Date of Award

Spring 2018

Degree Type

Thesis

Degree Name

Master of Science (MS)

First Advisor

Dr. Jeffrey Johansen

Abstract

This study validates a water quality index based on a diatom total phosphorus transfer function for monitoring of the Lake Erie’s pelagic zone. Lake Erie is again under the threat of cultural eutrophication, mostly due to the runoff of nutrients from the surrounding farmlands. Therefore, it is an issue of high importance to continue monitoring efforts in Lake Erie to assess further deterioration or progressive changes due to the restoration management practices. Diatom-biomonitoring represents one of the efficient and well-crafted tools to assess the actual conditions. However, indices of water quality based on diatom transfer functions are often used without a careful examination of their performance because model validation is, unfortunately, not an issue of primary funding importance. Therefore, we present a validation of a planktonic diatom index (PDI) based on a large, independent dataset from the Western and Central Basins of Lake Erie. The results of the regression model of PDI scores and measured total phosphorus were robust (r2=0.34, r2=0.63 after the removal of three high chloride-sulfate outliers). When considering performance of the index scores separately for each basin, differences in their morphology and nutrient loading are critically informative. The Western Basin is very shallow, with an irregular mixing regime and high nutrient and sediment load. Not surprisingly, due to the turbulent conditions in the Western Basin, it was not possible to find any significant relationship between PDI and single snapshot total phosphorus measurements. On the other hand, the Central Basin is deeper, with less nutrient enrichment, and stratification regularly occurring during the summer. The regular regime 8 with less variability enables more precise measurement of total phosphorus, hence, the regression model of PDI and measured total phosphorus was significant (r2=0.38; outliers removed, r2=0.66). Assuming that poor model performance in the Western Basin was caused by noise in measurements of total phosphorus due to continuous disturbances, we averaged the PDI scores and measured total phosphorus at each station over time. The resulting model was very robust (r2=0.52) and shows that the former lack of relationship was mostly due to large daily/seasonal variability of total phosphorus. However, the functionality of PDI relies on the assumption that total phosphorus is instrumental in the structuring of diatom communities. Using ordination methods, we show that although the effect of total phosphorus is often obscured by other variables (e.g. total nitrogen, alkalinity, conductivity), there is usually a high degree of correlation between total phosphorus and confounding variables, especially in the Western Basin. However, seasonal changes, such as mixing regime and changing light penetration, exert potentially a large effect on the ultimate diatom composition. Lake Erie is an ecologically and economically important waterbody that will likely continue to be an unstable ecosystem suffering from a mixture of severe anthropogenic impacts. It is, therefore, essential to be able to assess changes in water quality. The PDI, a tailor-made index for Lake Erie, represents a versatile tool that provides a way of monitoring of the pelagial.

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