Joseph C. Makarewicz

STRUCTURE AND FUNCTION OF THE ZOOPLANKTON COMMUNITY OF MIRROR LAKE, NEW HAMPSHIRE'

An intensive study of the zooplankton community of Mirror Lake, New Hampshire, was undertaken over a 3-yr period. Our objectives in the lake study have included measurements of a number of attributes of the zooplankton community that integrate structure and function at the ecosystem level; among these are dispersion, biomass, productivity, respiration, and nutrient cycling. Eight species of rotifers and 3 species of cladocerans were successfully cultured. Generation time for planktonic rotifers was -8-10 days (170C). The effect of higher food levels on rotifers was to shorten generation time and to increase brood size. In cladocerans, high food levels caused an increase in length and brood size. A curvilinear relationship existed between zooplankton community respiration and temperature in Mirror Lake. Mean monthly zooplankton community respiration ranged from 96.0 kg C/ha/mo in June of 1969 to a low of 20.5 kg C/ha/mo in April of 1970. Over a 3-yr period, respiration was 79.9% of assimilation. The 0 to 4.5-m strata (;epilimnion) contributed 68.5% and 46.5% of the annual zooplankton production and biomass. Zooplankton community production ranged from 22.3 kg C/ha/yr to 29.3 kg C/ha/yr with a 3-yr mean of 25.2 kg C/ha/yr. The annual zooplankton biomass ranged from 1.4 to 2.6 kg C/ha with a 3-yr mean of 2.0 kg C/ha. A linear relationship was found to exist between net phytoplankton and zooplankton production in various lakes of the world. Ecological efficiency apparently increases with the trophic status of the lake. It is recommended that the term ecological efficiency be refined to include both autochthonous and allochthonous inputs of reduced carbon into the lake. Rotifers assume a major role in intrasystem nutrient cycling and energy transfer within the lake ecosystem. Of the total amount of P incorporated into the organic matter of zooplankton community each year, 33.5% is assimilated in rotifer tissue. The annual turnover rate of P by rotifers is 30.9 and is high compared to crustaceans (10.1). Copepods comprise 55.4% of the total zooplankton biomass. However, the copepods, with their slow growth over an entire year, represent only 19.3% of the zooplankton production, while rotifers account for 39.8% of the zooplankton production annually in Mirror Lake. Also, evidence is presented that rotifers play a major role in energy transfer in lakes of varying trophic status (oligotrophic to eutrophic).

Phytoplankton Composition and Biomass in the Offshore Waters of Lake Erie: Pre- and Post-Dreissena Introduction (1983–1993)

Phytoplankton was collected in all basins of Lake Erie during 42 cruises during the spring and summer from 1983 to 1993—a period that spans the Dreissena mussel invasion. Two potential impacts of Dreissena on the phytoplankton community of the western, central, and eastern basins of Lake Erie were evaluated: Was selective feeding occurring as observed in Saginaw Bay and were reductions in biomass evident in the offshore regions of the three basins of Lake Erie? In the western basin, significant summer decreases in Chlorophyta, Bacillariophyta, Cyanobacteria, and total phytoplankton biomass were observed after Dreissena introduction. Similarly in the spring, Bacillariophyta and total phytoplankton biomass and chlorophyll a concentrations decreased significantly. Since several divisions of phytoplankton did not decrease in phytoplankton biomass in the western basin, and spring Cyanobacteria biomass increased significantly while other divisions decreased in biomass, selective feeding on the phytoplankton community was suggested. Where significant reductions in biomass were observed in the offshore waters of the western basin, they were approximately 50% of the reduction observed at the nearshore sites in Lake Erie by other workers. Dreissena impact on the phytoplankton community of the pelagic waters of the central and eastern basin appeared to be minimal. Pre- and post-Dreissena total phytoplankton biomass and chlorophyll a concentrations were not significantly different or increased significantly after the Dreissena invasion. Biomass of several divisions of phytoplankton significantly increased after Dreissena introduction in the central and eastern basins. These included Bacillariophyta (central basin), Cyanobacteria (central and eastern basin), Chrysophyta (eastern basin), Chlorophyta biomass (eastern basin) and phytoplankton biomass (central basin) and chlorophyll a (central basin) in the spring, and Chrysophyta (eastern basin) and Cryptophyta biomass (central basin) in the summer. Generally, a reduction in phytoplankton biomass would be expected as a result of Dreissena grazing, not an increase in biomass. Dreissena-mediated changes in phytoplankton have generally occurred in shallow, well-mixed lakes, ponds, and embayments, not in deeper waters such as the central and eastern basins of Lake Erie.

