Phytoplankton Composition,Abundance, and Distribution: Nearshore Lake Ontario and Oswego River and Harbor

Phytoplankton samples were collected during July, August, and October of 1981 from 16 stations located in the nearshore of Lake Ontario and in the Oswego Harbor and river. Phytoplankton assemblages observed represented many species widely recognized as associated with eutrophic environments and environments possessing high chloride levels. Picoplankton and diatoms were the dominant groups on a numerical and bioyolume basis, respectively. The eutrophic species Stephanodicus tenuis, Fragilaria capucina, and Scenedesmus spp. were present in the Oswego Harbor and river with substantially higher abundances than in the nearshore region of Lake Ontario. A decrease in abundance of the historically prevalent Asterionella and Tabellaria and an increase in cryptomonads is suggested for the Oswego area. Halophilic species dominated the diatom assemblage of the Oswego Harbor and mouth of the Oswego River. The abundance of halophilic species was correlated with high conductivity and chloride levels. Cyclotella atomus, Cyclotella cryptica, and Cyclotella meneghiniana, the dominant halophilic species at the harbor and river stations, accounted for 36.6% of the total diatom abundance.     

Rapid Increase and Subsequent Decline of Zebra and Quagga Mussels in Long Point Bay,Lake Erie: Possible Influence of Waterfowl Predation

Distribution and density of two introduced dreissenid species of mollusks, the zebra mussel Dreissena polymorpha and quagga mussel D. bugensis, were monitored in the Inner Bay at Long Point, Lake Erie, 1991–1995. Since populations of certain waterfowl species have been reported to alter their dietary intake and migration patterns in response to the ready availability of zebra mussels, the percent occurrence of zebra mussels in the diet of 12 duck species (552 birds) was studied concurrently, and several spring and fall aerial waterfowl surveys were flown between 1986 and 1997 (n = 75), to document changes in duck populations at Long Point. The first reproductive population of zebra mussels on the bay most likely appeared in 1990. After an initial rapid increase in density and colonization of the Inner Bay, zebra mussels began to steadily and consistently decline in absolute numbers, density per station and occupied area. Mean density per station in 1995 was 70% less than in 1991, the first year of rapid colonization, and 67% less than in 1992, the year of peak abundance in the bay (P < 0.05). Occupied area peaked in 1992, with 80% of sampling stations supporting mussels; the following 3 years showed consistent declines in the proportion of stations supporting mussels: 1993 = 75.9%, 1994 = 63.2% and 1995 = 57.1% (P < 0.05). Mussels in size class 0 to 5 mm were most abundant in 1991, 1993 and 1995, whereas those in size class 6 to 10 mm predominated in 1992 and 1994 (P < 0.05). Very few mussels over 15 mm were found. Lesser Scaup Aythya affinis (75.4 to 82.5 % occurrence), Greater Scaup A. marila (66.7 to 81.5 % occurrence), and Bufflehead Bucephala albeola (46.7 to 60 % occurrence) were the only three waterfowl species that consistently incorporated zebra mussels in their diet, and the mussel decline coincided with a substantial increase in the populations of these species at Long Point. Waterfowl days for Lesser and Greater Scaup combined increased rapidly from 38,500 in 1986 (prior to the zebra mussel colonization of Long Point) to 3.5 million in 1997 (P = 0.012). Bufflehead days increased from 4,700 to 67,000 during the same period (P = 0.001). Oligotrophication of Lake Erie, through reduced plankton and chlorophyll concentrations, has occurred since the invasion of zebra mussels, probably a result of filtering activities of introduced mussels. While a reduction in plankton availability may have contributed to the zebra mussel decline, high rates of waterfowl predation probably had the most substantial effect on mussel densities at Long Point. Waterfowl predation also probably influenced the size structure of the zebra mussel population, since waterfowl are size-selective foragers, and increased water clarity would have facilitated their ability to select preferred medium and large size classes of mussels. Quagga mussels, which were first detected in 1993, experienced a decline in both density and area occupied over the next two years. Quagga mussels rarely attached to soft substrates, and their decline is possibly related to the decline of suitable hard substrates, such as zebra mussels, as well as to predation by waterfowl.     

