Date of Award
Level of Access Assigned by Author
Master of Science (MS)
Ecology and Environmental Sciences
D. Bryan Dail
Second Committee Member
Katherine E. Webster
Third Committee Member
Susan H. Brawley
Excess nutrients from both point and non-point sources commonly impair rivers and contribute to the formation of nuisance algal blooms that can cause aesthetic issues and decrease dissolved oxygen concentrations. The Meduxnekeag River in Aroostook County, Maine has historically experienced nuisance filamentous algal blooms and low dissolved oxygen concentrations during summer months, raising concern about possible nutrient enrichment within the river. In addition to nutrient inputs from agriculture, the river receives effluent from a starch plant and then flows through downtown Houlton, where it receives stormwater runoff and effluent from a wastewater treatment plant (WWTP). The primary objectives were to determine spatial and temporal trends in nutrient concentrations and loads, assess algal coverage within the river and determine its relationship to nutrient concentrations, and to determine which nutrient sources within the watershed pose the greatest risk to the water quality.
I investigated the relationship between nutrients, algal growth, and land use within the Meduxnekeag River watershed. Fourteen sites in a 34 kilometer reach of the river were sampled biweekly in 2004 and 2005 from May until October. Sampling included the collection of water samples for analysis of nitrogen (N) and phosphorus (P) and assessments of filamentous algal coverage at each site. The C:N:P ratio was determined for water and algal samples. I used a general linear model to determine if nutrient concentrations accounted for a significant amount of the variability in algal coverage among sites. Nutrient loads were calculated for each site based on discharge information from three gauges within the study area, and loads were related to nutrient sources within the watershed.
Both TP and NO3- concentrations increased downstream. For all sites, the mean total P (TP) concentration was 11.8 μg/L and the mean nitrate (NO3-) concentration was 0.175 mg/L. Soluble reactive P was close to or below detection in more than half of the samples and ammonium was below detection in most samples. Area-weighted load data indicated that the WWTP was the single largest source of TP and NO3-, but unweighted data suggested that cumulatively, agricultural land within the watershed contributed more than three times the amount of TP as the WWTP.
A nuisance bloom did not occur in either study year; the highest mean algal coverage at a site was only 16% (0-43%) and much less than the 30% coverage typically considered as nuisance level. Despite the steady increase in both NO3- and TP concentrations downstream, the major pattern in algal coverage was for significantly higher values upstream than downstream. Neither N nor P accounted for a significant amount of variability in algal coverage. Although nutrient ratios suggested P limitation, longitudinal patterns in algal coverage suggested that nutrients were not the primary limiting factor. Other factors such as light, substrate, and flow may be limiting algal growth in the river.
The study determined the relative contribution from different sources within the watershed but also established that the entire study area had low average nutrient concentrations. Within the past few years, agricultural practices within the watershed have improved, and the WWTP decreased the P content of its effluent. These changes could have lowered nutrient concentrations below the river's threshold for nuisance blooms and highlight the importance of effective watershed management, although further monitoring is warranted. This suggests that while the WWTP and agricultural land are the dominant nutrient sources within the watershed, current management of point and non-point sources is effectively maintaining low nutrient concentrations within the river.
Fretwell, Elizabeth A., "The Temporal and Spatial Relationship Between Phosphorus and Nitrogen Concentrations, Algal Growth, and Nutrient Sources in the Meduxnekeag River Watershed" (2006). Electronic Theses and Dissertations. 1235.