Phytoplankton Biomass and Species Composition In Lake Erie, 1970 to 1987

Phytoplankton were collected at offshore sites during 33 cruises in the spring, summer, and autumn from 1983 to 1987. Forty-nine common species and varieties accounted for 83.3% of the total abundance and 83.1% of the biomass. Over the 5-year study period, the phytoplankton biomass (mean ± S.E.) of the western, central and eastern basins averaged 1.88 ± 0.12 g/m3, 1.04 ± 0.075 g/m3, and 0.63 + 0.071 g/m3, respectively. Depending on the basin, a 52 to 89% reduction in mean weighted algal biomass had occurred in the offshore waters of Lake Erie between 1970 and 1983-87. The historically highly productive western basin had a consistent decrease in biomass from 1958 to 1987. In general, occurrences of common species in 1970 and 1983-87 were similar. However, dramatic decreases in the abundances of many nuisance and eutrophic species were evident. A 70 to 98% reduction in biomass of Stephanodiscus binderanus, S. niagarae, S. tenuis, Aphanizomenon flos-aquae, and Rhodomonas minuta was observed. The decrease in biomass of the nuisance and eutrophic indicator species, the reappearance of mesotrophic species, such as Asterionella formosa and Rhizosolenia eriensis, common in the 1940s and 1950s, and the decrease in total phytoplankton biomass suggest a major improvement in the offshore waters of Lake Erie.

Chemistry of the offshore surface waters of Lake Erie: pre- and post-Dreissena introduction (1983-1993).

Major changes in ambient surface nutrient chemistry were observed after the introduction of Dreissena to Lake Erie. For example, statistically significant increases in spring soluble reactive phosphorus (SRP) (180%, 1.0 to 2.8 μg P/L), nitrate+nitrite (40%, 0.57 to 0.80 mg N/L), ammonia (131%, 15.1 to 34.9 μg N/L), silica (75%, 0.8 to 1.4 mg/L), N:P ratio and turbidity and a significant decrease in total Kjeldahl nitrogen (TKN) (25%, 0.24 to 0.18 μg N/L) were observed in the western basin from the 1983 to 1987 pre-Dreissena baseline period to the 1989 to 1993 Post-Dreissena period. In the summer, total phosphorus (TP) (13%, 20.1 to 17.5 μg P/L) and TKN (27%, 0.30 to 0.22 μg N/L) decreased, while nitrate+nitrite (122%, 0.18 to 0.40 mg N/L) and the N:P ratio increased significantly. Fewer chemical parameters changed significantly in the central and eastern basins, but major changes were observed. For example, spring SRP concentrations in the central and eastern basins increased 250% (0.8 to 2.8 μg P/L) and 92% (2.4 to 4.6 μg P/L), respectively. Silica in these basins increased 300% (0.1 to 0.4 mg/L) and 250% (0.2 to 0.7 mg/L), respectively. TKN decreased in all basins in both the spring and summer (range = 22 to 27%), while TP decreased in all basins in the summer (range = 13 to 24%) but not in the spring. Spatially, spring Post-Dreissena (1989 to 1993) ammonia, TP, and nitrate+nitrite concentrations were high in the western basin and decreased easterly, while chloride concentrations were variable with no downward or upward trend. In the central basin and eastward through the eastern basin, concentrations of ammonia, chloride, nitrate+nitrite, and total phosphorus were remarkably consistent during and between the pre- and Post-Dreissena periods. After the Dreissena invasion, a different spatial pattern of SRP, silica and phytoplankton biomass was observed. SRP and silica concentrations were high in the western basin and decreased into the central basin as in the pre-Dreissena period. Similarly, Post-Dreissena SRP and silica concentrations were low in the western portion of the central basin but then unexpectedly increased easterly by > 250% and > 1,000%, respectively, over the pre-Dreissena period. Phytoplankton biomass increased from within the west end of the western basin to a peak about halfway into the central basin, after which biomass decreased into the eastern basin. The increase in the dissolved fraction of nutrients in the western basin can be attributed to the excretion of dissolved fractions by Dreissena spp. after digestion of particulate matter, the remineralization of surficial organic sediments containing nitrogen and phosphorus-rich feces and pseudofeces and to a decrease in uptake of SRP by less abundant populations of phytoplankton in the western basin. In the western portion of the central basin, it is possible that SRP is being carried by the prevailing westerly current into the central basin stimulating phytoplankton population growth combined with minimal Dreissena grazing causing a peak in phytoplankton abundance. There does not appear to be a satisfactory explanation for the simultaneous increase in SRP and the lack of any change in phytoplankton pre- and Post-Dreissena in the eastern portion of Lake Erie.

