Paul Bertram

Phytoplankton Composition and Biomass in the Offshore Waters of Lake Erie: Pre- and Post-Dreissena Introduction (1983–1993)

Phytoplankton was collected in all basins of Lake Erie during 42 cruises during the spring and summer from 1983 to 1993—a period that spans the Dreissena mussel invasion. Two potential impacts of Dreissena on the phytoplankton community of the western, central, and eastern basins of Lake Erie were evaluated: Was selective feeding occurring as observed in Saginaw Bay and were reductions in biomass evident in the offshore regions of the three basins of Lake Erie? In the western basin, significant summer decreases in Chlorophyta, Bacillariophyta, Cyanobacteria, and total phytoplankton biomass were observed after Dreissena introduction. Similarly in the spring, Bacillariophyta and total phytoplankton biomass and chlorophyll a concentrations decreased significantly. Since several divisions of phytoplankton did not decrease in phytoplankton biomass in the western basin, and spring Cyanobacteria biomass increased significantly while other divisions decreased in biomass, selective feeding on the phytoplankton community was suggested. Where significant reductions in biomass were observed in the offshore waters of the western basin, they were approximately 50% of the reduction observed at the nearshore sites in Lake Erie by other workers. Dreissena impact on the phytoplankton community of the pelagic waters of the central and eastern basin appeared to be minimal. Pre- and post-Dreissena total phytoplankton biomass and chlorophyll a concentrations were not significantly different or increased significantly after the Dreissena invasion. Biomass of several divisions of phytoplankton significantly increased after Dreissena introduction in the central and eastern basins. These included Bacillariophyta (central basin), Cyanobacteria (central and eastern basin), Chrysophyta (eastern basin), Chlorophyta biomass (eastern basin) and phytoplankton biomass (central basin) and chlorophyll a (central basin) in the spring, and Chrysophyta (eastern basin) and Cryptophyta biomass (central basin) in the summer. Generally, a reduction in phytoplankton biomass would be expected as a result of Dreissena grazing, not an increase in biomass. Dreissena-mediated changes in phytoplankton have generally occurred in shallow, well-mixed lakes, ponds, and embayments, not in deeper waters such as the central and eastern basins of 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.

Chemistry of the offshore surface waters of Lake Erie: pre- and post-Dreissena introduction (1983-1993).

Major changes in ambient surface nutrient chemistry were observed after the introduction of Dreissena to Lake Erie. For example, statistically significant increases in spring soluble reactive phosphorus (SRP) (180%, 1.0 to 2.8 μg P/L), nitrate+nitrite (40%, 0.57 to 0.80 mg N/L), ammonia (131%, 15.1 to 34.9 μg N/L), silica (75%, 0.8 to 1.4 mg/L), N:P ratio and turbidity and a significant decrease in total Kjeldahl nitrogen (TKN) (25%, 0.24 to 0.18 μg N/L) were observed in the western basin from the 1983 to 1987 pre-Dreissena baseline period to the 1989 to 1993 Post-Dreissena period. In the summer, total phosphorus (TP) (13%, 20.1 to 17.5 μg P/L) and TKN (27%, 0.30 to 0.22 μg N/L) decreased, while nitrate+nitrite (122%, 0.18 to 0.40 mg N/L) and the N:P ratio increased significantly. Fewer chemical parameters changed significantly in the central and eastern basins, but major changes were observed. For example, spring SRP concentrations in the central and eastern basins increased 250% (0.8 to 2.8 μg P/L) and 92% (2.4 to 4.6 μg P/L), respectively. Silica in these basins increased 300% (0.1 to 0.4 mg/L) and 250% (0.2 to 0.7 mg/L), respectively. TKN decreased in all basins in both the spring and summer (range = 22 to 27%), while TP decreased in all basins in the summer (range = 13 to 24%) but not in the spring. Spatially, spring Post-Dreissena (1989 to 1993) ammonia, TP, and nitrate+nitrite concentrations were high in the western basin and decreased easterly, while chloride concentrations were variable with no downward or upward trend. In the central basin and eastward through the eastern basin, concentrations of ammonia, chloride, nitrate+nitrite, and total phosphorus were remarkably consistent during and between the pre- and Post-Dreissena periods. After the Dreissena invasion, a different spatial pattern of SRP, silica and phytoplankton biomass was observed. SRP and silica concentrations were high in the western basin and decreased into the central basin as in the pre-Dreissena period. Similarly, Post-Dreissena SRP and silica concentrations were low in the western portion of the central basin but then unexpectedly increased easterly by > 250% and > 1,000%, respectively, over the pre-Dreissena period. Phytoplankton biomass increased from within the west end of the western basin to a peak about halfway into the central basin, after which biomass decreased into the eastern basin. The increase in the dissolved fraction of nutrients in the western basin can be attributed to the excretion of dissolved fractions by Dreissena spp. after digestion of particulate matter, the remineralization of surficial organic sediments containing nitrogen and phosphorus-rich feces and pseudofeces and to a decrease in uptake of SRP by less abundant populations of phytoplankton in the western basin. In the western portion of the central basin, it is possible that SRP is being carried by the prevailing westerly current into the central basin stimulating phytoplankton population growth combined with minimal Dreissena grazing causing a peak in phytoplankton abundance. There does not appear to be a satisfactory explanation for the simultaneous increase in SRP and the lack of any change in phytoplankton pre- and Post-Dreissena in the eastern portion of Lake Erie.

