A Seasonal Comparison of Suspended Sediment Filtration by Quagga (Dreissena bugensis) and Zebra (D. polymorpha) Mussels

Field evidence suggests a shift in the dreissenid population from zebra (Dreissena polymorpha) to quagga (D. bugensis) mussels is occurring within the lower Great Lakes. This laboratory study directly compared per-mussel and per-dry-weight filtration rates (volume per time) of both species, gauged by the clearance of resuspended natural sediments (1 to 12 mg/L) from gently mixed, 1-L static vessels. Mussels of 15- and 20-mm lengths were collected together from the Lake Ontario drainage basin at Oak Orchard Creek, Medina, NY, and maintained and tested in ambient Niagara River water. A 2 × 4 factorial design was employed, with species and season as independent factors. Season significantly influenced filtration rate of both size classes, and winter rates were about half those measured during the rest of the year. Species significantly influenced filtration of 20-mm mussels. Quagga mussels of this size filtered up to 37% faster than zebra mussels (data for spring: 309 vs. 226 mL/h/mussel, n = 18 and 20 individuals, respectively). Species was not a significant factor alone for 15-mm mussels, but a species x season interaction was significant. The zebra mussels employed here had 16 to 22% more ash-free dry weight (AFDW) than the quagga mussels, accentuating filtration differences when expressed per-mg-AFDW.     

Laboratory Culture of Zebra (Dreissena polymorpha) and Quagga (D. bugensis) Mussel Larvae Using Estuarine Algae

Zebra (Dreissena polymorpha) and quagga mussel (D. bugensis) larvae were reared through and beyond metamorphosis in the laboratory on diets of the estuarine algae, Isochrysis galbana T-Iso and Pavlova (= Monochrysis) lutheri. Larvae were successfully spawned and raised in the laboratory for over 1 year with routine survival to settlement. Some adult males reared from an August 1994 spawn reached sexual maturity with active sperm by April 1995. Diets of dried Chlorella sp. and Synecococchus sp. were unable to support larvae. Settlement of pediveligers at 22°C occurred at ≈ 21 days for zebra mussel and 32 days for quagga mussels. Saturated fatty acids predominated in I. galbana and P. lutheri, whereas polyunsaturated fatty acids (PUFAs) predominated in Chlorella sp. The process to culture dreissenid larvae in the laboratory is very labor-intensive and requires continuous culture of live algae maintained in log growth phase. Larvae are fed every other day at a density of 2 × 10⁵ cells mL⁻¹ for the first week, and then daily densities of 3–5 × 10⁵ cells mL⁻¹ thereafter. Water quality is critical. Larvae and juveniles should be stocked at densities of ≈ l mL⁻¹ and the water must be changed at least 3× wk⁻¹ in static cultures. Water must be changed at least weekly in tanks holding adult brood stock. Methods of controlling protozoan and rotifer infestation of cultures are also important for successful culture.     

Sensitivity of post-settlement Dreissena rostriformis bugensis to UVB radiation at Earth surface intensity levels

As quagga mussels (Dreissena rostriformis bugensis) spread through North America, it is critical to understand environmental conditions that may affect their ability to thrive, and thus potentially lead to control measures. The sensitivity of D. r. bugensis to electromagnetic radiation in the Ultraviolet-B range (UVB) is considered through experiment. Post-settlement specimens were collected from Lake Mead (southern Nevada), acclimated to laboratory conditions, and then segregated into two size classes on the basis of length; small (6–9 mm) and large (13–19 mm). Test groups from both size classes were exposed to continuous UVB at one of three power levels (100, 300, or 500 μW/cm²) chosen to be consistent with maximal Earth surface conditions. Survivorship for both size classes was found to follow a logistic model and to scale with the square root of the applied UVB intensity. This result strongly suggests that a four-fold increase in applied power is required to double the mortality rate. Results also strongly suggest that resistance to UVB increases with size/maturity. Post-mortem measurements of shell thickness and UVB transmission demonstrate that the shells impede transmission of UVB with an effectiveness that is proportional to thickness.     

