Thomas G. Wolaver

The summer uptake and release of nitrogen by an intertidal oyster reef

Abstract It has long been speculated that oyster reefs process estuarine nutrients, but direct observation of this has been difficult. With a plastic tunnel (BEST - Benthic Ecosystem Tunnel), input/output concentrations of nitrogen and water velocities over an oyster reef were measured for 7 tidal cycles during summer conditions. Ammonia was always released from the oyster reef and average rates varied from 2825 to 15 304 μmol·m −2 ·h −1 . Ammonia release was usually higher on the ebb tide than the flood tide. It is speculated that nitrogen uptake and release may be a function of not only feeding and digestive behaviour, but also of water velocity. The magnitude of ammonia release is much higher than any previous observations on natural benthic systems, suggesting that oyster reefs may be important components in nitrogen cycling in estuaries.

The flume design — a methodology for evaluating material fluxes between a vegetated salt marsh and the adjacent tidal creek☆

An experimental flume is described which can be used as a tool to assess whether a vegetated marsh surface is a source or sink for nutrients via tidal inundation. An initial calibration study (two tidal cycles) was conducted to determine the optimum sampling design and aid in model development for flux calculations. A statistical analysis of the data showed a negligible concentration difference as a function of water depth for most of the constituents analyzed. This coupled with the low tidal velocities over the marsh surface (<1.5cm/s) suggested that a volumetric model was adequate for calculations of instantaneous discharge and nutrient flux through any station perpendicular to tidal flow. The resultant instantaneous mass flux calculations showed that water discharge was one of the dominant factors controlling the movement of material. A sine-cosine statistical model utilizing the main tidal periodicities was designed to: (1) model the instantaneous fluxes, (2) calculate the average net flux of suspended and dissolved materials, and (3) test the hypothesis that the average net flux equals zero versus a two-sided alternative using a standard regression t-test.

The exchange of phosphorus between a euhaline vegetated marsh and the adjacent tidal creek

Abstract Phosphorus exchange between a euhaline vegetated marsh and the adjacent tidal creek (North Inlet, South Carolina) was studied on 40 tidal cycles between 15 April 1983 and 19 June 1984. A flume was utilized to evaluate the net phosphorus exchange during tidal inundation, and a weir study was conducted to estimate the export of phosphorus from the marsh via runoff and seepage during low tide exposure (including storm events). Mean flood tide concentrations of PO 4 3− and particulate phosphorus (PP) varied seasonally between 0·1 and 1·3 μg at. P l −1 and 0·3 to 2·8 μg at. P l −1 , respectively, with the highest values observed during the summer. PO 4 3− and PP were consistently removed from the tidal water inundating the marsh especially during the warmer months. There was a statistically significant (α = 0·05) import of PO 4 3− and PP to the vegetated marsh during tidal inundation of 0·46 gP m −2 year −1 and 2·01 gP m −2 year −1 , respectively. For both constituents the removal rate on an areal basis was greater in the low marsh (tall Spartina alterniflora ) than on the high marsh (medium and short S. alterniflora ). The magnitude and direction of total phosphorus (TP) flux was similar to PP because the latter constituted up to 85% of the former. Export values of PO 4 3− , PP, and TP from the marsh via runoff and seepage during low-tide exposure (including rain events) were approximately 25% of the respective import values during tidal inundation. This study suggests a euhaline vegetated marsh is a sink for PO 4 3− , PP, and TP and therefore may not be the source of phosphorus outwelled in many marsh-estuarine systems.

The Bly Creek ecosystem study — organic carbon transport within a euhaline salt marsh basin, North Inlet, South Carolina

Dissolved and particulate organic carbon fluxes were measured within the Bly Creek basin, North Inlet area, Georgetown County, South Carolina. 34 tidal cycles were sampled between 20 June 1986 and 19 June 1984. Flux direction and magnitude were determined for each measured tidal cycle. Continuously measured auxiliary variables were used in a regression model to produce annual flux estimates with minimum se. Annual budgets were similarly developed for subsystems within the basin, and annual input to the basin by rain, groundwater, and freshwater runoff were estimated. Mean dissolved organic carbon (DOC) concentrations varied from 2.5 to 11.8 mg C·l−1, with lowest values in the summer and fall and highest values in late winter and spring. Net flux estimates varied similarly with greatest exports occurring during late winter and spring. On an annual basis 1.85 × 105 kg C·yr−1 (se 3.2 × 104 kg C·yr−1) was exported through the tidal creek. Freshwater inputs accounted for ≈ 11% of the observed export with 1.65 × 105 kg C originating within the marsh basin. Particulate organic carbon (POC) concentrations varied from 0.63 to 5.3 mg C·l−1 with largest concentrations in late summer. The annual basin flux estimate of 2.04 × 104 kg C·yr−1 imported was not statistically different from zero. However, there was a significant import of 2.1 × 104 kg C·yr−1 from the tidal creek to the marsh surface.

Factors affecting short-term variability in sediment pH as a function of marsh elevation in a Virginia mesohaline marsh

An analysis of 5 days of nearly continuously recorded sediment temperature, pH, and radiation measurements along a transect in a mesohaline marsh in Virginia suggests there was a shift in control over short-term pH variability from tidal inundation to radiation with height of the marsh surface at least in the surficial sediments. There was little evidence for tidal control of pH variability at depth in the sediment column. However, biological control of subsurface pH variability was evident both near the tidal creek and in the high marsh. Sediment pH means were generally highest toward the tidal creek and lower in the high marsh. The latter zone had the highest pH variability with diurnal pH excursions up to two units being observed during the experimental period. It is hypothesized that macrophyte transpiration or a series of interlocking mechanisms associated with photosynthesis and microbial activity were responsible for the pH excursions observed at depth in the marsh, since the rapid changes in pH were triggered at sunrise and sunset. The large excursions in sediment pH in the high marsh rhizosphere suggest that geochemical activity may be dynamic over the diurnal cycle.

The bly creek ecosystem study: Phosphorus transport within a euhaline salt marsh basin, North Inlet, South Carolina☆

Abstract Phosphorus transport through the tidal creek linking the Bly Creek basin (North Inlet, South Carolina) and the surrounding water body was studied on 34 tidal cycles between 20 June 1983 and 19 June 1984. Annual estimates of phosphorus input to the basin via streamwater, groundwater, and via streamwater, groundwater, and rain totalled 19.6 kg P·y −1 ; PO 4 export from the basin was not significant. Within the basin, the salt marsh was shown to be an important sink for PO 4 (207 kg P·y −1 ) while the oyster reef community exported a statistically insignificant 7.7 kg P·y −1 . The data suggest that the source of the PO 4 to the salt marsh is the water column or benthic sediments of the tidal creek. The salt marsh was a significant sink for particulate phosphorus, but those uptakes were less than the error on the insignificant fluxes into the basin via the water column. The oyster reef community was a statistically significant sink for total phosphorus (98 kg P·y −1 ).