Partitioning phosphorus concentrations and loads in tributaries of a recovering urban lake

The partitioning of phosphorus (P) loading to culturally eutrophic lakes according to sources is fundamental information to guide rehabilitation programs and support mathematical models. Patterns of concentrations and loading rates of forms of P are documented for the major tributaries of Onondaga Lake, New York, United States, an urban lake that has recently demonstrated marked recovery from extreme cultural eutrophy as a result of decreased P loading from a domestic wastewater treatment plant (WWTP). This analysis is based on long-term (19 yr) biweekly monitoring of total, dissolved, and soluble reactive forms of P at the mouths of the three largest tributaries, and shorter-term higher frequency monitoring of one of the streams at two sites to resolve rural versus urban contributions. Signatures of anthropogenic sources were identified, including: (1) combined sewer overflows, (2) leaky sewers or other dry weather discharges, and (3) hydrogeologic sediment sources. On an annual average basis, the prevailing tributary and WWTP contributions to the total P load are approximately 70 and 30%, respectively. However, in terms of effective P loading, that which can be used to support primary production during the critical summer months, the tributaries contributed substantially less (similar to 35%) of the P load. Under the prevailing WWTP loading rate, reductions in tributary loading from aggressive tributary loading management efforts (e. g., 20-30% decrease) would not be expected to yield conspicuous improvements in related features of lake water quality. However, noteworthy improvements are a reasonable expectation from such tributary management efforts following the mandated further reductions in WWTP loading.