Impact of Round Goby Predation on Zebra Mussel Size Distribution at Calumet Harbor,Lake Michigan

The size distribution of zebra mussels atop (exposed to predation) and beneath (protected from predation) rocks were compared at a round goby infested site (Calumet Harbor, IL/IN) and round goby-free site (Evanston, IL) in southern Lake Michigan. The largest zebra mussels were atop rocks at both sites and those from Calumet Harbor were significantly larger than those from Evanston. The smallest zebra mussels were beneath the rocks at both sites and those from Calumet Harbor were smaller than those from Evanston. Nearly all zebra mussels from Calumet Harbor rock tops were larger than the size range preferred by round gobies in published laboratory experiments. Conversely, most of the zebra mussels from rock tops from Evanston were within the size range preferred by round gobies. In a field experiment, glass sheets colonized with zebra mussels from Evanston rock tops were exposed to round goby predation. Predation was videotaped to determine what size zebra mussels were eaten by round gobies. Smaller zebra mussels were consumed more frequently and surviving zebra mussels were either very large or had refuge in the space between larger zebra mussels. The study showed that more effort is required to remove larger zebra mussels than smaller zebra mussels. The results indicate that, while round gobies prefer smaller zebra mussels, the underside of rocks, as well as proximity to larger zebra mussels, can provide refuge for the smaller zebra mussels. Hence round gobies are unlikely to totally remove zebra mussels from a habitat and their impact will likely vary between habitats.     

Occurrence and Food Habits of the Round Goby in the Profundal Zone of Southwestern Lake Ontario

Little is known about the ecology of round goby (Neogobius melanostomus), an invasive benthic fish, in the profundal zone of the Great Lakes. In April 2002–2005 we caught increasing numbers of round gobies with a bottom trawl in the 45–150 m depth range of southwestern Lake Ontario. In 2005, we examined gut contents of 30 round gobies from each of three depths, 55, 95, and 130 m, and qualitatively compared gut contents with density of benthic invertebrates determined by Ponar grabs. Round goby guts contained mostly Dreissena spp. and opposum shrimp, Mysis relicta (Mysis); the frequency of occurrence of dreissenids in guts decreased with depth, whereas the frequency of occurrence of Mysis in guts increased with depth. Abundance of these invertebrates in the environment followed the same pattern, although dreissenids of optimum edible size (3–12 mm) were still abundant (1,373/m²) at 130 m, where round gobies primarily consumed Mysis, suggesting that round gobies may switch from dreissenids to more profitable prey when it is available. Other food items were ostracods and fish, with ostracods generally eaten by smaller round gobies and fish eaten by larger round gobies. Occurrence and increasing abundance of round gobies in the profundal zone and predation on Mysis by round goby could have far-reaching consequences for the Lake Ontario fish community.     

Alternative Ecological Pathways in the Eastern Lake Ontario Food Web—Round Goby in the Diet of Lake Trout

Round goby (Neogobius melanostomus) range expansion and their possible inclusion in the diet of lake trout (Salvelinus namaycush) were investigated. Fish community index bottom trawls in eastern Lake Ontario (Kingston basin) during summer 2003 and 2004 indicated the presence of the round goby at relatively low densities (3.72 × 10^–2 ± 5.24 × 10^–3 fish/m²) in depths up to 30 m. Lake trout (mean fork length = 585 ± 78 mm and mean weight = 2,770 ± 1,134 g) stomach contents showed round goby to be the second most abundant diet item at almost 20% by number (36% by mass). Round goby ingested by lake trout ranged in total length from 50 to 110 mm. The most important prey species in terms of abundance (68%) and mass (56%) was alewife (Alosa pseudoharengus). Alewives were the most important diet item for all sizes of lake trout sampled, except those in the 550 - 650 mm size class, which ingested more round goby by mass than alewife. Round goby range expansion to deep water and prominence in the diet of lake trout signal significant change in the eastern Lake Ontario food web.     

