Effects of seagrass beds (Zostera noltii and Z. marina) on near-bed hydrodynamics and sediment resuspension

Abstract

The main objectives of this flume study were to (1) quantify density dependent effects of the short-leaf seagrass Zostera nolti on hydrodynamics and sediment resuspension from a sandy bed, and (2) measure the erodability of 2 contrasting sediments (sandy and muddy) and the extent to which this is modified by the presence of 2 seagrass species, Z. noltii (sandy) and Z. marina (muddy). Field measurements of near-bed tidal currents, turbulence and suspended particulate matter at 2 different Z. noltii locations (low energy [sheltered] and higher energy [exposed] environments) were interpreted in the context of the flume results. Skimming flow above the high density bed of Z. noltii was accompanied by a 40% reduction in near-bed flow, but this was offset by a 2-fold increase in turbulent kinetic energy (TKE) and bed shear stress (τ 0 ). Despite this increase in τ 0 there was an increase in sediment stabilisation with increasing seagrass density (10-fold increase in critical bed shear stress for erosion [τ e ] from 0.1 [bare sediment] to 1.0 Pa at the highest shoot density). This was largely explained by the increased microphytobenthos abundance (reflected in the higher chlorophyll a and carbohydrate contents) and a lower density of the grazer and bio-destabiliser Hydrobia ulvae. In contrast, the muddy site was more easily eroded (10-fold higher), with Z. marina having little effect on sediment erodability (bare: τ e = 0.05 Pa; Z. marina: τ e = 0.07 Pa). This higher erodability was due to differences in hydrodynamics and the physical/biological properties of the sediment. KEY WORDS: Seagrass beds · Zostera noltii · Zostera marina · Hydrodynamics · Sediment resuspensionResale or republication not permitted without written consent of the publisher

Lugworm (Abarenicola affinis) in seagrass and unvegetated habitats

“…In the present study, sediment organic content was similarly low in both habitat types, whereas microphytobenthos abundance (measured as sediment chlorophyll a concentration) was higher in the unvegetated habitat than in the seagrass habitat. In contrast to other studies that have documented increased organic matter accumulation and microphytobenthos growth in seagrass areas due to attenuation of water flow (Widdows et al 2008), these effects appear to be less distinct in the seagrass habitat of the present study. As detritus of Z. muelleri represented an additional available food source for A. affinis (Leduc et al 2006), food availability was not considered to be a limiting factor for lugworm growth in the seagrass habitat.…”

“…The second core (2.5 cm diameter, 2 cm depth) was taken for sediment chlorophyll a analysis. The latter two parameters indicate potential food sources for lugworms in the form of available organic material (including seagrass detritus) and microphytobenthos (Longbottom 1970;Leduc et al 2006;Widdows et al 2008). In addition, in the seagrass habitat, seagrass within the lugworm sampling core was cut off at the sediment surface prior to excavation and retained.…”

Avian communities inSpartina maritimarestored and non-restored salt marshes

“…S. maritima and Z. noltii act as marsh-structuring halophytes (Castellanos et al 1994, Figueroa et al 2003, Bouma et al 2009), increasing habitat diversity, providing organic matter and stabilizing sediments (Salgueiro & Caçador 2007, Widdows et al 2008. Thus, plantations would increase environmental heterogeneity in restored marshes, increasing the diversity of birds' foraging habitats (Weller & Spatcher 1965) because microhabitats are home for different invertebrates and fish species in low marshes (Nienhuis & Groenendijk 1986, Cardoso et al 2007, MacKenzie & Dionne 2008, Parker et al 2008.…”

Long-term displacement of intertidal seagrass and mussel beds by expanding large sandy bedforms in the northern Wadden Sea

“…Furthermore, it is unlikely that seagrass could be able to prevent large sandy bedforms from intruding. Although the intertidal Z. noltii is perennial and may stabilise sediment (Widdows et al, 2008), the leaves are shed in autumn and thus during winter the seagrass sites appear to be bare. Winter is the season when almost all storms happen to occur which could contribute to large sandy bedform development or enhancement.…”