Joann F. Cavaletto

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 μmax; there was no apparent change in photosynthetic parameters a or Pmax 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 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.

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).

Nutrient Limitation of Heterotrophic Bacteria in Florida Bay

We examined heterotrophic bacterial nutrient limitation at four sites in Florida Bay, U. S. in summer 1994 and winter 1995. Bacterial growth and biomass production in this system were most limited by inorganic phosphorus (P) in the eastern and southern regions of the bay. Nutrient additions stimulated productivity and biomass accumulation mostly in summer. The magnitude of growth responses (thymidine incorporation) to nutrient additions was nearly an order of magnitude less in winter than summer. Biomass-normalized alkaline phosphatase activity in the northeast and south-central region was 5–20 times greater than in the northwest and north-central regions, suggesting that P is most limiting to planktonic growth in those areas. Chlorophyll levels were higher in the northwest and north-central regions and P-uptake into particles >1 μm, primarily phytoplankton, was also higher in these regions. Consistent with these observations, others have observed that P is advected into the bay primarily in the northwestern region. Abundant seagrasses in Florida Bay may promote heterotrophic bacterial production relative to phytoplankton production by releasing dissolved organic carbon that makes bacteria more competitive for limiting quantities of inorganic phosphate, especially in the eastern bay where turbidity is low, P is most limiting, and light levels reaching the benthic plants are high.

Seasonal and Annual Variation in Weight and Biochemical Content of the Zebra Mussel, Dreissena polymorpha, in Lake St. Clair

Abstract Zebra mussels, Dreissena polymorpha, were collected monthly from April/May to November in 1990 and 1991 from two sites in Lake St. Clair. The sites were characterized by relatively high and low mussel densities. The following variables were measured: ash-free dry weight (AFDW) per unit shell length (SL), lipid content and classes, carbon content, and nitrogen content. Mussels from the high-density site had a lower AFDW:SL relationship, lower lipid content, and a lower C:N ratio than mussels from the low-density site. Seasonal trends in these variables were consistent between sites and years. AFDW:SL, lipid, and carbon content were highest in the spring and then declined to minimum levels in late summer/fall. The mean seasonal decline in weight from spring to late summer for a standard 15-mm mussel was 60%. This decline was greater than might be expected from gamete release alone and was likely a result of nutritional stress from warm summer temperatures and limited food supplies. Between 1990 and 1991, the mean AFDW of a 15-mm mussel declined 34% and 50% at the high- and low-density site, respectively. However, when C:N ratios and lipid levels in 1990 and 1991 were compared, C:N ratios were only lower in fall 1991 compared to fall 1990, and lipid levels for the 2 years were generally similar.

Mineralization of Organic Material and Bacterial Dynamics in Mississippi River Plume Water

Net remineralization rates of organic matter and bacterial growth rates were observed in dark-bottle incubation experiments conducted in July–August and February with water samples collected from sites in the Mississippi River plume of the Gulf of Mexico. Our objectives were to measure site-specific degradation rates of labile dissolved and particulate organic matter, quantify the potential importance of bacteria in these processes, and examine the kinetics of degradation over time. Unfiltered samples, and samples treated to remove (or dilute out) particles larger than bacteria, were enclosed in 9-1 bottles and incubated in the dark for 3–5 d. Respiration rates and inorganic compound accumulation rates were higher in summer than in winter and were highest in unfiltered surface samples at sites of intermediate salinities where phytoplankton were most abundant. The ratio of ammonium accumulation to oxygen removal in summer experiments suggested that the mineralized organic material resembled “Redfield” stoichiometry. Chemical fluxes were greater in bottles containing large (>1–3 μm) particles than in the bottles with these particles removed, but bacterial activities were generally similar in both treatments. These results suggest that particle consumers were an important component of total organic matter degradation. However, these experiments may have underestimated natural bacterial degradation rates because the absence of light could affect the production of labile organic substrates by phytoplankton. In agreement, with this hypothesis, bacterial growth rates tended to decrease over time in summer in surface plume waters where phytoplankton were abundant. In conjunction with other data, our results indicate that heterotrophic processes in the water column are spatially and temporally dependent on phytoplankton production.