Changes in Seasonal Nearshore Zooplankton Abundance Patterns in Lake Ontario following Establishment of the Exotic Predator Cercopagis pengoi

Cercopagis pengoi, a zooplanktivore first discovered in Lake Ontario in 1998, may reduce availability of prey for planktivorous fish. Cercoapgis pengoi is most abundant in late summer and fall. Therefore, we hypothesized that abundance of small zooplankton (bosminids and cyclopoids) species would decrease at that time. To determine if the establishment of C. pengoi was followed by changes in the zooplankton community, seasonal patterns in nearshore zooplankton collected from May to October 1995–2000 were examined. Early summer density of small zooplankton was similar in all years while late summer and fall densities were significantly lower in 1998–2000 than in 1995–1997. The declines of small zooplankton coincided seasonally with the peak in C. pengoi density. Other possible causes for the observed changes in small zooplankton are less likely. High levels of fish predation should have resulted in smaller zooplankton in 1998–2000 than in 1995–1997 and larger declines in Daphnia than other groups. This was not observed. There was no significant decline in chlorophyll-a concentrations or changes in temperature between 1995–1997 and 1998–2000. Therefore, the declines in density of small zooplankton were most likely the result of C. pengoi predation. The effect of C. pengoi establishment on alewives is increased competition for zooplankton prey but C. pengoi has replaced a portion of the zooplankton biomass and adult alewife diet formerly dominated by Diacyclops thomasi and Bosmina longirostris.     

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.     

The Effects of Zebra Mussels on the Lower Planktonic Foodweb in Lake Champlain

Selective grazing by zebra mussels has altered phytoplankton communities in many North American lakes, but the specific changes are not the same in each ecosystem. Because of this variation in response, we investigated the impacts of zebra mussels on the plankton community of Lake Champlain with two objectives: first to determine whether zebra mussels increased the dominance of potentially toxic cyanobacteria in the phytoplankton, and second to explore the impact of zebra mussels on protozoans, rotifers, copepod nauplii, and other microzooplankton in the lower food web. Experiments were conducted in 200-L mesocosms filled with Lake Champlain water filtered through a 150-μm sieve to remove macrozooplankton. Zebra mussels were added to half of the mesocosms while the others were maintained as controls. Over a 96-hour experimental period, we tracked nitrogen and phosphorus concentration, chlorophyll a, microcystin concentration, and both phytoplankton and microzooplankton composition and abundance. We found an increase in SRP and total nitrogen concentration and a decrease in the ratio of TN:TP in the zebra mussel treatments over time. Microcystin was undetectable throughout the experiment using the ELISA assay. Phytoplankton biovolume, including cyanobacteria biovolume, declined significantly in the zebra mussel treatments, as did rotifer, protozoan and nauplii abundance. By both direct (consumption) and indirect (altered nutrient availabilities and increased competition) means, zebra mussels clearly seem capable of strongly influencing the lower planktonic foodweb in the many shallow water habitats of Lake Champlain.     