Distribution, Fecundity, and Genetics of Cercopagis pengoi (Ostroumov) (Crustacea, Cladocera) in Lake Ontario

Two distinctive forms of cercopagids, first detected in 1998 and identified as Cercopagis pengoi and C. ossiani using taxonomic keys, were observed to co-occur in Lake Ontario. C. ossiani was the predominant form in western Lake Ontario in mid-June 1999 but was then replaced by C. pengoi-like animals over the rest of the season. Mitochondrial DNA analyses revealed that these forms were genetically identical at the ND5 gene and that they are morphologically distinctive forms of C. pengoi. In 1999, Cercopagis reached a maximum abundance of 1,759 individuals/m3 (average abundance = 281 individuals/ m3, average biomass = 5.2 mg/m3). In August, Cercopagis biomass was lowest at nearshore and embayment sites and highest at offshore sites. Body length of parthenogenetic females was lower at nearshore (1.16 mm) and embayment (1.19 mm) sites relative to offshore (1.32 mm) ones. Maximal clutch size of parthenogenetic females was 24 embryos per individual. Cercopagis has already spread to Lake Michigan and five Finger Lakes. Although waterfowl may disperse Cercopagis, these invasions likely resulted from human activities.

The Impact of Agricultural Best Management Practices on Downstream Systems: Soil Loss and Nutrient Chemistry and Flux to Conesus Lake, New York, USA

ABSTRACT Six small, predominantly agricultural (>70%) watersheds in the Conesus Lake catchment of New York State, USA, were selected to test the impact of Best Management Practices (BMPs) on mitigation of nonpoint nutrient sources and soil loss from farms to downstream aquatic systems. Over a 5-year period, intensive stream water monitoring and analysis of covariance provided estimates of marginal means of concentration and loading for each year weighted by covariate discharge. Significant reductions in total phosphorus, soluble reactive phosphorus, nitrate, total Kjeldahl nitrogen, and total suspended solids concentration and flux occurred by the second year and third year of implementation. At Graywood Gully, where Whole Farm Planning was practiced and a myriad of structural and cultural BMPs were introduced, we observed the greatest percent reduction (average = 55.8%) and the largest number of significant reductions in analytes (4 out of 5). Both structural and cultural BMPs were observed to have profound effects on nutrient and soil losses. Where fields were left fallow or planted in a vegetative type crop, reductions, especially in nitrate, were observed. Where structural implementation occurred, reductions in total fractions were particularly evident. Where both were applied, major reductions in nutrients and soil occurred. After 5 years of management, nonevent and event concentrations of total suspended solids in streams draining agricultural watersheds were not significantly different from those in a relatively “pristine/reference” watershed. This was not the case for nutrients.

A Decade of Predatory Control of Zooplankton Species Composition of Lake Michigan

