Date of Award

Summer 8-20-2021

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

Jasmine Saros

Second Committee Member

Amanda Klemmer

Third Committee Member

Lee Karp-Boss

Abstract

Concentrations of dissolved organic carbon (DOC) increased across lakes of Maine for several decades before stabilizing or decreasing in recent years. To investigate the seasonal effects of DOC on phytoplankton habitat structure, I assessed vertical gradients of temperature, oxygen, light, and chlorophyll in four lakes in Acadia National Park from under ice through fall turnover in 2020. Lake DOC concentrations ranged from low (~2 mg L-1) to moderate (~4 mg L-1). Low-DOC lakes were clearer, with greater mean Secchi depths (9-15 m) than moderate-DOC lakes (5-6 m). Moderate-DOC lakes experienced hypolimnetic anoxia in the summer and had more variable concentrations and vertical gradients of chlorophyll a. Seasonal variance of vertical habitat gradients were similar between low- and moderate-DOC lakes, but there was greater stability of chlorophyll a biomass and consistently deeper chlorophyll fluorescence peaks in the low-DOC lakes. Phytoplankton community structure was also assessed at three depths in one low- and one moderate-DOC lake on four dates over the sampling period. While the concentration and depth of chlorophyll a varied more in the moderate-DOC lake, there was greater turnover of phytoplankton community in the low-DOC lake, with Morisita-Horn dissimilarity slightly higher in low-DOC Jordan Pond (r2 = 0.66, p < 0.01) than in moderate-DOC Seal Cove Pond (r2 = 0.61, p < 0.01). Long-term trends in DOC concentration were correlated with Secchi depth across several lakes in Acadia National Park (r2 = 0.76, p < 0.001), and DOC concentration was also related to changes in phytoplankton pigment assemblages in the sediment record of Seal Cove Pond (r2 = -0.38, p = 0.09). DOC concentration was not related to diatom-inferred mixing depth in Seal ii Cove Pond’s sediment record, suggesting that DOC has a greater impact on light environment than on mixing depth in the moderate-DOC lake. Differences in the vertical habitat gradients and phytoplankton community structure of lakes ranging in DOC concentration from 2-4 mg L-1 indicates that lakes are highly sensitive to changes in light environment, thermal structure, and other environmental drivers, particularly at low DOC concentrations. The greater sensitivity of low- to moderate-DOC lakes has important implications for broader ecosystem processes, including nutrient cycling, productivity, and water quality.

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