Biogeochemical processes and phytoplankton nutrient limitation in the inflow transition zone of a large eutrophic lake during a summer rain event

We used high‐frequency sampling paired with a three‐dimensional hydrodynamic–ecological model application to study how dynamic variations in water, nutrient and sediment transport in a stream inflow interacted with circulation processes in a receiving eutrophic lake to affect water quality during a 5‐day period of high rainfall in the summer. Wind‐driven horizontal circulations in the lake caused deflection of the inflowing stream, strongly influencing water quality in the littoral zone over distances >1 km from the stream mouth and constraining sediment and nutrient flux to the pelagic zone. The nutrient limitation status of phytoplankton varied both temporally and spatially within the lake in relation to nutrient transport processes and, potentially, variation in assemblage composition and environmental history. The occurrence of active processes that caused net removal (e.g. deposition) or accumulation (e.g. resuspension) of nutrients and sediments in the water column was characterized by comparing concentrations of suspended sediments and dissolved nutrients measured in the transition zone with concentrations estimated assuming only conservative mixing between the stream and the lake. Chlorophyll a concentrations in the transition zone were generally diluted by the stream, although ‘hot spots’ (≈10–30 m wide) of high concentrations occurred in the nutrient‐rich plume on a day when phytoplankton at a pelagic site were nutrient limited. This study highlighted how characteristics of littoral‐pelagic coupling can influence fine‐scale spatial and temporal variations in the water quality of large lakes.