David M. Dolan

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.

Trends in Temperature, Secchi Depth, and Dissolved Oxygen Depletion Rates in the Central Basin of Lake Erie, 1983–2002

We examined temperature trends in a 20-year set of monitoring records collected at multiple deep-water stations in the central basin of Lake Erie. Data collected were statistically corrected (“deseasonalized”) to remove biases resulting from irregular sampling intervals within years. Depthintegrated summer temperature has increased by an average (±SE) of 0.037 ± 0.01°C per year. An observed reduction of Secchi depth (SD) by 7 ± 3 cm/y seems to be unrelated to variation in either total phosphorus (TP) or chlorophyll a concentrations. Midsummer midbasin SD values varied widely between 4 and 10 m, possibly depending on whether phytoplankton were concentrated in the epilimnion (giving shallow SD), or whether phytoplankton had settled out of the epilimnion into the lower layers, giving deeper SD values. Hypolimnetic volume-corrected oxygen depletion (HVOD) rates have also been highly variable, ranging from 2.68 to 4.72 mg/L/mo. These rates are sensitive to production of oxygen in the thermocline and hypolimnion by photosynthetically active phytoplankton that have settled from the epilimnion. The HVOD rate in any year was correlated with the previous years TP loading into Lake Erie. Since TP loading trends largely reflect the consequences of improving water treatment through the 1980s and increasing contributions from tributary run-off sediments through the 1990s, there is little direct evidence to suggest that the appearance of dreissenids has directly influenced hypolimnial oxygen depletion rates in the central basin. The observation that central-basin HVOD tracked the reductions in TP loadings through the 1980s may be the first affirmation that central basin hypolimnetic oxygen dynamics can be regulated by phosphorus inputs. This implies that TP loads must continue to be regulated if we wish to minimize oxygen depletion rates as a strategy to reduce the frequency of episodic central basin anoxia.

Lake Erie Total Phosphorus Loading Analysis and Update: 1996–2002

Abstract The Lake Erie basin remains one of the most intensely monitored areas in the Great Lakes, largely because of continued interest by government agencies and the public in its trophic status. Total lake phosphorus loading estimates require data from three essential pathways: tributaries, point sources, and the atmosphere. Point source and atmospheric deposition monitoring results are available to allow continued estimation of these components. Several key watersheds are still being monitored, making some tributary load estimation possible. The problem is to make estimates for unmonitored areas, which are now substantially greater than encountered previously. Except for 2 years, the total annual load estimates for 1996–2002 (11,584, 16,853, 12,710, 6,608, 8,456, 7,333, and 9,733 metric tonnes per year, respectively) were near or substantially below the target load set by the Great Lakes Water Quality Agreement of 11,000 metric tonnes per year. The estimates for 1997 and 1998 markedly exceeded the target load due mainly to elevated tributary loads because of heavy precipitation. The margin of error or half-width of approximate 95% confidence intervals varied from 4% to 11% of the total estimated load depending on year. Detailed tables of the yearly (1996–2002) estimates are provided, as well as summaries by Lake Erie sub-basin for 1981–2001.

Long-Term and Recent Changes in Southern Lake Michigan Water Quality with Implications for Present Trophic Status

ABSTRACT Southern Lake Michigan has changed in response to alterations in nutrients and invasive species. NOAA and EPA monitoring results are used to examine those changes. NOAA provides detailed seasonal resolution, but limited spatial coverage, whereas the EPA provides more spatial coverage, but limited seasonal resolution. We compare changes in total phosphorus (TP), silica, nitrate plus nitrite, and chlorophyll concentrations from before and after the invasion by the quagga mussel (Dreissena rostriformis bugensis). Although TP at NOAA stations was consistently higher than at EPA stations, both confirm declines in spring and summer surface mixed layer (SML) conditions. Chlorophyll differed at EPA and NOAA stations before quagga mussel invasion, but not after the invasion. Spring chlorophyll decreased at NOAA stations after the invasion, but summer conditions did not change at either set of stations. Pre-invasion silica at NOAA stations was slightly higher than at EPA stations, and the lakes Si reservoir increased over the study period. Basin-scale spring Si increased gradually, whereas summer SML Si increased dramatically after 2003, likely reflecting reduced diatom production. Basin-scale nitrate increased significantly from pre- to post-invasion in both spring and summer. Summer nitrate utilization declined drastically in recent years, likely reflecting reduced phytoplankton production. TP loads decreased; however, the timing of changes in chlorophyll and Si and nitrate utilization suggest the recent increase in dreissenid filtering dramatically reduced spring phytoplankton abundance and production across the entire southern basin. The offshore pelagic zone of the historically mesotrophic southern Lake Michigan is now similar to oligotrophic Lake Superior.

A hydrodynamic approach to modeling phosphorus distribution in Lake Erie.

