Oxygen Exchange and Hydrodynamics of Tidal Flat Ecosystems Along the Virginia, USA Coast

Bare tidal flats along much of the eastern USA coast are increasingly undergoing state changes to one of two systems: human-restored Crassostrea virginica oyster reefs or invasive Gracilaria vermiculophylla macroalgal mats, with little known about how these changes impact ecosystem metabolism and flow hydrodynamics. In this dissertation, I present results from a combination of aquatic eddy covariance (AEC), acoustic Doppler velocimetry (ADV), and bioacoustic sound measurements, which collectively demonstrate how these changes have altered the metabolism, turbulence, and soundscapes of tidal flats on the Virginia coast.
Chapters 1 and 2 are focused on the benthic oxygen flux of oyster reefs, which I measured in situ using the non-invasive AEC technique. In Chapter 1, I present results from summer AEC measurements at 4 sites: a bare mudflat, 2 restored oyster reefs of differing age and oyster density, and a high-density natural oyster reef. Both respiration (R) and primary production increased with oyster density, while flow speed and light were important drivers of oxygen flux. The reefs were all highly heterotrophic, indicating that they are net consumers of carbon, and all had a similar R per oyster, showing that ecosystem function is enhanced with oyster density. In Chapter 2, I present results from additional seasonal (fall, winter, spring, summer) AEC measurements from the natural reef that were combined with data from Chapter 1 to create a 4-year record of reef metabolism that included an oyster die-off. Seasonality significantly impacted R, likely due to temperature effects, but had less impact on gross primary production (GPP). As a result, there was a clear shift in reef net ecosystem metabolism (NEM), from nearly balanced in the winter to highly heterotrophic in the summer. Over the 4-year record summer values of R, GPP, and NEM were all correlated to oyster density, while R and GPP were closely coupled, indicating a tight internal cycling of carbon on the reef. Following the die-off R, GPP, and NEM were all substantially diminished, indicating a loss of ecosystem function.
In Chapter 3, I describe AEC oxygen flux and profiling ADV measurements over high- and low-density sites within a Gracilaria mat that colonized the previously bare mudflat from Chapter 1. There were significant increases in both nighttime oxygen uptake and daytime oxygen release between bare, low-density, and high-density measurements, indicating enhanced ecosystem metabolism at increasing algal density. The high-density site in particular had substantial GPP, yet was overall metabolically balanced. There were also significant impacts of this invasion on hydrodynamics. Flow speed and turbulence were significantly diminished within the algal canopy, likely stabilizing tidal flat sediments and inhibiting mass transport at the sediment-water interface, while turbulence above the canopy was enhanced. These changes to the hydrodynamic environment are likely beneficial to Gracilaria, suggesting a positive feedback that facilitates its colonization of tidal flats.
Chapter 4 includes results from bioacoustic sound recordings, conducted in collaboration with E. M. Stine from the UVA music department, combined with ADV turbulence measurements over 2 of the oyster reefs from Chapter 1 and a nearby bare mudflat. Oyster reef sound and turbulence act as cues for larval settlement, with both stimuli detected by the statocyst organ. We found strong correlations between turbulence dissipation and low frequency (f < 100 Hz) sound over the sites, indicating that turbulent pressure fluctuations are equivalent to low frequency sound waves. Therefore, it is possible that oyster larvae are not responding to separate stimuli, but rather to low frequency pressure fluctuations detectable in the soundscape.
Taken collectively, this research demonstrates the substantial impact that these state changes from bare mud to oyster reefs or macroalgal mats have had on tidal flats. Whether the primary management goal is to promote (oyster reefs) or prevent (Gracilaria mats) these state changes, it is critical for resource managers to understand their effects on native ecosystem function.