William J. Edwards

Temporal Trends in Lake Erie Plankton Biomass: Roles of External Phosphorus Loading and Dreissenid Mussels

Abstract We compare the results of lakewide plankton studies conducted during 1996–2002 with data reported in the literature from previous years to evaluate the effectiveness of continued nutrient control, the relationship between external phosphorus loading and plankton abundance, and the many predicted outcomes of the dreissenid invasion. We found that although recent external annual phosphorus loading has not changed since reaching mandated target levels in the early- to mid-1980s, phytoplankton communities have. Total phytoplankton biomass, measured through enumeration and size-frequency distributions has increased since minima were observed in 1996 or 1997, with summer (July–September) biomasses generally greater than before the dreissenid establishment in the late 1980s. Cyanobacteria biomass also increased during summer in all basins after the dreissenid invasion. In contrast, chlorophyll a concentration has decreased in all basins during both spring and summer. However chlorophyll a concentration was poorly correlated with total phytoplankton biomass. Relative to the mid-1980s, crustacean zooplankton biomass during the years 1996–2002 increased in the western basin during spring and summer, increased in the central basin during spring but remained the same during summer, and decreased to low levels in the eastern basin. Several of these observations are consistent with predictions made by previous researchers on the effects of reduced total external phosphorus loading and the stimulatory or inhibitory effects of dreissenid mussels. However, several were not. Results from this study, particularly the inconsistencies with tested predictions, highlight the need for further research into the factors that regulate plankton community dynamics in Lake Erie.

Hypolimnetic Oxygen Depletion Dynamics in the Central Basin of Lake Erie

Hypolimnetic oxygen depletion has been recognized as a problem in the central basin of Lake Erie since the 1970s. However, recent expansion in distribution of the depletion after several years of low depletion rates in the 1990s has led investigators to explore the factors that influence the extent of the depletion. We have investigated the vertical oxygen budget in the central basin, which is influenced by the following factors: 1) vertical mixing; 2) exchange across the air-water interface; 3) photosynthesis; 4) respiration of plankton; and 5) sediment oxygen demand. We tested the importance of these factors using a 1-D vertical oxygen budget and transport simulations through sensitivity analysis and by estimating vertical mixing parameters using a temperature gradient microprofiler. Epilimnetic factors were found to be robust and the present monitoring efforts are sufficient; while epilimnetic production is ultimately the source of the hypolimnetic oxygen depletion, epilimnetic factors do not directly influence on hypolimnetic oxygen depletion. However, hypolimnetic depletion was sensitive to sediment oxygen demand and hypolimnion respiration, which are the results of primary production in the epilimnion, and hypolimnetic mixing, which is not related to eutrophication. These parameters, especially the physical mixing measurements, and their links with eutrophication and primary production require greater monitoring and analysis because of their influence on the expansion of oxygen depletion in the central basin of Lake Erie.

Prevention of the Spread of Zebra Mussels during Fish Hatchery and Aquaculture Activities

Abstract Fish hatchery and aquaculture activities present numerous opportunities for accelerating the spread of zebra mussels Dreissena polymorpha to new locations. We tested aquaculture chemicals including NaCl, KCl, CaCl2, formalin, and the lampricide TFM (3-trifluoromethyl-4-nitrophenol) for efficacy against zebra mussel veligers and tolerance by several species of juvenile fish. Our short-term exposures simulated the high-stress environment of fish transport. High concentration of NaCl was the most effective treatment against veligers but also caused unacceptably high mortality in most fish species. A dilute solution of formalin (25 mg/L) in conjunction with KCl was effective against veligers and was safe for all fish taxa tested. However, concurrent exposures to 5,000 mg NaCl/L, which is commonly used to counteract fish shock during transport, decreased the toxicity of the formalin to veligers.

Field Testing of Protocols to Prevent the Spread of Zebra Mussels Dreissena polymorpha during Fish Hatchery and Aquaculture Activities

Abstract The zebra mussel Dreissena polymorpha, an introduced, aquatic bivalve species, causes changes in the ecology and negatively impacts the economic value of infected water bodies. Unfortunately, the spread of the zebra mussel can be facilitated by aquaculture activities. We field-tested the effectiveness of chemical treatment protocols established to prevent the spread of microscopic zebra mussel larvae (veligers) during fish stocking. We achieved more realistic exposure conditions by performing fish and veliger toxicity tests in aerated 750-L tanks on fish hatchery tanker trucks using the chemical treatments established in earlier laboratory work. Posttreatment survival was measured for veligers in the 750-L tanks, followed by an assessment of the survival of walleye Stizostedion vitreum and saugeye (sauger S. canadense × walleye) 5 d after stocking into 4,000-m2 fish ponds. A 1-h pretreatment of 750 mg/L KCl followed by a 2-hr treatment of 25 mg/L formalin was effective in killing veligers without...

Abiotic and biotic controls of phytoplankton biomass dynamics in a freshwater tributary, estuary, and large lake ecosystem: Sandusky Bay (Lake Erie) chemostat

Abstract The classic view of tributary–lake interactions solely considers tributaries as sources of nutrients for phytoplankton growing offshore. We tested how river input of phytoplankton affected dynamics in a lacustrine estuary and how these phytoplankters could, upon discharge, affect offshore phytoplankton dynamics. In addition, we explored whether phytoplankton biomass was controlled by bottom-up abiotic factors (stream flow, temperature, light attenuation, and nutrient concentrations) or top-down biotic factors (zooplankton herbivory) and the role the lacustrine estuary played in modifying the quality of the water discharged into the lake. We tested these possibilities using field sampling over 2 years in the Sandusky River, Sandusky Bay, and the Sandusky subbasin of Lake Erie in conjunction with river flow and nutrient monitoring and multivariate statistical analysis of the relationships among the various datasets. Nutrient loading to the estuary corresponded with watershed input; nutrients were used within the estuary, decreasing nutrient loads to the lake. Phytoplankton biomass, however, was extremely high in the estuary and was much higher at the bay–subbasin interface than offshore, indicating that the estuary likely discharges phytoplankton to the nearshore area, not offshore. An information-theoretic analysis of 15 candidate models showed that phytoplankton biomass was best predicted by the ratio of total inorganic nitrogen to total phosphorus, indicating bottom-up control. The lacustrine estuary therefore functions as a chemostat, using incoming nutrients to grow large amounts of phytoplankton. Our results indicate that the classic model of tributary–bay interactions needs revision to include phytoplankton load and its subsequent effects within the lake.