Great Lakes Environmental Research Laboratory
Nonindigenous Species Research Program 1996/1997 Update

The objective of this program is to expand our knowledge of the biology and ecological effects of nonindigenous species in the Great Lakes. Research involves field investigations on Saginaw Bay, Lake Huron, and other sites to monitor ecosystem changes and community response to invading species, and examines ecology of the organisms themselves. Research also includes laboratory experiments to examine the biology (feeding, development, physiology) and ecological interactions of the invading organisms, including toxicokinetics and bioaccumulation of toxics. The focus of this program has been limited primarily to the zebra mussel by funding priorities. Some of the early work on the ecological implications of the spiny water flea (Bythotrephes cedarstroemi) was conducted under this program prior to the rise of the zebra mussel as a major significant problem for the Great Lakes.

This program incorporates ERL Research Task: GLERL 10 - Nonindigenous Species Research (Task 10 Leader: Henry Vanderploeg, 734-741-2284, Henry.Vanderploeg@.noaa.gov)

A particular emphasis of this program over the last two years has been examining the role of the zebra mussel in promoting nuisance blooms of the potentially toxic blue-green alga Microcystis on Saginaw Bay and the effects of these blooms on the ecosystem and the mussels themselves. Part of this work has examined the roles of the mussels' selective filtering and nutrient excretion in promoting blooms. Preliminary analyses of data have suggested that mussel selective filtration may promote Microcystis dominance, but that a source of P needed to be found to explain the high concentrations that develop. Therefore, emphasis has been placed on examining the sediments as a source of P, particularly the deep sediments, which may have become a site of P regeneration because of (hypothesized) accumulation there of unutilized organic matter promoted directly and indirectly by mussel activities. These deep sites are also being examined for hypoxia or anoxia. Our experience on Saginaw Bay has led to our participation on the "Microcystis team" sponsored by the Lake Erie Protection Fund to explain the causes and consequences of Microcystis blooms on Lake Erie.

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Project Index

GLERL 10 - Nonindigenous Species Research

Examining the impacts of the zebra mussel, Dreissena polymorpha, on the lower food web of Saginaw Bay
Metabolic physiology of the zebra mussel and density changes in native mussels (Unionidae)
Observations on the Trophic Ecology of Dreissena Early Life Stages: The Critical Planktonic Period
Influence of the zebra mussel (Dreissena polymorpha) on the accumulation of organic contaminants in the Saginaw Bay food web.
Long Term Changes in the Resuspendible Sediments of Saginaw Bay
Effects of the Zebra Mussel on Nutrient and Microbial Dynamics in Saginaw Bay, Lake Huron
Selection and Utilization of Algal Resources by Dreissena: Unstable Interactions Between Zebra Mussels and the Algal Community of Saginaw Bay

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ERL Research Task: GLERL 10 - Nonindigenous Species Research

Examining the impacts of the zebra mussel, Dreissena polymorpha, on the lower food web of Saginaw Bay

Principal Investigator: Thomas Nalepa (734-741-2285; Tom.Nalepa@noaa.gov).

Collaborating Scientists: Henry Vanderploeg, Brian Eadie, Patricia Van Hoof, Gary Fahnenstiel, David Fanslow, Joann Cavaletto, James Liebig (GLERL); Thomas Johengen, Gerald Gostenik, Amy Gluck (CILER)

The objectives of this project are (1) to identify and understand changes in the abundance, biomass, and composition of the lower food web of Saginaw Bay that have resulted from the invasion of the zebra mussel (Dreissena polymorpha), (2) to construct a model of carbon flow through the system and determine major changes in pathways, which may have been caused by the zebra mussel disrupting the ecosystem, and (3) to monitor changes in the abundance and distribution of the zebra mussel in the bay. This project was started in 1990 and has accumulated the most detailed and longest multi-year set of data on the changes in an aquatic ecosystem following an infestation by zebra mussels.

FY96 Progress and Accomplishments

Collected samples monthly from May to October at 13 sites within the bay. Variables measured included nutrients, secchi depth, chlorophyll, phytoplankton, zooplankton (4 sites only), and benthos (10 sites). This year we also conducted a survey of zebra mussel populations at 9 sites. Mussels were counted and sized to get estimates of abundance and biomass. This represents the seventh consecutive year of sampling.

