Abstract
Abstract. The net ecosystem productivity (NEP) of two seagrass meadows within one of the largest seagrass ecosystems in the world, Florida Bay, was assessed using direct measurements over consecutive diel cycles during a short study in the fall of 2018. We report significant differences between NEP determined by dissolved inorganic carbon (NEPDIC) and by dissolved oxygen (NEPDO), likely driven by differences in air–water gas exchange and contrasting responses to variations in light intensity. We also acknowledge the impact of advective exchange on metabolic calculations of NEP and net ecosystem calcification (NEC) using the “open-water” approach and attempt to quantify this effect. In this first direct determination of NEPDIC in seagrass, we found that both seagrass ecosystems were net heterotrophic, on average, despite large differences in seagrass net above-ground primary productivity. NEC was also negative, indicating that both sites were net dissolving carbonate minerals. We suggest that a combination of carbonate dissolution and respiration in sediments exceeded seagrass primary production and calcification, supporting our negative NEP and NEC measurements. However, given the limited spatial (two sites) and temporal (8 d) extent of this study, our results may not be representative of Florida Bay as a whole and may be season-specific. The results of this study highlight the need for better temporal resolution, accurate carbonate chemistry accounting, and an improved understanding of physical mixing processes in future seagrass metabolism studies.
Highlights
Seagrass ecosystems are often net autotrophic, producing more organic matter (OM) than they consume (Duarte et al, 2005, 2010; Barrón et al, 2006; Unsworth et al, 2012; Long et al, 2015a; Ganguly et al, 2017; Perez et al, 2018)
There was a difference between CO2 fluxes derived using the RC01 and Ho06 k600 parameterizations, but this difference was small in magnitude compared to net ecosystem productivity (NEP) and net ecosystem calcification (NEC); so for the sake of simplicity, we only present results using the Ho06 parameterization in the main text of this paper
We present the first direct NEP determined by dissolved inorganic carbon (NEPDIC) measurements in a representative seagrass meadow by combining rigorous carbonate system analysis with a diel sampling approach
Summary
Seagrass ecosystems are often net autotrophic, producing more organic matter (OM) than they consume (Duarte et al, 2005, 2010; Barrón et al, 2006; Unsworth et al, 2012; Long et al, 2015a; Ganguly et al, 2017; Perez et al, 2018). Florida Bay is a well-studied seagrass-dominated ecosystem and is assumed to be net calcifying given the vast autochthonous sedimentary deposits of CaCO3 that have accumulated in the bay in the last 3 millennia (Stockman et al, 1967; Bosence et al, 1985). While much of this CaCO3 was produced by other photoautotrophic or non-photoautotrophic calcifiers (Frankovich and Zieman, 1994), it is likely that some unknown fraction was de-
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