Dissolved oxygen fluxes and ecosystem metabolism in an eelgrass (Zostera marina) meadow measured with the eddy correlation technique

Dissolved oxygen (DO) fluxes were measured by eddy correlation to estimate net ecosystem metabolism (NEM) during summer in a restored eelgrass (Zostera marina) meadow and a nearby, unvegetated sediment. This technique measures benthic fluxes under true in situ light and hydrodynamic conditions, integrates over a large area (typically . 100 m2), and captures short-term variations. DO fluxes measured through eight 24-h periods showed pronounced temporal variation driven by light and local hydrodynamics on multiple scales: hour-to-hour, within each daily cycle, and between deployments. The magnitude of variation between hours during single deployments equaled that between deployments, indicating that short-term variation must be included for metabolism estimates to be accurate. DO flux variability was significantly correlated to mean current velocity for the seagrass site and to significant wave height for the unvegetated site. Fluxes measured in low-flow conditions analogous to many chamber and core incubations underestimated those measured in higher-flow conditions typical of in situ conditions by a factor of 2–6. Rates of gross primary production (GPP), respiration (R), and NEM varied substantially between individual deployments, reflecting variations in light and hydrodynamic conditions, and daily values of GPP and R for individual deployments were tightly linked. Average daily NEM of the seagrass site was higher than that of the unvegetated site; the seagrass site was in metabolic balance, and the unvegetated site showed a tendency toward net heterotrophy during this midsummer period. Benthic metabolism is the key component of overall system metabolism and nutrient cycling in shallow bays and lagoons where most of the seafloor lies within the photic zone (McGlathery et al. 2007). Benthic autotrophs, including seagrasses and microand macroalgae, fix substantial amounts of carbon, are a temporary sink for nutrients, and influence bacterial processes in the sediment (e.g., mineralization, nitrification–denitrification, anaerobic ammonium oxidation [Annamox]) by modifying redox conditions and competing with bacteria for limiting nutrients. The status of the benthos in shallow coastal systems as a source or a sink of carbon and nutrients is largely dependent on the metabolism of these primary producers and the associated heterotrophs in the community (Eyre and Ferguson 2005; Sundback and McGlathery 2005; McGlathery 2008). Seagrasses are the foundation of benthic communities in many shallow systems, and their metabolism often exceeds that of nearby unvegetated sediments by several fold (Barron et al. 2006; Stutes et al. 2007; Apostolaki et al. 2010). However, accurate estimates of metabolism are difficult to obtain using conventional methods because of the relatively large stature of seagrasses, their often dense root system, the complex hydrodynamics associated with the canopy structure, and the spatial heterogeneity characteristic of these communities. There are many methods that have been used to determine benthic metabolism, including extrapolation from photosynthesis–irradiance curves (Kraemer and Alberte 1993), changes in primary producer biomass (Hasegawa et al. 2007), mass balance models (Kemp et al. 1997; Kaldy et al. 2002), diel changes in water-column concentrations of either dissolved oxygen (DO) or dissolved inorganic carbon (‘‘open-water’’ method; D’Avanzo et al. 1996; Ziegler and Benner 1998), and direct benthic flux measurements of DO or dissolved inorganic carbon in either in situ chambers or laboratory incubations of substrate cores (McGlathery et al. 2001; Gazeau et al. 2005; Yarbro and Carlson 2008). These conventional methods all have significant limitations. For example, in situ chambers or laboratory-incubated cores may bias flux measurements, especially in permeable sediments, because they do not replicate natural hydrodynamic forcing and light levels (Cook and Røy 2006; Berg and Huettel 2008). Core incubations are typically done at a single irradiance level or in complete darkness to determine daytime and nighttime metabolism, and flow rates are generally very low compared to in situ conditions. The chamber incubation technique captures some of the natural variation in light, although the chamber itself reduces light penetration, and flows are also generally lower than in situ conditions (Tengberg et al. 2004). For the open-water method, there are inaccuracies associated with difficulties in correcting for oxygen exchange across the air–water interface and changes in oxygen concentrations due to horizontal flow (Ziegler and Benner 1998). Studies comparing the different methods have shown that the rates of ecosystem metabolism can vary significantly depending on the type of method used due to these problems (Kemp and Boynton 1980; Ziegler and Benner 1998). In this study, we used the eddy correlation technique (Berg et al. 2003) to determine benthic DO fluxes for two benthic communities—a temperate seagrass meadow, and, for reference, a nearby ‘‘unvegetated’’ sediment that contained benthic microalgae. We used these fluxes to estimate gross primary production (GPP), community * Corresponding author: [email protected] Limnol. Oceanogr., 56(1), 2011, 86–96 E 2011, by the American Society of Limnology and Oceanography, Inc. doi:10.4319/lo.2011.56.1.0086