Completed identification of all benthos collected between 1987 and 1995, and of zooplankton collected between 1990 and 1995; compiled and formatted all physical and chemical data collected in 1991-93.

To begin characterizing the soft sediments, a continuous-recording oxygen sensor was deployed in the water column just above the sediment-water interface, and P availability and P (and N) flux rates from the sediments were measured.

Completed and published a NOAA Technical Memorandum that provides all physical and chemical data collected in Saginaw Bay in 1991-93. Besides giving all raw data, this publication provides a detailed account of all field collection methods, laboratory analytical methods, and procedures for quality assurance.

Data representing changes in the zooplankton and benthos since the zebra mussel invaded the bay were tabulated and preliminary interpretations were initiated. These preliminary data were presented at two conferences (Zebra Mussel Conference, February, 1996, and Annual Meeting of the North American Benthological Society, June, 1996).

Compiled data and analyzed trends in zebra mussel populations in the inner bay. The 1995 data indicate that abundances and biomass have not changed since 1993. This may indicate that the population has stabilized and assumed an "equilibrium" with the surrounding environment.

FY97 Plans

Complete processing (counting, sorting, species identification) of benthic invertebrates collected in Saginaw Bay in 1996 and initiate analysis of population trends from 1987-1996. This data set will document impacts of the zebra mussel on benthic invertebrates in the bay.
Process all samples taken in 1996 to assess the status of the zebra mussel population in the bay. This involves counting and measuring all mussels in 27 diver-collected samples (9 sites x 3 replicates).
Complete input of zooplankton data into a file and begin analysis of trends in abundances of the major groups. Again, the purpose is to document changes resulting from the invasion of the zebra mussel into the bay.
Organize and check monitoring data collected in Saginaw Bay in 1994, 1995, and 1996. Monitoring data consists of all physical, chemical, and biological data collected monthly from May to October during each year.

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Metabolic physiology of the zebra mussel and density changes in native mussels (Unionidae)

Principal Investigator: Thomas Nalepa (734-741-2285; Tom.Nalepa@noaa.gov).

Although information is available from Europe about the biological and ecological characteristics of the zebra mussel, little is known about its physiological characteristics or variation in its biochemical content. Such data serve as indicators of the relative "health" of the population. Thus, comparisons of such data from different areas and during different times of the year may lead to a better understanding of the ecological requirements of the organism and, over time, may provide an understanding of fluctuations in the populations. This project will to determine the seasonal oxygen consumption and nitrogen (ammonia) excretion, and soft-tissue lipid content and C:N ratios of zebra mussels collected from Lake St. Clair.

FY96 Progress and Accomplishments

Zebra mussels were collected monthly from two sites in Saginaw Bay. The sites represent contrasting conditions - one site was in the eutrophic inner bay and the other site was in the more oligotrophic outer bay. The relative health of the mussels was examined by measuring the following variables: lipid level, length/weight, ETS, and reproductive status. Similar measurements were also obtained on mussels from the outer bay site that were translocated to the inner bay site, and mussels from the inner bay site that were translocated to the outer bay site. These will be compared to measurements made on mussels collected in 1995.

A paper was published that documented changes in the mussel community in Lake St. Clair over the 1986-1994 period. The native mussel population in the lake was extirpated, while the zebra mussel population expanded from the southeast region of the lake to the northwest region. While the lakewide biomass of the zebra mussel population in 1994 was lower than the biomass of the unionid population in 1986, filtering capacity was 12 times greater. This change has likely led to the dramatic changes observed in the lake (increased water clarity, increased plant growth and shifts in fish communities).

Two papers were presented that described differences in the relative condition of mussels from the inner bay site and the outer bay site during 1995. Mussels from the former site has a lower weight per unit length and lower lipid concentrations than mussels from the latter site. Also, mussels from the inner bay were smaller than those from the outer bay. These differences are likely related to the poorer food quality in the inner bay.

FY97 Plans

Complete measurements of growth, length-weight, size-frequency, glycogen, lipids, ETS, and reproductive status in mussels collected monthly from an inner bay site (eutrophic) and an outer bay site (oligotrophic) in Saginaw Bay. These parameters will be used to assess variation in population structure and relative condition of mussels from two different environments.
Conduct a survey to estimate presence/absence of native mussels from select sites in Lake St. Clair using the transect method. While previous studies have shown that densities of native mussels have declined to near zero over the past 8 years, this more intensive survey will establish whether any individuals remain.

