This project investigates phosphorus dynamics in Androscoggin Lake at a higher time resolution than current steady-state models, using automated and manual sampling to resolve events at timescales of minutes to weeks. It focuses upon (1) external loading from the 30-Mile River watershed as a function of stormflow, (2) water-column dynamics in Androscoggin Lake in relation to river inputs and weather, and (3) the potential for internal phosphorus loading from sediments in relation to changing dissolved-oxygen concentrations in overlying waters. Integration of these records provides insight into the controls on phosphorus loading and their sensitivity to potential future climate and land-use change.
Androscoggin Lake is an unusual lake because, during floods, the lake receives water and sediment from the Androscoggin River via the connecting Dead River. Over thousands of years these floods have built a reverse delta that contains rare ecosystems of high conservation value, preservation of which must be balanced against other water-quality goals for the lake. Compared to other lakes within its watershed, Androscoggin Lake exhibits relatively high values of phosphorus, part of which is attributed to reversed flow up the Dead River. A 2003 study by the Maine Department of Environmental Protection (MDEP, 2003, Androscoggin Lake – Dead River Phosphorus and Hydrologic Loading Analysis), presented a steady-state model for total phosphorus (TP) concentrations in Androscoggin Lake, based on yearly average conditions and calibrated by water samples collected at weekly to monthly intervals. This study (Table 5) suggested that under present conditions, average TP loading is dominated by inputs from the upstream (“30-Mile River”) watershed (~8.2 ppb), internal loading from sediments (~1.5 ppb) and back flooding water (~0.85 ppb), but noted that these estimates are “approximate at best,” and indeed under-predict late summer lake TP concentrations (~12-20 ppb).
Preliminary data from Bowdoin College studies in the past two years suggest that Androscoggin Lake and its watershed are dynamic, with significant stratification/mixing events within the water column (see Figure 1) and variations in through-flow of water that influence TP concentrations directly, and potentially indirectly by impacting rates of internal loading from bottom sediments.
This project will investigate these phosphorus dynamics in Androscoggin Lake, using automated and frequent manual sampling to characterize water-column characteristics and phosphorus levels. In addition, I will relate internal phosphorus loading rates to the lake’s water column dynamics by collecting and incubating sediment cores to measure internal loading fluxes under controlled aerated and anoxic conditions as well as extracting phosphorus fractions from the cores. Although additional factors are known to control internal P loading in Maine lakes (e.g., Lake et al., 2007, Factors contributing to the internal loading of phosphorus from anoxic sediments in six Maine, USA, lakes: Science of the Total Environment 373, 534-541), they lie beyond the scope of this study.
Impact of Project
The project will provide insights into phosphorus dynamics within Androscoggin Lake, an unusual and threatened lake designated as a highest priority lake on Maine’s Nonpoint Source Priority Watersheds List. By combining measurements of water and phosphorus inflow to the lake, lake water-column stability in response to weather and stormflow, and sediment phosphorus fractionation and fluxes, this study will describe the movement and fate of phosphorus at higher time resolution and in much greater detail than in previous studies. Results will augment existing steady-state models for lake response, and suggest further approaches to understanding this dynamic system.
Luke Salvato, M.S. Candidate
Department of Geology, Bowdoin College
610A Smith Union
Brunswick, ME 04011
Peter Lea (faculty advisor
Bowdoin College Geology Department
Image Description: lake temperature image