Distribution and dynamics of nitrogen and microbial plankton in southern Lake Michigan during spring transition 1999–2000

Isotope dilution experiments showed similar light and dark NH4 regeneration rates at lake (6 versus 5 nM N h 1 ) and river-influenced (20 versus 24 nM N h 1 ) sites. Ammonium uptake rates were similar to regeneration rates in dark bottles. Dark uptake (attributed mainly to bacteria) accounted for 70% of total uptake (bacteria plus phytoplankton) in the light at most lake sites but only 30% of total uptake at riverinfluenced sites in or near the St. Joseph River mouth (SJRM). Cluster analysis grouped stations having zero, average, or higher than average N-cycling rates. Discriminant analysis indicated that chlorophyll concentration, oligotrich ciliate biomass, and total P concentration could explain 66% of N-cycling rate variation on average. Heterotrophic bacterial N demand was about one third of the NH4 regeneration rate. Results suggest that, with the exception of SJRM stations, bacterial uptake and protist grazing mediated much of the N dynamics during spring transition. Since NH4 is more available to bacteria than NO3 , regenerated NH4 may have a strong influence on spring, lake biochemical energetics by enhancing N-poor organic matter degradation in this NO3 -replete ecosystem. INDEX TERMS: 1845 Hydrology: Limnology; 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 4845 Oceanography: Biological and Chemical: Nutrients and nutrient cycling; KEYWORDS: nitrogen, microbial food web, Lake Michigan

Lake Michigan Bythotrephes Prey Consumption Estimates for 1994–2003 using a Temperature and Size Corrected Bioenergetic Model

ABSTRACT Bythotrephes were collected on a regular basis at a 110 m deep reference station in Lake Michigan over a 10-year period 1994–2003. The measured population structure in conjunction with an updated bioenergetic model was used to estimate daily predation demands by Bythotrephes on the Zooplankton community. The bioenergetic model incorporated the effect of temperature on growth and respiration and used a scalable size structure to adjust for a dynamic range in size across the season. A general linear model was developed to apply the bioenergetic results to routinely collected field data for estimating predation needs. Daily population consumption needs were estimated to be approximately equal to Bythotrephes standing biomass but varied as a function of water temperature and percent instar composition. At a temperature of 18 °C the predation needs of the population were equal to the population biomass. At warmer temperatures (22– 24 °C) the daily needs were up to 35% above the population biomass. Within and across years the population was variable, while trend lines from the long-term data indicated biomass and predation needs had an initial peak in mid-August followed by a plateau period with a seasonal high peak mid-October. A decrease in the midseason long-term average size structure suggests that Bythotrephes may experience prey limitation during this time period of the year. Over the course of the 10-year period population cycles and peak biomass were fairly stable with no indication of a change in predation needs.

Seasonal Dynamics of Lipids in Freshwater Benthic Invertebrates

Although there is a wealth of information on lipids in marine organisms, especially zooplankton (Kattner and Hagen, 1995; Tande and Henderson, 1988;Clarke et al., 1987; Lee, 1975; Lee and Hirota, 1973), relatively little information is available on freshwater benthic organisms. The benthos of large temperate lakes often experience a seasonal variation in food supply that is similar to the high to midlatitudes of the ocean (Lee, 1975). For instance, in temperate large lakes the diatom bloom that eventually settles to the profundal zone of the lake may be the main source of high-quality food for the benthos during the year (Fitzgerald and Gardner, 1993; Johnson and Wiederholm, 1992;Gardner et al., 1990; Fahnenstiel and Scavia, 1987).

Temporal and spatial separation allow coexistence of predatory cladocerans: Leptodora kindtii, Bythotrephes longimanus and Cercopagis pengoi, in southeastern Lake Michigan

ABSTRACT The predatory cladocerans, Leptodora kindtii, Bythotrephes longimanus, and Cercopagis pengoi coexist in the waters of southeastern Lake Michigan near Muskegon, Michigan. Leptodora is indigenous, whereas Bythotrephes and Cercopagis are nonindigenous and became established in 1986 and 2000, respectively. To observe seasonal changes in their abundances, and relationships to each other, cladocerans were collected from 1994 to 2008 at an offshore (110-m) site, from 1998 to 2008 at a transitional (45-m) site and from 1999 to 2008 at a nearshore (15-m) site. Bythotrephes was most abundant at the offshore site compared to Leptodora and Cercopagis. Bythotrephes peak abundances usually occurred in autumn at all sites. Cercopagis tended to be more abundant at the nearshore site, and peak densities occurred in summer. At the mid-depth site, similar abundances occurred for all three predatory cladocerans, however, the date of peak abundance was usually earliest for Cercopagis, followed by Leptodora, and latest for Bythotrephes. In recent years, 2007 and 2008, densities of all three predatory cladocerans have increased. Temperature preference, fish predation, and competition between the invertebrate predators may all be important in allowing the dominance of one species over the other seasonally or spatially.