of a complex cological system. Both wet and dry hydrological conditions were associated with reduced surface Chl-a relative to intermediate inflow states, revealing a non-linear response in which hydrological variation tended to suppress, rather than uniformly enhance, phytoplankton biomass. However, the direction and magnitude of responses varied substantially among lakes. Together, these findings challenge the assumption that intensifying hydrological variation will consistently exacerbate eutrophication. Instead, hydrological variation interacts with lake structure and watershed characteristics to shape ecosystem responses under changing hydroclimatic conditions.