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Monday, January 28 • 3:20pm - 3:40pm
(FISHERIES: HABITAT) Hypoxia Alters Spatial Overlap of Primary and Secondary Consumers in the Pelagic Food Web of Reservoirs

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AUTHORS: Rebecca A. Dillon, Aquatic Ecology Laboratory, Department of Evolution, Ecology, and Organismal Biology, The Ohio State University; Joseph D. Conroy, Inland Fisheries Research Unit, Division of Wildlife, Ohio Department of Natural Resources; Stuart A. Ludsin, Aquatic Ecology Laboratory, Department of Evolution, Ecology, and Organismal Biology, The Ohio State University

ABSTRACT: Hypolimnetic hypoxia has been shown to affect individual behavior, food web structure and interactions, and ecosystem function in aquatic ecosystems worldwide. While recent research has explored the impact of hypolimnetic hypoxia on coastal marine and large-lake food webs, less is known about the effects of hypoxia on reservoir food webs, especially pelagic ones. To address this gap, we examined how the spatial distribution of primary consumers (zooplankton) and secondary consumers (i.e., zooplanktivorous fish, clupeids; vertically migrating, hypoxia-tolerant, macroinvertebrates, Chaoborus spp.) varied between periods of normoxia (spring) and hypoxia (summer) in two small (surface area = 13.5, 11.7 km<sup>2</sup>), shallow (average depth = 6.6, 5.7 m) Ohio reservoirs. We tested the hypothesis that hypolimnetic hypoxia increases spatial overlap among zooplanktivorous fish, macroinvertebrates, and their potential zooplankton at night, whereas it reduces their overlap during the day because hypoxia-tolerant macroinvertebrates can use the hypoxic hypolimnion (and their zooplanktivorous fish predators cannot). We used net tows and hydroacoustics to describe the distribution and spatial overlap of zooplankton, Chaoborus, and zooplanktivorous fish during both day and night, and simultaneously measured physiochemical attributes (e.g., temperature, dissolved oxygen concentration, light levels). We found partial support for our hypothesis, as the overlap (determined from visual examination of net tow and hydroacoustics data) between fish and zooplankton was always high during periods with hypoxia, and was only high at night during normoxia. The overlap between Chaoborus and zooplankton was higher at night than during the day during periods of both normoxia and hypoxia, as Chaoborus were found at all depths during the day. Fish, Chaoborus, and zooplankton had the greatest spatial overlap at night during hypoxic periods. Our findings highlight the potential for hypoxia to alter pelagic food-web interactions in reservoir ecosystems.

Monday January 28, 2019 3:20pm - 3:40pm

Attendees (5)