In situ microcosms of a planktonic community were exposed to the triazine herbicide simazine. Randomly selected sets of microcosms were collected and sampled each week for three weeks (plus Week 0). Samples of ambient water were collected each week for evaluation of enclosure effects. Physical and chemical parameters were measured per microcosm. Simazine was measured at Weeks 0 and 3 only. The following organisms were preserved and quantified: phytoplankton, bacteria, and zooplankton, including ciliates, copepod nauplii, cladocerans and rotifers. Simazine decreased dissolved oxygen and pH, but increased nitrate and ammonia concentrations compared to control microcosms. A temporary decrease in temperature occurred at Week 1. Phytoplankton were differentially affected by simazine. Sensitive taxa included Trachelomonas, Glenodinium, diatoms and several species of relatively minor significance. Dinobryon and miscellaneous coccoids were not significantly affected. Phytoplankton ≥9 um were more affected by simazine than phytoplankton <9 um. Many cells <9 um may be facultative or obligate heterotrophs and not susceptible to simazine. Although data were variable, bacteria were also not affected by phytoplankton changes or simazine. Rotifers were the major zooplankters and the two dominant species, Kellicottia bostoniensis and Keratella cochlearis, were reported to graze exclusively on cells <9 um. Polyarthra vulgaris and Synchaeta pectinata also graze in this size range but are not limited to it. Copepod nauplii/copepodids were present, but adult copepods and cladocerans were rare. The tintinnid ciliate Codonella exhibited a temporary population increase during the study. Zooplankton were not affected by simazine-induced changes in the phytoplankton. Kellicottia bostoniensis was the only zooplankter affected by simazine: it had lesser mortality in higher concentrations of simazine. Possible reasons for this enhanced survival were discussed. The zooplankton (primarily rotifers) appeared to feed more on heterotrophic cells than on autotrophic cells, largely as a function of food size, and may have been more closely associated with the detrital food chain than the autotrophic food chain. / M.S.

The plankton community of the Gulf of California exist in a transition region from oceanic to neritic habitats, tropical to warm-temperature waters that is strongly influenced by a complex hydrography and bathymetry. Hyperiid Amphipods were chosen as a focus of this study to elucidate the various forces that shape the distributions of the entire community. To test how strongly hyperiids select water of a particular type (the "Water Mass" hypothesis), multiple regression analysis was applied to species' distributions and station hydrography. The quantitative results indicate that there is a strong relation between the distribution of a given hyperiid and the location of discrete water bodies in the Gulf of California. Three linked gyral currents, powered by a tidally-driven interval wave have been hypothesized to influence phytoplankton distributions in the Gulf. Through both qualitative and quantitative statistical analysis, these gyres are shown to be quite important in structuring species' distributions and have a strong effect on the character of the Gulf hydrography. Significant change in community diversity are found to occur only at the boundaries of these gyres. Many authors consider hyperiid amphipods as obligate parasites upon gelatinous zooplankton, and not worthy of distributional analyses. Both qualitative and quantitative test of this hypothesized relationship between hyperiid and "host" offer little support for the concept of hyperiid amphipods as parasites. Instead, there is considerable evidence that hyperiids are "substrate-bound," as are most amphipods, and use gelatinous zooplankton as facultative, transient hosts. Being able to switch hosts as desired, hyperiids can select for optimum conditions, and can serve to model the zooplankton community as a whole.

Zooplankton samples collected May through August, 1990 to 1996 at site M, 15 m depth in the Indiana waters of Lake Michigan were examined for microcrustaceans, rotifers, and zebra mussel (Dreissena polymorpha) veligers. Abundance numbers per m3 were combined with Coffman's (1995) data to quantify recent trends in the zooplankton.Rotifers dominated the community, with Polyarthra vulgaris, Colurella, Keratella crassa, and K. cochlearis the most prevalent species. Copepods, primarily nauplii and copepodid forms, were the next most abundant taxa. Except for 1994 and 1995, Cladocera abundance was higher than zebra mussel veliger abundance. The cladoceran assemblage was dominated by Bosmina Iongirostris, with very few Daphnia noted. A precipitous decrease in zebra mussels veligers from 1994 to 1996 warrants further investigation to determine the nature and the extent of the population decline.Regression analysis of zebra mussel veliger abundance against abiotic and biotic factors demonstrated veligers varied directly with rotifer abundance and water column temperature and indirectly with cladoceran and yellow perch (Perca flavescens) abundance. A similar regression model of rotifer abundance revealed direct correlation with calanoid, cladoceran, zebra mussel veliger, and yellow perch abundance. / Department of Biology

