Date of Award
Master of Science in Coastal Marine and Wetland Studies
Coastal and Marine Systems Science
College of Science
Erin E. Hackett
Richard N. Peterson
Richard F. Viso
The impairment of regional water quality in Long Bay is an episodic occurrence that has been documented for over a decade. According to one explanation, the occurrence of these events is hypothesized to be the combination of local, terrestrially derived inputs and water-column stratification in the nearshore zone. A portion of these inputs may discharge as surface run-off through estuaries ending in sandy transitional environments termed "swashes". An investigation into the fate of land-derived materials through swashes utilize a linear conservative mixing model to describe the non-conservative behavior of materials in the overlying water and pore-water. This model relies on two endmembers to form a conservative mixing line across an environmental gradient along a transect located within the primary channel of the swash. Measured concentrations are plotted against the conservative mixing line to assess the chemical changes in properties which either result in the generation or removal of these properties with respect to the transect. The non-conservative behavior of these properties forms the basis for assessment on the role of a permeable sand column in modifying land-derived inputs. Highly permeable sediments (2.9 +/- 1.1 x 10^(-11) m^2) compose the sandy transect with which visually observed tidally-driven currents interact. Higher pore-water nutrient concentrations (dissolved inorganic nitrogen = 3-30 umol/L) compared to overlying-water concentrations (3-30 umol/L vs. 0-20 umol/L, respectively, for dissolved inorganic nitrogen) support the assumption that organic matter, filtered by the sandy sediment of the primary channel, is respired and nutrients are generated. Sedimentary chlorophyll concentrations are at least ten times as high as overlying-water chlorophyll concentrations (0-3 ug/cm^3 vs. 0-0.02 ug/cm^3, respectively), consistent with a "benthic nutrient filter", whereby microphytobenthos are intercepting pore-water nutrients that are being transported upwards towards the overlying water. During periods of expected high primary productivity, the swash is a source of overlying-water oxygen and chlorophyll a, while at periods of expected low productivity, the swash is a source for nutrients and a sink for chlorophyll a. This transect study of the shoreline segment of Singleton Swash documents the role of a permeable sand column functioning as a biofilter. Inputs of nutrients from surface run-off (35 umol/L and 4 umol/L for dissolved inorganic nitrogen and dissolved phosphate, respectively) resulted in high sedimentary chlorophyll concentrations but also a sink in both sedimentary chlorophyll and overlying-water oxygen, as expected in eutrophic conditions. These findings imply that the presence of a highly permeable sand column may be serving as an effective mitigation strategy for land-derived inputs of nutrients prior to reaching the coastal ocean, and must be given consideration in coastal engineering designs.
Legut, Nicholas Anthony, "The role of a permeable sand column in modifying tidal creek geochemistry and land-derived inputs to the coastal ocean" (2019). Electronic Theses and Dissertations. 112.