On Determining the Principal Source of Phosphorus Causing Summer Algal Blooms in Western Washington Lakes

Abstract

ABSTRACT Internal loading of total phosphorus (TP) calculated from mass balance averaged 68±21% of the total (internal + external) summer loading in 14 of 17 western Washington lakes that had internal loading. Moreover, whole-lake mean TP for the 16 lakes with complete mass balance data was 50 ±9 μg·L−1, which was similar to TP (51±18 μg·L−1) predicted from summer internal P loading determined by the maximum change in lake TP. Lake TP predicted from summer external loading, assuming a nominal retention coefficient, was only 30±9 μg·L−1. In-lake management measures to control internal loading had been either implemented or recommended in 14 of the 17 lakes. Summer P was substantially reduced in 10 of 12 lakes (or separate lake basins) receiving in-lake treatments, substantiating the dominant effect of internal loading. These observations indicate that emphasis on reducing external loads only, through watershed best-management practices (BMPs), without in-lake measures, would not have been the most cost-effective approach to control summer algal blooms in these lakes, especially if external loads were nonpoint and of minor importance during summer when algal problems are most prevalent. Comparison of external and internal P loads during summer, as illustrated here, is recommended so that in-lake controls are considered with watershed controls in a balanced approach to cost-effective lake management.