Causes of phytoplankton changes in Saginaw Bay,Lake Huron,during the zebra mussel invasion

Colonization of the Laurentian Great Lakes by the invasive mussel Dreissena polymorpha was a significant ecological disturbance. The invasion reached Saginaw Bay, Lake Huron, in 1991 and initially cleared the waters and lowered algal biomass. However, an unexpected result occurred 3 years after the initial invasion with the return of nuisance summer blooms of cyanobacteria, a problem that had been successfully addressed with the implementation of phosphorus controls in the late 1970s. A multi-class phytoplankton model was developed and tested against field observations and then used to explore the causes of these temporal changes. Model scenarios suggest that changes in the phytoplankton community can be linked to three zebra mussel-mediated effects: (1) removal of particles resulting in clearer water, (2) increased recycle of available phosphorus throughout the summer, and (3) selective rejection of certain Microcystis strains. Light inhibition of certain phytoplankton assemblages and the subsequent alteration of competitive dynamics is a novel result of this model. These results enhance our understanding of the significant role of zebra mussels in altering lower trophic level dynamics of Saginaw Bay and suggest that their physical re-engineering of the aquatic environment was the major force driving changes in the phytoplankton community composition.     

Modeling PCB Bioaccumulation by the Zebra Mussel (Dreissena polymorpha) in Saginaw Bay,Lake Huron

Zebra mussels were collected from navigation buoys in Saginaw Bay and the lower Saginaw River and analyzed for PCBs. Total PCB concentrations ranged from 0.076 to 1.2 μg/g, with the highest values for zebra mussels collected near the mouth of the Saginaw River. These data were combined with PCB concentration measurements in water and suspended solids to calculate congener-specific partition coefficients and bioaccumulation factors. Lipid-normalized bioaccumulation factors for PCB congeners ranged from 10^5.6 to 10^7.0. A model based on three-phase equilibrium partitioning and a bioaccumulation mass balance were applied to simulate the concentration ratios. Model results provided an unbiased best fit of the observed partition coefficients and bioaccumulation factors, as well as optimum parameter estimates. PCB fluxes mediated by zebra mussels were calculated from the model. Transfer across the gut was found to be the predominant route of PCB uptake for zebra mussels. Egestion and fecal excretion were estimated to generate a PCB flux of 1 to 30 μg/m²/d at zebra mussel densities reported for the Great Lakes. This flux exceeds the gross settling flux of PCBs to inner Saginaw Bay, demonstrating the potential of zebra mussels to alter particle and contaminant fluxes in the Great Lakes ecosystem. Zebra mussels may contain as much PCBs as the surficial sediments in Saginaw Bay.     

Circular Rep encoding single stranded (CRESS) DNA virus-like sequences detected in quagga mussels (Dreissena rostriformis bugensis) and sediments from the central Lake Michigan benthos

Circular Rep Encoding Single Stranded (CRESS) DNA viruses are a diverse group of viruses that have been identified in both terrestrial and aquatic ecosystems. Recent work in the Laurentian Great Lakes characterized the ecology and diversity of CRESS-DNA viruses associated with amphipods, Diporeia spp. In the last 20 years the Lake Michigan benthos has changed considerably with drastic population declines of Diporeia spp. concurrent with an increase in invasive quagga mussel (Dreissena rostriformis bugensis) abundance. The purpose of this study was to characterize CRESS-DNA virus-like elements (VLES; which could represent complete, partial/defective, endogenized or satellite viruses) associated with both invasive quagga mussels and in sediment collected in central Lake Michigan. Viral metagenomic libraries were prepared from two size classes (>25 mm and <15 mm shell length) of Lake Michigan quagga mussels and for two different sediment layers (136 mm and 290 mm below the lake floor) in a sediment core extracted from the lake. Viral metagenomes were different between quagga mussels and sediment cores. Nine VLE sequences were present in both the quagga mussel tissues and the sediment core layers analyzed. Cs¹³⁷ radiometric dating results indicate that these VLEs were present in the sediment prior to arrival of quagga mussels in Lake Michigan. These data suggest quagga mussels may interact with CRESS-DNA VLEs and algal DNA VLEs historically present in the Lake Michigan benthos. Overall, these data suggest that quagga mussels interacted with CRESS-DNA VLES present in Lake Michigan benthos since at least 1952.     

