Phosphorus Dynamics in Androscoggin Lake, Wayne and Leeds, Maine

Project Synopsis
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.

Introduction
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.

 Objectives

• To relate mixing and stratification events in Androscoggin Lake’s water column, and their influence on phosphorus dynamics, to weather and stormflow variations.
• To characterize sediment phosphorus fractions and phosphorus flux rates to the overlying water under controlled settings of fully aerated and anoxic conditions.
• To estimate external loading of total phosphorus to Androscoggin Lake from its watershed, with emphasis on stormflow dynamics.

Methods Outline

• I will establish two monitoring stations within the lake, one in its central deep basin (38’ deep) and a second in shallower water (~15’) near its outlet. Each site will have sediment traps and Hobo submersible temperature and light data-loggers deployed near the bottom and surface of the lake to determine when the lake is stratified or mixed and the stability of the water column. The sediment traps will be sampled weekly to evaluate how much sediment is stirred from the lake bottom during mixing events and/or is transported to the sites during stormflows.
• I will install a Campbell Scientific weather station at the lake to measure solar radiation, wind speed, rainfall and air temperature at 15-minute intervals and convert them into time-series plots that can be compared to mixing and stratification events in the lake’s water column.
• I will make use of an existing Bowdoin data-logger at the Wayne Village Dam, with a rating curve calibrated by ADCP, as well as a U.S. Geological Survey stream gauge on the Dead River near the lake outlet, to monitor inflow from the 30-Mile watershed and outflow from the lake. ISCO automated pumping samplers will collect samples for total phosphorus and suspended sediment at these sites, to provide first-order estimates of external TP loads and throughput.
• At least twice a week, I will use a Van Dorn sampler to collect water at 1-meter depth intervals at the two monitoring stations, which will be analyzed for phosphorus and chlorophyll concentrations. At the same time, I will use a YSI 6600 water-quality sonde to measure dissolved oxygen and temperature in the water from surface to bottom, providing a more detailed view (but a less-frequently sampled view compared to the submersible temperature loggers) of stratification and of oxygen concentrations near the lake bottom.
• I will collect sediment cores from the lake (KB corer) and extract their phosphorus fractions using the methods of Lukkari et al. (2007, Limnology and Oceanography Methods 5, 433-444).  Duplicate cores will be incubated under lake water to measure phosphorus fluxes: one set with air gently bubbled into the overlying water, and another set with nitrogen gas to induce anoxia (cf. Steinman et al., 2004, Journal of Environmental Quality 33, 2040-2048).  By periodically drawing off overlying water and measuring its phosphorus concentration, I can monitor release rates under these end-member conditions and relate them back to dissolved oxygen conditions measured at the monitoring stations.
• Phosphorus concentrations will be measured in the lab with a Lachat autoanalyzer (MDL of 1 ppb) according to the species of interest. Chlorophyll a concentrations will be measured with a Turner Instruments fluorometer, corrected for pheophytin.

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.

Contact Information
Luke Salvato, M.S. Candidate
Department of Geology, Bowdoin College
610A Smith Union
Brunswick, ME 04011
lsalvato@bowdoin.edu

Peter Lea (faculty advisor
Bowdoin College Geology Department
plea@bowdoin.edu

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