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Observations on the Trophic Ecology of Dreissena Early Life Stages: The Critical Planktonic Period

Principal Investigator: Henry Vanderploeg (734-741-2284; Henry.Vanderploeg@noaa.gov).

Collaborating Scientists: James Liebig (GLERL); Amy Gluck (CILER)

The pelagic phase (eggs and larvae) of the zebra mussel is a weak link in its life cycle, with mortalities of nearly 100% depending on environmental conditions. This objectives of this project are to (1) observe feeding mechanisms, particle choice, and feeding rates of Dreissena larvae, (2) determine nutritional requirements of Dreissena larvae, and (3) determine the vulnerability of Dreissena eggs and larvae to zooplankton.

FY96 Progress and Accomplishments

Experimental work on culturing Dreissena larvae with Rhodomonas and other freshwater algae was completed. A paper describing a general culture method and the importance of algal size and polyunsaturated fatty acid (PUFA) concentration on development and growth of zebra mussel larvae was published. This paper argued that PUFA concentration was an important factor in nutrition of the larvae, and that low survival rates reported in nature may be related to the low concentrations of long-chain n-3 PUFAs in blue-green and some green algae that dominate eutrophic lakes in summer.

We presented results of our experiments of direct video observations of copepod attacking Dreissena larvae at the 6th Annual Zebra Mussel Conference. These experiments showed the copepods attacked the larvae, but many of the larvae were protected by their shells when attacked by 2 of the 3 species of copepods examined. It would appear that copepod predation is not a major check on larval abundance.

Products

VANDERPLOEG, H.A., J.R. Liebig, and A.A. Gluck, 1996. Evaluation of different phytoplankton for supporting development of zebra mussel larvae (Dreissena polymorpha): the importance of size and polyunsaturated fatty acid content. J. Great Lakes Res., 22, 36-45.

FY97 Plans

Project was completed in FY96.

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Influence of the zebra mussel (Dreissena polymorpha) on the accumulation of organic contaminants in the Saginaw Bay food web.

Note: this project was formerly titled: Toxicokinetics and Bioaccumulation of Organic Contaminants by the Zebra Mussel.

Principal Investigator: Patricia Van Hoof (313-741-2286; vanhoof@cyberrealm.net).

Collaborating Scientists: Brian Eadie, Margaret Lansing (GLERL); Sander Robinson, Jeffrey Johnson (CILER).

The goal of this project is to assess the impact of the zebra mussel on the distribution of contaminants in ecosystems dominated by this organism. The feeding activities of zebra mussels may result in faster deposition of sediments and may also change the composition and mobility of materials on the bottom. The selected chemicals, primarily PCBs and PAHs, are representative of both their class and the physical and chemical characteristics embodied in these chemicals as model compounds. Compounds from other chemical classes will be used where specific characteristics will be helpful to determine specific processes or mechanisms of action.

