We employed a comparative approach to review the vulnerability of the trophic interactions within aquatic systems to global threats associated with anthropogenic activities. The goal of this chapter was to identify and characterize mechanisms by which human-mediated environmental threats may modulate trophic dynamics across aquatic ecosystems. Trophic dynamics include some of the most obvious and pervasive factors influencing ecosystems and were used as a metric because of their importance and commonality across all aquatic environments. Our use of trophic dynamics proved to be insightful, illustrating that the flow of energy through aquatic food webs will be (or already has been) altered by invasive species, land use change, nutrient loading, exposure to ultraviolet radiation, overharvesting, acidification, and increasing global temperatures. The response of trophic dynamics to these threats was often similar across oceans, estuaries, lakes, and rivers. This similarity proved to be interesting given the differences in both the level of concern expressed by scientists and the predicted variability in environment-specific responses. As the trophic interactions of an ecosystem are at the root of its function and structure, examining trophic dynamics could be an informative method for evaluating the response of aquatic environments to global threats. If future analyses validate the use of trophic dynamics as a metric, it is our hope that trophic dynamics can be used by scientists and politicians to mitigate the effects of human actions. *Corresponding author: E-mail: jessie.clasen@gmail.com; Present address: Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada Acknowledgments †These authors contributed equally to this chapter. We gratefully acknowledge the Ecological Dissertations in the Aquatic Sciences (Eco-DAS) VIII (2008, Hawaii) participants and organizers for the opportunity. J.L.C. would like to thank Jennifer B.H. Martiny for her encouragement and wise advice. J.K.L. thanks Robert Cowen for all the support. C.E.H.K is grateful for Craig Williamson¢s encouragement. D.M. would like to thank Jay Pinckney and Tammi Richardson for their support and encouragement. D.L.P. thanks Gabe Filippelli and Andrew M. Rusiniak for their advice and encouragement. Eco-DAS VIII was financially supported by the National Science Foundation, the National Oceanic and Atmospheric Administration, the American Society of Limnology and Oceanography, the Office of Naval Research, and the National Aeronautics and Space Administration. We also thank two anonymous reviewers for their helpful comments and suggestions on earlier drafts of this chapter. Publication was supported by NSF award OCE0812838 to P.F. Kemp ISBN: 978-0-9845591-1-4, DOI: 10.4319/ecodas.2010.978-0-9845591-1-4.47 Eco-DAS VIII Chapter 4, 2010, 47-66 © 2010, by the American Society of Limnology and Oceanography, Inc. Eco-DAS VIII Symposium Proceedings environments, which are beyond the direct influence of nutrient-rich terrestrial runoff. However, all aquatic ecosystems are connected and as such are susceptible to these global threats. Investigating the commonalities that do exist may be useful in shedding new light on the effects of human activities across ecosystems, and making progress toward mediating them. We will examine the commonality of trophic dynamics to illustrate the similarities and differences in how anthropogenic threats impact aquatic ecosystems. Trophic dynamics (also called trophodynamics) is the study of how energy flows through an ecosystem. It includes some of the most obvious (e.g., predator-prey relationships) and pervasive factors influencing ecosystem structure and function. Because of the commonality and importance of trophic dynamics across aquatic environments, using trophic dynamics as a proxy for ecosystem vulnerability is particularly useful, because it encompasses both the direct and indirect ecosystem responses to anthropogenic threats. The goal of this chapter is to identify and characterize mechanisms by which human-mediated environmental threats may modulate trophic dynamics across aquatic ecosystems. Unfortunately there are many examples of global threats to aquatic ecosystems. In this chapter we highlight just a few that fall within our areas of expertise and were identified by our Eco-DAS colleagues as imminent and particularly worrisome (see the chapter by Keister et al. 2010, this volume). These threats include invasive species, land use changes, global temperature alterations, nutrient loading, ultraviolet (UV) radiation, overfishing, and acidification. Each global threat is discussed below in the context of how it affects trophic dynamics in different ecosystems. This comparative approach examines the vulnerability of trophic interactions to anthropogenically induced global changes. Ultimately, we aim to synthesize results, highlight areas of special concern that deserve future attention, and determine if we can use this approach to develop effective mitigation strategies against future changes.