Zooplankton Grazing During the Zebra Mussel (Dreissena polymorpha) Colonization of Saginaw Bay,Lake Huron

Weight-specific zooplankton filtering rates were determined at three sites in Saginaw Bay during the period of maximum zooplankton abundance prior to and after the establishment of zebra mussel colonies (June 1991 and June 1992). Biomass-specific filtering rates were similar in both years (inner bay: 0.24–0.33 mL μg dry wt.⁻¹ d⁻¹; outer bay: 1.27–1.83 mL μg dry wt.⁻¹ d⁻¹), but large decreases in zooplankton biomass resulted in a decrease, on average, of 58% in community filtering rates between years. As part of a large-scale monitoring program, zooplankton abundance and biomass estimates were also recorded at 13 sites during May-August of both years. Mean biomass in the inner bay was 40% lower in 1992 than in 1991, and in the outer bay, mean biomass was 70% lower in 1992 than in 1991. Zooplankton community composition was the same in both years. We estimated the time required to clear the water volume of the inner bay during the May–June period to be 17 days in 1991 and 37 days in 1992. For these two periods, decreased zooplankton numbers and community filtering rates indicate that grazing by zooplankton was likely not responsible for noted declines in phytoplankton abundance and productivity.     

Phytoplankton Productivity in Saginaw Bay,Lake Huron: Effects of Zebra Mussel (Dreissena polymorpha) Colonization

Phytoplankton photosynthesis-irradiance parameters, chlorophyll concentrations, underwater extinction coefficients (kPAR), and surface irradiance were determined at 8–10 sites on 27 occasions in Saginaw Bay from spring 1990 through fall 1993 corresponding to a period before and after the establishment of large zebra mussel populations (began in summer 1991). Similar measurements, with the exception of the photosynthetic parameter, α, had also been made in 1974/75 at eight sites on nine occasions. In inner Saginaw Bay where zebra mussels were primarily found, chlorophyll and kPAR values decreased, while the photosynthetic parameters, P_max and α, increased after zebra mussel colonization. At sites in the outer bay where no zebra mussels were found, chlorophyll and kPAR values did not change after zebra mussel colonization, whereas photosynthetic parameters increased. Decreases in chlorophyll and kPAR in the inner bay were related to the zebra mussel, but increases in photosynthetic parameters in both the inner and outer bay were not. Areal-integrated and volumetric phytoplankton productivity decreased by 38% and 37%, respectively, in inner Saginaw Bay after the establishment of zebra mussels; phytoplankton productivity at outer bay control sites was similar during the same period. Decreased phytoplankton productivity in the inner bay was attributable to the large decrease in chlorophyll as increases in underwater irradiance (increased kPAR) and photo synthetic parameters could not compensate for the chlorophyll effect. Increase in underwater irradiance produced a significant increase in light to the benthic region and contributed to increased benthic primary productivity; ratio of photic zone to station depth increased in inner Saginaw Bay, from 0.6–0.8 before the zebra mussel colonization (1974–1990) to 1.1–1.3 after colonization (1992–1993). Overall, primary productivity in the inner bay did not exhibit a notable change after zebra mussel colonization as decreases in phytoplankton productivity were accompanied by increases in benthic primary productivity. Thus, zebra mussels altered inner Saginaw Bay from a pelagic-dominated system to a benthic/pelagic system which will have long-term effects on food web structure and productivity at higher trophic levels.     

Analysis of Factors Affecting Zebra Mussel (Dreissena polymorpha) Growth in Saginaw Bay: A GIS-Based Modeling Approach

A statistical model was developed using a Geographical Information System (GIS) to investigate the spatial relationships between limnological variables and the growth of zebra mussels (Dreissena polymorpha) in Saginaw Bay, Lake Huron, as defined by the change in biomass over a specified time. The presence of suitable substrate was an important factor for zebra mussel habitat, making inner Saginaw Bay an area of high mussel abundance. Temperature, phytoplankton biomass (measured as chlorophyll a), and total suspended solids (TSS) as food particles were considered to be the most important limnological variables affecting growth of zebra mussels. Three layers of attributes were developed from these variables, and were overlaid in a GIS environment according to their respective weighting factors, which were calculated in statistical analysis of spatially-matched data. The model results showed that chlorophyll a contributed the most to mussels’ growth. The effect of chlorophyll a on Dreissena growth was 9 times more important than that of temperature. The shallow portions of the inner bay and the areas in proximity to the shorelines were found to be the most suitable growth regions. A comparison of model predictions with field data on growth of mussels at various spatial locations of the inner bay demonstrated the value of this model. In addition, the model was field-tested for temporal variation in the rate of change of Dreissena biomass at one sampling station where data were available. The developed GIS-based statistical model provides a rapid, objective, reliable and cost effective tool to prioritize locations of Dreissena growth.     

