Date of Award

2011

Document Type

Honors Thesis (Open Access)

Department

Colby College. Environmental Studies Program

Advisor(s)

F. Russell Cole

Second Advisor

Philip Nyhus

Third Advisor

Liam O'Brien

Abstract

The purpose of this study was to assess the abundance and family diversity of zooplankton communities in the Belgrade Lakes, and to identify the broad scale and local variables that structure zooplankton communities in this region. The local effects of shoreline development and the presence of macrophyte patches were compared to larger scale variables, such as watershed wide residential development. Zooplankton are an intermediate link in the freshwater food web, and communities respond both to predation pressures as well as nutrient inputs. Shoreline development was expected to influence zooplankton densities by the increased nutrient inputs via erosion off developed sites with no buffer. The presence of macrophytes was expected to increase the densities of zooplankton since macrophytes serve as a refuge for zooplankton from zooplanktivorous fish.

Zooplankton were sampled in all seven of the Belgrade Lakes from macrophyte patches, in the water adjacent to undeveloped shorelines, and in the water adjacent to developed shorelines with no vegetated buffer. 142 sites were sampled in total, and at least five replicates of each of the three primary habitat types were assessed from all of the Belgrade Lakes. Each zooplankton specimen was identified to the Family level, with the exception of copepods, which were identified to the Order. Abundance of each family was established for each site as number of individuals per liter. Aggregate density and the density of each Family were tested for significant differences between sites with no buffer and undeveloped sites, as well as between sites with macrophytes and sites without macrophytes. The relative influence of all variables on density were analyzed using a two step model. The first step was a zero inflated negative binomial regression which predicts the log odds of observing a density of zero individuals. This accounted for the high number of zero densities observed in each family. The second step was a logarithmic regression to predict the non-zero densities of each family.

No significant differences were observed between sites with no buffer and undeveloped sites for the density of any Family. Aggregate density and the Cladoceran Families Chydoridae, Daphniidae and Sididae all had significantly higher densities within macrophyte patches compared to outside of macrophyte patches.

Flushing rate was linked to decreases in several families of zooplankton, including Chydoridae, Daphniidae, and the Order Cyclopoida. Increased flushing rate is generally associated with better water quality and lower nutrient levels in lakes, which is consistent with our prediction that nutrient levels will be important determinants of zooplankton abundance. Residential development was linked with decreases in several families of zooplankton, including Bosminidae, Chydoridae, Sididae and the Order Cyclopoida. While this is counter-intuitive to the logic that increased nutrient loading will increase zooplankton densities, nutrient and chlorophyll levels were held constant in several models. Another mechanism, such as changes in pH may be responsible for the declines predicted. The decline in densities in the presence of Northern Pike was not expected, since the presence of an aggressive piscivore was predicted to lower the grazing pressure of zooplanktivores on zooplankton, and an increase in zooplankton abundance. The opposite effect was observed, suggesting that predation controls on zooplankton abundance are not as important as other environmental variables in the Belgrade Lakes. Management of lake water quality should focus on controlling nutrient inputs into the lakes to improve water quality, because top-down drivers are unlikely to be structuring the zooplankton communities of the Belgrade Lakes.

Keywords

Bosminidae, Calanoida, Chydoridae, Cyclopidae, Daphniidae, Sididae, Polyphemidae, Asplanchnidae

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