Transition zones in small lakes: the importance of dilution and biological uptake on lake-wide heterogeneity

Stream inflows are frequently the dominant route for nutrients from catchments to lakes. Studies on large, deep reservoirs and lakes have shown the importance of the fate of inflow plumes for nutrient accessibility to phytoplankton. However, few studies have considered shallow water transition zones between streams and lakes, often a feature of small lakes. This study examined the spatial and temporal dynamics of phosphorus in a shallow stream-lake transition in a small lake to improve our understanding of how phosphorus reaches the pelagic zone. Despite the high discharge levels, and the importance of dilution in explaining observed spatial gradients in soluble reactive phosphorus (SRP), total phosphorus (TP) and chlorophyll a, we found evidence for significant biological uptake of SRP in the inflow embayment during the growing season. This may represent an additional mechanism for the dispersal of phosphorus from the embayment into the lake. The length scale for the transition zone was short (~150 m) which indicated that the direct influence of the inflow on the wider lake was small. However, SRP measurements taken only from the pelagic site underestimated mean lake-wide concentrations when including transition zones by up to 18% during the growing season.