Ecosystem-Level Effects of Zebra Mussels (Dreissena polymorpha): An Enclosure Experiment in Saginaw Bay,Lake Huron

We examined the short-term effects of zebra mussels (Dreissena polymorpha) on ecosystem processes in late August 1991 in Saginaw Bay, Lake Huron. Four 1,600-L enclosures, made of Fabreen with a diameter of 1 m, a depth of 2 m, and closed at the bottom, were used to enclose natural plankton communities. These communities were dominated by diatoms with some chlorophytes, chrysophytes, and cyanophytes. Phytoplankton growth was limited by P-availability. Two enclosures were held as controls, and zebra mussels encrusting unionid shells were suspended in two of the enclosures: one enclosure (HZ) contained approximately four-fold greater numbers of mussels than the other (LZ). The concentration of suspended particles, chlorophyll, and algal biomass in HZ and LZ declined over a 6-day interval. Diatom numbers declined more than other taxa. Phytoplankton growth rates in HZ and LZ increased to near μ_max; there was no apparent change in photosynthetic parameters a or P_max scaled for chlorophyll. Soluble reactive P (SRP) increased significantly (p < 0.05) in HZ but not LZ. Dissolved organic P (DOP) and ammonium ion were elevated; dissolved organic carbon (DOC) was unchanged in HZ and LZ. The rate of phosphate uptake by bacteria and algae declined to less than 2% of controls; this rate decrease could not be explained simply by grazing losses or isotope dilution. The rate of ammonium regeneration by the plankton and the potential rate of ammonium uptake by the plankton did not differ significantly in HZ or LZfrom the control enclosures. Our findings indicate that the zebra mussel can have significant short-term effects on phytoplankton abundance, water transparency, water chemistry and phosphorus dynamics. We propose a model of zebra mussel effects that suggests high densities of zebra mussels may indirectly alter and control those processes that are rate-limited or concentration-limited by nutrient availability.     

Effects of Zebra Mussel (Dreissena polymorpha) Colonization on Water Quality Parameters in Saginaw Bay,Lake Huron

A large-scale study of Saginaw Bay was initiated in 1990 and continued through 1993 to examine the effects of the zebra mussel colonization which began in summer/fall 1991. Saginaw Bay responded quickly to the zebra mussel colonization, as fall 1991 values of chlorophyll were similar to 1992 and 1993 values. In inner Saginaw Bay, where most zebra mussels were found, chlorophyll, kPAR, and total phosphorus values decreased, and Secchi disk depth increased during the study period, regardless of the presence or absence of zebra mussels at a specific station. At outer bay control stations no significant differences were found for chlorophyll, kPAR, and Secchi disk values. In order to examine longer-term trends, water quality data from 1979–1980 (STORET) were combined with our 1990 data (pre-zebra mussel period) and compared to values from the post zebra mussel period (fall 1991, all 1992 and 1993). At stations with high densities of zebra mussels, chlorophyll and total P decreased by 66% and 48%, respectively, and Secchi disk values increased 88%. At outer bay control stations no significant differences were found for chlorophyll or Secchi disk. When parameters were averaged throughout inner Saginaw Bay, zebra mussels caused a 59% and 43% decrease in chlorophyll and in total phosphorus and a 60% increase in Secchi disk transparency. Although zebra mussels significantly altered water quality parameters in the pelagic region of Saginaw Bay, they did not necessarily change system trophic state; rather they altered the spatial partitioning of resources.     