Total Phosphorus and Dissolved Oxygen Trends in the Central Basin of Lake Erie, 1970–1991

Five yearly estimators of total phosphorus (TP) concentrations in the central basin of Lake Erie from 1970 to 1986 were calculated and compared to evaluate their utility in the assessment of long term trends. Data were selected from stations within a defined polygon for those years in which at least four surveys were conducted, including spring and fall isothermal conditions and at least two surveys during stratification. Included were an arithmetic average of all survey means during each year, a time-weighted average of all survey means, a spring (isothermal) average, a summer (stratified) average, and an autumn (isothermal) average. The rate of decline in TP concentrations from the spring-only estimator (−0.276 μg TP L-1 yr-1) was similar to that of the time-weighted multiple survey estimator (−0.214 μg TP L-1 yr-1). Additional spring survey data from 1987 through 1991 confirmed that the negative trend continued. The stratified season and the autumn-only estimators exhibited no significant trends, but were biased low and high, respectively, relative to the spring and multiple survey estimators. Despite the reductions in TP concentrations since 1970, the annual normalized rate of oxygen depletion in the hypolimnion of the central basin showed only a slight downward trend (-0.03 mg L-1 mo-1). In 1988 and 1989, however, the depletion rates were lower than any reported during the previous 20 years.

A Decade of Predatory Control of Zooplankton Species Composition of Lake Michigan

Abstract From 1983 to 1992, 71 species representing 38 genera from the Calanoida, Cladocera, Cyclopoida, Mysidacea, Rotifera, Mollusca and Harpacticoida comprised the offshore zooplankton community of Lake Michigan. Our data demonstrate that the composition and abundance of the calanoid community after 1983 is not unlike that of 1960s and that species diversity of the calanoid community is more diverse than the cladoceran community in the 1990s as compared to the early 1980s. Even though the relative biomass of the cladocerans has remained similar over the 1983–1993 period, the species diversity and evenness of the Cladocera community in the early 1990s is unlike anything that has been previously reported for Lake Michigan. Cladocera dominance is centered in one species, Daphnia galeata mendotae, and only three species of Cladocera were observed in the pelagic region of the lake in 1991 and 1992. Nutrient levels, phytoplankton biomass, and the abundance of planktivorous alewife and bloater chub and Bythotrephes are examined as possible causes of these changes in zooplankton species composition. The increase in Rotifera biomass, but not Crustacea, was correlated with an increase in relative biomass of unicelluar algae. Food web models suggest Bythotrephes will cause Lake Michigans plankton to return to a community similar to that of the 1970s; that is Diaptomus dominated. Such a change has occurred. However, correlational analysis suggest that alewife and bloater chubs (especially juveniles) are affecting size and biomass of larger species of zooplankton as well as Bythotrephes .

The U.S. EPA Lake Erie Indicators Monitoring Program 1983–2002: Trends in Phosphorus, Silica, and Chlorophyll a in the Central Basin

During the past 20 years, Lake Erie has exhibited a series of complex chemical changes resulting from changing anthropogenic influences and introductions of exotic species. Since 1990, some apparent trends in nutrient concentrations have been inconsistent with the predictions of models originally used to guide Lake Erie water quality management. We performed time trend analysis on total phosphorus (TP), chlorophyll a (Chl a), and dissolved reactive silica (DRS) measurements collected during spring and summer in the central basin of Lake Erie between 1983 and 2002. Three distinct time trends in basin-specific, station-averaged open-water TP concentrations were observed over the 20-year period: 1983–1989 decreases, 1990–1997 increases, and 1997–2001 decreases. Exceptionally high levels of turbidity and TP were observed in spring 2002, possibly reflecting increasing frequency of winter storm events. Open-water concentrations of TP declined during the 1980s as annual TP loadings to Lake Erie declined below the 11,000 metric ton target level that had been expected to reduce central basin eutrophication. This was accompanied by a significant increase in available silica and in Chl a concentrations. During the period 1990–2002, when dreissenid mussels were abundant, spring TP concentrations increased but summer TP concentrations declined. Unexpected increases in spring central basin concentrations of DRS (exceeding 1 mg/L) were observed. During the same period, Chl a concentrations declined in the spring and rose slightly during summer.

The development and implementation of indicators of ecosystem health in the great lakes basin

This paper describes a process for the selection of a suite of ecosystem health indicators for the Great Lakes, as called for in the Great Lakes Water Quality Agreement. The paper also presents some preliminary data on status and trends in ecosystem components based on those indicators. The indicator selection process was carried out by over 150 scientists and managers from both Canada and the USA, and involved the presentation of the proposed indicators at the State of the Lakes Ecosystem Conferences (SOLECs). An open period for comment followed the conferences where input from scientists involved in Great Lakes programs was received. The suite of indicators will, over time, present information in an understandable format that will allow for more informed management decisions.