Benthic Invertebrate Community Responses to Round Goby (Neogobius melanostomus) and Zebra Mussel (Dreissena polymorpha) Invasion in Southern Lake Michigan

The round goby (Neogobius melanostomus Pallas), a fish native to eastern Europe, recently has become established in southwestern Lake Michigan. Because round gobies prey on zebra mussels (Dreissena polymorpha Pallas) and other benthic invertebrates, the effects of round gobies on invertebrates within zebra mussel colonies was investigated. Using a 2 × 3 factorial design, the effects of round gobies (present or absent) and zebra mussel densities (zero, low, and high) on non-mussel invertebrates was examined. Ten ceramic tiles of each mussel density were colonized in the laboratory and then anchored in Calumet Harbor, IL for 10 weeks. Round gobies had access to half the tiles while half were covered with coarse mesh screening that excluded round gobies, but allowed invertebrates to move into and out of the exclosures. Low and high zebra mussel density tiles supported significantly greater numbers of non-mussel invertebrates (p < 0.001) than zero density tiles, particularly amphipods (p < 0.001), hydroptilid caddisflies (p < 0.05), isopods (p < 0.05), and chironomids (p < 0.001). Chlorophyll a concentrations were highest (p < 0.001) at low zebra mussel densities. The presence of round gobies significantly reduced densities of total non-mussel invertebrates (p < 0.01) and leptocerid caddisflies (p < 0.05), resulting in a significant increase in chlorophyll a (p < 0.01) concentrations. A significant zebra mussel density x round goby interaction showed that total invertebrate biomass responded positively to the combined effect of high zebra mussel density and round goby absence. These results demonstrate that round gobies and zebra mussels are altering benthic invertebrate community structure and algal resources in nearshore rocky areas of southwestern Lake Michigan.     

Univariate Step-trend and Multivariate Assessments of the Apparent Effects of P Loading Reductions and Zebra Mussels on the Phytoplankton of the Bay of Quinte,Lake Ontario

The purpose of this work was to identify the apparent effects on the phytoplankton of the Bay of Quinte (northeastern Lake Ontario) of two environmental alterations—a point-source phosphorus loading reduction in the late 1970s and the establishment of invading zebra mussels, Dreissena spp., about 1995. After the P loading reduction, there were significant declines in biovolume of total phytoplankton (−51%), Chlorophyceae (−66%), Dinophyceae (−58%), Bacillariophyceae (−56%), Cryptophyceae (−52%), and Cyanophyceae (−26%). The cyanophyte decline was attributed mainly to non-nitrogen-fixing species. Two dominant diatom genera, Aulacoseira (Melosira) and Stephanodiscus were 53 and 78% lower, respectively. In contrast, after Dreissena spp. establishment, total phytoplankton, Bacillariophyceae, Chrysophyceae, and Cryptophyceae biovolumes did not change significantly; however, total Chlorophyceae, Cyanophyceae, and Dinophyceae showed declines of 42, 51, and 55%, respectively. There were major declines in the diatoms Stephanodiscus (−82%), Synedra (−84%), and Tabellaria (−98%) after the arrival of zebra mussels. Average biovolume of the cyanophyte Microcystis (a potential “bloom”-forming toxin producer) increased 13-fold after the establishment of zebra mussels. Multivariate analyses (ANOSIM tests of within-vs between-group differences), based on phytoplankton community structures defined by the quantitative contributions from 20 phytoplankton taxa, revealed statistically significant differences among communities representing pre- and post P control and pre-and post Dreissena conditions. The shifts in phytoplankton community composition between the identified time periods were attributed to several different taxa, whose relative contributions to the inter-community dissimilarities differed between the upper and lower bay stations.     

Modeling the Role of Zebra Mussels in the Proliferation of Blue-green Algae in Saginaw Bay,Lake Huron

