Exacerbated deep-water hypoxia, brought about by anthropogenic-derived dissolved organic carbon (DOC) input, has potentially detrimental impacts on organisms residing in freshwater ecosystems. Oxygenation of the water column is a potential management strategy to mitigate this impact, and used to alleviate DOC induced oxygen depletion. In this thesis, I investigated the effects of hypolimnic oxygenation on benthic macroinvertebrate and zooplankton biomass and size assemblages in an experimental dystrophic lake. The experiment, which took place between 2017-2020 in a dystrophic brown-water lake consisting of two interconnected basins, was performed according to a Before-After-Control-Impact design. One basin was continuously monitored under reference conditions while the oxygen concentrations of the other basin was experimentally increased halfway through the experimental period. Macroinvertebrates were sampled from the epi-, meta- and hypolimnion while zooplankton were sampled from throughout the water column (0-6 m) and, during 2020, also the hypolimnion (3-6 m) separately. Collected macroinvertebrate and zooplankton samples were analyzed for biomass, chironomid body lengths and zooplankton community compositions per basin and treatment. Mean benthic biomass, chironomid biomass and chironomid body length increased in response to oxygenation in the hypolimnion but no effect was found in the epi- or metalimnion habitats. Observed biomass responses were mainly driven by the increase in chironomid biomass. No differences were found in neither zooplankton community assemblages nor biomass following oxygenation. These results suggest that the lack of oxygen-rich habitats act as a major limiting factor for hypolimnic macroinvertebrates and especially for chironomids. Zooplankton, on the other hand, appear to be limited by resource availability as opposed to the abundance of suitable, oxygen-rich habitats only. The results suggest that oxygenation may have clear benefits for deep-water macroinvertebrates. However, as a management tool, it is ineffective as a means to increase zooplankton abundances.

Abstract Eutrophication and salinization are both major threats to lake ecosystems and the contaminants can alter the structure of freshwater lake ecosystems, resulting in a loss of biodiversity and ecosystem services. Zooplankton and phytoplankton are key components in all aquatic ecosystems as they maintain a healthy quality of the water and they are also an important part of the food chain. Studies show that eutrophication and salinization have severe negative effects on the zooplankton community. This study compares zooplankton communities in 36 mesocosms of eutrophicated waters with nine treatments of salinity and light exposure. The dataset was provided by Associate Professor Lovisa Lind-Eirell, using a randomized design with three levels salt concentrations (15, 250, and 1000 mg of chloride/L of highly eutrophicated waters, in combination with three levels (10%, 35% and 70%) of ambient sunlight. The hypothesis and expected results were a significant decline in zooplankton species richness and population sizes, in treatments with high salinity and low light exposure. The hypothesis was partly correct as nauplii and copepod populations declined treatments of high salinity, whereas cladocerans and rotifers seem to have a saline tolerance which corresponds with other studies. More research is required regarding the combined effects pollutants have on our environment and our ecosystem services. If we wish to keep freshwater ecosystems healthy, prevent further loss of diversity and keep utilizing ecosystem services such as drinking water of good quality, a change of human actions is necessary. More research of interactive effects is needed to better understand the extent of the damage to freshwater ecosystems. Actions to prevent further loss of biodiversity and ecosystem services are necessary but the threats have been acknowledged and finding solutions is currently a work in progress on a global scale. / Sammanfattning Idag är övergödning och förhöjda kloridnivåer två av de största hoten mot ekosystemen i sötvattensjöar. Föroreningarna kan förändra strukturen i dessa ekosystem, vilket leder till förlust av ekosystemtjänster, såsom drickbart vatten. Zooplankton och växtplankton är nyckelkomponenter i alla akvatiska ekosystem eftersom de upprätthåller vattenkvaliteten och utgör en viktig del av näringskedjan. Studier visar att övergödning och förhöjda kloridnivåer har negativa effekter på zooplankton. Denna studie jämför zooplankton i 36 vattentankar med eutrofa vatten med nio behandlingar av varierande nivåer av salthalt och ljustillgänglighet. Datauppsättningen tillhandahölls av docent Lovisa Lind-Eirell på Karlstad universitet och utgjordes av en randomiserad design som bestod av tre olika saltkoncentrationer (15, 250 och 1000 mg klorid/L) av starkt eutrofierade vatten,  i kombination med tre ljusnivåer (10 %, 35 % och 70 %). Hypotesen och det förväntade resultatet var en signifikant minskning av både arter och populationer av zooplankton i behandlingar med hög salthalt och låg ljustillgänglighet. Hypotesen var delvis korrekt eftersom nauplii och copepoda populationer minskade i behandlingar med höga kloridhalter, medan cladocera och rotifera verkade ha en tolerans mot höga kloridhalter. Mer forskning om interaktiva effekter behövs för att bättre förstå omfattningen av skadorna på ekosystemen i våra sötvatten. Åtgärder för att förhindra ytterligare förlust av biologisk mångfald och ekosystemtjänster är nödvändiga, och att hitta lösningar på dessa problem pågår för närvarande på global skala.

