Lake Modeling Project Part II: An integrated assessment of the effects of internal phosphorus cycling and sediment resuspension on the eutrophication of lakes and reservoirs in the Central Plains
This is a continuation of Lake Modeling Project Part I. Reservoirs in agriculturally dominated watersheds are particularly vulnerable to eutrophication. External nutrient loading has been considered to be the main source of enrichment. However, it is becoming increasingly clear that internal mechanisms including direct flux from the sediment and re-suspension of surficial sediments can contribute significant amount of phosphorus (P) back into the water column. As a result, reductions in external nutrient loading alone may not effectively curb the negative processes associated with eutrophication.
Relatively little is known about internal nutrient recycling within reservoirs and streams in the Central Plains region. At the Central Plains Center for Bioassessment (CPCB) we studied internal P-recycling in several TMDL reservoirs. Our initial research indicated that sediment P-recycling rates can be high, and vary both within and between reservoirs. Therefore, the CPCB proposed a comprehensive analysis of internal nutrient recycling in a series of TMDL reservoirs and their associated inflow streams in order to improve our understanding of relationships between sediment nutrient release, re-suspended sediments, and eutrophication in agriculturally devoted reservoirs.
To accomplish the primary objectives of this study, internal nutrient recycling in four TMDL reservoirs and their associated inflow streams were studied over a one-year period using several complimentary approaches. First, sediment P-release studies were conducted to determine nutrient release rates. In conjunction, the nutrient content of the sediment was measured and entire sediment nutrient pools were estimated for each reservoir. We also initiated a research program to quantify sediment re-suspension rates within reservoirs, and the ecological impacts of these sediments on lake processes. A self-cleaning turbidity meter that collects measurement at 15-minute intervals was placed near the bottom surface of each reservoir to document seasonal patterns and rates of sediment re-suspension. Based on these results, we conducted bioassay sediment addition experiments to determine the direct effects of sediment re-suspension on water column nutrient concentrations and algal production. Finally, the comprehensive data collected from this proposed research was combined with data from a current study of external nutrient loading in the same TMDL reservoirs. Collectively, these data provide an accurate estimate of the relative contribution that internal nutrient recycling makes towards whole lake nutrient budgets. We believe that this research provides a protocol for determining internal nutrient recycling rates within reservoirs of the Central Plains region.