FY96 Progress and Accomplishments

Analyses of approximately two-thirds of the field and QA/QC samples were completed.
A third year of sampling for contaminant analysis of the Saginaw Bay food chain was completed.
Analyses of all zooplankton, zebra mussel, gammarid amphipods, sediment and fish samples were completed.
Water resin and filter samples, feces/pseudofeces and a third of algae remain to be analyzed.
We developed a data management system that can handle the large amount of data, flag results according to quality control criteria, and format results into a useful reporting standard.
For the fourth consecutive year, we analyzed zebra mussel tissue on a seasonal basis to compare with filtering rate measurements made in the zebra mussel-algal interactions project. We also arranged to have fish samples collected for a second year by Michigan DNR.
Results from this project were presented at three conferences this year (SETAC, 6th Zebra Mussel and Aquatic Nuisance Species Conference and IAGLR). At the Zebra Mussel Conference we discussed the dramatic seasonal cycle of PCB congener concentrations in zebra mussels and how the cycle coincided with the mussels' filtration activity. During spring and fall when food quality is high, tissue PCB levels are high; in summer PCB levels drop by a factor of six coincident with the almost complete shutdown of mussel filtration due to a Microcystis bloom. Benthos at a zebra mussel infested site do not appear to be exposed to enhanced PCB contamination relative to benthos at an uninfested site.
At the IAGLR conference, exponential relationships of food-chain biomagnification for individual PCB congeners versus trophic level, as represented by enrichment of stable isotope N-15 relative to N-14, were presented (1993 collections only). Interestingly, the slopes of these relationships when log normalized were independent of PCB hydrophobicity for congeners with log K_ow values greater than 5.5. For compounds with log Kow values less than 5.5, biomagnification was dramatically reduced, if evident at all. Lipid normalized tissue PCB concentrations were not as strongly correlated with N-15 enrichment. These results demonstrate that enrichment of each of the more hydrophobic PCB congeners within the food chain is similar despite having a two order-of-magnitude range in hydrophobicity (log K_ow 5.8-7.4).
Two other comparable studies in the literature did not look at individual congeners but at total PCB residue versus N-15 enrichment. Our Saginaw Bay food chain slope for total PCB (0.19+0.02) was comparable to that reported for a Lake Ontario pelagic food chain (0.14+ 0.01). Biomagnification of other hydrophobic contaminants (Mercury, dioxin, DDT, toxaphene) in a variety of aquatic food webs track Del N-15 enrichment similarly with slopes of 0.2-0.4. These relationships support the hypothesis that trophic level and structure are important determinants of ecological partitioning of persistent organic contaminants. The more links in a food chain, the greater the exposure of higher trophic predators. Thus, N-15 is a valuable descriptor of trophic status and can be used to predict contaminant exposure. Field collections from 1994 and 1995 will be compared to results from 1993 to assess seasonal and annual variability of these relationships. (Click here to view a related FY97 Operating Plan Milestone)
Toxicokinetic studies of the transfer of selected organic contaminants into crayfish from aqueous exposures and through feeding on contaminated zebra mussels were measured kinetically. The accumulation rates from water were ranged from 14-44 ml g^-1 h^-1 for large organisms. This is comparable to that for fish of similar size. Elimination rates ranged from 0.002 to 0.02 h^-1 with the ovaries exhibiting the most rapid elimination of all tissues. The assimilation efficiency for accumulation from zebra mussel tissue increased with the hydrophobicity of the food ranging from a low of 36% for pyrene to a high of 81% for hexachlorobiphenyl. With the relatively slow uptake rate from water, food will be a significant source of accumulation and the bioaccumulation potential for the crayfish is predicted to be 45 times greater than that of the zebra mussel.
Two approaches were investigated to determine the efficiency of contaminant transfer from algae to the zebra mussel. This information is required to better predict the impact of the zebra mussel on food web transfer of contaminants. Two approaches for measuring assimilation efficiency were investigated, a mass balance approach and a kinetic approach. The two methods set the bounds on the assimilation efficiency with the kinetic approach setting the lower limit. For selected polychlorinated biphenyl compounds and polycyclic aromatic hydrocarbon compounds, the assimilation efficiencies are in the range of 70 - 90 %. The assimilation of these compounds from suspended sediments ranges form 20 - 50%. Thus, with the high filtration and ingestion rates, accumulation of sorbed contaminants will contribute significantly to the total contaminant introduction into the food web.

Products

GOSSIAUX, D.C., P.F. LANDRUM, and S.W. Fisher. Effect of temperature on the accumulation kinetics of PAHs and PCBs in the zebra mussel, Dreissena polymorpha. J. Great Lakes Res. (In Press)

Berg, D.J., S.W. Fisher and P.F. LANDRUM. Clearance and Processing of Algal Particles by zebra mussels (Dreissena polymorpha). J. Great Lakes Res. (In Press).

FY97 Plans

Analyze 1995 samples and additional fish and zebra mussel samples collected in 1996, as resources allow.
Document the results of our study of the bioaccumulation of PCBs in Saginaw food chains and the role of zebra mussels in contaminant transfer.

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Long Term Changes in the Resuspendible Sediments of Saginaw Bay

Principal Investigator: Brian Eadie (734-741-2281; Brian.Eadie@.noaa.gov).

Collaborating Scientists: Thomas Nalepa, Henry Vanderploeg, Gary Fahnenstiel (GLERL); Thomas Johengen (CILER); Paul Bertram, Glenn Warren (USEPA-Great Lakes National Program Office)

During feeding, adult zebra mussels capture suspended particulate matter and redeposit it as feces and pseudofeces on the bottom, thereby affecting the composition and mobility of materials in the sediment resuspendible pool in Saginaw Bay, and resulting in changes in residence times of particle-associated constituents. This project focuses on sampling resuspendible sediments and analyzing them for gross compositional changes, nutrients, carbon, and cesium-137, to estimate the resuspension flux of these constituents, and to examine the carbon and nitrogen pathway changes caused by the mussel.