Local adaptation and phenotypic plasticity are main mechanisms of organisms’ resilience in changing environments. Both are affected by gene flow and are expected to be weak in zooplankton populations inhabiting large continuous water bodies and strongly affected by currents. Lake Baikal, the deepest and one of the coldest lakes on Earth, experienced epilimnion temperature increase during the last 100 years, exposing Baikal’s zooplankton to novel selective pressures. We obtained a partial transcriptome of Epischura baikalensis (Copepoda: Calanoida), the dominant component of Baikal’s zooplankton, and estimated SNP allele frequencies and transcript abundances in samples from regions of Baikal that differ in multiyear average surface temperatures. The strongest signal in both SNP and transcript abundance differentiation is the SW-NE gradient along the 600+ km long axis of the lake, suggesting isolation by distance. SNP differentiation is stronger for nonsynonymous than synonymous SNPs and is paralleled by differential survival during a laboratory exposure to increased temperature, indicating directional selection operating on the temperature gradient. Transcript abundance, generally collinear with the SNP differentiation, shows samples from the warmest, less deep location clustering together with the southernmost samples. Differential expression is more frequent among transcripts orthologous to candidate thermal response genes previously identified in model arthropods, including genes encoding cytoskeleton proteins, heat-shock proteins, proteases, enzymes of central energy metabolism, lipid and antioxidant pathways. We conclude that the pivotal endemic zooplankton species in Lake Baikal exists under temperature-mediated selection and possesses both genetic variation and plasticity to respond to novel temperature-related environmental pressures.

Copepods in high elevation lakes and ponds in Colorado are exposed to significant levels of ultraviolet radiation (UV), necessitating development of UV avoidance behavior and photoprotective physiological adaptations. The copepods are brightly pigmented due to accumulation of astaxanthin, a carotenoid which has photoprotective and antioxidant properties. Astaxanthin interacts with a crustacyanin-like protein, shifting its absorbance from 473 nm (hydrophobic free form, appears red) to 632 nm (protein-bound complex, appears blue). In six sites in Colorado, habitat-specific coloration patterns related to carotenoprotein complex have been observed. The objective of this study was to determine whether pigment accumulation or carotenoprotein expression has a greater effect on resistance to UV exposure. For each site, copepod tolerance to UV was assessed by survivorship during UV exposure trials. Average UV exposure was determined for each habitat. Astaxanthin profiles were generated for copepods in each site. Ability to withstand UV exposure during exposure trials was significantly different between color morphs (p < 0.0001). Red copepods were found to tolerate 2-fold greater levels of UVB than blue or mixed copepods. Additionally, red copepods have much higher levels of total astaxanthin than blue or mixed copepods (p < 0.0001) and receive a higher daily UV dose (p < 0.0003). Diaptomid carotenoprotein sequence is not homologous with that of other crustaceans in which crustacyanin has been characterized which prevented quantification of carotenoprotein transcript expression. Overall, diaptomid color morph may be an important indicator of UV conditions in high elevation lentic ecosystems.

