MEPS 601:15-32 (2018)  -  DOI: https://doi.org/10.3354/meps12697

ABSTRACT: Long-term deterioration of water quality is known to reduce the importance of benthic ecosystem metabolism in shallow coastal ecosystems, but drivers of spatial and short-term variability in ecosystem metabolism are poorly understood. We addressed this knowledge gap through detailed seasonal measurements of ecosystem metabolism across depth gradients from shallow (2 to 3 m) eelgrass-dominated to deeper (4 to 5 m) muddy regions of a shallow, productive estuary. Combined measurements of gross primary production (GPP), respiration (R) and, by difference, net ecosystem production (NEP) by the open-water diel oxygen technique and in situ chamber incubations showed high importance of shallow eelgrass habitats for metabolism at the system scale. Seasonal variations in GPP, R and NEP increased with light availability and temperature with highest NEP in all habitats during the warm and sunny mid-summer. The shallow eelgrass-dominated and neighboring habitats were seasonally net autotrophic (NEP = 0.54 and 0.31 mg O₂ m^-2 d^-1, respectively), compared to net heterotrophy (NEP = -0.26 mg O₂ m^-2 d^-1) at the deeper muddy site. Detailed studies along depth gradients further confirmed the role of eelgrass as a key driver of spatial differences in ecosystem metabolism across the estuary. Strong northerly winds (>8 m s^-1) caused short-term (<24 h) periods of similar oxygen dynamics and similar apparent productivity in shallow and deeper waters, indicative of efficient lateral mixing, while calm periods (<4 m s^-1) enabled formation of ‘pockets’, i.e. water masses with limited connectivity, which exacerbated the metabolic differences between shallow and deep sites.

KEY WORDS: Intra-estuarine heterogeneity · Benthic habitat · Pelagic habitat · Temporal variability · Eelgrass