Application of Multivariate Trophic State Index Tool for Lake Nutrient TMDL Development in Iowa

Algae bloom and turbidity issue are common impairment to many lakes on Clean Water Act 303(d) list. A technically-sound nutrient target to establish a TMDL endpoint is a pressing need, but deriving nutrient target has proven to be challenging. Carlson's Trophic State Index (TSI) is a numeric indicator of the continuum of the biomass of suspended algae in lakes and thus reflects a lake's nutrient condition and water transparency. The level of plant biomass is estimated by calculating the TSI value for chlorophyll-a. TSI values for total phosphorus (TP) and Secchi depth serve as surrogate measures of the TSI value for chlorophyll. In order to establish TMDL endpoint for lake TMDL, the most stringent water quality indicator among TP, chlorophyll-a and secchi depth that cause impairment needs to be selected. However, because the modeling tool "Lake Phosphorus Worksheet" by Iowa Department of Natural Resources can only explicitly simulate TP, the potential reduction target in chlorophyll-a or secchi depth will need to be translated into reduction goal in terms of TP. Therefore, the objective of this paper is to demonstrate this procedure combining Carlson's TSI tool with multivariate TSI comparison chart for establishing TMDL target through a lake TMDL case study in Iowa.Typically, a total phosphorus TSI of less than 70, which is related through the trophic state index to chlorophyll-a and secchi depth, defines the nutrient-loading target for TMDLs previously developed in Iowa. Thus the Phase I targets for lake TMDL in Iowa are normally a median TSI value of less than 70 for TP, median TSI value of less than 65 for both chlorophyll and secchi depth. These values are equivalent to TP and chlorophyll concentrations of 96 and 33 μg/L, respectively, and a secchi depth of 0.7 meters. In the case of Five Island Lake, because only chlorophyll-a is in non-compliance (i.e. TSI value of 66 for median water quality data is greater than target TSI of 65), the objective of this TMDL is to improve chlorophyll by 1.5%. Assuming a 1:1 response between TSI for secchi depth and TSI for TP, the target TSI for total phosphorus is set at 98.5% of its current TSI level, which corresponds 68. TSI of 68 for TP corresponds to in-lake TP target as 82 μg/L.In the Lake Phosphorus Worksheet, there is a suite of three watershed loading and eleven in-lake models. How to balance the estimate between lake watershed loading model and in-lake water quality response model and make the best selection is technically challenging. After comparison of the estimated TP loads based on watershed loading model and in-lake water quality models and verification of all model parameters in prespecified range, the best-fit empirical model for existing loads is selected. The allowable TP loads at the target concentration (TP=82 μg/L) for the lake, using the selected empirical model as a linkage to nutrient target, can thus be calculated.In summary, Carlson's TSI tool combining with multivariate comparison chart is a useful tool for problem identification and establishment of nutrient target for lake TMDL. Lake Phosphorus Worksheet that includes a series of model options is an effective modeling tool that is well suited for lake nutrient TMDL analysis.