Nutrient Changes in Saginaw Bay,Lake Huron,After the Establishment of the Zebra Mussel (Dreissena polymorpha)

Concentrations of particulate and dissolved nutrients in Saginaw Bay, Lake Huron, were examined relative to zebra mussel colonization which occurred summer 1991. The magnitude and spatial pattern of changes indicate that mussels had a significant impact on nutrients in Saginaw Bay. Annual means for total suspended solids, particulate organic carbon, particulate phosphorus, and particulate silica in the inner bay were significantly lower in 1992 and 1993 (post-zebra mussel) than in 1991 (pre-zebra mussel). Annual means decreased from 11.5 mg L^−l, 1.45 mg C L^−l (121 μM), 20.4 μg P L⁻¹(0.66 μM), and 1.52 mg SiO₂ L⁻¹ (24 μM) respectively in 1991 to 4.4 mg L⁻¹, 0.79 mg C L⁻¹(66 μM), 11.2 μg P L^−l (0.36 μM), and 0.77 mg SiO₂ L⁻¹(12 μM) in 1993. In contrast, there were no significant differences among years for these parameters at control stations, which were located in the outer bay and had no known populations of mussels. Annual means for nitrate, ammonium, and silica were significantly higher in the inner bay in 1992 than in 1991, but not significantly different in 1993. Means increased from 0.39 mg N L⁻¹, 21.0 μg N L^−], and 1.11 mg SiO₂ L⁻¹ respectively in 1991 to 0.47 mg N L⁻¹, 30.9 μg N L⁻¹, and 1.71 mg SiO₂ L⁻¹ in 1992. No significant differences were observed for these parameters in the control group. Differences between 1992 and 1993 may reflect differences in the amount of runoff and circulation between Saginaw Bay and Lake Huron.A phosphorus budget indicated that zebra mussels were a significant sink for phosphorus. Mussels from the inner bay accumulated 108, 682, and 52 t respectively in 1991, 1992, and 1993. Comparatively, the annual pool of phosphorus in the water column of the inner bay decreased from a pre-zebra mussel (1979–1980) average of 712 t to 421 and 382 t in 1992 and 1993 respectively.     

Effects of the Zebra Mussel (Dreissena polymorpha Pallas) on Protozoa and Phytoplankton from Saginaw Bay,Lake Huron

Direct effects of the grazing activities of the zebra mussel, Dreissena polymorpha, on the natural assemblage of planktonic protozoa and algae from Saginaw Bay, Lake Huron, were studied in September and October 1994. Water and mussels collected from two eutrophic sites were incubated in an outdoor “natural light” incubator at ambient temperature for 24 hours. Experiments were conducted in 4-L bottles with screened (40 or 53-μm net) or unscreened water and with and without mussels. Despite relatively high growth rates of protozoa on both dates, mussels lowered protozoan numbers by 70–80% and reduced the species richness of the protozoan community by 30–50%. Large heterotrophic flagellates were reduced up to 100% while peritrichous ciliates attached to the colonies of blue-greens were reduced only by 50%. Dreissena selectively removed nanoplanktonic Cryptomonas and Cyclotella, but had no significant effect on the predominant phytoplankton species, Microcystis. Overall, Dreissena clearance rates were low in the presence of this cyanophyte species. We conclude that zebra mussels, in regions where they are abundant, can cause significant changes in composition of both the protozoan and phytoplankton communities.     

Impact of Round Gobies (Neogobius melanostomus) on Dreissenids (Dreissena polymorpha and Dreissena bugensis) and the Associated Macroinvertebrate Community Across an Invasion Front

We studied the impact of round gobies (Neogobius melanostomus) on lithophilic invertebrates (having an association with a stony substrate) across an invasion front along the Door Peninsula, which flanks eastern Green Bay, Lake Michigan. We conducted both a cross-invasion front field survey and a rock-transfer experiment. For the field survey, we collected pairs of rocks from ten sites, including sites north of the invasion front and south of the invasion front. Zebra mussels (Dreissena polymorpha), quagga mussels (D. bugensis), and non-mussel invertebrates were removed from the rocks and enumerated. The rocks were measured and the algae removed and weighed. Round gobies were censused by videotaping along transects. There was a statistically significant negative relationship with round goby abundance for most invertebrates, including zebra mussels, quagga mussels, isopods, and snails, with the result for amphipods being suggestive. For the experiment, we transferred 20 rocks in bags from a round goby “absent” site with 10 going to a round goby abundant site and 10 being returned to the original site. The rocks incubated overnight, invertebrates were removed the next day, and the rocks were measured. There were significantly fewer zebra mussels, quagga mussels, isopods, amphipods, and snails from the rocks incubated at the round goby abundant site compared to those returned to the round gobyfree site. Thus, the results of the survey and rock-transfer experiment suggest that round gobies are influencing the benthic macroinvertebrate abundance through predation. The negative impact on mussels is probably due to direct predation while the negative impact on the other invertebrates may be a combination of direct predation and indirect effects due to the loss of the microhabitat or food that zebra mussels produce.     