ABSTRACT The purpose of this paper is to show how a high-resolution numerical circulation model of Lake Erie can be used to gain insight into the spatial and temporal variability of phosphorus (and by inference, other components of the lower food web) in the lake. The computer model simulates the detailed spatial and temporal distribution of total phosphorus in Lake Erie during 1994 based on tributary and atmospheric loading, hydrodynamic transport, and basin-dependent net apparent settling. Phosphorus loads to the lake in 1994 were relatively low, about 30% lower than the average loads for the past 30 years. Results of the model simulations are presented in terms of maps of 1) annually averaged phosphorus concentration, 2) temporal variability of phosphorus concentration, and 3) relative contribution of annual phosphorus load from specific tributaries. Model results illustrate that significant nearshore to offshore gradients occur in the vicinity of tributary mouths and their along-shore plumes. For instance, the annually averaged phosphorus concentration can vary by a factor of 10 from one end of the lake to the other. Phosphorus levels at some points in the lake can change by a factor of 10 in a matter of hours. Variance in phosphorus levels is up to 100 times higher near major tributary mouths than it is in offshore waters. The model is also used to estimate the spatial distribution of phosphorus variability and to produce maps of the relative contribution of individual tributaries to the annual average concentration at each point in the lake.

Analysis of the Impacts of the Zebra Mussel, Dreissena polymorpha, on Nutrients, Water Clarity, and the Chlorophyll-Phosphorus Relationship in Lower Green Bay

ABSTRACT Zebra mussels, Dreissena polymorpha, invaded Green Bay, Lake Michigan in the early 1990s. In 1986, prior to zebra mussel invasion, the Green Bay Metropolitan Sewerage District initiated a long-term water quality monitoring program involving 12 stations in three distinct zones along a trophic gradient in lower Green Bay. We analyzed this data set pre and post invasion using various regression models to determine the impacts of the zebra mussel on water clarity, nutrient concentrations, and the relationship between chlorophyll and phosphorus in this system. Following zebra mussel invasion, Secchi depths did not change in all three zones. Chlorophyll a concentrations decreased post zebra mussels in all zones. These differences were attributed to the filter feeding abilities of zebra mussels. Lower Green Bay exhibits a strong trophic gradient and zebra mussel impacts on the chlorophyll-phosphorus relationship differed between the three zones. We saw no changes in the chlorophyll-phosphorus relationship in zone 1, zone 2 appeared to be a transition zone with slight changes in the chlorophyll-phosphorus relationship, and in zone 3 there was evidence of an altered chlorophyll-phosphorus relationship post zebra mussels. These results indicate that the impact of zebra mussels on water quality parameters and on chlorophyll-phosphorus dynamics may differ depending on initial trophic status and on zebra mussel densities.

Predicting benthic counts in Lake Huron using spatial statistics and quasi-likelihood

Spatial statistics have been applied to many types of problems in the environmental sciences, mostly dealing with continuously distributed data from Gaussian or near-Gaussian processes. There is a need for methods capable of handling discrete, non-Gaussian data, such as species counts from biological processes. This paper applies the method of quasi-likelihood from general linear models to the problem of spatial prediction of benthic invertebrate counts. Variogram models are fitted to quasi-likelihood residuals with two alternative distance metrics. The models are compared using cross-validation and predictions are made using the classical estimator of the variogram and distance determined from a Geographic Information System (GIS). A brief simulation study is included that verifies the applicability of asymptotic results to the estimation of model parameters. Copyright

Factors Affecting Recruitment of Age-0 Muskellunge in Escanaba Lake, Wisconsin, 1987–2006

Abstract We modeled variation in recruitment (R) of age-0 muskellunge Esox masquinongy to identify factors influencing their abundance in Escanaba Lake, Wisconsin. Muskellunge R declined over the study period and ranged from 0.00 to 1.85 age-0 fish/km of shoreline (mean = 0.42 age-0 fish/km of shoreline). A Ricker stock–recruitment model determined that the following factors explained 88% of the variation in annual R of age-0 muskellunge between 1987 and 2006: abundance and age structure of the adult muskellunge population, abundance of bluntnose minnow Pimephales notatus, abundance of age-3 and older (age-3+) walleyes Sander vitreus, abundance of age-0 white suckers Catostomus commersonii, and coefficient of variation (CV) of May water temperatures. Abundance of adult muskellunge (≥76.2 cm total length) accounted for only 1% of the variation in R and showed no significant relationship with R. Abundance of bluntnose minnow improved the model fit to 40% of the variation in R and indicated that higher R was...

Effect of Temperature, Zinc, and Cadmium Ions on the Removal of Cr(VI) from Aqueous Solution via Ion Exchange with Hydrotalcite

Experiments were conducted to determine and model the effect of cadmium and zinc ions in solution on the removal of Cr(VI) via ion exchange with hydrotalcite, a clay mineral media. Because many locations that have contaminated groundwater are located in colder climates, the ion exchange process was first characterized as a function of temperature. Binary solutions of Cr(VI) with either cadmium or zinc were investigated followed by a factorial design of ternary systems. Isotherms were modeled and used to compare removal of Cr(VI) in a single ion solution to multi-ion systems. Fixed-effect analysis of variance, PROC GLM (SAS Version 9), was used to analyze main and interactive effects. Results indicated that, while temperature did not significantly affect removal of Cr(VI), both Zn and Cd did reduce Cr(VI) ion exchange, most likely due to the formation of ionic complexes. Also, interactions between all three metal species ion the ternary system negatively affected removal of Cr.