Water flow in tide- and wave-dominated beds of the seagrass Thalassia testudinum

Abstract

Biological processes in seagrass meadows are regulated by the exchange of momentum, heat and mass between the surrounding water and the plants and thus may strongly depend on the characteristics of water flow and turbulence. Comparisons of mean flow profiles, turbulence distribution and mixing in meadows of the seayrass Thalassia testudinum colonizing 2 hydrodynamically different sites (wave-dominated and tide-dominated) suggest that the hydrodynamic microclimate and consequently mixing within seagrass beds strongly depend on the hydrodynamic forces (waves and currents) acting on the plants. Unidirectional flows deflect the water over the meadow (skimming flows). which potentially leads to lower mixing between the water above and within the meadow. In contrast, waves cause the blades to move back and forth, increasing the exchange between the water column and that within the meadow. Therefore, the hydrodynamic conditions prevailing in the seagrass habitat (waves, tides) change the pattern of flow attenuation and mixing within the vegetation.

Nutrient dynamics of seagrass ecosystems: 15N evidence for the importance of particulate organic matter and root systems

“…Moreover, seagrass canopies strongly enhance sedimentation by reducing current velocity and wave energy. Canopy trapping has been demonstrated in temperate areas (Koch 1999, Koch & Gust 1999, Terrados & Duarte 2000, Granata et al 2001 as well as in tropical ones (Agawin & Duarte 2002, Gacia et al 2003. In a seagrass meadow dominated by Thalassia hemprichii, Agawin & Duarte (2002) showed that particle loss is 4 times higher than in unvegetated areas, suggesting an important transfer of planktonic production to the seagrass meadow.…”

Role of two contrasting ecosystem engineers (Zostera noltii and Cymodocea nodosa) on the food intake rate of Cerastoderma edule

“…density, stiVness, length, etc.…; see Bouma et al 2005 andKoch andGust 1999 andreferences therein). Hence, we speculate that contrasting results between studies on the growth and/or survival of Wlter feeder in seagrass meadows might perhaps be partly explained by contrasting seagrass canopy properties by having diVerent eVects on food supply.…”

Spatial structure of communities on dead pen shells (Atrina rigida) in sea grass beds

“…In these sites, A. zelandica densities are an order of magnitude greater than the densities found in St. Joe Bay; these high densities may be the reason behind the modiWcation of water Xow by pen shells and their inXuence on settling organisms in nearby areas (Norkko et al 2006). The New Zealand sites have water velocities ranging from 0.05 to 0.35 m s ¡1 (Hewitt et al 2002) while in St. Joe Bay water Xows much slower, at 0.027-0.045 m s ¡1 (Koch and Gust 1999), which would also help explain the inXuence of A. zelandica density on the surrounding macrofauna diversity in New Zealand. However, these studies focused on species living in the substrate adjacent to pen shells and not on dead pen shell inhabitants.…”

“…This connection between sea grass beds and pen shell communities may be direct (e.g., inhabitants of pen shells directly beneWting from the surrounding sea grass) or indirect (e.g., pen shells and sea grass both beneWting from similar factors such as current and nutrients). St. Joe Bay's T. testudinum meadows can reduce water turbulence (Koch and Gust 1999), which would enhance propagule settlement on to pen shells. One explanation for this variation is that shell density seems to aVect motile but not sessile species community structure.…”