This research was conducted to provide baseline data on the population dynamics of microcrustacean zooplankton at Prairie Creek Reservoir in Delaware County, Indiana from 24 January 1996 to 30 December 1996. This lake is a shallow lake (average depth <8m) used for recreational fishing and water supply for the local community. No published studies of zooplankton have previously been conducted at this location. I intended to test the hypothesis that Prairie Creek Reservoir has the same microcrustacean populations of shallow lakes in this region. The findings were compared to other studies in several North American lakes and reservoirs. Biotic and abiotic factors were considered as related to the population dynamics of zooplankton. Zooplankton serve as a critical forage base for many fish species, including several of economic importance. Increases and changes in species composition of zooplankton organisms associated with eutrophic conditions may also be an indication of deteriorating water quality (Gannon 1972). The data from this research suggest that Prairie Creek Reservoir has the typical zooplankton populations of a shallow, eutrophic North American lake. The presence of relatively high numbers of organisms generally associated with eutrophic environments, such as Bosmina longirostris, Leptodiaptomus siciloides, Acanthocyclops vernalis, and others supports this idea. The data from this study will be helpful to fisheries and environmental managers and will provide a baseline for future studies. / Department of Biology

Eighteen lakes around Mt. St. Helens (MSH) were sampled for zooplankton from September '92 until September '94. Samples were enumerated and identified to the species level in most cases. Standard deviation and t-tests were performed to determine the precision of enumeration methods and replication of duplicate tows. Palatability indexes based upon predator preferences were developed and coupled with length-frequency analyses as indicators of predation pressure. The weighted means of the summer samples were then subjected to cluster analysis in an attempt to categorize lakes with respect to zooplankton community structure. Lastly, the community compositions and abundances of MSH lakes were compared to those in lakes on Mt. Rainier and Mt. Hood in an attempt to assess recovery of MSH lakes from the 1980 eruption. Results of analyses indicate the presence of three distinct groups of lakes: 1) A group of lakes with heavy predation resulting in simplified zooplankton communities dominated by Keratella, Ke/licottia, and sometimes cyclopoid species. Predation in these instances can be attributed to extremely high fish or Chaoborus abundance. 2) A second group of lakes characterized by great depth, high transparency, significant abundances of Diaptomus kenai, and moderate to light fish predation. These lakes support balanced zooplankton communities with substantial proportions of Daphnid and calanoid specimens attaining large size. Significant indications of size-specific niche differentiation among the cladocerans are notably absent from these first two groups. 3) A third group consists of lakes which appear to be more productive than the other two groups. This group has higher biovolumes of zooplankton in general, coexistence of several different sized cladoceran species, the highest diversity indices of all the lakes sampled, and moderate predation as indicated by length-frequency analysis. Two conclusions are drawn from the data. First, it appears that predation and primary productivity are both significant factors affecting the abundance and composition of MSH zooplankton communities. Additionally, these data document a significant overlap in zooplankton species in lakes near Mt. Rainier and Mt. Hood, suggesting that the zooplankton communities in lakes around MSH have recovered from the effects of the 1980 eruption.

Freshwater ecosystems are subject to a wide variety of stressors, which can have complex interactions and result in ecological surprises. Non-native fish introductions have drastically reduced the number of naturally fishless lakes and have resulted in cascading food web repercussions in aquatic and terrestrial habitats. Additional anthropogenic influences that result from increases in global airborne emissions also threaten wildlife habitat. Atmospheric nitrogen deposition has been recognized as an anthropogenic contributor to acidification and eutrophication of wilderness ecosystems. Planktonic communities have shown declines in response to predation and shifts in composition as a result of nutrient inputs and acidification, both of which are potential fates of nitrogen deposition. This study identified the response of zooplankton communities from two lakes (fish present vs. absent) in Mount Rainier National Park to manipulations simulating an episodic disturbance event in mesocosms. The experiment used a 2 x 2 factorial design with acid and nitrogen treatments. Treatments resulted in significantly elevated nitrogen and decreased pH conditions from control mesocosms over 42 days, indicating that the treatment effects were achieved. Results indicate that zooplankton communities from lakes with different food web structure respond differently to the singular effects of acid and nitrogen addition. Surprisingly, the interaction of the two stressors was related to increases in community metrics (e.g., abundance, biomass, body size, richness, and Shannon-Weiner diversity) for both lake types. This work can aid management decisions as agencies look to restore more aquatic montane habitats to their historic fishless states, and assess their abilities to recover and afford resistance to atmospheric pollution.