David A. Culver

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.

Isolation and Characterization of Microcystins, Cyclic Heptapeptide Hepatotoxins from a Lake Erie Strain of Microcystis aeruginosa

Cyanobacteria (blue-green algae) dominated by Microcystis aeruginosa were detected in phytoplankton samples collected from several sites in the western basin of Lake Erie during the summer of 1996 and one site in 1995. Toxin content of the samples was measured using ELISA and found to be equivalent to as much as 1 μg microcystin/L. Microcystins are potent liver toxins and tumor promoters produced by several genera of cyanobacteria. The Lake Erie M. aeruginosa colony strain (LE-3) was isolated and cultured to characterize chemically the specific microcystins (MCYSTs) present. Structures were assigned based on analytical high performance liquid chromatography (HPLC) retention times, amino acid analyses, and mass spectrometric data, revealing the leucine-arginine analog, microcystin-LR as the predominant product.

Bioenergetics Model for Larval and Juvenile Walleyes: An in Situ Approach with Experimental Ponds

Abstract We used an in situ approach to redefine allometric parameters of consumption and metabolism in a percid bioenergetics model for young-of-year (age-0) walleyes Stizostedion vitreum (0.0021–0.8237 g wet weight; 8–49 mm total length) in ponds across a range of densities (10–50 fish·m–3). Of three methods used to estimate evacuation rates, only the one based on whole-gut fullness of larvae and stomach fullness of juveniles in food-free enclosures proved reliable, The intercepts for allometry of maximum consumption and resting metabolism for age-0 walleyes were higher than the intercepts for adults; however, slopes of age-0 and adult allometry were similar. Activity costs (ACT = ratio of active metabolism to resting metabolism) for age-0 walleyes varied across body weights and with fish density; patterns of activity costs at different fish densities reflected patterns of food availability. Activity and p-values (proportion of maximum consumption realized) were correlated with powers of zooplankton bio...

Effects of hormonal treatment on induced spermiation and ovulation in the yellow perch (Perca flavescens)

Abstract Yellow perch cultured in the laboratory were used to examine gonad maturation and the effectiveness of hormonal treatments on sperm production and synchronization of ovulation. Females (54.4±8.3 g (mean±s.d.) average weight) and males (26.6±6.6 g) were individually tagged in December and fed pelleted commercial diet from December through to May. Body weight increased in females but decreased in males until the end of the experiment in May when natural spawning occurred. The gonadosomatic index increased in females from 7.5±0.7 in December to 30.9±3.1% in May, whereas it remained constant in males (5.6±0.6 to 5.1±0.7%) during the same period. Separate groups of males and females were injected at monthly intervals with a luteinizing hormone-releasing hormone analog (LHRHa) starting in January (males) or February (females). Hormone-injected males produced significantly ( P

Zooplankton Grazing and Phytoplankton Abundance: an Assessment Before and After Invasion of Dreissena Polymorpha

The introduction of the zebra mussel Dreissena polymorpha into Lake Erie raises the question of whether it competes with the existing zooplankton community for phytoplankton. In this study, we approached this question by examining the role of zooplankton grazing in influencing Lake Erie abiotic conditions and biotic interactions during 2 years of varying zebra mussel abundances. In situ grazing experiments (32P-labeled yeast cells) revealed that cladoceran species contributed >85% of zooplankton community grazing rate, whereas copepods accounted for 200 mL L−1 d−1) calculated from the models between June and early July coincided with the high Secchi transparencies found during these periods (clear-water phase) in both the year with few zebra mussels (1988) and the year with abundant mussels (1989). The impact of Daphnia grazing on the phytoplankton community is negative, directly reducing edible algal volume to <5 mL m−3. The release of Daphnia grazing pressure in August was followed by an increase in edible phytoplankton in both nearshore and offshore areas of both years. High edible algal volumes and low Secchi transparencies in August were in contrast to what we would expect if adult zebra mussels that were present throughout 1989 or veligers that were abundant in late July and August were able to overgraze phytoplankton. We concluded that the pulsed Daphnia grazing still controlled edible algal density and water transparency in 1989, even though Dreissena polymorpha was abundant in the lake in 1989.