Shifts in Benthic Algal Community Structure and Function Following the Appearance of Zebra Mussels (Dreissena polymorpha) in Saginaw Bay,Lake Huron

Zebra mussel, Dreissena polymorpha (Pallas), proliferation in Saginaw Bay, Lake Huron is associated with increased water clarity and increased light levels on benthic substrata in the littoral zone. We hypothesized that the filtering activities of Dreissena and associated increases in light penetration should affect the structure and function of benthic algae in the bay. Monthly quantitative benthic algal samples were collected from natural substrata by SCUBA in the littoral zone of the bay through the growing seasons of 1991 (initial Dreissena colonization), 1992 and 1993 (post-Dreissena colonization). Algal community structure was examined microscopically and productivity rates were measured using carbon-14 in sealed acrylic chambers in situ. Our data demonstrate that, following Dreissena proliferation, light penetration, benthic algal biomass, chlorophyll concentrations and rates of benthic primary productivity have increased. These changes coincided with a shift from diatom domination of the benthic algal community to a flora dominated by filamentous green algae (Zygnematales). We suggest that these shifts have the potential to affect benthic food webs within littoral zones of the Great Lakes.     

Initial Colonization of the Zebra Mussel (Dreissena polymorpha) in Saginaw Bay,Lake Huron: Population Recruitment,Density, and Size Structure

The various life stages of the zebra mussel (Dreissena polymorpha) were examined during the initial years (1991–93) of the mussel's invasion into Saginaw Bay, Lake Huron. Yearly trends in densities of larvae, newly-settled juveniles, and adults were poorly related. Larval densities were lowest in 1991 and increased each year, but the number of settled juveniles was highest in 1991. Adults increased between 1991 and 1992 and then declined in 1993. Mean adult densities at sites with hard substrates were 11,700, 33,200, and 4,100/m² in each of the 3 years, respectively. Year-to-year variation at individual sites was high and likely a result of recruitment dynamics and spatial patchiness of available substrate. By 1993, densities on hard substrates were generally similar throughout the bay, but length-frequency distributions in the inner and outer bay were quite different. The 1991-cohort was not distinguishable in the inner bay in 1993 either because of poor growth or a limited life span, but this cohort was readily distinguishable in the outer bay. In addition, ash-free dry weight of a standard 15-mm mussel in the inner bay declined 65% between 1991 and 1993. Although food concentrations (chlorophyll and particulate organic carbon) declined to low levels in 1993 and both densities and soft-tissue weight of Dreissena declined, it is not clear whether populations in the bay have peaked and are now at equilibrium with the surrounding environment.     

Effects of the Zebra Mussel,Dreissena polymorpha,on Community Nitrogen Dynamics in Saginaw Bay,Lake Huron

The effects of the zebra mussel, Dreissena polymorpha, on chlorophyll and nutrient concentration changes and community ammonium uptake and regeneration rates were determined in bottle experiments on waters collected from a eutrophic site and an oligotrophic site in Saginaw Bay, Lake Huron in 1992. Our objectives were to estimate nitrogen cycling rates and to determine the direct (excretion) and indirect (foodweb) effects of the zebra mussel on these rates. Isotope labeling experiments with added ¹⁵NH₄⁺ were conducted on waters collected on five sampling dates between April and October. Direct effects of zebra mussels on ammonium regeneration and potential uptake were examined by comparing results from bottles incubated with (15 individuals in 4 L lake water) and without added zebra mussels. Indirect foodweb effects were examined by measuring regeneration and potential uptake rates in subsamples of water that had previously been incubated in the presence or absence of zebra mussels.Zebra mussels removed a large fraction of chlorophyll from the oligotrophic site on all sampling dates and from the eutrophic site in October, but had a negligible effect on chlorophyll levels in waters from the eutrophic site in June, July, August, and September when cyanophytes were abundant. Community ammonium regeneration rates and uptake rates both followed seasonal patterns resembling those for chlorophyll concentrations in control treatments at the eutrophic site. Rates for water from the oligotrophic site were low (usually not significantly different from zero) and are not reported here. Community ammonium regeneration rates were consistently enhanced in the presence of zebra mussels, indicating that zebra mussel excretion could have a dominant effect on nitrogen regeneration in regions where it is abundant. Zebra mussels appeared to decrease community uptake rates of ammonium in August and September but did not predictably affect nitrogen remineralization rates by other lower foodweb organisms (e.g. bacteria, protozoans, zooplankton).     