Changes in Phytoplankton Size-class Abundance and Species Composition Coinciding with Changes in Water Chemistry and Zooplankton Community Structure of Lake Michigan, 1983 to 1992

Phytoplankton and zooplankton were collected at offshore sites of Lake Michigan during 37 cruises in the spring and summer from 1983 to 1992. For the period, 39 common phytoplankton species accounted for 96.0% of the total abundance and 85.6% of the biomass. Over the 10-year study, the lake-wide average spring and summer phytoplankton biomass in the pelagic waters ranged from 0.27 to 1.2 g/m3 (mean ± S.E. = 0.54 ± 0.03 g/m3) and phytoplankton abundance ranged from 5,132 to 39,780 cells/mL (mean ± S.E. = 18,291 ± 822 cells/mL). Mesotrophic diatoms accounted for 47.2% of the total phytoplankton biomass. The lack of a trend in the ratio of mesotrophic to eutrophic diatom indicator species suggested that no change in trophic status of the pelagic region occurred during the 1983 to 92 period; that is, the water quality of the offshore of Lake Michigan did not change. A year-to-year shift in dominance from one mesotrophic diatom species to another was evident. Over the 10-year period, flagellates accounted for 52.2% of the summer phytoplankton biomass. The data support the concept of a shift in summer species composition away from blue-green algae dominance to flagellates prior to 1982. However after 1987, the relative importance of the > 70 μm size class increased to over 21% of the phytoplankton abundance in the spring and over 55% in the summer. In particular, the colonial blue-greens, Anacystis and Aphanothece, became substantially more prevalent during the summer, while the flagellates, Chromulina and Ochromonas, decreased in abundance. The changes in the relative abundance of phytoplankton size classes and species composition were examined to determine if they correlated with either the top-down mediated changes in the zooplankton community during the 1980s and 1990s or with any bottom-up variability in nutrient chemistry. Canonical correspondence analysis (CCA) suggested a significant portion of the variability of the spring and summer phytoplankton size classes from 1983–1992 (88.0% and 99.2%, respectively) was explained by environmental axis 1. Primary determinants of relative abundance of phytoplankton size class included nutrients (silica, total phosphorus, and N:P ratio) and abundance of some species of zooplankton. Specifically, Diaptomus minutus, D. ashlandi, D. sicilis, and Daphnia galeata mendotae were inversely correlated with the 0 to 10 μm size class, which we interpret as a grazing effect on phytoplankton size structure, and positively correlated with an increase in abundance of the > 70 μm size class of phytoplankton. CCA suggested that top-down and bottom-up effects were affecting phytoplankton size composition and abundance simultaneously.

Developing ecosystem objectives for the Great Lakes: policy, progress and public participation

Under the Great Lakes Water Quality Agreement of 1978 between the United States and Canada, as amended in 1987, an ecosystem objective with associated indicators for Lake Superior was adopted, and a commitment was made to develop ecosystem objectives and indicators for each of the other Great Lakes. Building upon a history of activities within the International Joint Commission related to the development of ecosystem indicators for Lake Superior and for mesotrophic waters, a binational Ecosystem Objectives Work Group (EOWG) has been established by the U.S. and Canada and charged with developing ecosystem objectives for the Great Lakes, beginning with Lake Ontario. These objectives are primarily biological in nature, in contrast to chemical objectives. The approach of the EOWG is to identify in sequence: (1) broad ecosystem goals, (2) a suite of objectives whose attainment would ensure achievement of the goals, and (3) one or more measurable indicators of progress toward meeting each objective. Societal values are reflected in the goals and objectives following consultation with competing users of ecosystem resources. Identification of appropriate indicators requires the assistance of technical experts. The experience of the EOWG in developing ecosystem objectives for Lake Ontario illustrates the application of this process.

Indicators for Human and Ecological Risk Assessment: A U.S. Environmental Protection Agency Perspective

Assessment of risk to public health or environmental resources requires competent characterization of stressors and corresponding effects. Because of the complexity of most stressor-response relationships, it is impossible to completely characterize all the variables, so a select set of measurements is made to reflect the most critical components. Such measurements, or indicators, are included in monitoring programs to estimate trend, stressor source, or magnitude of effects and lead to thresholds for management action or restoration. Although a wide variety of programs and program objectives exists, there are some common challenges for indicator development, including a strong link to management actions. Indicator measurements used in U.S. Environmental Protection Agency (USEPA) risk assessment activities must stem from collaboration among managers, risk assessors, scientists and stakeholders. The primary objective of the USEPAs Fifth Symposium of the National Health and Ecological Effects Research Laboratory was to improve health and ecological risk assessment through dedicated sessions that maximized interaction and discussion among these groups. Existing measurements were challenged for appropriateness, efficiency and scientific validity. Emerging science was explored for greater understanding, better interpretation, and improved methodology. A secondary objective was to uncover and exploit common indicators and supporting data for human health and ecological models.