Se. Worcester, EFFECTS OF EELGRASS BEDS ON ADVECTION AND TURBULENT MIXING IN LOW CURRENT AND LOW SHOOT DENSITY ENVIRONMENTS, Marine ecology. Progress series, 126(1-3), 1995, pp. 223-232
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).