Date of Award

12-2019

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Ocean Engineering and Marine Sciences

First Advisor

Kevin B. Johnson

Second Advisor

Kelli Hunsucker

Third Advisor

Jonathan Shenker

Fourth Advisor

Richard Aronson

Abstract

The Indian River Lagoon (IRL) has experienced a loss in seagrasses due to organic sediment accumulation and harmful algal blooms. Seagrasses are an essential nursery habitat for many fishes and benthic invertebrates. IRL fish known to utilize benthic habitat and feed on invertebrates include Atlantic croaker (Micropogonias undulates), juvenile Red Drum (Sciaenops ocellatus), and Mojarra (Eucinostomus spp.). The most abundant major infaunal taxa of the IRL are crustaceans, polychaetes, and mollusks. Environmental dredging in the IRL’s Turkey Creek is intended to improve water quality, remove harmful sediments and restore benthic habitat. Such dredging may impact benthic invertebrate populations and, indirectly, their predatory fish species. This study explores the impact of environmental dredging on benthic invertebrates and fish, and attempts to quantify how those impacts affect the food web. This was accomplished by examining how the most abundant infaunal community groups changed in dredged vs. undredged sediments, and whether those sediments had low, medium, or high organic content (“muck” for the latter). Benthic invertebrate populations were sampled and quantified by collecting sediments using a Petite Ponar grab and counting animals >500µm. Abundant taxa found to colonize dredged Turkey Creek muck sediments include amphipods (e.g., Cerapus tubularis and Grandierdiella bonnieroides), bivalves (e.g., Mulinia lateralis and Parastarte triquetra), polychaetes (e.g., Capitella capitata and Glycera americana), and tanaids (e.g., Leptochelia dubia). Densities in dredged muck sites ranged from 0-3,897 ind. m-2 (±0-3,662 SE, p=0.05) for amphipods, 0- 637 ind. m-2 (±0-178 SE, p<0.001) for bivalves, 0-492 ind. m-2 (±0-234 SE, p<0.01) for polychaetes, and 0-148 ind. m-2 (±0-119 SE) for tanaids. Abundant taxa found to colonize dredged Turkey Creek intermediate sediments include amphipods (G. bonnieroides and C. tubularis), bivalves (M. lateralis and P. triquetra), polychaetes (G. americana), and tanaids (Leptochelia dubia). Densities in dredged intermediate sites ranged from 0-23,703 ind. m-2 (±0-6,418 SE, p<0.02) for amphipods, 0-7,170 ind. m-2 (±0-3,192 SE, p<0.05) for bivalves, 0-10,251 ind. m-2 (±0-6,078 SE) for polychaetes, and 0-118 ind. m-2 (±0-118 SE) for tanaids. To explore the impacts on trophic relationships, gut content analyses of predatory fish collected via seining were compared with the composition and abundances of available prey via a modified Costello method in conjunction with Ivlev’s selection index. These were contrasted before and after dredging to discover changes in dietary patterns or feeding strategies related to dredging. Mojarra were found to feed on prey other than their preferred type (amphipods) before dredging, presumably due to low amphipod availability. However, after dredging mojarra were selective predators on amphipods and this appears to be due to increased amphipod availability. The sciaenids (juvenile Atlantic croaker and red drum) also focused on prey other than their preferred type (amphipods and mysids, respectively) before dredging. Dredging yielded conditions where all predators could shift to feeding on their preferred prey type based on higher Ivlev’s selectivity values. Increases in fish prey selectivity, abundances in guts, and environmental infaunal abundances suggest environmental dredging can be a positive step in restoring benthic trophic relationships.

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