Significant seasonal variation in size at settlement has been observed in newly settled larvae of Dreissena polymorpha in Lake Constance. Diet quality, which varies temporally and spatially in freshwater habitats, has been suggested as a significant factor influencing life history and development of freshwater invertebrates. Accordingly, experiments were conducted with field-collected larvae to test the hypothesis that diet quality can determine planktonic larval growth rates, size at settlement and subsequent post-metamorphic growth rates. Larvae were fed one of two diets or starved. One diet was composed of cyanobacterial cells which are deficient in polyunsaturated fatty acids (PUFAs), and the other was a mixed diet rich in PUFAs. Freshly metamorphosed animals from the starvation treatment had a carbon content per individual 70% lower than that of larvae fed the mixed diet. This apparent exhaustion of larval internal reserves resulted in a 50% reduction of the postmetamorphic growth rates. Growth was also reduced in animals previously fed the cyanobacterial diet. Hence, low food quantity or low food quality during the larval stage of D. polymorpha lead to irreversible effects for postmetamorphic animals, and is related to inferior competitive abilities.

Individuals within a single population can vary widely in their phenotype e.g. in their body shape. These differences are an important source of biodiversity and they can precede evolutionary divergence within a population. In this thesis we use the biological invasion of the zebra mussels into Swedish lakes to investigate which processes create or maintain phenotypic diversity within populations of the two native fish species perch and roach and the mussel itself. Both fishes have specially adapted body shapes that depend on whether they feed in the near-shore or open-water habitat of lakes. This habitat-specific divergence was more pronounced in lakes with zebra mussels, probably because resources in both habitats were in higher supply due to the mussels’ effects on the lakes. Divergence in perch body shapes between habitats was also higher in lakes with a higher water clarity, suggesting that visual conditions can affect the resource use and thus also the expression of a habitat-specific body shape. When investigating the diversity of body shapes in the mussel itself we found that mussels from one lake changed their shell shape when exposed to different predators: fish predators induced a more elongated shell shape while crayfish predators induced a rounder shell. These specific shell shapes probably serve as two alternative predator defenses protecting the mussel from predation. We conclude that the availability and use of distinct resources is an important source of diversity within populations. Abiotic conditions can play a previously underappreciated role by promoting or impairing the use of the distinct resources thus affecting the divergence. The diversity of shell shapes we found in the zebra mussels complements our study by demonstrating that not only consumer responses to resources but also resources’ responses to predators can generate phenotypic diversity.

The number of species extinctions is increasing at an alarming rate. Long-lived freshwater mussels of the order Unionoida, which include a parasitic stage on a host fish, are highly threatened. Habitat degradation by turbidity and sedimentation is thought to be one major reason for their decline. The objective of this thesis was to examine recruitment patterns and identify the causes of the lack of recruitment in the threatened unionoid freshwater pearl mussel (Margaritifera margaritifera). In addition, I investigated the effects of turbidity on non-endangered dreissenid mussels, where turbidity was manipulated through use of bioturbating mayflies. In a survey of 107 Swedish streams, mussel population size and trout density were both positively correlated to recruitment probability of M. margaritifera. A more in-depth study of the age-structure of nine populations revealed that four of these populations showed no signs of recruitment over the last ten years. Within-stream variation in recruitment was high as both mussels and trout had patchy distribution, and may be important for population regulation. Moreover, examination of different life stages revealed no differences in the gravid mussel stage or the stage when mussels infect salmonid fish. Instead, differences were observed for the juvenile, benthic stage, presumably related to differences in turbidity and sedimentation. High turbidity may affect filter-feeding efficiency of mussels and high sedimentation may reduce survival by clogging sediments, thereby altering, for example, oxygen and food conditions. In the study of the effects of turbidity, bioturbating mayflies increased turbidity and filter-feeding dreissenid mussels reduced turbidity. Mussel growth both decreased and increased with increasing turbidity, depending on sediment type. Turbidity and sedimentation often impact entire stream systems, and a holistic, catchment-based management strategy may be needed to reduce the effects of sedimentation on freshwater pearl mussels. The effects of restoration take a long time and must start soon if recruitment of mussels is to be re-established. Restoration may also be more urgent in some streams than in others, as the maximum age of M. margaritifera populations in my study differed by as much as 60 years. As mussel and trout densities seem to be important for recruitment success, one conservation method may be to concentrate mussels into sites where trout density is high.

Non-indigenous species can profoundly alter the ecosystems they invade and impact local economies. Growth and body condition declines of commercially fished Great Lakes lake whitefish coincide with the establishment of non-native dreissenid mussels and the cladoceran Bythotrephes longimanus. Declines in lake herring abundance—a key prey item for other commercially important species—have also been reported. Though additional stressors such as climate change may have contributed to changes in coregonid populations, they have not been thoroughly evaluated. Here, I present data that condition and contaminant declines in coregonids are associated with increasing density or warming climate, but growth declines in lake whitefish are likely due to ecosystem changes associated with dreissenids and Bythotrephes. In South Bay, Lake Huron, changes in lake whitefish diet composition and stable isotope signatures were consistent with increased reliance on nearshore resources after dreissenid establishment; lake whitefish occupied shallower habitats and experienced declines in mean diet energy densities post-dreissenid invasion. Growth of South Bay lake whitefish declined after environmental effects were statistically removed, whereas condition declines were explained best by changes in lake whitefish density. Among four lake whitefish populations, growth declined after dreissenids established, but not in uninvaded reference populations. Growth also declined among four lake whitefish populations after the establishment of Bythotrephes relative to reference populations. In contrast with growth, condition of lake whitefish did not change as a result of dreissenid or Bythotrephes invasion. Bioenergetic models revealed that activity rates increased and conversion efficiencies decreased in lake whitefish populations exposed to dreissenids, despite higher consumption rates in populations with dreissenids present. Condition declines among many lake whitefish and lake herring populations (and declines in mercury among herring populations) reflected regional differences and were not related to the presence of Bythotrephes or Mysis relicta. Declines in condition were more pronounced in northwest Ontario populations where climate has changed more dramatically than in southern Ontario. This work suggests that projected range expansions of dreissenid mussels and Bythotrephes will likely affect native fisheries, and their effect on these fisheries may be exacerbated by declining fish condition associated with climate change.

