Enhancement of sediment suspension and nutrient flux by benthic macrophytes at low biomass

Abstract

In shallow coastal ecosystems where most of the seafloor typically lies within the photic zone, benthic autotrophs dominate primary production and mediate nutrient cycling and sediment stability. Because of their different structure and metabolic rates, the 2 functional groups of benthic macrophytes (seagrasses, macroalgae) have distinct influences on benthic−pelagic coupling. Most research to date in these soft-bottomed systems has focused on mature seagrass meadows where shoot densities are high and on dense macroalgal mats that accumulate in response to eutrophication. Relatively little is known about the influence of low-biomass stands of seagrass and macroalgae on nutrient fluxes and sediment suspension. Using an erosion microcosm with controlled forcing conditions, we tested the effects of the eelgrass Zostera marina L. and the invasive macroalga Gracilaria vermiculophylla on sediment suspension and nutrient fluxes under high-flow conditions. At low densities, G. vermiculophylla increased sediment suspension and increased the nutrient flux from the sediment to the water column. For macroalgae, increased sedi ment suspension is likely due to dislodgement of sediment particles by bedload transport of the algae. In this case, the increase in sediment transport was reflected in an increase in nutrient flux from the sediment, showing that modification of physical forcing by benthic primary producers can also affect nutrient flux. The presence or absence of Z. marina did not have a significant effect on nutrient flux. However, the results suggest that there may be a range of low shoot densities for which storm-like flows increase sediment suspension to values higher than those expected for a bare sediment bed.

Hydrodynamics of vegetated channels

“…Consistent with this, Moore (2004) observed that resuspension within a seagrass meadow was reduced, relative to bare-bed conditions, only when the above-ground biomass per area was greater than 100 g/m 2 (dry mass), which corresponds to ah = 0.4 (Luhar et al 2008). In a similar study, Lawson et al (2012) measured sediment erosion in beds of different stem densities. Using the blade length (8 cm) and width (3 mm) provided in that paper, we convert the stem density into a roughness density ah.…”

Strong and weak, unsteady reconfiguration and its impact on turbulence structure within plant canopies

“…In addition, the penetration of turbulence through the canopy to the bed determines the likelihood of sediment resuspension, an important feedback to vegetation health. 10,11 Specifically, resuspension negatively impacts light availability for photosynthesis and associated erosion may destabilize shoots. Dense canopies that reduce near-bed turbulence can enhance the supply of nutrients to the plants by promoting the retention of nutrient-rich fine sediment and organic matter.…”

INTRODUCTION Eelgrass recovery in the coastal bays of the Virginia Coast Reserve, USA

“…They suggest that the greater tidal range and proximity of the coastal bays to cooler ocean waters ameliorates the stress from high air temperature periods during the summer. Using controlled microcosm experiments, Lawson et al (2012) show that at low shoot densities Z. marina increases sediment suspension due to the horizontal deflection of flow around eelgrass shoots, but that past a threshold density, eelgrass reduces sediment suspension. In field studies, Hansen & Reidenbach (2012) quantify the relative effects of meadow structure on tidal currents, waves, and near-bed turbulence, and the resulting sediment suspension: expansion of Z. marina within the coastal bays has shifted the seafloor from an erosional to depositional environment, leading to enhanced light penetration through the water column and creating a positive feedback for Z. marina growth.…”