Spatiotemporal variability of light attenuation and net ecosystem metabolism in a back-barrier estuary

Abstract

Abstract. Quantifying system-wide biogeochemical dynamics and ecosystem metabolism in estuaries is often attempted using a long-term continuous record at a single site or short-term records at multiple sites due to sampling limitations that preclude long-term monitoring. However, differences in the dominant primary producer at a given location (e.g., phytoplankton versus benthic producers) control diel variations in dissolved oxygen and associated ecosystem metabolism, and they may confound metabolic estimates that do not account for this variability. We hypothesize that even in shallow, well-mixed estuaries there is strong spatiotemporal variability in ecosystem metabolism due to benthic and water-column properties, as well as ensuing feedbacks to sediment resuspension, light attenuation, and primary production. We tested this hypothesis by measuring hydrodynamic properties, biogeochemical variables (fluorescent dissolved organic matter – fDOM, turbidity, chlorophyll a fluorescence, dissolved oxygen), and photosynthetically active radiation (PAR) over 1 year at 15 min intervals at paired channel (unvegetated) and shoal (vegetated by eelgrass) sites in Chincoteague Bay, Maryland–Virginia, USA, a shallow back-barrier estuary. Light attenuation (KdPAR) at all sites was dominated by turbidity from suspended sediment, with lower contributions from fDOM and chlorophyll a. However, there was significant seasonal variability in the resuspension–shear stress relationship on the vegetated shoals, but not in adjacent unvegetated channels. This indicated that KdPAR on the shoals was mediated by submerged aquatic vegetation (SAV) and possibly microphytobenthos presence in the summer, which reduced resuspension and therefore KdPAR. We also found that gross primary production (Pg) and KdPAR were significantly negatively correlated on the shoals and uncorrelated in the channels, indicating that Pg over the vegetated shoals is controlled by a feedback loop between benthic stabilization by SAV and/or microphytobenthos, sediment resuspension, and light availability. Metabolic estimates indicated substantial differences in net ecosystem metabolism between vegetated and unvegetated sites, with the former tending towards net autotrophy in the summer. Ongoing trends of SAV loss in this and other back-barrier estuaries suggest that these systems may also shift towards net heterotrophy, reducing their effectiveness as long-term carbon sinks. With regards to temporal variability, we found that varying sampling frequency between 15 min and 1 d resulted in comparable mean values of biogeochemical variables, but extreme values were missed by daily sampling. In fact, daily resampling minimized the variability between sites and falsely suggested spatial homogeneity in biogeochemistry, emphasizing the need for high-frequency sampling. This study confirms that properly quantifying ecosystem metabolism and associated biogeochemical variability requires characterization of the diverse estuarine environments, even in well-mixed systems, and demonstrates the deficiencies introduced by infrequent sampling to the interpretation of spatial variability.

Biogeochemical states, rates, and exchanges exhibit linear responses to large nutrient load reductions in a shallow, eutrophic urban estuary

“…Comparable time series of metabolic rates in response to longterm nutrient load reductions are relatively rare, but metrics of net ecosystem metabolism have shown seasonal shifts in response to warming (Staehr et al 2017), positive relationships to nutrient loading (Oviatt et al 1986;Caffrey 2004), and a strong association of autotrophy with relative increases in the ratio of dissolved nitrogen to total organic matter loads (Kemp et al 1997;Herrmann et al 2015). In the case of the Back River estuary, primary production rates approaching 1000 mmol O 2 m À2 d À1 are 3-5 times larger than rates typically reported for estuaries (e.g., Cloern et al 2013;Caffrey et al 2014;Ganju et al 2020), consistent with this estuary having rates of nutrient loading among the highest globally (Boynton et al 2018). In the years following treatment upgrades, overall metabolic rates, the frequency of elevated rates, and the seasonal variability in rates declined to levels now comparable, if not slightly higher, than other relatively eutrophic estuaries (300-500 mmol O 2 m À2 d À1 ).…”

Stressing over the Complexities of Multiple Stressors in Marine and Estuarine Systems

“…The shallow coastal lagoons of Maryland's Coastal Bays and Delaware's inland bays provide further contrasts. They have spatially limited but localized diel cycling hypoxia and acidification, as they are shallower, and biogeochemical interactions with the benthos become more important in these shallow systems, as do high frequency changes in CO 2 , pH, and nutrient fluxes [170]. Retentive zones are also important for HABs, especially those that form cysts, as the cysts accumulate and can lead to blooms whenever favorable conditions occur.…”

Modeling Impacts of Nutrient Loading, Warming, and Boundary Exchanges on Hypoxia and Metabolism in a Shallow Estuarine Ecosystem

“…Due to the nature of their bathymetry and proximity to land, shallow estuarine systems have several unique characteristics compared to larger, deeper systems. Whereas deep estuaries have production and respiration cycles dominated by water-column plankton (e.g., Fennel and Testa 2019), shallow estuaries can be dominated by benthic metabolism from submerged aquatic vegetation (Ganju et al 2020), microphytobenthos (McGlathery et al 2007), or subtidal sediments. One consequence of this distinction is that oxygen depletion in deeper systems tends to be a seasonal, kilometer-scale phenomenon supported by sinking phytoplanktonderived organic material, whereas shallow ecosystems can generate local diel cycling hypoxia over 6-12 h as a result of high rates of benthic metabolism or high rates of water-column respiration associated with high phytoplankton biomass (>100 μg/L; e.g., Tyler et al 2009).…”

“…Whereas deep estuaries have production and respiration cycles dominated by water‐column plankton (e.g., Fennel and Testa 2019), shallow estuaries can be dominated by benthic metabolism from submerged aquatic vegetation (Ganju et al. 2020), microphytobenthos (McGlathery et al. 2007), or subtidal sediments.…”