Jagjit Kaur

Modeling the Role of Zebra Mussels in the Proliferation of Blue-green Algae in Saginaw Bay, Lake Huron

Between 1991 and 1993, Saginaw Bay experienced an invasion by zebra mussels, Dreissena polymorpha, which caused a significant perturbation to the ecosystem. Blooms of Microcystis, a toxin-producing blue-green alga, became re-established in the bay after the zebra mussel invasion. Microcystis blooms had all but been eliminated in the early 1980s with controls on external phosphorus loadings, but have re-occurred in the bay most summers since 1992. An apparent paradox is that these recent Microcystis blooms have not been accompanied by increases in external phosphorus loadings. An ecosystem model was used to investigate whether the re-occurrence of Microcystis could be due to changes caused by zebra mussels that impacted phytoplankton community structure and/or internal phosphorus dynamics. The model was first used to establish baseline conditions in Saginaw Bay for 1991, before zebra mussels significantly impacted the system. The baseline model was then used to investigate: (1) the composite impacts of zebra mussels with average 1991–1995 densities; (2) sensitivity to changes in zebra mussel densities and external phosphorus loadings; and (3) three hypotheses on potential causative factors for proliferation of blue-green algae. Under the model assumptions, selective rejection of blue-green algae by zebra mussels appears to be a necessary factor in the enhancement of blue-green production in the presence of zebra mussels. Enhancement also appears to depend on the increased sediment-water phosphorus flux associated with the presence of zebra mussels, the magnitude of zebra mussel densities, and the distribution of zebra mussel densities among different age groups.

Diet Choice by the Exotic Round Goby (Neogobius melanostomus) as Influenced by Prey Motility and Environmental Complexity

This laboratory study examined the influence of substratum complexity and water clarity/visibility on non-indigenous round goby (Neogobius melanostomus) diet choice between dreissenid mussels (Dreissena polymorpha and D. bugensis, 6 to 9 mm length) and the exotic amphipod Echinogammarus ischnus. When both prey items were offered simultaneously in bare 20-L aquaria holding clear ambient water, 6.5 to 8-cm round gobies chose primarily amphipods (> 85% of diet numerically) and consumed fewer dreissenids (< 2/h) than when mussels were offered alone (5.2/h). Round gobies could ingest substantially more biomass when feeding on a mixed diet (∼17 to 24 mg/h dry weight, not including dreissenid shells) or on amphipods alone (∼26 mg/h), than feeding on dreissenids alone (∼12 mg/h). Longer handling time of mussels may thus have influenced the round gobies’ preference for amphipods. Added substrata (stones or gravel) and/or diminished visibility (turbid water or darkness) shifted round goby diet markedly towards sessile dreissenids as motile amphipods found refuge. Two-way ANOVA indicated that both substratum and water clarity/visibility significantly influenced round goby diet, but did not interact. It is possible that the large contribution of dreissenids to round goby diet in the Great Lakes may not necessarily reflect a preference for them, but rather lower encounter rates with more profitable prey.

Analysis of Factors Affecting Zebra Mussel (Dreissena polymorpha) Growth in Saginaw Bay: A GIS-Based Modeling Approach

A statistical model was developed using a Geographical Information System (GIS) to investigate the spatial relationships between limnological variables and the growth of zebra mussels (Dreissena polymorpha) in Saginaw Bay, Lake Huron, as defined by the change in biomass over a specified time. The presence of suitable substrate was an important factor for zebra mussel habitat, making inner Saginaw Bay an area of high mussel abundance. Temperature, phytoplankton biomass (measured as chlorophyll a), and total suspended solids (TSS) as food particles were considered to be the most important limnological variables affecting growth of zebra mussels. Three layers of attributes were developed from these variables, and were overlaid in a GIS environment according to their respective weighting factors, which were calculated in statistical analysis of spatially-matched data. The model results showed that chlorophyll a contributed the most to mussels’ growth. The effect of chlorophyll a on Dreissena growth was 9 times more important than that of temperature. The shallow portions of the inner bay and the areas in proximity to the shorelines were found to be the most suitable growth regions. A comparison of model predictions with field data on growth of mussels at various spatial locations of the inner bay demonstrated the value of this model. In addition, the model was field-tested for temporal variation in the rate of change of Dreissena biomass at one sampling station where data were available. The developed GIS-based statistical model provides a rapid, objective, reliable and cost effective tool to prioritize locations of Dreissena growth.

Modeling Dissolved Oxygen in a Dredged Lake Erie Tributary

ABSTRACT A two-dimensional numerical model was developed to study dissolved oxygen (DO) kinetics in a dredged Lake Erie tributary. The model design was aimed to specifically address the fact that many tributaries to the Great Lakes are dredged periodically for navigation, and that resultant changes in morphology and hydraulics can have significant impacts on DO. Due to the greater depths caused by dredging, river velocities slow considerably and vertical mixing is not as effective, leading to thermal stratification and potential short-circuiting of warmer upstream flow. The model solves the two-dimensional (laterally averaged) hydrodynamic and mass balance equations to simulate transport and transformation relevant to dissolved oxygen using an alternating direction, implicit finite difference method. Effects of oxygen-demanding pollutants from municipal and industrial discharges and also from nonpoint sources are included. A model application was developed for the Black River (Ohio), a tributary of Lake Erie. The river is dredged periodically, becomes stratified during the low flow summer months, and is affected by changing lake levels associated with seiching in Lake Erie. After calibration and confirmation, the model was used as a diagnostic tool to understand the impact of various loading sources on low DO levels observed along the bottom of the river. It is shown that sediment oxygen demand (SOD), combined with the river hydraulics, is the primary cause for low DO levels in the Black River.