Date of Award

Spring 2011

Document Type

Legacy Thesis

Degree Name

Master of Science in Coastal Marine and Wetland Studies

Department

Coastal and Marine Systems Science

College

College of Science

First Advisor

Jane Guentzel

Second Advisor

James O. Luken

Third Advisor

Kehui (Kevin) Xu

Abstract

The Waccamaw River is a coastal plain blackwater system located along the eastern coastline of South Carolina, USA. The fish in this river contain concentrations of mercury that have resulted in the issuance of many fish consumption advisories. Large areas of the shoreline contain water hyacinth (Eichhornia crassipes) mats during the warmer months that may affect the chemistry of the water column and microbial populations living on and around these mats. Water hyacinth plants can trap organic matter, inhibit light and oxygen penetration into the water, and serve as a substrate for bacterial colonization, which may provide a site conducive to the transformation of inorganic mercury to methyl mercury. The purpose of this study was to quantify the chemical and biological parameters of the water column that may promote the formation of methyl mercury and to quantify the amount of total and methyl mercury in water hyacinth plants throughout growth and decay. This study also addressed how nutrients and metal concentrations in water hyacinth plants changed during mat growth and decomposition. Water hyacinth plant samples and water samples were collected on a monthly basis from May to November 2010 from the Waccamaw River, South Carolina. Water samples were collected from an open water site and under a water hyacinth mat. The slightly acidic pH conditions (5.65- 6.93), moderate conductivity measurements (106.5-143.5 µS), low oxygen levels under the mat (1.9-4.3 mg/L), presence of coliform and sulfate reducing bacteria, high concentrations of organic carbon (14.7-57 mg/L), and sulfate concentrations (3-19.9 mg/L) suggest that the water in this river is an environment conducive to the formation of methyl mercury. Concentrations of total and methyl mercury and percentages of methyl mercury in water samples were not significantly different between the open water and under the mat sites and ranged from 2.9-8.9 ng/L, 0.2-2.5 ng/L, and 9-31%, respectively. The blackwater and large water hyacinth mats in the Waccamaw River directly block sunlight penetration and may prevent photodegradation of methyl mercury from occurring, which may help explain the minimal differences observed between sites. Sediment samples from the open water and under the mat had concentrations ranging from 205-358 ng/g- dry for total mercury and 0.291-0.551 ng/g-dry for methyl mercury. The percentages of methyl mercury ranged from 0.14-0.16%. The low concentrations and percentages of methyl mercury in the sediments suggest that net mercury methylation may occur primarily in the water column and around the water hyacinth roots. This is the first study that has determined the speciation of mercury in water hyacinth shoots and roots throughout growth and decay in a natural environment. The concentrations of total mercury in water hyacinth shoots and roots ranged from 2.2-58.7 ng/g-dry and from 29.7-71.9 ng/g-dry, respectively. Methyl mercury ranged from 0.84- 2.24 ng/g-dry in the shoots and from 8.95-37.1 ng/g-dry in the roots. The percentages of methyl mercury in the plants varied from 2-63% and were approximately 1,000-15,000 times higher in total and methyl mercury relative to the water column. The amount of mercury measured in a square meter of water hyacinth ranged from 8,833-52,459 ng/m² dry for total mercury and from 2,686-15,786 ng/m² dry for methyl mercury. Water hyacinth plants may represent another methyl mercury pathway to aquatic food webs and may serve as an important link in the bioaccumulation of methyl mercury in aquatic ecosystems. There were observable increases in the concentrations of total P, total N, other N, total Hg, and methyl Hg in water samples after plant decomposition in October and November 2010. These results suggest that water hyacinth plants release nutrients and mercury into the water when they die either from natural senescence or increased plant decomposition from herbicide application. Water hyacinth has the fastest growth rate of any vascular plant and has become globally distributed. The data from this study may be applicable to other coastal plain systems that have been invaded by water hyacinth and may have implications for aquatic plant management.

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