Nitrogen Dynamics in Sandy Freshwater Sediments (Saginaw Bay,Lake Huron)

Sediment-water nitrogen fluxes and transformations were examined at two sites in Saginaw Bay, Lake Huron, as a model for sandy freshwater sediments. Substantial ammonium release rates (74 to 350 μmole NH₄⁺/m²/h¹) were observed in flow-through cores and in situ benthic chamber experiments. Sediment-water ammonium fluxes were similar at the inner and outer bay stations even though inner bay waters are enriched with nutrients from the Saginaw River. The high net flux of remineralized ammonium into the overlying water from these sandy sediments resembles typical data for marine systems (11 to 470 μmole NH₄⁺/m²/h¹) but were higher than those reported for depositional freshwater sediments (0 to 15 μmole NH₄⁺/m²/h¹; Seitzinger 1988). Addition of montmorillonite clay (ca. 1 kg dry weight/m²) to the top of the sandy cores reduced ammonium flux. Mean “steady-state” ammonium flux following clay addition was 46 ± 2 (SE) % of the initial rates as compared to 81 ± 8% of the initial rates without clay addition. Zebra mussel excretion dominanted ammonium regeneration in the inner bay where the bivalve was abundant, but addition of zebra mussel feces/psuedofeces (3.0 g dw/m²) to sediments did not increase ammonium or nitrate flux. Partial nitrification of ammonium at the sediment-water interface was suggested by removal of added ¹⁵NH₄⁺ from lake water passing over dark sediment cores. Sediment-water fluxes of nitrogen obtained from flow-through sediment cores resembled those from in situ benthic chambers. However, extended static incubations in gas-tight denitrification chambers caused more of the regenerated nitrogen to be nitrified and denitrified than occurred with the other two measurement systems.     

Phytoplankton community composition of Saginaw Bay,Lake Huron,during the zebra mussel (Dreissena polymorpha) invasion: A multivariate analysis

The colonization of the zebra mussel (Dreissena polymorpha) in Saginaw Bay dramatically altered the phytoplankton community composition resulting in exclusion of light sensitive species and dominance of species with oligotrophic preferences and light resistance. In 1990, the NOAA Great Lakes Environmental Research Laboratory initiated a 7-year survey program to monitor changes in the lower food web of Saginaw Bay, where zebra mussels became established in the fall of 1991. To investigate shifts in the phytoplankton community composition over the 7-year period from 1990 to 1996 we searched for clusters of similar composition using multivariate principal component analysis (PCA) on proportions of 22 taxonomic groupings of the total phytoplankton density (cells per milliliter). We then used an agglomerative hierarchical clustering analysis of the PCA scores. We identified five characteristic phytoplankton communities in configurations that allowed recognizing four distinct periods in Saginaw Bay linked to the zebra mussel invasion. Significant changes were indicative of increased water clarity and eutrophic conditions being replaced by more oligotrophic conditions as clusters dominated by light sensitive species, such as the cyanobacteria Oscillatoria redekii, became immediately rare and clusters dominated by diatoms such as Cyclotella spp. became common. Microcystis spp., a light tolerant cyanobacteria not grazed by zebra mussel, dominated assemblages after 1994. The shifts in phytoplankton composition confirm that zebra mussels effects on phytoplankton communities are mediated by both direct (filtration) and indirect (nutrient cycling) mechanisms and also suggests that increased light penetration is an important mechanism behind some changes.