The dissolution of gypsum or plaster of Paris has been widely used as an inexpensive integral measure of ‘water motion’ in the field and in laboratory tanks for studies of physical‐biological interactions. Commonly, gypsum‐dissolution rates have been calibrated to steady flow speed or velocity in the laboratory and the calibrations have been applied to dissolution (i.e., mass‐transfer) rates in the field or in tanks. We evaluated the gypsum‐dissolution technique in a steady‐flow, a fluctuating‐flow, and a mixed‐flow environment by comparing dissolution rate to direct flow measurements with an acoustic Doppler velocimeter. We found that dissolution rates were related to steady flow and to fluctuation intensity in the exclusively steady‐flow and fluctuating‐flow environments, respectively. The relationships were weak in the mixed‐flow environment. Finally, dissolution and thus mass‐transfer relationships were different in each flow environment, and the effects of steady flow and fluctuation intensity were not additive. Providing that it is rigorously checked and appropriately calibrated, the dissolution technique can be used to measure steady flow speed or fluctuation intensity in a steady‐flow or fluctuating‐flow environment, respectively. However, comparisons of dissolution rates between steady‐flow, fluctuating‐flow, and mixed‐flow environments or within environments that change over time to determine water motion will be misleading. The gypsum‐dissolution tech‐nique can be used as a good direct indicator of mass‐transfer rates. However, mass‐transfer rates are different in different flow environments. The gypsum‐dissolution technique is not a universal integrator of ‘water motion.’

Quantifying system-wide biogeochemical dynamics and ecosystem metabolism in estuaries is often attempted using a long-term continuous record at a single site or short-term records at multiple sites due to sampling limitations that preclude long-term monitoring. However, differences in the dominant primary producer at a given location (e.g., phytoplankton versus benthic producers) control diel variations in dissolved oxygen and associated ecosystem metabolism, and they may confound metabolic estimates that do not account for this variability. We hypothesize that even in shallow, well-mixed estuaries there is strong spatiotemporal variability in ecosystem metabolism due to benthic and water-column properties, as well as ensuing feedbacks to sediment resuspension, light attenuation, and primary production. We tested this hypothesis by measuring hydrodynamic properties, biogeochemical variables (fluorescent dissolved organic matter – fDOM, turbidity, chlorophyll a fluorescence, dissolved oxygen), and photosynthetically active radiation (PAR) over 1 year at 15 min intervals at paired channel (unvegetated) and shoal (vegetated by eelgrass) sites in Chincoteague Bay, Maryland–Virginia, USA, a shallow back-barrier estuary. Light attenuation (K_dPAR) at all sites was dominated by turbidity from suspended sediment, with lower contributions from fDOM and chlorophyll a. However, there was significant seasonal variability in the resuspension–shear stress relationship on the vegetated shoals, but not in adjacent unvegetated channels. This indicated that K_dPAR on the shoals was mediated by submerged aquatic vegetation (SAV) and possibly microphytobenthos presence in the summer, which reduced resuspension and therefore K_dPAR. We also found that gross primary production (P_g) and K_dPAR were significantly negatively correlated on the shoals and uncorrelated in the channels, indicating that P_g over the vegetated shoals is controlled by a feedback loop between benthic stabilization by SAV and/or microphytobenthos, sediment resuspension, and light availability. Metabolic estimates indicated substantial differences in net ecosystem metabolism between vegetated and unvegetated sites, with the former tending towards net autotrophy in the summer. Ongoing trends of SAV loss in this and other back-barrier estuaries suggest that these systems may also shift towards net heterotrophy, reducing their effectiveness as long-term carbon sinks. With regards to temporal variability, we found that varying sampling frequency between 15 min and 1 d resulted in comparable mean values of biogeochemical variables, but extreme values were missed by daily sampling. In fact, daily resampling minimized the variability between sites and falsely suggested spatial homogeneity in biogeochemistry, emphasizing the need for high-frequency sampling. This study confirms that properly quantifying ecosystem metabolism and associated biogeochemical variability requires characterization of the diverse estuarine environments, even in well-mixed systems, and demonstrates the deficiencies introduced by infrequent sampling to the interpretation of spatial variability.

