Biodiversity, biological integrity, and population ecology in disturbed aquatic environments
Declining water quality is of great concern in the United States. In 2009 the US Environmental Protection Agency reported that 44% of rivers and streams and 64% of lakes, reservoirs, and ponds are impaired or not clean enough for their designated water use categories. One consequence of degraded aquatic ecosystems include the rapid disappearance of many aquatic organisms, among which freshwater gastropods are the most vulnerable. Chapter 2 presents findings from a field investigation of 39 Iowa lakes and demonstrates that one-third of study lakes do not support any gastropods and overall gastropod richness was limited to 10 species, including one invasive. Aquatic gastropod richness is greatly reduced compared to 49 species previously observed in Iowa. A lake’s habitat quality characteristics were found to be key factors determining diversity and densities of aquatic gastropods.
To address water degradation problems, the Clean Water Act of 1972 was enacted and one of the objectives was to “restore and maintain the chemical, physical, and biological integrity of the nation’s waters.” Chapter 3 of this dissertation presents two indices to assess the biological integrity of Iowa lakes: the Multi-taxon Multimetric Index, which includes biological metrics from phytoplankton, zooplankton, macroinvertebrates, and fish communities, and the Planktonic Multimetric Index, which includes biological metrics from phytoplankton and zooplankton communities.
The study lakes have extended profundal zones due to eutrophic conditions and low water transparency. The main macroinvertebrate dwellers of the profundal zone are organisms that can tolerate hypoxic conditions, such as Oligochaeta, Chironomidae larva, and Chaoboridae larva. Chapter 4 of this dissertation covers Chaoborus punctipennis Say and Chaoborus albatus Johnson, phantom midges with coexisting small transparent aquatic larva that conduct daily vertical migrations from the hypolimnion to the epilimnion to feed on zooplankton during the night and to reduce mortality by fish. We found that these species reach their highest densities in dissimilar conditions. Chaoborus punctipennis reached higher abundance in deeper frequently stratified lakes with a substantial volume of the lake below the thermocline and the high frequency of formation of the hypoxic zone. Conversely, C. albatus gained higher abundance in smaller shallower and less voluminous lakes.