Non-indigenous species can profoundly alter the ecosystems they invade and impact local economies. Growth and body condition declines of commercially fished Great Lakes lake whitefish coincide with the establishment of non-native dreissenid mussels and the cladoceran Bythotrephes longimanus. Declines in lake herring abundance—a key prey item for other commercially important species—have also been reported. Though additional stressors such as climate change may have contributed to changes in coregonid populations, they have not been thoroughly evaluated. Here, I present data that condition and contaminant declines in coregonids are associated with increasing density or warming climate, but growth declines in lake whitefish are likely due to ecosystem changes associated with dreissenids and Bythotrephes. In South Bay, Lake Huron, changes in lake whitefish diet composition and stable isotope signatures were consistent with increased reliance on nearshore resources after dreissenid establishment; lake whitefish occupied shallower habitats and experienced declines in mean diet energy densities post-dreissenid invasion. Growth of South Bay lake whitefish declined after environmental effects were statistically removed, whereas condition declines were explained best by changes in lake whitefish density. Among four lake whitefish populations, growth declined after dreissenids established, but not in uninvaded reference populations. Growth also declined among four lake whitefish populations after the establishment of Bythotrephes relative to reference populations. In contrast with growth, condition of lake whitefish did not change as a result of dreissenid or Bythotrephes invasion. Bioenergetic models revealed that activity rates increased and conversion efficiencies decreased in lake whitefish populations exposed to dreissenids, despite higher consumption rates in populations with dreissenids present. Condition declines among many lake whitefish and lake herring populations (and declines in mercury among herring populations) reflected regional differences and were not related to the presence of Bythotrephes or Mysis relicta. Declines in condition were more pronounced in northwest Ontario populations where climate has changed more dramatically than in southern Ontario. This work suggests that projected range expansions of dreissenid mussels and Bythotrephes will likely affect native fisheries, and their effect on these fisheries may be exacerbated by declining fish condition associated with climate change.

Invasive species such as quagga mussel (Dreisseina bugensis) alter native ecosystems around the world. This study uses genetic markers to examine historical lineages for quagga mussels in lakes (reservoirs) of the Colorado River System. Specimens were collected from Lake Mead, Lake Mohave, Lake Pleasant, Lake Havasu, Lower Otay Reservoir, Yuma Area, and two Central Arizona Project pumping stations. Objectives of this project were to perform analyses of genetic variability within populations and determine if relatedness among individuals could resolve whether they originate from a single, or multiple, invasion events and genetically distinguish the populations at each water body. Analyses examined the mitochondrial DNA COI region and eight microsatellite DNA markers. Three populations were characterized in the study area and compelling information gathered about gene flow between them. Results indicate that microsatellite markers are useful to track quagga mussel invasions and provide insights into migration patterns that would otherwise be missed.

abstract: Quagga mussels are an aquatic invasive species capable of causing economic and ecological damage. Despite the quagga mussels’ ability to rapidly spread, two watersheds, the Salt River system and the Verde River system of Arizona, both had no quagga mussel detections for 8 years. The main factor thought to deter quagga mussels was the stratification of the two watersheds during the summer, resulting in high temperatures in the epilimnion and low dissolved oxygen in the hypolimnion. In 2015, Canyon Lake, a reservoir of the Salt River watershed, tested positive for quagga mussel veligers. In this study, I used Landsat 7 and Landsat 8 satellite data to determine if changes in the surface temperature have caused a change to the reservoir allowing quagga mussel contamination. I used a location in the center of the lake with a root mean squared error (RMSE) of 0.80 and a correlation coefficient (R^2) of 0.82, but I did not detect any significant variations in surface temperatures from recent years. I also measured 21 locations on Canyon Lake to determine if the locations in Canyon Lake were able to harbor quagga mussels. I found that summer stratification caused hypolimnion dissolved oxygen levels to drop well below the quagga mussel threshold of 2mg/L. Surface temperatures, however were not high enough throughout the lake to prevent quagga mussels from inhabiting the epilimnion. It is likely that a lack of substrate in the epilimnion have forced any quagga mussel inhabitants in Canyon Lake to specific locations that were not necessarily near the point of quagga veliger detection sampling. The research suggests that while Canyon Lake may have been difficult for quagga mussels to infest, once they become established in the proper locations, where they can survive through the summer, quagga mussels are likely to become more prevalent. / Dissertation/Thesis / Masters Thesis Civil, Environmental and Sustainable Engineering 2018

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