Date

Fall 2017

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)

Department

Moss Landing Marine Laboratories

Abstract

The Elkhorn Slough, in the heart of the Monterey Bay, includes water bodies that have been isolated from tidal flushing cycles by dikes, levees, roads and train tracks. This partitioning has changed the functionality of these systems primarily through reduced circulation and increased eutrophication. The Elkhorn Slough is surrounded by a patchwork of lands under varied land-uses, including open space, developed properties, and extensive agriculture, that results in high and variable nutrient loading into the surrounding aquatic habitat. Water bodies of restricted flow disproportionately exhibit impacts due to this loading. In this study, nutrient concentrations were measured using both discrete water column sampling methods and in situ continuous osmotic sampling techniques. Fluxes were measured using water column nutrient determinations, flow measurements, ground water flux correlations, benthic chambers, modeled pore water gradients and estimates of Ulva uptake. These measurements were used to quantify the cycling of nitrate, ammonium and phosphate in these pocket regions and have identified systemic nutrient drivers to be surface water flow, Ulva uptake and groundwater inputs. A box model approach was used to determine the degree to which these drivers contributed to overall nutrient concentrations on a seasonal timescale. The systems of study were all in dynamic disequilibrium, rather than steady-state. Nitrate varied from 355.5 μM to 0.0 μM on timescales as short as one month. Ammonium varied from 199.3 μM to 0.0 μM and phosphate varied from to 22.7 μM to 0.0 μM on similar timescales. These variations were large compared to the same nutrients in the adjacent Elkhorn Slough. Such variability is significant when characterizing these systems, as it is indicative of the nature of nutrient flux in tidally restricted ecosystems and the rapid extremes in chemical composition experienced by the resident biota. The dominant sources of nutrients in these restricted areas also varied in time with surface runoff dominating in the wet season, and ground water inputs (possibly due to agricultural irrigation) dominating in the dry season. Ulva uptake and advective flow were the largest loss terms and these too varied significantly in time. Understanding the key nutrient drivers, as well as the degree to which these drivers influence biogeochemical cycling of nutrients in these systems, informs mitigation projects for best management of estuarine regions with structural barriers inhibiting natural flow, an increasingly common feature of the coastal landscape. In addition, in this thesis, I suggest that variability itself is a niche/ecosystem characteristic that forces the structure of resident biota such as Ulva.

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