Date of Award

12-2018

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Forestry and Environmental Conservation

Committee Member

J. Drew Lanham, PhD, Committee Chair

Committee Member

Richard M. Kaminski, PhD

Committee Member

Patrick D. Gerard, PhD

Abstract

Widgeongrass (Ruppia maritima) is a cosmopolitan submersed aquatic vegetation (SAV) of brackish wetlands. Management of SAV species is practiced in impounded tidal wetlands in coastal South Carolina to provide forage for waterfowl and other waterbirds. Widgeongrass also provides habitat and associated periphyton for aquatic invertebrates. I conducted an observational study to test effects of complete drawdown (CD) to dried substrate versus partial, shallow water (0–10 cm) drawdown (PD) during May–June 2016 on aquatic invertebrate and SAV biomasses and aquatic invertebrate diversity in managed brackish tidal impoundments (MTI) in the Ashepoo, Combahee, and Edisto Rivers Basin, South Carolina, because such data were lacking to inform managers of best practices to promote standing crops of invertebrates and SAV. I sampled sediments and SAV in 20 MTIs (8 complete and 12 partial drawdown MTIs) and three natural tidal marsh sites (control) during August 2016, November 2016, January 2017, and April 2017. I used mixed model analysis of variance to test (∝ = 0.10) effects of drawdown on SAV and benthic invertebrate biomasses (g[dry]/m2) and benthic invertebrate diversity (Shannon H′). I used mixed model analysis of covariance to test (∝ = 0.10) effects of drawdowns and SAV biomass (covariate) on total invertebrate biomass (benthic and epifaunal combined; g[dry]/m2) and diversity (Shannon H′). I detected a drawdown effect on SAV (P = 0.014) and benthic (P = 0.063) and total (P = 0.014) invertebrate biomasses for August 2016, benthic and total invertebrate biomasses for November 2016 (P = 0.022 and P = 0.041, respectively) and April 2017 (P = 0.042 and P = 0.030, respectively), and benthic invertebrate biomass for January 2017 (P = 0.079). I also detected a drawdown effect on benthic invertebrate diversity for August 2016 (P =0.007), November (P = 0.056) and April 2017 (P = 0.013) and total invertebrate diversity for August (P = 0.025) and April 2017 (P = 0.012). Additionally, I detected a significant positive effect of SAV biomass on total invertebrate biomass (0.089 ≤ P ≤ 0.002) and diversity (0.014 ≤ P ≤ 0.001) for November 2016 and April 2017, and benthic invertebrate biomass for August 2016, November 2016, and April 2017 (0.054 ≤ P ≤ 0.001). Submersed aquatic vegetation and benthic and total invertebrate biomasses at peak production in August 2016 were greatest and less variable in partially drawndown impoundments before Hurricane Matthew devastated SAV communities in October 2016. Partially drawndown MTIs exhibited greater benthic and total invertebrate biomasses and diversity for all sampling periods with detected treatment effects except January 2017 wherein both PD and CD MTIs exhibited greater benthic invertebrate biomass than unmanaged marsh (control) and April 2017 wherein benthic invertebrate diversity was greater in PD MTIs than both CD MTIs and unmanaged marsh. I present bioenergetic carrying capacity estimates (energetic use days [EUD/ha]), derived from my SAV and invertebrate biomass estimates by treatment for each sampling period, for dabbling ducks associated with South Carolina MTIs. Across sampling periods, EUDs for PD MTIs averaged 2.7 times greater than EUDs for CD MTIs. I recommend partial drawdowns to maximize invertebrate and SAV biomasses and MTI foraging carrying capacities for migratory ducks and other waterbirds in coastal South Carolina. However, I also recommend periodic complete drawdowns to consolidate flocculent soils and decompose organics to promote rooting by SAV.

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