insignificance of urban run off on 

 T-N. 



IMPLICATIONS FOR LAKE WATER QUALITY 



An assessment of predicted im- 

 pacts on the lakes of the T-N and T-P 

 loads may be performed using critical 

 loading rate estimates developed by 

 Vollenweider (1975) and Dillion and 

 Rigler (1975), and specifically for 

 Florida lakes, by Brezonik and Shan- 

 non (1971) and Kratzer (1979). Two 

 sets of critical rates are given: 

 those below which the lakes should 

 remain oligotrophic ; and those above 

 which the lake should tend to eutro- 

 phy or suffer degraded water quality. 

 These are compared in Table 4 with 

 the loadings of Table 3. 



On the basis of N:P ratios for 

 the lakes, phosphorus is probably the 

 most important relative to the pre- 

 diction of trophic conditions. Phos- 

 phorus limitation is in fact typical 

 of most lakes , with nitrogen limita- 

 tion occurring only in unusual geo- 

 logical circumstances or for lakes 

 receiving large loadings of sewage 

 effluent (which typically has very 

 low N:P ratios). Units 24 and 30 

 will be on central sewers and the 

 proposed lakes will receive no sewage 

 effluent. 



The phosphorus loading rates for 

 the lakes are at or below the exces- 

 sive levels given by both Vollen- 

 weider (1975) and Brezonik and Shan- 

 non (1971). The lakes of Unit 24 

 receive higher loadings than those of 

 Unit 30 in part because they have 

 more deepwater with a large flux due 

 to vertical diffusion. On the basis 

 of the phosphorus loading rates, 

 lakes in both units are expected to 

 be mesotrophic with fair to good 

 water quality. 



The T-P concentrations in the 

 lakes may be predicted using mass 

 balance approaches in which the 

 steady state concentration is a 

 function of loading rate, detention 

 time and mean depth (Dillion and 

 Rigler 1975; Kratzer 1979). The 

 predicted average T-P concentrations 

 are 0.020 to 0.026 mg/1 for Unit 24 

 and 0.014 to 0.019 mg/1 for Unit 30, 

 respectively, where the range results 

 from using several different pre- 

 dictive relationships . These ranges 

 are consistent with observed T-P 

 concentrations in existing Lake 

 Marco Shores. 



On the basis of the nutrient 

 loadings and T-P concentrations other 

 parameters can be predicted by vari- 

 ous regression relationships. On the 

 whole, predicted water quality is 

 good with Secchi disk transparencies 

 on the order of 1.2 to 1.5 m and T-N 

 of about 1 . 3 mg/1 . 



Chlorophyll a levels in the 

 lakes have been predicted using cor- 

 relations of average T-P vs. chloro- 

 phyll a reported in the literature 

 (Dillion and Rigler 1974 and Kratzer 

 1979), and from reported predictive 

 relationships between phosphorous 

 loading rates and chlorophyll a 

 levels (Kratzer 1979). Predicted 

 chlorophyll a concentrations range 

 from about 6 to 13 nig/m in Unit 24 

 and from 3 to 13 mg/m in Unit 30. 

 These values are in the mesotrophic 

 to slightly eutrophic range. Chloro- 

 phyll a is a commonly used trophic 

 indicator, and it serves as a measure 

 of algal biomass in lakes. 



In summary, the predicted water 

 quality parameters indicate that 

 water quality in the proposed lakes 

 will be satisfactory and that nutri- 

 ent-overenrichment will not be a 

 problem. 



245 



