The Spatiotemporal Variation of Sediment Characteristics and Nutrient Deposition in Two Coosa River Reservoirs

Abstract

Reservoirs create biogeochemical hotspots along river systems by trapping sediments and nutrients from their river watersheds. This study aims to understand sediment transport and nutrient deposition through time and space for two Coosa River reservoirs, Lay Lake and Weiss Lake. Lay Lake was built in 1914 and has historically suffered from poor water quality and eutrophication; Weiss Lake was built in 1961 and lies on the border of Alabama and Georgia receiving inputs from NW Georgia. Paleolimnological and sediment analytical techniques were applied to surface sediments and sediment cores to accomplish two primary objectives: 1) examine Lay Lake for aging reservoir storage and the impacts of upstream dam construction, as well as to characterize algal and cyanobacteria responses to material inputs over time, and 2) identify drivers of material delivery and spatial deposition for key nutrients (C, N, P) and heavy metals (As, Pb, Zn) for both Weiss Lake and Lay Lake. Cores and surface sediment samples were collected and analyzed for bulk density, organic matter, photosynthetic pigments, nutrients and heavy metals. Cores were dated using key differences between reservoir and riverine sediments, as well as the BACON model in R. Results reveal Lay Lake has been relatively eutrophic since its creation in 1914, and has maintained a relatively diverse phytoplankton community through time. While change linked to age was not evident, a change in 1964 due to upstream dam construction did occur causing decreased flows and alterations to deposition. Although, this change did not alter phytoplankton community structure. Weiss Lake and Lay Lake also appeared to be distributing materials differently according to individual reservoir characteristics and patterns in land use, hydrology, and material inputs. Results emphasize the complexity and heterogeneity of reservoirs and long-term eutrophication, and suggest that management for hydroelectric reservoirs should be focused on individual needs of each system.