EFFECTS OF EELGRASS BEDS ON ADVECTION AND TURBULENT MIXING IN LOW CURRENT AND LOW SHOOT DENSITY ENVIRONMENTS

The effects of eelgrass Zostera marina beds on the rates of horizontal transport (advection) and turbulent mixing in the water column were m easured in the field. The dye-tracking technique used in this study di rectly measures mixing within the canopy rather than estimating mixing from velocity profiles. Tomales Bay, California, and False Bay, Washi ngton, USA, were characterized by slow currents (generally <0.05 m s(- 1)) and low shoot densities (100 to 200 shoots m(-2)). At these sites, the eelgrass canopy reduced advection, but had no effect on turbulent mixing relative to areas without eelgrass. At sites that had nearly c ontinuous eelgrass cover, there was greater advection and mixing above the eelgrass canopy than there was at the same height in the water co lumn over bare substratum. In contrast, sites with discrete eelgrass b eds surrounded by sand showed no evidence of such accelerated now over the canopy. In addition, the eelgrass canopy damped out the variation s in advection and mixing due to wind and tidal currents that occurred in adjacent areas without eelgrass. Previous studies have focused on the effects of seagrasses on flow in relatively fast currents (typical ly >0.10 m s(-1)) and high shoot densities (>400 shoots m(-2)). This s tudy has shown that only parts of the paradigm from these previous stu dies holds in sluggish flow and sparse grass (i.e. reduced advection w ithin the canopy) whereas other parts of the paradigm differ (i.e. tur bulence within the canopy).