Light availability is of primary importance to the ecological function of shallow estuaries. For example, benthic primary production by submerged aquatic vegetation is contingent upon light penetration to the seabed. A major component that attenuates light in estuaries is colored dissolved organic matter (CDOM). CDOM is often measured via a proxy, fluorescing dissolved organic matter (fDOM), due to the ease of in situ fDOM sensor measurements. Fluorescence must be converted to CDOM absorbance for use in light attenuation calculations. However, this CDOM–fDOM relationship varies among and within estuaries. We quantified the variability in this relationship within three estuaries along the mid-Atlantic margin of the eastern United States: West Falmouth Harbor (MA), Barnegat Bay (NJ), and Chincoteague Bay (MD/VA). Land use surrounding these estuaries ranges from urban to developed, with varying sources of nutrients and organic matter. Measurements of fDOM (excitation and emission wavelengths of 365 nm (±5 nm) and 460 nm (±40 nm), respectively) and CDOM absorbance were taken along a terrestrial-to-marine gradient in all three estuaries. The ratio of the absorption coefficient at 340 nm (m⁻¹) to fDOM (QSU) was higher in West Falmouth Harbor (1.22) than in Barnegat Bay (0.22) and Chincoteague Bay (0.17). The CDOM : fDOM absorption ratio was variable between sites within West Falmouth Harbor and Barnegat Bay, but consistent between sites within Chincoteague Bay. Stable carbon isotope analysis for constraining the source of dissolved organic matter (DOM) in West Falmouth Harbor and Barnegat Bay yielded δ¹³C values ranging from −19.7 to −26.1 ‰ and −20.8 to −26.7 ‰, respectively. Concentration and stable carbon isotope mixing models of DOC (dissolved organic carbon) indicate a contribution of ¹³C-enriched DOC in the estuaries. The most likely source of ¹³C-enriched DOC for the systems we investigated is Spartina cordgrass. Comparison of DOC source to CDOM : fDOM absorption ratios at each site demonstrates the relationship between source and optical properties. Samples with ¹³C-enriched carbon isotope values, indicating a greater contribution from marsh organic material, had higher CDOM : fDOM absorption ratios than samples with greater contribution from terrestrial organic material. Applying a uniform CDOM : fDOM absorption ratio and spectral slope within a given estuary yields errors in modeled light attenuation ranging from 11 to 33 % depending on estuary. The application of a uniform absorption ratio across all estuaries doubles this error. This study demonstrates that light attenuation coefficients for CDOM based on continuous fDOM records are highly dependent on the source of DOM present in the estuary. Thus, light attenuation models for estuaries would be improved by quantification of CDOM absorption and DOM source identification.

Air–sea fluxes of momentum and heat were measured simultaneously with surface wind waves and near-surface currents in mid Chesapeake Bay during summer 1998 under low wind conditions. The data were collected using a Gill sonic anemometer and a Sontek Acoustic Doppler Velocimeter with a pressure sensor mounted on a temporary fixed tower in 8.8 m of water. The analyzed data show that the neutral drag coefficients depend upon both wind speed and wave age. They are better correlated to wave age than to wind speed. Data scatter is significantly larger in low winds than in high winds. Under light winds, the neutral drag coefficients increase with decreasing wind speed and have values much higher than those for relatively higher wind speeds. At higher wind speeds, neutral drag coefficients increase with increasing wind speed. Regardless of wind speed, neutral drag coefficients always decrease with increasing wave age. Neutral drag coefficients are lower than the results of similar field studies when fit ... Abstract Air–sea fluxes of momentum and heat were measured simultaneously with surface wind waves and near-surface currents in mid Chesapeake Bay during summer 1998 under low wind conditions. The data were collected using a Gill sonic anemometer and a Sontek Acoustic Doppler Velocimeter with a pressure sensor mounted on a temporary fixed tower in 8.8 m of water. The analyzed data show that the neutral drag coefficients depend upon both wind speed and wave age. They are better correlated to wave age than to wind speed. Data scatter is significantly larger in low winds than in high winds. Under light winds, the neutral drag coefficients increase with decreasing wind speed and have values much higher than those for relatively higher wind speeds. At higher wind speeds, neutral drag coefficients increase with increasing wind speed. Regardless of wind speed, neutral drag coefficients always decrease with increasing wave age. Neutral drag coefficients are lower than the results of similar field studies when fit ...

The shoaling transformation from generally linear deep-water waves to asymmetric shallow-water waves modifies wave shapes and causes near-bed orbital velocities to become asymmetrical, contributing to net sediment transport. In this work, we used two methods to estimate the asymmetric wave shape from data at three sites. The first method converted wave measurements made at the surface to idealized near-bottom wave-orbital velocities using a set of empirical equations: the “parameterized” waveforms. The second method involved direct measurements of velocities and pressure made near the seabed: the “direct” waveforms. Estimates from the two methods were well correlated at all three sites (Pearson’s correlation coefficient greater than 0.85). Both methods were used to drive bedload-transport calculations that accounted for asymmetric waves, and the results were compared with a traditional excess-stress formulation and field estimates of bedload transport derived from ripple migration rates based on sonar imagery. The cumulative bedload transport from the parameterized waveform was 25% greater than the direct waveform, mainly because the parameterized waveform did not account for negative skewness. Calculated transport rates were comparable to rates estimated from ripple migration except during the largest event, when calculated rates were as much as 100 times greater, which occurred during high period waves.

U.S. Geological Survey scientists and technical support staff measured oceanographic, waterquality, seabed-elevation-change, and meteorological parameters in Chincoteague Bay, Maryland and Virginia, during the period of August 13, 2014, to July 14, 2015, as part of the Estuarine Physical Response to Storms project (GS2–2D) supported by the Department of the Interior Hurricane Sandy recovery program. These measurements provide time series data that quantify the response and can be used to better understand the resilience of this back-barrier estuarine system to storms. The Assateague Island National Seashore (National Park Service) and the Chincoteague National Wildlife Refuge (U.S. Fish and Wildlife Service) are on the east side of Chincoteague Bay.