FY96 Progress and Accomplishments

A third year of sediment trap collections was completed and the results supported the increased mass flux found in our 1993 data. Samples were analyzed for mass flux and organic and nutrient content. The results from these trap studies confirmed that large quantities of organic-rich materials were reaching the bottom of the deepest portion of the bay. Thus the significant reduction in the biomass of infauna observed by Nalepa could not be attributed to low organic food flux. Two alternatives for the depletion are being considered; 1) low quality or toxic food from Microcystis blooms, and 2) development of anoxia/hypoxia due to the large flux of organic-rich material into the deepest part of the bay.

An array, consisting of a near-bottom dissolved oxygen probe (sampling hourly) sediment trap (serviced monthly), and a deployable water sampler (8 hr-3d intervals) was launched in mid-June and will collect samples/data through mid-September. This effort is being conducted in collaboration with EPA-GLNPO, who have provided 3 days of Lake Guardian time for this project. Initial analysis of the DO data indicates that the O₂ depletion rate is rapid (approximately 1 mgO₂/L/d) but that re-aeration occurs frequently; hypoxia has not been observed. The data will be of value in attempting to determine the probability of the development of anoxia/hypoxia. The sequential water samples will be examined for phytoplankton and total P to correlate Microcystis blooms with events such as P runoff or P injected with sediment resuspension.

A paper describing the relationship between PCB concentration and stable isotopes (C-13 and N-15) in Saginaw Bay food webs was presented at the 1996 meeting of the International Association of Great Lakes Research. N-15 was a useful predictor of food web structure that, in turn, was useful for predicting PCB concentration.

All of the stable isotope measurements have been completed.

FY97 Plans

Complete all analyses of sediment trap samples collected 1991-96.
Document the use of stable isotopic tracers in clarifying the Saginaw Bay food web.
Document the rates of DO depletion in Saginaw Bay and the connection with zebra mussels.

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Effects of the Zebra Mussel on Nutrient and Microbial Dynamics in Saginaw Bay, Lake Huron

Principal Investigator: Wayne Gardner (new address: University of Texas at Austin, Marine Science Institute, Port Aransas, Texas)

Collaborating Scientists: Joann Cavaletto (GLERL); Peter Lavrentyev, Thomas Johengen (CILER)

By altering the biomass of phytoplankton and bacteria, both of which compete for phosphorus, and removing suspended particulates, zebra mussels may affect nutrient cycles and microbial dynamics in an ecosystem. The goals of this project are to (1) determine the direct and indirect effects of the zebra mussel on nutrient regeneration and uptake by various trophic components in the lower food web, (2) determine the effects of the zebra mussel on bacteria, (3) determine how the sources, cycling rates, and fate of labile dissolved organic matter are affected by the presence of the zebra mussel, (4) collect and determine the "nutritional composition" of feces and pseudofeces produced by the zebra mussel, and (5) compare field observations of standing stocks and process rates with those predicted from bottle and mesocosm experiments.

FY96 Progress and Accomplishments

Field work was continued to examine the biogeochemical dynamics of nitrogen in Saginaw Bay and the effects of the zebra mussel on nutrient cycling and food web dynamics in the Bay. Examination of results from intact-core studies of sediment nitrogen dynamics at two sites in the absence of zebra mussel revealed that about 30 - 40 % of the total sediment-regenerated nitrogen was lost from the system via nitrification/denitrification. Addition of zebra mussels to sediment cores significantly increased the percentage (from 8 to 27%) of added ¹⁵N-labeled ammonium that was nitrified in a July experiment, presumably by removing protozoans that would otherwise have grazed down populations of nitrifying bacteria. Nitrification results in the presence and absence of zebra mussels were not significantly different in August during a heavy bloom of the cyanobacterium Microcystis aeruginosa. The above results agree with the concept that zebra mussel can affect nutrient dynamics in the bay by exerting a cascading effect on lower food web organisms.

Results on cascading trophic effects on aquatic nitrification in Great Lakes coastal waters were documented. These results indicate that previously-neglected bacterivory of nitrifiers by heterotrophic protozoa and their trophic interactions with larger zooplankton may be an important factor mediating nitrification rates in aquatic ecosystems.

