Trophic State, Tripton, Pelagic Versus Near-Shore, and Modeling Issues for Cayuga Lake, NY.

Abstract

An analysis of limnological and input monitoring data for Cayuga Lake, NY is presented that addresses differences in metrics of trophic state and turbidity between pelagic waters and a shallow (< 6 m) near-shore area (the shelf) that receives multiple inputs, within the context of the effects of tripton and mixing processes and modeling needs. The analysis is based on a combination of long-term monitoring and shorter-term studies, including: (1) 10 to 20 years of measurements of concentrations of chlorophyll a [Chl], total phosphorus [TP], and other forms of P; (2) 10 years of measurements of Secchi disc depth (SD) and surrogates of light scattering, including turbidity [Tn], and the beam attenuation coefficient at 660 nm [c(660)]; (3) P and Tn measurements for point sources and tributaries that enter the shelf (4 to 10 y) and related constituent loading calculations; (4) a 40 site transect along the length of the lake (> 50 km) with rapid profiling instrumentation that resolves spatial patterns in thermal stratification, fluorometric chlorophyll a, and c(660); (5) light scattering versus gravimetric features of minerogenic tripton particles from tributary, shelf and pelagic sites; and (6) extent of mixing between the shelf and pelagic waters. Despite the P loading received from local sources, summer average [Chl] levels are not significantly higher on the shelf compared to bounding pelagic waters because of the high flushing rate of the shelf promoted by mixing with pelagic waters. The generally higher [TP], c(660), and Tn, and lower SD on the shelf compared to pelagic waters is shown to reflect inputs of clay minerals. The particle sizes of this material, which diminished SD and increased Tn and c(660) on the shelf, are shown to be in the 1 to 10 ?m range. Two water quality modeling initiatives are recommended to guide related management deliberations: (1) a lake-wide seasonal P or nutrient-phytoplankton model, with a twodimensional transport framework that would provide longitudinal and vertical resolution, and (2) a shorter-term three-dimensional model for the tripton component of c(660) that would simulate the dynamics and spatial details of the impacts of runoff events on clarity levels on the shelf.