Between 1991 and 1993, Saginaw Bay experienced an invasion by zebra mussels, Dreissena polymorpha, which caused a significant perturbation to the ecosystem. Blooms of Microcystis, a toxin-producing blue-green alga, became re-established in the bay after the zebra mussel invasion. Microcystis blooms had all but been eliminated in the early 1980s with controls on external phosphorus loadings, but have re-occurred in the bay most summers since 1992. An apparent paradox is that these recent Microcystis blooms have not been accompanied by increases in external phosphorus loadings. An ecosystem model was used to investigate whether the re-occurrence of Microcystis could be due to changes caused by zebra mussels that impacted phytoplankton community structure and/or internal phosphorus dynamics. The model was first used to establish baseline conditions in Saginaw Bay for 1991, before zebra mussels significantly impacted the system. The baseline model was then used to investigate: (1) the composite impacts of zebra mussels with average 1991–1995 densities; (2) sensitivity to changes in zebra mussel densities and external phosphorus loadings; and (3) three hypotheses on potential causative factors for proliferation of blue-green algae. Under the model assumptions, selective rejection of blue-green algae by zebra mussels appears to be a necessary factor in the enhancement of blue-green production in the presence of zebra mussels. Enhancement also appears to depend on the increased sediment-water phosphorus flux associated with the presence of zebra mussels, the magnitude of zebra mussel densities, and the distribution of zebra mussel densities among different age groups.     

Distribution and Abundance of Larval Fish in the Nearshore Waters of Western Lake Huron

Ichthyoplankton was collected at 17 nearshore (bottom depth ≥5 m but ≤10 m) sites in western Lake Huron during 1973–75 with a 0.5-m net of 351-micron mesh towed at 99 m/min. Larvae of rainbow smelt (Osmerus mordax) dominated late spring and early summer catches and larvae of alewives (Alosa pseudoharengus) the midsummer catches. Larval yellow perch (Perca flavescens) were caught in early summer but were rarely the dominant species. The time of spawning and hatching, and thus occurrence of larvae, differed between areas but was less variable for alewives than for yellow perch. The appearance of larvae in Saginaw Bay was followed successively by their appearance in southern, central, and northern Lake Huron. Rainbow smelt were most abundant in northern Lake Huron and yellow perch and alewives in inner Saginaw Bay. Densities of either rainbow smelt or alewives occasionally exceeded 1/m³, whereas those of yellow perch never exceeded 0.1/m³. Abundance of alewives was usually highest 1 to 3 m beneath the surface and that of rainbow smelt 2 to at least 6 m beneath the surface. Important nursery areas of rainbow smelt were in bays and off irregular coastlines and those of yellow perch were in bays. All nearshore waters seemed equally important as nursery areas of alewives.     

Abundance and Biomass of the Meiobenthos in Nearshore Lake Michigan with Comparisons to the Macrobenthos

The meiobenthos of nearshore southeastern Lake Michigan was quantified by taking cores from three depths (11,17, and 23 m) at monthly intervals from May to November 1976–79. Total meiobenthic abundance ranged from 69,700/m² to 1,300,000/m² and total biomass ranged from 0.03 to 0.87 g/m². Nematodes accounted for 80% of all individuals and 66% of the biomass. Most of the major groups peaked in late spring/summer, but some peaked in early spring and fall. With the exception of nematodes, mean annual densities varied from two-fold to twelve-fold at a given station. Harpacticoids, tardigrades, and ostracods tended to be more abundant at the deeper depths, while cyclopoids and cladocerans were less abundant. There was no consistent relationship between sampling depth and the abundance of nematodes and rotifers. Temporal and spatial variation in many of the groups appeared related to changes in the amount of bottom detritus. The overall macrobenthos:meiobenthos biomass ratio was 15:1 and ranged from 5:1 to 45:1 on an annual basis.     

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.     

Invasion History,Proliferation, and Offshore Diet of the Round Goby Neogobius melanostomus in Western Lake Huron,USA

We used data from three trawl surveys during 1996–2003 to document range expansion, population trends, and use of offshore habitats by round gobies in the U.S. waters of Lake Huron. Round gobies (Neogobius melanostomus) were not detected in any survey until 1997, but by 2003 they had been recorded at 18 of the 28 sites sampled. The only areas not colonized were offshore habitats in northern Lake Huron. Round goby abundance increased during 1997–2001, thereafter overall abundance either increased (offshore) or became variable (near shore and Saginaw Bay). Mean lengths varied among surveys primarily due to high abundance of age-0 gobies in Saginaw Bay samples. Round gobies were found up to 34 km offshore at depths of 73 m. Round gobies consumed a wide range of invertebrate prey, but focused on dreissenids in shallow water (27–46 m), and native invertebrates at greater depths. The pattern of round goby dispersal was consistent with a pattern of simultaneous initial introductions at shipping ports followed by natural dispersal, and lake wide population size has probably not stabilized.     

