Date of Award

Summer 8-22-2018

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

Sarah Nelson

Second Committee Member

Ivan Fernandez

Third Committee Member

Jasmine Saros

Abstract

As the rate of sulfate (SO42-) deposition continues to decline and climate is trending towards warmer and wetter conditions, the biogeochemical and physical response of small, north temperate lakes is variable. In this study, we observed long-term chemical trends combined with seasonal water temperature patterns in the context of climate change and recovery from acidification in two remote lake populations in Maine: 29 high elevation lakes and eight low elevation lakes. Small, temperate lakes are the most abundant type of lake, making them a widely representative study sample to consider. Maine’s high elevation lakes (>600m) could potentially provide unique insight into the response of surface water chemistry to declining acidic deposition and interannual climate variability. The geochemical response in 29 lakes was analyzed during 30 years of change in sulfate (SO42-) deposition and climate. All 29 lakes exhibited positive trends in DOC from 1986-2015, and 19 of 29 lakes had statistically significant increases in DOC throughout the study period. These results illustrate a region-wide change from low DOC lakes (/L) to moderate DOC lakes (5-30 mg/L). Increasing DOC trends for these high elevation lakes were more consistent than for lower elevation lakes in the northeastern US. A linear mixed effects model demonstrated that lakewater SO42- and climate variables describe most of the variability in DOC concentrations (r2 = 0.78), and the strongest predictor of DOC concentration was an inverse relationship with SO42-. Due to SO42- concentrations trending towards pre-acidification levels and projections of a warmer, wetter, and more variable climate, there is uncertainty for the future trajectory of DOC trends in surface waters. Long-term monitoring of Maine’s high elevation lakes is critical to understand the recovery and response in surface water chemistry to a changing chemical and physical environment in the decades ahead. DOC trends in lower elevation lakes were more variable. We hypothesize this is attributed to different levels of SO42- deposition and availability of DOC in these watersheds, which varied in size and landscape character. We used high frequency temperature arrays to observe stratification dynamics during summer 2017. We hypothesized that lakes with higher DOC concentrations will exhibit a larger ratio of hypolimnion volume: total lake volume, and therefore have a larger volume of cold-water refugium that is less likely to warm or disappear over the course of a summer stratification period. We found the strongest predictors of percent change of hypolimnion volume ratio (HVR) were both the interaction of DOC concentration and maximum lake depth and just DOC concentration. This suggests that deeper and darker lakes are more likely to maintain larger hypolimnia and cold-water refugia over the course of summer stratification than shallower lakes. Schmidt stability quantifies the strength of stratification, and a significant, inverse relationship between mean Schmidt stability and percent change in (HVR) suggests that morphometry matters; lakes with greater stratification stability and simple basin morphometry are more likely to maintain larger hypolimnia throughout the course of summer stratification The disproportionately large contribution of small lakes to the global carbon cycle deem them an ideal system to observe and quantify changing DOC dynamics. Disentangling the effects of climate, acidification, and individual lake characteristics on small, north temperature lakes is critical to understanding the biogeochemical response of freshwater to a changing physical and chemical environment in the decades to come.

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