Growth, survival, and quality of juvenile walleye Stizostedion vitreum as influenced by n−3 HUFA enriched Artemia nauplii

Abstract We quantified how addition of n −3 highly unsaturated fatty acids (HUFA) to brine shrimp ( Artemia sp.) nauplii influenced growth, survival, osmotic challenge and fatty acid composition of juvenile walleye. Four groups of 6-day-old walleye were fed enriched Artemia . Lipids were altered to provide four treatments: (1) 100% cod liver oil (CLO), (2) 60% CLO/40% n −3 HUFA concentrate, (3) 40% CLO/60% n −3 HUFA concentrate, and (4) 100% n −3 HUFA concentrate. Different enrichments influenced neutral lipids of Artemia , but not their phospholipids. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) concentrations in Artemia neutral lipids in treatment 4 were doubled compared to treatment 1. Fatty acid composition of walleye body lipids, in both neutral and phospholipid fractions, reflected the changes in EPA and DHA concentrations found in enriched Artemia . After 3 weeks, walleye fed Artemia enriched with a combination of CLO and n −3 HUFA concentrate grew faster than those fed 100% n −3 HUFA enriched Artemia . Walleye fed Artemia nauplii enriched with 100% CLO survived better than all other treatments (25.0±4.5 vs. 11.4–13.1%). Walleye recovery after osmotic challenge was ordered treatment 1 through treatment 4; as n −3 HUFA in Artemia increased, so did susceptibility to this challenge. Furthermore, both survival and vitality of walleye were negatively correlated with the EPA/(arachidonic acid) AA ratio in their whole body phospholipids. The present work extends our knowledge on the effect on n −3 HUFA at higher levels in walleye juveniles diets (12.5% EPA, 5% DHA), and alterations in whole body fatty acids balance. We conclude that high EPA/DHA diet can be responsible for a decreased survival and higher susceptibility to stress in freshwater fish.

Daphnia Population Dynamics in Western Lake Erie: Regulation by Food Limitation and Yellow Perch Predation

Two Daphnia species exhibited similar dynamic patterns in western Lake Erie. Populations peaked in early summer, declined to minimum abundance in mid-July, and then disappeared after August. To determine relative importance of food limitation and young-of-year (YOY) fish predation in regulating Daphnia dynamics, we examined relationships (1) between edible phytoplankton abundances and Daphnia fecundity and birth rates, and (2) between Daphnia biomass consumed by YOY yellow perch (Perca flavescens) and Daphnia death rates. The population peak was created by a burst of parthenogenetic reproduction. Suppression of birth rates (<1 individual d−1) by low edible phytoplankton resources (<4 g•m−3 wet wt) and increased consumption by YOY yellow perch caused a midsummer decline of Daphnia populations. Once the Daphnia populations were reduced, the predation from age-1 and older planktivorous fish in western Lake Erie was likely to keep the populations at low densities in spite of increased food resources in late summer. An increased white perch (Morone americana) population and the invasion of zebra mussels (Dreissena polymorpha) in western Lake Erie may alter the cycle of Daphnia dynamics, hence influencing yellow perch population in the lake.

Do Dreissenid Mussels Affect Lake Erie Ecosystem Stability Processes

Abstract Ecosystem stability processes such as constancy, resilience and persistence are important, but often neglected, topics of invasive species research. Here we consider how invasive dreissenid mussels affect ecosystem stability processes in Lake Erie through both consumptive and excretory processes using the stability landscape heuristic (Gunderson, 2000). Consumption of phytoplankton by dreissenid mussels adds complexity to the system and potentially slows energy transfer from lower to higher trophic levels decreasing system constancy and lowering system resiliency. Excreting soluble waste products at low nitrogen to phosphorus ratios exacerbates these impacts on stability processes because low nutrient ratios favor growth of cyanobacterial blooms, less preferred food of zooplankton, further decreasing the transfer of energy from lower to higher trophic levels. We also provide evidence for recent changes in Lake Eries stability landscape including a return towards eutrophy.

Cylindrospermopsis in Lake Erie: Testing its Association with Other Cyanobacterial Genera and Major Limnological Parameters

ABSTRACT We report the first documented observation of the potentially toxic cyanobacterium Cylindrospermopsis in Lake Erie and Sandusky Bay in 2005 (0.043–1.326 mg L−1 wet weight, 16–1,942 trichomes mL−1) and quantify the physical and chemical parameters and the cyanobacterial community composition contemporaneous to its occurrence. We hypothesize that the high temperature, low light intensity, and high nutrient content of Sandusky Bay, a shallow, drowned river mouth along the southwestern shore of Lake Erie, provides an ideal habitat for Cylindrospermopsis. This is consistent with published laboratory and field studies that show these physical and chemical conditions facilitate Cylindrospermopsis growth. Using multivariate statistics, we found that Cylindrospermopsis biomass correlated with high temperatures and shallow depths, conditions often found in Sandusky Bay. Light climate and nutrient concentrations were not associated with Cylindrospermopsis biomass, most likely because the light climate did not systematically change during the season and because nutrients exceeded demand. We propose that Cylindrospermopsis will increase in importance in Lake Erie, as previous research on climate change in the Great Lakes region predicts future higher water temperatures and lower water levels.

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.