Date of Award

2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biological Science

First Advisor

Mark Stolt

Abstract

Coastal ecosystems continue to be negatively impacted by increased development and anthropogenic inputs resulting in nutrient enrichment, reduced water quality, loss of seagrasses, sedimentation, and coastal acidification. These stressors, along with historic over harvest and disease, have resulted in the collapse of commercial oyster fisheries in many estuaries worldwide. Expansion of oyster aquaculture has reversed this trend, creating a growing market for oysters as a food resource. This growth however, is being constrained by a number of issues, particularly access to the coastal zone, and identification of productive locations for aquaculture lease development in areas free of conflicting uses.

Coastal ecosystem managers have identified a critical need for a support tool that can guide development of bivalve aquaculture, and site selection for restoration while avoiding user conflicts in the coastal zone. To evaluate the potential of subaqueous soil maps as a tool for managing aquaculture development and restoration site selection, my dissertation focused on three areas of research. Firstly, I conducted in-situ sampling of surface soil pH within several mapped soil types found within coastal lagoons and embayments, to characterize pH variability and determine if coastal acidification may influence bivalve recruitment. Secondly, I identified the soil properties that related to oyster productivity for on-the-bottom aquaculture systems by conducting oyster growth trials for dominant soil landscapes within both coastal lagoons and embayments. Lastly, I developed a decision support tool that combined the results from the previous experiments along with conflicting use information to quantify the spatial extent of conflicting uses and potential development of bivalve aquaculture within the coastal salt pond region using standardized subaqueous soil maps.

I used a hydropedological approach to assess the spatial variability of coastal acidification within two coastal lagoons and embayments in Rhode Island by measuring oyster shell dissolution, pH within the water column, and pore water pH within the upper 5 cm of the underlying subaqueous soils. Sampling and monitoring sites were stratified based on submerged soil-landscape types mapped at the Great Group level as Haplowassents, Sulfiwassents, and Psammowassents. Using a linear mixed modeling approach, we found that pore water pH varied significantly among soils and with depth. Median pore water pH was significantly greater in sandy, low organic matter content Psammowassents (7.97) than the finer textured, higher soil organic matter content Sulfiwassents (7.35), and the Haplowassents (6.57) that receive groundwater discharge from the surrounding subaerial soils. Juvenile calcifying organisms can experience acidic stress at pH values below 7.6; thus, current pH values within the upper few centimeters of Sulfiwassents and Haplowassents may be low enough to impact recently set juvenile calcifying organisms inhabiting these soils. Consequently, mean shell loss during a 4-wk period was significantly greater in the Sulfiwassents (1.54) than the Psammowassents (0.96%), with the greatest shell loss (18.62%) in one of our Haplowassent sites with groundwater discharge.

I compiled growth rate and survival data from growth trials conducted with juvenile eastern oysters (Crassostrea virginica) in dominant subaqueous soil types over five growing seasons within Rhode Island coastal estuaries (two years of these growth trials were conducted by a former graduate student in the Laboratory of Pedology and Soil Environmental Science using the same study design). Using a linear mixed modeling statistical approach, I found that oysters grown in sandy firm substrates (Haplowassents and Psammowassents) showed increased growth rates and survival when compared to oysters grown in silty substrates with low bearing capacity (Sulfiwassents). These results suggest that substrate type may assist in identifying portions within estuaries that exhibit greater seston flux without having to conduct extensive hydrodynamic modeling. Sites with increased seston flux have been shown to positively influence growth rate.

Using the results from the previous studies, I developed a GIS-based support tool to couple subaqueous soils data with spatial data of non-compatible uses. I found that between 43% to 70% of the coastal salt pond region represents non-compatible uses for aquaculture development, including boating and navigation, submerged aquatic vegetation, and recreational shellfishing. Of the remaining available area, soil landscapes that can support productive on-bottom culture ranges from 2% - 34%, depending on the coastal salt pond. Currently, 2% of the coastal salt ponds are leased for aquaculture, leaving 3% (143 acres) available for lease development, given current regulations.

Our research suggests that subaqueous soil maps are a good way to stratify estuarine substrates to identify preferred soil landscapes for on-the-bottom oyster aquaculture development and restoration site selection, as well as areas prone to acidification. As the extent of subaqueous soil survey continues to expand along the Atlantic coast, subaqueous maps will increasingly be available as a planning tool to guide use and management of the coastal zone.

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