Distribution,Abundance, and Range of the Round Goby,Apollina melanostoma,in the Duluth-Superior Harbor and St. Louis River Estuary, 1998–2004

Round gobies were first discovered in the Duluth-Superior Harbor, Lake Superior, in 1995. Anecdotal sightings by anglers and others suggested that the infestation was growing and expanding; however, direct evidence of the distribution and expansion rate in the harbor was largely unknown. Distribution and range of the round goby, Apollonia melanostoma, (formerly Neogobius melanostomus) was assessed using bottom trawl sampling throughout the Duluth-Superior Harbor, and portions of the lower St. Louis River from 1998 to 2004. Previous to 1998, round gobies only were reported to occupy the harbor between the two shipping entries (river kilometer 1 to 7). By 2004, they expanded throughout the harbor and upstream to river kilometer 13, but remained absent in western Lake Superior. The number of round gobies captured per 5 minutes of trawling (catch per unit effort, CPUE) increased from less than 1 fish in 1998 to an average 5.4 ± 1.2 SE fish in 2004, indicating a large increase in the population. The median yearly fish total length varied from 56.0 to 81.5 mm and wet weight varied from 2.3 to 7.0 g. As nest guarding male round gobies were located in rocky habitats inaccessible to trawling, the initial years were dominated by female round gobies with a 16:1 female to male ratio, but by 2002 the maximum ratio was 2:1. The ratio change may be indicative of the increasing population forcing males from their preferred rocky habitat onto open substrates that were more accessible to trawling.     

Recruitment Failure of Mottled Sculpin Cottus bairdi in Calumet Harbor,Southern Lake Michigan,Induced by the Newly Introduced Round Goby Neogobius melanostomus

This study documents a local extinction of mottled sculpins, apparently due to round gobies, and presents data pertinent to the mechanism of extinction. Mottled sculpins, Cottus bairdi, were assessed using SCUBA standardized diving transects during the invasion of the round goby, Neogobius melanostomus, into Calumet Harbor, southern Lake Michigan. Laboratory stream studies were conducted in which gravid male and female mottled sculpins were allowed to nest, then were exposed to one male round goby. Diet studies were conducted to assess the potential for competition for food at small sizes of both species. The SCUBA surveys showed that mottled sculpin populations rapidly declined, after the first round gobies were found in the area in 1994, despite the presence of a well established population prior to the round goby arrival. Mottled sculpins have been almost totally extirpated from the area in 1998 due to three proposed mechanisms: competition for food resources at small sizes, for space at intermediate sizes, and for spawning space at large sizes. The laboratory stream study confirmed that round gobies interfered with nest-guarding male mottled sculpins, seized their spawning shelters, changed to spawning coloration in preparation for spawning, and caused near loss of all the mottled sculpin eggs. It is concluded that recruitment failure and subsequent demise of mottled sculpins was most likely caused by spawning interference by round gobies.     