Abstract From 1983 to 1992, 71 species representing 38 genera from the Calanoida, Cladocera, Cyclopoida, Mysidacea, Rotifera, Mollusca and Harpacticoida comprised the offshore zooplankton community of Lake Michigan. Our data demonstrate that the composition and abundance of the calanoid community after 1983 is not unlike that of 1960s and that species diversity of the calanoid community is more diverse than the cladoceran community in the 1990s as compared to the early 1980s. Even though the relative biomass of the cladocerans has remained similar over the 1983–1993 period, the species diversity and evenness of the Cladocera community in the early 1990s is unlike anything that has been previously reported for Lake Michigan. Cladocera dominance is centered in one species, Daphnia galeata mendotae, and only three species of Cladocera were observed in the pelagic region of the lake in 1991 and 1992. Nutrient levels, phytoplankton biomass, and the abundance of planktivorous alewife and bloater chub and Bythotrephes are examined as possible causes of these changes in zooplankton species composition. The increase in Rotifera biomass, but not Crustacea, was correlated with an increase in relative biomass of unicelluar algae. Food web models suggest Bythotrephes will cause Lake Michigans plankton to return to a community similar to that of the 1970s; that is Diaptomus dominated. Such a change has occurred. However, correlational analysis suggest that alewife and bloater chubs (especially juveniles) are affecting size and biomass of larger species of zooplankton as well as Bythotrephes .

Occurrence of Bythotrephes Cederstroemi in Lake Ontario Offshore Waters

The Cladocera Bythotrephes cederstroemi appeared first in the nearshore waters of Lake Ontario in 1985 and had established itself in the offshore by 1987. Maximum offshore abundance in Lake Ontario in 1987 was 1,321 m-2 (102 m-3). During the autumn, the period of highest abundance, Bythotrephes accounted for 2% of the zooplankton biomass. The appearance of Bythotrephes in the offshore waters of Lake Ontario in 1987 is coincidental with a 65% decline in adult alewife abundance. In 1988, alewife abundance increased 50% and Bythotrephes was not present in zooplankton tows.

Production, Biomass, and Trophic Interactions of Mysis Relicta in Lake Ontario

Mysis relicta was sampled in Lake Ontario biweekly with vertical tows from May through November 1984. Two sites were studied off Hamlin Beach State Park, one station at 35 m and the other at 100 m. At the 35-m station, total abundance ranged from near zero in late September to 207 m−2 in mid-August, with a mean abundance of 48 organisms m−2 for the study period. Average biomass and production values at the 35-m station were 0.08 g dry weight m−2 and 0.13 g dry weight m−2 per 7 months, respectively. At the 100-m station, total abundance ranged from 110 m−2in late September to 860 m−2 in mid-July, with a mean abundance of 378 m−2 for the study period. Average biomass and total production at the 100-m station was 0.68 g dry weight m−2 and 1.23 g dry weight m−2 per 7 months, respectively. Mysis relicta abundance increased from 1971 to 1984 in the offshore and nearshore of Lake Ontario. This increase in abundance may be related to the stocking of salmonines in Lake Ontario and/or to the significant die-off of adult alewives in the winter of 1983–984.

A Lakewide Comparison of Zooplankton Biomass and Its Species Composition in Lake Erie, 1983–87

From 1983 to 1987, 118 species representing 53 genera from the Calanoida, Cladocera, Cyclopoida, Rotifera, and Harpacticoida comprised the offshore zooplankton community of Lake Erie. Twenty-eight common species plus their juvenile stages accounted for 94.4% of the total biomass and 92.5% of the total abundance. The eutrophic indicators Brachionus caudatus, B. calyciflorus, B. angularis, Filinia longiseta, Trichocerca multicrinis, and Trichocerca cylindrica had abundances restricted to or significantly higher in the western basin. Between 1983 and 1987, essentially smaller species (Daphnia galeata mendotae, Daphnia retrocurva, Eubosmina coregoni, Bosmina longirostris, and Diaphanosoma leuchtenbergianum) were dominant as in 1948 and 1970. In 1984 a large cladoceran new to Lake Erie, Daphnia pulicaria (mean = 755 individuals/m3, maximum abundance of 3,752/m3), became prevalent. Another cladoceran, Bythotrephes cederstroemi, was first observed in Lake Erie in 1985 and continued to be present in 1986 and 1987. Cyclops vernalis, present throughout the lake in 1967, was observed only in the western basin during 1983 to 1987 similar to the distribution in the 1930s. Rotifer composition in 1967 and the 1983-87 period was similar. Cladocera abundance during the 1983-87 period was comparable to abundances observed in the’30s and’40s. Zooplankton abundance and biomass decreased from the western basin to the eastern basin except in 1985. This decrease correlated with the decrease in phytoplankton biomass (r = 0.81).