Spatiotemporal variability of light attenuation and net ecosystem metabolism in a back-barrier estuary
Authors: Ganju Neil K. , Testa Jeremy M. , Suttles Steven E. , Aretxabaleta Alfredo L. 
Published: May 2020

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.