Eddy correlation measurements of oxygen fluxes in permeable sediments exposed to varying current flow and light

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 m22 d21. 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 (R2 5 0.992) and revealed a 4-fold increase in oxygen uptake when current velocity increased from , 0 to 20 cm s21. 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 m22 d21, 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 m22 d21. 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 s21, 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. The oxygen flux between sediments with their biotic communities and the overlying water is the most commonly used proxy for benthic carbon mineralization and primary production (Canfield et al. 1993; Glud 2008). As a result, benthic oxygen exchange is a key component in carbon budgets on both local and global scales. The continental shelf contributes about 50% of benthic carbon mineralization in the ocean, and the highest rates are found near shorelines, where shallow water depths and nutrient and organic input from land boost primary production (Middelburg and Soetaert 2004; Middelburg et al. 2005). As a result, a quantitative understanding of carbon transformations on the inner shelf—an environment dominated by permeable sands (Emery 1968)—is essential for carbon cycle assessments. A similar argument can be made for freshwater systems that have recently been recognized as transforming or storing more than 50% of the carbon inputs from land rather than serving as passive conduits to the ocean (Cole et al. 2007; Aufdenkampe et al. 2011). Thus, numerous rivers and creeks lined with highly permeable sands likely play an important role in carbon cycling. Despite the large number of studies that have focused on transport and biogeochemical processes in permeable sediments (Huettel and Webster 2000), there is less known about biogeochemical cycling in these sediments relative to muddy cohesive sediments. One reason for this discrepancy is that it is more challenging to quantify processes in sands under realistic in situ conditions. Permeable sediments allow substantial advective transport of water that carries both dissolved and solid substances into and out of the open pore space between the sand grains (Huettel et al. 1996). This gives rise to a very different reaction scheme for biogeochemical processes—processes to which benthic fluxes are tightly linked—compared to what is typically found in muddy sediments. For example, the filtering of particulate organic matter by sands when flushed with bottom water and the transport of dissolved organic matter effectively transport ‘food’ for mineralization processes deep into the sands. This supply of organic matter, combined with the advective transport of oxygen into sands, stimulates oxic mineralization (Reimers et al. 2004; Cook et al. 2007). It is this flushing of sands and their exchange of water-bound constituents with the water column that are difficult to capture in benthic flux measurements. The main drivers of advective transport in permeable sediments are well known. Pressure differences arising from current flows (Thibodeaux and Boyle 1987) and wave action (Riedl et al. 1972) over rippled or otherwise uneven sand surfaces can induce significant pore-water flow in the upper sand layers. Also, pumping activity of tube-building macrofauna can result in pore-water movement in permeable sands (Foster-Smith 1978). However, only a few studies have reported exchange and flow rates and their effects on biogeochemical cycling. Precht and Huettel * Corresponding author: [email protected] Limnol. Oceanogr., 58(4), 2013, 1329–1343 E 2013, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2013.58.4.1329