Time scales of external loading and spatial heterogeneity in nutrients-chlorophyll a response: Implication on eutrophication control in a large shallow lake

Nutrient over-enrichment of phosphorus (P) and nitrogen (N) makes phytoplankton proliferate, and is closely linked to algal blooms. Identifying effects of nutrients on algal biomass is necessary for eutrophication control. Long term (1994–2018) monitoring data of chlorophyll a (Chl-a), in-lake nutrient concentrations, and external phosphorus loading were leveraged in four ecological zones of Lake Okeechobee. Generalized Additive Models (GAMs) were developed to detect the response of Chl-a to nutrients. Term of cumulative loading with different time scales was added for the first time into GAMs to study the legacy effect of P accumulation. We find that the added term of cumulative total phosphorus (TP) loading integrated over four years to one decade produced the GAM model with the best explanatory power. This result suggests that the long-term TP loading, rather than short-term external loading, has a more significant role on algal biomass in Lake Okeechobee. Our study also demonstrated the inhibition effect of ambient TP concentration and cumulative TP loading on Chl-a at their high values, while a positive impact of total nitrogen concentration in most circumstances. In addition, a threshold value of nitrate and nitrite in the response curve indicated the nitrogen fixing ability of some algae species. These findings imply that the delayed effects of restoration strategies can be up to one or two decades after external phosphorus loading reduction, and that inhibition phenomenon during high TP concentrations and cumulative loadings should be paid attention to when dealing with eutrophication problems in large and shallow lakes.