Date of Award

5-2008

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

Katherine E. Webster

Second Committee Member

David Halliwell

Third Committee Member

Peter Vaux

Abstract

Both bottom-up and top-down controls on water quality dynamics were examined in two hydrologically connected, shallow, mesotrophic lakes, East and North ponds. East Pond experienced algal blooms for seven often years between 1998-2006, while downstream lake, North Pond did not experience the same frequency, intensity or duration of algal blooms. Major foodweb differences between the two lakes were a larger white perch (Morone americana) population in East Pond (174,000 ± 11,000) compared to North Pond (77,000 ± 23,000) and the presence of northern pike {Esox lucius). Direct influences on lake water quality such as nutrients (total phosphorus) and zooplankton grazing pressure (cladoceran biomass) were monitored in East and North Ponds during the open water season from 2004 to 2006. During the study, algal blooms occurred in East Pond, with chlorophyll a concentrations ranging from 2.6 to 88 [xg L"1, while downstream North Pond chlorophyll a concentrations ranged from 2.2 to 10 fig IS with no nuisance algal blooms (as defined by Maine Department of Environmental Protection as a Secchi depth measurement of less than 2m). Low oxygen levels during July of 2005 and 2006 were observed in East Pond coinciding with increased late summer TP, suggesting phosphorus release from the sediments. Mean cladoceran biomass was greater in East Pond (31.4 jug L"1) compared to North Pond (23.4 pig L"1). From June to early July mean cladoceran body size in North Pond (0.72mm ± 0.1) was similar to East Pond (0.74mm ± 0.03), but was much lower from mid-August-October (0.41mm ± 0.1) compared to East Pond (0.65mm ± 0.05). The dramatic decline in body size and abundance in late summer in North Pond may be related to alewife ( Alosa psuedoharengus) predation; this species is not present in great numbers in East Pond. The potential for indirect effects on lake water quality through predation pressure on grazing cladocerans was evaluated in August and September 2005 and September 2006. Juvenile white perch, black crappie (Pomoxis nigromaculatus) and yellow perch {Perca flavescens) diets were examined along with carbon and nitrogen isotope signatures. Young of the year white perch in both ponds consumed primarily large-bodied cladocerans, such as Daphnia. Additionally, white perch consumed more cladocerans compared to juvenile yellow perch (p =0.001). Juvenile white perch in East Pond positively selected for large-bodied zooplankton such as Daphnia and cyclopoids. 515N results demonstrated that juvenile white perch fed at similar trophic levels to yellow perch and black crappie, consistent with diet data. As part of a proposed biomanipulation on East Pond, a large biomass (50-70%) of white perch biomass is being removed from the lake in 2007 and 2008. Results from this study suggest that the removal of white perch will have a positive effect on the cladoceran population in East Pond, thus increasing cladoceran grazing pressure on algae in the lake. However, this study highlights the need to maintain lower densities of zooplanktivores in order to realize continued benefits of a biomanipulation and to continue to control external sources of nutrients to the lake.

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