For the first time in the Great Lakes, quantitative data were collected on bottom dwelling protozoa and on the effects of zebra mussels on their populations and on nutrient fluxes at the sediment-water interface. In regions where zebra mussels were present, common algivorous species of microbenthos were replaced by opportunistic omnivorous and bacterivourous species. In general, community abundances tended to increase at zebra mussel sites, but the diversity within those communities decreased.

Water samples were collected in biweekly intervals from five standard monitoring stations in Saginaw Bay in May-June 1996 to characterize spring microbial communities of the bay and are being processed.

Ammonium recycling experiments were conducted in 1995 on surface and nepheloid water in the presence and absence of zebra mussels to evaluate the potential role of zebra mussels, nepheloid particles, and Microcystis on nitrogen cycling rates in Saginaw Bay. Experimental bottles were incubated at ambient temperature under "natural light" and dark conditions. Ammonium concentrations and regeneration rates increased in bottles that contained zebra mussels due to zebra mussel excretion. Ammonium concentrations in

bottles without zebra mussels remained about the same or declined, probably due to ammonium uptake by phytoplankton in the light. Treatment differences in ammonium concentrations over time were not apparent among bottles that contained surface, nepheloid or screened water. August experiments revealed higher ammonium uptake and regeneration rates in bottles incubated in the light vs. the dark. Screening (53 micron mesh) the water had no apparent effect on regeneration and uptake rates, except for slightly lower uptake rates in the dark, probably due to removal of some of the larger phytoplankton such as Microcystis. These results indicate that both zebra mussel excretion and Microcystis uptake can be important factors affecting nitrogen dynamics in waters where these organisms are abundant.

Experiments were initiated in FY96 to examine nutrient (P and N) release from sediments collected from a depositional region in Saginaw Bay under different conditions of oxygen demand. Intact cores are collected and lake water is passed over them at different rates to create varying oxygen conditions and provide insights about how oxygen status affects release and transformations of nutrients at the sediment-water interface and whether

FY97 Plans

Due to the departure of the Principal Investigator to another institution, this project is being terminated.

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Selection and Utilization of Algal Resources by Dreissena: Unstable Interactions Between Zebra Mussels and the Algal Community of Saginaw Bay

Principal Investigator: Henry Vanderploeg (734-741-2284; Henry.Vanderploeg@noaa.gov).

Collaborating Scientists: James Liebig (GLERL); David Culver (Ohio State University); Wayne Carmichael (Wright State University); J. Rudi Strickler (University of Wisconsin-Milwaukee); Thomas Johengen, Amy Gluck, Megan Agy (CILER)

Because of their high abundance and very high filtering rates in shallow aquatic systems, zebra mussels remove a significant portion of the primary production. In some shallow experimental lakes in Europe, introduced zebra mussels have improved water clarity and quality of the algal resource base so that food web efficiency and fish production have increased. As a result, some scientists have advocated introduction into other aquatic systems. Improvement of water clarity has been seen in lakes Erie and St. Clair; however, in Lake St. Clair, the increased water clarity may have contributed to massive blooms of vascular macrophytes that have washed up on shore and fouled beaches. In the inner portion of Saginaw Bay, water clarity improved in midsummer of 1991 and 1993; but in 1992 and 1994 there were marked decreases in water clarity owing to massive blooms of Microcystis and other large phytoplankton. Also, there have been outbreaks of near-bottom blooms of the filamentous alga Spirogyra, which have later washed up on beaches.

Our observations on the response of Dreissena to a potentially toxic alga strongly suggest that, through a mechanism of selective rejection of the toxic alga, the mussels may increase the probability of blooms of nuisance algae in areas they infest heavily. It also seems likely that zebra mussels have been a major contributor to Spirogyra blooms by increasing water clarity (by filtering out small algae) and metabolizing the ingested biomass into nutrients used by Spirogyra. To explain these unexpected and unstable responses in Saginaw Bay requires us to carefully examine Dreissena's selection, rejection, and utilization of algal resources of different quality and size.

