Incomplete recovery of a shallow lake from a natural browning event

Abstract

Abstract Terrestrial loads of dissolved organic carbon (DOC) have increased in recent years in many north temperate lakes, leading to widespread brownification. This process can profoundly alter the relative strength of planktonic and benthic primary production in lakes due to changes in light and nutrient availability. While most existing in situ studies of brownification focus on gradual decadal trends, short‐term flooding can induce rather sudden natural browning events in lakes. We investigated the recovery of a small, temperate, groundwater‐fed shallow lake from a strong natural flooding‐induced browning and nutrient loading event. We hypothesised that along with a return to pre‐flood lake water levels, DOC concentrations would return to pre‐flood concentrations due to bacterial and photolytic mineralisation and flocculation, while total phosphorus (TP) concentrations would show a delayed and potentially incomplete recovery due to sustaining effects of phytoplankton dominance on nutrient dynamics in the lake. Phytoplankton gross primary production (GPP), which surged in response to higher nutrient concentrations and a compressed mixing depth, was expected to follow the same recovery pattern as TP. Benthic periphyton GPP was expected to recover in tandem with improving light conditions. We measured post‐flood water levels, water chemistry, and summer pelagic and benthic GPP. The lake water level declined gradually, returning to pre‐flood levels within 4 years. As hypothesised, DOC concentrations initially declined rapidly, but remained 1.5‐fold higher (c. 22 mg/L) than pre‐flood values 3 years after peak browning. TP concentrations fluctuated greatly between seasons but remained generally elevated in subsequent years, being roughly double pre‐browning values (c. 130 µg/L) 3 years post‐flood. Measurements taken in 2019, 7 years after the initial browning event, showed both DOC and TP concentrations remaining at these elevated levels. Across the intensive study period 3 years before and after peak browning (2010–2015), TP concentrations were positively correlated to summer phytoplankton biomass, while DOC and TP concentrations were negatively correlated to summer periphyton biomass and GPP. Accordingly, the lake's recovery period exhibited a decline of pelagic GPP and a partial recovery of summer periphyton biomass and production, although a full return to pre‐flood values was not recorded in either case. Whole‐lake areal summertime GPP increased due to browning and remained higher than before the browning event. This study provides a rare examination of differences in response rates between DOC, TP, and autotrophic structure in a shallow, temperate lake following a natural flood‐induced extreme browning event. The available data provide compelling evidence that browning events can produce lasting changes in the water chemistry, trophic status, and productive pathways of a lake, persisting beyond the lake's return to pre‐flood water levels. Such delayed or incomplete recoveries from flooding‐induced browning and nutrient loading may have consequences for regional and long‐term temporal trends in lake biogeochemistry, potentially contributing to the widespread phenomenon of incremental brownification.