Zooplankton of the littoral and limnetic zones of Bear Lake, Idaho- Utah, were collected over a 17 -month period. Twenty-three species of zooplankton were recorded, as well as nine other species represented by both flora and fauna. At the time of sampling 17 physical and chemical parameters were a lso measured. Methods of analysis for the plankton and the environment are described and discussed. The objectives of this project were fourfold: To establish a record of the current zooplankton population before changes due to the increase o f organic nutrients occur. To compile a qualitative and quantitative standard against which future populations can be compared. To assess the source and the amount of present nutrients which are being contributed to the lake. To investigate the effect the present addition of nutrients is having on zooplankton productivity. The physical and chemical analysis of the Bear Lake waters showed a number of interesting trends. The measurement of total dissolved solids, when compared with previous investigations, showed a decline: i.e., it appears that a great dilution had taken place in the lake waters since 1912. The measurement of salinity revealed two chemical patterns in Bear Lake waters. First: chloride, sulfate, sodium, and potassium showed a decrease in concentration on a long-term basis. Second, calcium showed a slight rise with a leveling off, and magnesium showed a decline with a leveling off over the same period. Calcium is being precipitated as calcium carbonate, and the removal of sodium, potassium , sulfate, magnesium, and chloride is achieved through the pumping of lake waters outside the basin. Nitrogen was measured with respect to three of its forms--ammonia, nitrite, and nitrate. Ammonia was comparatively evenly distributed throughout the water column at all depths. The cycling of ammonia appeared to follow that of known oligotrophic lakes. Generally, the nitrite was low in the limnetic and allochthonous waters and higher in the littoral zone. The littoral and limnetic water exhibited lower nitrate levels than the allochthonous sources. This is believed to be due to the prevailing land use patterns. The production of cattle and sheep and the cultivation of extensively fertilized crops appear to give the most plausible answer. The littoral waters appeared to be higher with respect to ammonia, nitrite, and nitrate, in the microhabitats that are natural, and in others that are man-made: i.e., harbors, breakwaters, and shore based homes. The distribution of phosphate-phosphorous in the waters varied little during the study. The mean values were quite similar for the littoral and limnetic waters. The allochthonous waters had approximately four times the phosphate concentration of the lake waters. The chemical analysis of the bottom muds revealed that phosphorous as phosphate and as P2O5 were significantly higher in concentration between the 50- and 200-foot contours than at lesser depths. The low levels of soluble phosphate and the slight variation encountered in the water column seem to be related to the orthograde nature of the oxygen curve found in the lake. Relationships between chemical analysis and the zooplankton associated within a particular station in the lake were completed by correlation and regression analysis. Associations between independent and dependent variables apparently defined various environmental preferences or requirements necessary for the maintenance of particular individual species. In light of the basic taxonomic considerations these proposals appear to be within reason. Individual correlation and regression analysis were completed for three lake zones investigated: littoral, limnetic, and the haptobenthos. Individual analysis of variances were completed within the three Bear Lake zones in order to assess the affect of habitat on the plankton population. The statistical analyses were compared to the means for these several stations within one zone, and biological and statistical explanations were made. Supplementary water quality analyses were conducted in order to explain some abnormal chem1cal and biological results. Bacteriological testing of the Bear Lake waters revealed that a large percentage of the littoral and allied limnetic zone presented definite problems with respect to water quality and public use.

It is the purpose of this study to present a quantitative survey of the family Peridiniidae in Tomales Bay over the six month period between October thirtieth and May sixth. Emphasis has been placed on variation in the total Peridiniidae population for this period, on the seasonal variation of individual species, and on the possible effects of hydrographic and climatic conditions on the population.

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