An invasive macroalga alters ecosystem metabolism and hydrodynamics on a tidal flat

Invasive macroalgae represent a growing global concern, yet there has been little research detailing their effects on native ecosystem metabolism and flow hydrodynamics. In this in situ study, we made aquatic eddy covariance oxygen flux and profiling acoustic Doppler velocimetry measurements over high and low density sites within an invasive Gracilaria vermiculophylla (also referred to as Agarophyton vermiculophyllum) macroalgal mat located on a tidal flat along the Virginia (USA) coast. Mean +/- SE (n) submerged dark and light oxygen flux over the high density site were -178 +/- 9 (144) and 139 +/- 26 (104) mmol m(-2) d(-1), respectively, vs. -71 +/- 7 (45) and 32 +/- 15 (48) mmol m(-2) d(-1) over the low density site. The high density site was highly productive, with gross primary production of 169 +/- 12 mmol m(-2) d(-1) (mean +/- SE), yet maintained overall metabolic balance, as net ecosystem metabolism was -22 +/- 39 mmol m(-2) d(-1). Close agreement with literature values for air-exposed flux measurements suggests that our results are good first-order approximations of true daily values, demonstrating that this invasion has significantly enhanced tidal flat productivity. Flow and turbulence were significantly attenuated by Gracilaria, resulting in decreased bed shear stress within the algal canopy, likely stabilizing tidal flat sediments and inhibiting mass transport at the sediment-water interface. Turbulence above the canopy was enhanced, but there was no increase in downward momentum transfer. These changes to the hydrodynamic environment decrease the risk of detachment and subsequent advective loss of algae, suggesting a positive feedback that facilitates algal invasion.