Previous research implicates gelatinous zooplankton grazing for reducing microzooplankton populations and influencing pelagic carbon fluxes. The principal objective of this study was to determine in situ ingestion rates and assimilation efficiencies for seasonal hydromedusae populations in North Inlet, SC, USA. Hydromedusae collected during July 2005, 2006 and January 2006 were exposed to natural plankton communities that were previously radiolabeled with 14C. Ingestion rates and assimilation efficiencies were calculated from measurements of hydromedusae tissue radioactivity after ingestion and egestion, respectively. Net tows determined that the numerically dominant gelatinous predators in this system were Bougainvillia muscus (July 2005, 2006) and Nemopsis bachei (January 2006). Summer ingestion rates for B. muscus significantly increased with increased temperature, but there was no effect of temperature on winter ingestion rates for N. bachei, suggesting temperature responses are species specific. Moreover, there was a significant species effect on ingestion rates for July 2006. Hydromedusae assimilation efficiencies are relatively constant throughout the year and are not significantly affected by temperature, species, or season. Seasonal differences in grazing are likely a function of the pelagic community and temperature-dependent metabolism. These results coupled with seasonal hydromedusae abundance data imply summer populations of hydromedusae graze more heavily on prey than winter populations, while also assimilating more carbon into predator tissue. Effects of hydromedusae predation may include modification of trophic structure, shifts in algal community composition, and decline in ecosystem stability, leading to impacts on sustainable commercial fisheries.

The objective of this study was to quantify the respiratory requirements and oxygen consumption rates for seasonal hydromedusae populations in the North Inlet estuary, Georgetown, SC, USA. Respiration rates were determined for hydromedusae collected in January 2006, 2007 and July 2006 using microrespirometry. The numerically dominant gelatinous predators were Nemopsis bachei (January 2006 and 2007) and Bougainvillia muscus (July 2006). Seasonal oxygen consumption rates were low (0.80-1.27 mu mol O-2 g(-1) h(-1)). Q(10) values were high during January 2006 and July 2006 (4.7), but low during January 2007 (0.5), suggesting hydromedusae were sensitive to the exposure temperatures (7-34 degrees C) in this study and well adapted to the temperature range encountered in this estuary. Moreover, the oxygen consumption rates during July 2006 showed significant differences between the two species. The allometric relationship between respiration rate and hydromedusae weight differed seasonally. Our findings suggest hydromedusae can persist in hypoxic conditions and may partially explain the frequently observed increase in these cnidarians under conditions of low dissolved oxygen concentrations.

Determination of growth rates of harmful algae is critical to understanding bloom dynamics. However, we have few reliable methods of directly determining in situ growth rates on natural populations. One available method is photopigment radiolabelling, where C-specific growth rates are based on synthesis rates of chlorophylls and carotenoids using (14)C-bicarbonate as a tracer. Here, we examined radiolabelling of the biomarker pigment gyroxanthin-diester as a tool for determining in situ growth rates of the toxic dinoflagellate Karenia brevis. We also characterized growth responses of K. breris to various nitrogenous (N) nutrient sources and the presence or absence of a natural plankton community (including grazers) at two irradiances. We found that gyroxanthin radiolabelling may be used successfully to determine C-specific growth rates of K. brevis during monospecific blooms. However, the approach may be of limited use in mixed assemblages when there is low Karenia biomass because low concentrations of gyroxanthin per cell and low turnover rates result in poor signal for the gyroxanthin pigment. Growth rates of K. brevis generally ranged between 0.1 and 0.4d(-1). Cells in batch culture grew equally well on inorganic and organic forms of N, so they may be physiologically capable of using the major N forms present in agricultural run-off, atmospheric deposition, and other anthropogenic inputs. This has implications for the control of K. brevis blooms in nutrient-impacted coastal waters in that organic N inputs should be considered (along with inorganic N) in nutrient management strategies. Our highest C-specific growth rates (0.7 d(-1)) were measured in the presence of microzooplankton grazers, also indicating that the release of dissolved compounds may play a role in stimulating the growth of K. brevis in nature.