General empirical models for estimating nutrient load limits for Florida's estuaries and inland waters.

We developed empirical models that can predict maximum allowable, nutrient loading rates or load limits (LM) for maintenance of mesotrophy in Florida’s fresh and estuarine waters. Water residence time (tw) explains most of the variation in LM previously set by the U.S. Environmental Protection Agency and Florida’s environmental agencies for total phosphorus (TP) and for total nitrogen (TN). We found that a single model for TP and for TN fit data from both estuarine and freshwater systems. Further, we show that the models are consistent with or analogous to the critical loading model of the Organization of Economic Cooperation and Development (OECD 1982): Lc = [TP or TN]c x z/Τw(1 + Τw0.5). The LM - Τw model for TP is sufficiently similar to the OECD model as to indicate that either model could provide a reasonable fit to mesotrophy in both temperate and subtropical systems. The models can facilitate the development of total maximum daily loads for estuaries and inland waters of Florida and may have broader regional utility. They may also aid the development of nutrient concentration criteria.