<p>Phosphorus load reduction and dreissenid invasion are the two most important factors that influence europhication dynamics in the Great Lakes. The 1978 amendments to the Great Lakes Water Quality Agreement (GLWQA) between the United States and Canada established target phosphorus loads for the lakes, leading to reductions in external phosphorus loading to the Great Lakes. With diminished phosphorus levels, further nutrient management was a minor concern until the proliferation of invasive dreissenid mussels. Dreissenid mussels were first documented in the Laurentian Great Lakes in the late 1980s. Zebra mussels (<italic>Dreissena polymorpha</italic>) spread quickly into shallow, hard-substrate areas; quagga mussels (<italic>Dreissena rostriformis bugensis</italic>) spread more slowly and are currently colonizing deep, offshore areas. These mussels have the potential to modify biogeochemical processes and food web structure, altering nutrient cycling and availability. Following the mussel invasion, cyanobacterial blooms and nuisance benthic algal growth have reappeared in many nearshore areas of the Great Lakes.</p><p>This dissertation characterizes long-term patterns of phosphorus loading and mussel populations for Saginaw Bay, and estimates the effects of load reductions and dreissenid invasion on several aspects of pelagic water quality, focusing on phosphorus flux and cycling in Saginaw Bay. Bayesian approaches were used to quantify the impacts of load reduction and mussel invasion, while at the same time addressing model parameter uncertainty and prediction uncertainty associated with long-term observational data. Annual total phosphorus load estimates suggest a decreasing trend until the late 1970s to early 1980s, reflecting the effectiveness of point source controls implemented pursuant to GLWQA. Despite the decrease, however, the annual loads have not likely met the 440 tonne yr-1 target established for Saginaw Bay. In 1990 zebra mussels were discovered in the bay and by 1992 they were widespread and peaked with densities of >30,000 m<super>-2</super>. Following the peak, mean densities dropped and modeling results predict that the density will reach equilibria at ~600 m<super>-2</super>. When mussels appeared, the proportion of tributary phosphorus retained in Saginaw Bay increased from ~0.5 to ~0.7, reducing phosphorus export to the main body of Lake Huron. The combined effects of increased phosphorus retention and decreased phosphorus loading have caused an ~60% decrease in phosphorus export from Saginaw Bay to Lake Huron. The analysis of long-term patterns of pelagic water quality highlights the sustained effects of mussel invasion on altering water quality parameters in Saginaw Bay; there was a consistent decrease in chlorophyll concentrations by ~46%, and total phosphorus concentrations by ~25%, and an increase in secchi depths by ~15% over ~20 year invasion of mussels. A comparison of chlorophyll-phospohrus relationship between pre- and post-invasion periods suggest the reduced chlorophyll yield for a given phosphorus concentration after the mussel invasion. Further, decreases in both total phosphorus and chlorophyll concentrations were found in the majority of 24 mussel-invaded US lakes in addition to Saginaw Bay, and modeling results predict less chlorophyll yields per unit phosphorus level that ranges from oligo- to mesotrophic conditions. All lines of evidence presented in the dissertation point to the important roles of load reductions and invasive mussels affecting eutrophication dynamics in lake ecosystems.</p> / Dissertation

Thesis (D. Min.)--Trinity Evangelical Divinity School, 2008. / Abstract. Description based on print version record. Includes bibliographical references (leaves 170-176).

<p>Natural and constructed rocky reef habitats constitute important areas for lithophilic spawning fishes and their embryonic and larval offspring. Interstitial spaces created by the structure of rocky reefs create micro-environments where incubating embryos and juvenile fishes are potentially protected from predators. However, if interstitial spaces are filled or blocked by sediment deposition or biofouling, the reef structure may lose the protective benefits for embryonic and larval fish survival. Lake whitefish (<i>Coregonus clupeaformis</i>) and walleye (<i>Sander vitreus</i>) are native Great Lake lithophilic broadcast spawning fish that use rocky spawning habitats that are vulnerable to degradation caused by deposition of suspended sediments. To restore degraded rocky reef habitat, common practices include addition of material to existing reef structures or construction of new reefs, but both of these practices can be costly and time intensive. In this study, we measured the effect of different types and amounts of sediment cover on hatching success of walleye eggs and assessed if differences in female walleye (female length and egg size) account for tolerance to sediment cover. Additionally, we explored an alternative approach for reef restoration, custodial maintenance, in which we created two novel devices to potentially clean rocky reef habitat. We carried out two laboratory experiments in 2018 and 2019 to test the effect of sediment cover on hatching success of walleye eggs (2018) and to test how female identity and female length or egg size may interact with sediment cover to influence hatching success (2019). We exposed walleye eggs to instantaneous sediment cover (0 mm – 7mm) of either sand (course) or silt (fine) sediments from fertilization until day 15 of incubation. Our results indicated that walleye eggs were sensitive to silt cover (71% mortality- 2 mm cover silt) but not sand (47% mortality- 7mm cover sand). While there was an indication that hatching success was marginally related to female length and egg size, we concluded that sediment cover seemed to have similar effects on eggs, regardless of female length or egg size. The susceptibility of walleye eggs to mortality caused by sediment cover underscores the need for non-degraded spawning habitat. Our two cleaning devices used either propulsion or pressurized water jets to clean sediments from the rocky structure as they were towed behind a small vessel (i.e., did not require the use of SCUBA divers). We used devices to clean two natural rocky reefs in Saginaw Bay, Lake Huron in 2018 and 2019. We measured relative hardness before and after use of devices on cleaned and uncleaned study plots to determine effectiveness of devices. In addition, we measured egg deposition by fall (lake whitefish) and spring (walleye) lithophilic spawners on study plots to determine potential differences in fish usage of cleaned and uncleaned areas. We found that cleaning devices contributed to changes in relative hardness among study plots. Egg deposition was also variable on study plots but in general, egg deposition was consistently highest on treatment plots cleaned by our device that used propulsion. The practicality of cleaning devices was seemingly related to the magnitude of degradation of rocky reefs, nevertheless, our results show that the use of these or similar devices may potentially increase egg deposition by creating areas of higher-quality habitat. While more testing is necessary to fully understand the potential of our reef cleaning devices, this two-year study suggests that these devices may be capable of restoring degraded rocky spawning habitat which could potentially minimize the negative effects associated with sediment degradation on lithophilic spawning fish.</p>