Changes in Marsh Area Along the Canadian Shore of Lake Ontario

Baseline marsh area is approximated for the Canadian shoreline of Lake Ontario west of the Bay of Quinte using maps dated from 1789 to 1962. The measured difference between that and the area of marsh indicated on topographical maps dated from 1977 to 1979 reveals a net loss of about 4,000 acres or 57% of the baseline area for 38 marshes. Using extrapolation for another 24 marshes, the net loss for 62 marshes is estimated at approximately 4,745 acres or 43% of the baseline area. Losses were particularly high for marshes west of and including Toronto. Estimated net loss was lower in absolute terms, but similar in proportion to that from a previous study which was based on waterfowl production habitat. Relative losses of marsh area along the Canadian shoreline of Lake Ontario are consistently higher than previous estimates for inland wetlands in counties fronting the same shoreline. This and other studies suggest that heavily settled Great Lakes environments have generally lost up to 75% of their baseline wetland area and almost 100% in a few cases. The use of historic maps provides an early baseline estimate of wetland area, in some cases the only estimate possible, but it is less subject to standardization, probably reflects emergent vegetation, and is somewhat cumbersome.     

A Comparison of Total Phosphorus,Chlorophyll a,and Zooplankton in Embayment,Nearshore, and Offshore Habitats of Lake Ontario

Lower trophic levels were compared in embayment, nearshore, and offshore habitats of Lake Ontario, 1995 to 1997, in the context of oligotrophication and invasion of dreissenid mussels. Total phosphorus (TP), chlorophyll a (chl a), Secchi disk depth, temperature, and zooplankton were measured to spatially and temporally contrast these habitats and to test for recently hypothesized “decoupling” of chl a from TP (lower than predicted chl a per unit TP, consistent with dreissenid mussel grazing). The embayment habitat had higher concentrations of TP and chl a, greater volumetric zooplankton density and biomass, and higher springtime water temperatures than both nearshore and offshore habitats, while overall areal zooplankton biomass was highest in the offshore. Furthermore, concentrations of TP and volumetric zooplankton density in nearshore habitats are now more similar to the offshore pelagia than they were three decades ago. Finally, a lower yield of chl a per unit TP was found in nearshore habitat compared to offshore and embayment habitats. The current lower yield of chl a per unit TP in nearshore habitat can be attributed more to Dreissena than to erosion and/or resuspension of sediments.     

Sculpins and Crayfish in Lake Trout Spawning Areas in Lake Ontario: Estimates of Abundance and Egg Predation on Lake Trout Eggs

Crayfish (Orconectes spp.) and sculpins (Cottus spp.) were collected at eight lake trout spawning reefs in Lake Ontario to assess abundance and potential to consume lake trout eggs. Abundance of crayfish ranged from a high of 9.5/m² in eastern Lake Ontario to 0/m² in western Lake Ontario where the absence or near absence at four reefs sampled was attributed to cold water upwelling. Sculpin abundance ranged from 4.2 to 50.1/m². Mean daily egg consumption (eggs/stomach) for sculpins 50 to 75 mm in length, ranged from 0 to 0.9 but differences among reefs were not significant. At one reef, significantly more eggs (2.5 eggs/stomach) were consumed by large sculpins (> 75 mm) than by small (44–49 mm) sculpins (0.2 eggs/stomach). Estimated egg consumption (eggs/stomach/m²) for sculpins > 43 mm for the eight reefs for the period between estimated date of peak lake trout spawning and a standardized 30-d period post spawning, ranged from 0 to 496 eggs/m² consumed or from 0 to 54% of estimated egg abundance. No lake trout eggs were found in crayfish stomachs, because of their mode of feeding. Estimated egg consumption by crayfish was indirectly estimated from a relationship developed between carapace length and egg consumption using published literature and experimental work. Using this procedure, estimated egg consumption by crayfish for a standardized 30-d period after the date of peak spawning ranged from 0 to 65 eggs/m² consumed, or from 0 to 82% of potential egg abundance for the eight reefs. At low egg abundance (< 100/m²), the density of crayfish and sculpin observed in Lake Ontario could result in sufficient egg consumption to cause almost 100% mortality of lake trout eggs. At higher egg abundance, however, mortality due to crayfish and sculpins appears to be relatively low. Deposition was sufficiently low at 5 of 8 sites to suggest the possible importance of sculpin and crayfish predation on lake trout recruitment failure in Lake Ontario.