FY96 Progress and Accomplishments

We continued to make direct simultaneous measurements of seasonal N and P excretion by Saginaw Bay zebra mussels in the laboratory under field-simulated conditions to evaluate the possibility that a low N:P ratio in the mussel-excreted nutrients was contributing to favorable nutrient conditions for blue-green algal dominance. Our results indicate that very little P is excreted by the mussels in Saginaw Bay, which is in contrast to results reported by other investigators, who have shown high P excretion for Lake Erie mussels. The difference seems to be related to the low P concentration of the seston in Saginaw Bay, which is a reflection of the bay's greater P limitation. This makes the case stronger for mussel filtering being the driving force behind blue-green promotion, at least in Saginaw Bay. These results have led us to focus (in other projects) on finding the source of P that fuels the bloom.

Seasonal measurements of mussel filtering, pseudofecal production, and assimilation were continued using the special modified technique that allows evaluation of these variables on different size fractions including the large size fraction dominated by Microcystis colonies. These experiments have shown that the colonial Microcystis which dominates Saginaw Bay is not ingested or assimilated. Experiments with laboratory cultures of small algae showed that Dreissena can selectively remove small algae while leaving the Microcystis behind. This is strong evidence that Dreissena can, through its selective grazing, promote Microcystis blooms. There may be some size/toxicity interactions we do not understand yet. Mussels readily and rapidly ingested a reportedly very toxic Microcystis strain from a culture collection (PCC 7820) having a small colony size. In contrast, when Microcystis covered a broad size spectrum, as in Saginaw Bay, mussels rejected all size classes of algae. It is possible the mussels may have selected for a strain in Saginaw Bay and Lake Erie different from the strains available from culture collections. All experiments have been correlated with video observations of the mussels to explain rejection mechanisms.

We completed our first grazing and nutrient experiment on Lake Erie just as some blue-greens were beginning to develop. We saw higher P excretion than in Saginaw Bay, and we could see that despite a lot of ingestion of algae, there was also a lot of rejection of blue-greens. This experiment again reinforces the notion that mussel filtering/rejection is a major driver of bloom promotion. Possibly in Lake Erie, mussel nutrient excretion is another factor in bloom promotion.

Toxicity (microcystin content) of algae from Saginaw Bay collected in 1995 were analyzed by Wayne Carmichael (Wright State University). The Microcystis strain was toxic, and on one occasion the microcystin content of the water exceeded safe drinking water standards. Within a week of the formation of a bloom this August, we began to collect samples from throughout the bay for microcystin analyses. We are also monitoring microcystin concentrations in the mussels..

Results of the experiments described above have been reported at the 6th International Zebra Mussel Conference, at the 44th Annual North American Benthological Meeting, at a Sea Grant Regional workshop, at a public meeting of Saginaw Bay residents in Bangor Township, at the Michigan DNR sponsored Saginaw Bay Watershed Conference, on radio, on local TV, and in newspapers throughout the Great Lakes' region. The role of mussels in promoting the blooms is of great concern to the public and has potentially serious consequences for water quality management.

FY97 Plans

Complete experiments to determine role of toxicity and colony size on zebra mussel rejection of Microcystis to evaluate how mussel selective feeding may be promoting toxic blooms of Microcystis in Saginaw Bay and in Lake Erie.
Finish experiments of nutrient excretion by mussels to determine significance of zebra mussel P and N excretion in promoting blooms of Microcystis in Saginaw Bay and in Lake Erie.
Complete analyses of mussel filtering experiments on Saginaw Bay seston that were performed at monthly intervals during May-November in 1995 and 1996 to evaluate the effects of mussel filtering on the algal community of Saginaw Bay.
Do spatially and temporally intense surveys of algae and nutrients during the Microcystis bloom period to determine if P loading for Microcystis blooms is internal from sediments or external from the Saginaw River.
Improve resolution of Microcystis spatial surveys by using towed and deck-mounted versions of an instrument package consisting of optical plankton counter, fluorometer, and CTD.
Improve spatial resolution of Microcystis surveys by correlating ground truth measurements of Microcystis and surface reflectance with new multi-band satellite observations (Sea WiFS and ADEOS-OCTS).
Evaluate role of storm events on P loading for fueling Microcystis blooms by deploying a mooring which includes a sequential water sampler that can be remotely activated (by cellular telephone) to sample for P and phytoplankton during a storm event.
Measure spatial distribution of microcystin ( the toxin of Microcystis) on Saginaw Bay.
Compare pelagic and benthic biomass (and estimate production) of plants in Saginaw Bay in 1995 and 1996 from our pelagic work and from the benthic work of Bowling Green State University to evaluate the significance of benthic primary production to Saginaw Bay ecosystem function since the zebra mussel invasion.