The nutrient dynamics in two contrasting estuaries and in the adjacent nearshore environment along the south-east coast of South Africa was investigated seasonally. Due to an inter-basin transfer of water from the Gariep Dam to the Great Fish River, the Great Fish estuary is a fresh water dominated, terrestrially driven system with an annual fresh water inflow of 250 – 650 x 10⁶ m³ per year. In contrast, the Kariega estuary is a fresh water deprived, marine dominated system with a fresh water inflow estimated at 2.5 – 35 x 10⁶ m³per year. The reduced fresh water inflow into the estuary is attributed to regular impoundments along the Kariega River. Water samples were collected from surface and subsurface layers along the length of the estuaries as well as from a series of transects occupied in the nearshore environment. Samples were analysed for nitrate, nitrite, ammonium, phosphate and silicate. Temperature and salinity were recorded at each station. A Land-Ocean Interactions in the Coastal Zone (LOICZ) budget was constructed for each estuary to describe the role of ecosystem-level metabolism as either a sink or a source of phosphorus, nitrogen and carbon. Seasonal variation in physico-chemical properties and nutrient concentrations in the Kariega estuary was minimal due to constant low inflow, while in the Great Fish estuary, concentrations varied in response to changes in flow rate. Nutrient concentrations were consistently higher in the Great Fish estuary than in the Kariega estuary, largely reflecting differences in fresh water inflow. During periods of high flow (32.92 m³.s⁻¹in the Great Fish River) dissolved inorganic nitrogen (DIN) concentrations in the Great Fish estuary were an order of magnitude higher than those recorded in the Kariega estuary. Results of the LOICZ budgeting procedures revealed that in spite of the contrasting hydrodynamic features, the estuaries behave in largely the same manner – both predominantly sources of nutrients with heterotrophic processes dominating over autotrophic actions and both were net denitrifyers during all surveys. This was, however, due to different sets of processes operating in the two estuaries, namely low nutrient concentrations resulting in microbial activity in the Kariega estuary, and riverine influx of nutrients and phytoplankton combined with a short residence time of the water in the Great Fish estuary. In the marine nearshore environment, higher nutrient concentrations were recorded adjacent to the Great Fish estuary than offshore of the Kariega estuary. This was due to a surface plume of less saline water leaving the Great Fish estuary, which acted as an ‘outweller’ of nutrients. Offshore of the Kariega estuary, on the other hand, the nutrient concentrations were characteristic of marine waters due to a lack of fresh water outflow from the estuary. Nutrient concentrations in the marine environment adjacent to the Kariega estuary were, at times, higher than those recorded within the estuary. This observation supports previous statements which suggest that the Kariega estuary is not an ‘outweller’ of dissolved nutrients and particulate material, but rather an extension of the marine environment.

Pathogens and parasites are increasingly recognized as important components within host populations, communities, and ecosystems. Parasite contributions to ecosystem function most likely manifest as density-mediated impacts of parasites on their hosts, the direct contributions of parasite biomass to a system, and via parasite-induced changes in host behavior and physiology (trait-mediated impacts). Here, a framework was constructed that can be used to conceptualize parasite contributions to ecosystem function (Chapter 1). Then the influence of parasite attack on host movement was explored to further evince the mechanistic underpinnings of trait-mediated parasite impacts (Chapter 2). Additionally, mesocosms were created across a gradient of parasitism to examine how these mechanisms are likely to unfold at larger biological scales (Chapter 3). Lastly, a series of differential equations was created to model host-parasite-ecosystem interactions and generate theoretical predictions about how and when parasites are likely to influence ecosystem processes (Chapter 4). Parasites have many characteristics of ecosystem engineers, but their role has historically been ignored. These studies begin to explore the role that parasitism may have as one of the drivers of ecosystem processes.

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<p>Mayflies are important macroinvertebrate members of steam communities and are highly sensitive to agricultural insecticides. In this dissertation I sought to explore whether Heptageniidae<i> </i>mayflies in Indiana have evolved greater tolerance to clothianidin, a neonicotinoid insecticide that has been widely applied over the predominant corn and soybean crops of the state for the last 20 years. Over the five chapters of this dissertation, I investigate different aspects of the effects of clothianidin on mayfly populations. First, I collected mayfly populations from streams around the state across a gradient of agricultural landscape to see if proximity to corn and soybean crops was associated with tolerance to the insecticide. I found significant variation in tolerance between the populations but only connected it to land use for one species and one insecticide. Second, I investigated the potential for increases in predation rates due to exposure to low concentrations of clothianidin by both a vertebrate and invertebrate predator. Sublethal exposure did increase the risk of predation by the invertebrate predator but not the vertebrate. Third, I collected four populations of mayflies and placed them in a common stream which is heavily impacted by agricultural runoff. The most tolerant of the four populations had the highest survival rates and was native to the polluted stream, suggesting that it was best adapted to the local conditions. Finally, the last chapter is a discussion of the factors influencing the evolution of contaminant tolerance, the implications for biomonitoring, and a series of recommendations for incorporating evolved tolerance into biomonitoring practices. Collectively, this work suggests that the lethal and sublethal effects of clothianidin exposure create conditions suitable for the evolution of tolerance in Heptageniid mayflies. While there are already differences in tolerance between populations, it seems likely that mayfly pesticide tolerance will continue to increase as pesticides continue to impact aquatic ecosystems. </p>

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