No abstract

Based on noninvasive eddy correlation measurements at a marine and a freshwater site, this study documents the control that current flow and light have on sediment-water oxygen fluxes in permeable sediments. The marine sediment was exposed to tidal-driven current and light, and the oxygen flux varied from night to day between 229 and 78 mmol m 22 d 21 . A fitting model, assuming a linear increase in oxygen respiration with current flow, and a photosynthesis-irradiance curve for light-controlled production reproduced measured fluxes well (R 2 5 0.992) and revealed a 4-fold increase in oxygen uptake when current velocity increased from , 0 to 20 cm s 21 . Application of the model to a week-long measured record of current velocity and light showed that net ecosystem metabolism varied substantially among days, between 227 and 31 mmol m 22 d 21 , due to variations in light and current flow. This variation is likely typical of many shallow-water systems and highlights the need for long-term flux integrations to determine system metabolism accurately. At the freshwater river site, the sediment-water oxygen flux ranged from 2360 to 137 mmol m 22 d 21 . A direct comparison during nighttime with concurrent benthic chamber incubations revealed a 4.1 times larger eddy flux than that obtained with chambers. The current velocity during this comparison was 31 cm s 21 , and the large discrepancy was likely caused by poor imitation by the chambers of the natural pore-water flushing at this high current velocity. These results emphasize the need for more noninvasive oxygen flux measurements in permeable sediments to accurately assess their role in local and global carbon budgets.

Accurate light measurements are important in the analysis of photosynthetic systems. Many commercial instruments are available to determine light; however, the comparison of light estimates between studies is difficult due to the differences in sensor types and their calibrations. The measurement of underwater irradiance is also complicated by the scattering and attenuation of light due to interactions with particulates, molecules, and the bottom. Here, three sensor types are compared to evaluate the calibration of light intensity loggers to estimate photosynthetically active radiation (PAR). We present a simple calibration of light intensity loggers that agree within 3.8% to factory-calibrated scalar PAR sensors under a wide range of environmental conditions. Under the same range of conditions, two identical factory-calibrated PAR sensors showed a similar difference of 3.7%. The light intensity loggers were calibrated to a high-quality PAR sensor using an exponential fit (r 2 = 0.983) that accounts for differences in sensor types with respect to the angle of incoming light, scattering, and attenuation. The light loggers are small, robust, and simple to operate and install, and thus well-suited for typical subsurface research. They are also useful for small-scale measurements, when broad spatial coverage is needed, or in research requiring multiple sensors. Many studies have used these simple light intensity sensors to estimate PAR, yet their limitations and advantages in mimicking PAR have not been well defined previously. We present these small and user-friendly loggers as an excellent alternative to more sophisticated scalar PAR sensors.

The Virginia coastal bays experienced local extinction of eelgrass (Zostera marina) during the early 1930s, and restoration beginning in 2001 has generated an ecosystem state change from bare to vegetated sediments. Oxygen fluxes were measured seasonally using the eddy correlation technique at three sites representing different stages of seagrass colonization: unvegetated (bare), 5 yr, and 11 yr since seeding. Derived seasonal ecosystem respiration (R) and gross primary production (GPP) increased up to 10-fold and 25-fold, respectively, with meadow age. Although hourly oxygen (O 2 ) fluxes were highly correlated with light at the vegetated sites, no identifiable trends with light were observed at the bare site. The light compensation point where O 2 production and respiration are in balance increased from 46 mmol photons m 22 s 21 to 257 mmol photons m 22 s 21 and 63 mmol photons m 22 s 21 to 472 mmol photons m 22 s 21 at the 5 yr and 11 yr seagrass sites, respectively, with increasing seasonal temperatures from 12.3uC to 27.9uC and 9.3uC to 30.5uC, respectively. This suggests that more light, and thus more O 2 production, is required to offset increasing respiration with both temperature and meadow age. Photosynthesisirradiance curves generated from hourly O 2 fluxes throughout the seasons were used to estimate annual net ecosystem metabolism (NEM). Annual NEM rates at the bare, 5 yr, and 11 yr sites were 27.6, 8.6, and 27.0 mol O 2 m 22 yr 21 , respectively. Although the system went through a period of net autotrophy during early stages of colonization, the ecosystem state change from unvegetated sediments to dense seagrass meadows changed the magnitude of both GPP and R, but not the overall metabolic balance of the system.