High Net Primary Production of Mediterranean Seagrass (Posidonia oceanica) Meadows Determined With Aquatic Eddy Covariance

Abstract

We report primary production and respiration of Posidonia oceanica meadows determined with the non-invasive aquatic eddy covariance technique. Oxygen fluxes were measured in late spring at an open-water meadow (300 m from shore), at a nearshore meadow (60 m from shore), and at an adjacent sand bed. Despite the oligotrophic environment, the meadows were highly productive and highly autotrophic. Net ecosystem production (54 to 119 mmol m −2 d −1) was about one-half of gross primary production. In adjacent sands, net primary production was a tenth-to a twentieth smaller (4.6 mmol m −2 d −1). Thus, P. oceanica meadows are an oasis of productivity in unproductive surroundings. During the night, dissolved oxygen was depleted in the open-water meadow. This caused a hysteresis where oxygen production in the late afternoon was greater than in the morning at the same irradiance. Therefore, for accurate measurements of diel primary production and respiration in this system, oxygen must be measured within the canopy. Generally, these measurements demonstrate that P. oceanica meadows fix substantially more organic carbon than they respire. This supports the high rate of organic carbon accumulation and export for which the ecosystem is known.

Technical note: Estimating light-use efficiency of benthic habitats using underwater O<sub>2</sub> eddy covariance

“…A key requirement of the LUE approach is high-quality GPP data. Despite there being numerous potential obstacles to obtaining this data (Table 1), a growing number of eddy covariance studies document tight relationships between hourly fluxes and sunlight availability in a wide array of aquatic 10 habitats such as in sediment deposits, seagrass canopies, coralline algal beds and coral reefs Chipman et al, 2016;Attard et al, 2014;Attard et al, 2015;Rheuban et al, 2014;Long et al, 2013;Long et al, 2015;Koopmans et al, 2020;Rovelli et al, 2017). In this study, R 2 values for lightsaturation curves ranged from 0.83 to 0.94 indicating a predominant primary production signal, and this gives credence to applying the LUE approach.…”

“…flow-induced advective pore water exchange in highly permeable sediments (Table 1). Despite there being numerous sources of variability, high-quality EC fluxes often show a tight coupling to sunlight (photosynthetic active radiation, PAR) availability on the hourly timescale, indicating a dominant 15 primary production signal in many aquatic systems Chipman et al, 2016;Attard et al, 2014;Attard et al, 2015;Rheuban et al, 2014;Long et al, 2013;Long et al, 2015;Koopmans et al, 2020;Rovelli et al, 2017).…”

“…conservative estimates of GPP, since R typically is higher during daytime in the presence of photosynthesis (Fenchel and Glud, 2000;Hotchkiss and Hall, 2014). Indeed, several EC studies have documented lower O2 effluxes in the evening than in the morning under similar light intensities (a socalled 'hysteresis'), and high R rates at the onset of darkness (Rovelli et al, 2017;Rheuban et al, 2014;Koopmans et al, 2020). It is generally understood that R is stimulated by GPP; it increases 5 progressively throughout the day as labile photosynthates accumulate (Epping and Jørgensen, 1996;de Winder et al, 1999), and the magnitude of the hysteresis is related to the light history (Adams et al, 2016).…”

Carbon Balance in Salt Marsh and Mangrove Ecosystems: A Global Synthesis

“…Global seagrass area from [195], macroalgae area from [196] and area of other ecosystems from [188], except the global coastal ocean [194], which encompasses estuaries, other wetlands, and continental shelves. Seagrass data [188] updated from [197][198][199][200][201][202][203][204][205][206][207][208][209][210] and macroalgae data from . Plankton GPP and R in salt marshes from [237] and in mangroves from [52].…”

Technical note: Estimating light-use efficiency of benthic habitats using underwater O<sub>2</sub> eddy covariance

“…when EC O 2 flux = benthic O 2 flux) and (2) "true" temporal variability in the benthic O 2 exchange rate (i.e. when EC O 2 flux = benthic O 2 flux) EC O 2 flux = benthic O 2 flux Reference EC O 2 flux = benthic O 2 flux Reference Non-steady-state conditions within the benthic boundary layer Holtappels et al (2013), Brand et al (2008) Changes in diffusive boundary layer thickness in cohesive sediments Kuhl et al (1996) Sensor stirring sensitivity Holtappels et al (2015) Pore-water advection in permeable sediments Cook et al (2007), McGinnis et al (2014) Surface wave influence Berg et al (2015), Reimers et al (2016) Diel fauna activity Wenzhofer and Glud (2004) Sensor response time McGinnis et al (2008), Berg et al (2015) Sediment resuspension Toussaint et al (2014) Internal plant O 2 storage, canopy storage, or bubbling Attard et al (2019a), Rheuban et al (2014), Long et al (2020) Oxidation of anaerobic metabolites in sediments Fenchel and Glud (2000) Nutrient availability Elser et al 2007Photosynthesis-coupled respiration Epping and Jørgensen (1996) Acclimation of the photosynthetic system Ralph et al (2002) 1.2 Eddy covariance estimates of benthic primary production…”