Investigation of the effects of current velocity on mussel feeding and mussel bed stability using an annular flume

Abstract

An annular flume was used to measure the effect of increasing current velocity on mussel (Mytilus edulis) feeding rate and the stability of mussel beds sampled from the mouth of the Exe estuary (SW England). It was found that, in contrast to earlier flume studies, the feeding rates of mussels from open coast sites were unaffected by current velocities up to 0.8 m s -1 . Algal cell depletion in the water column above mussels was a function of current velocity, increasing with declining currents below 0.05 m s -1 . The erodability/stability of the mussel bed, measured in terms of critical erosion velocity, sediment mass eroded and mean erosion rate, was found to be a function of the nature of the substrate and the density of the mussels. Erosion of mussel beds on sandy substrate showed a non-linear relationship with mussel bed density. In comparison with the sand (0% mussel cover), sediment resuspension was about five and four times higher for 25% and 50% cover, respectively. This was due to the increased turbulence and scouring around the clumps of mussels in low-density parts of the bed, and this resulted in some mussels detaching from the bed. At ~100% mussel cover, the sandy bed was more protected by the dense surface layer of mussels, and none became detached during erosion due to the high number of byssal attachments between individuals. The sediment resuspension from the 100% mussel cover was about three times lower than the 0% cover. Erosion of the bed with 50% cover resulted in burial of a large proportion of the mussels, with a 6 cm increase in sediment level. However, the mussels returned to the surface and recovered in 1-2 days, due to a combination of migration upwards and substrate settlement. Channels on the edge of the main Exmouth mussel bed were characterised by a more stable substrate comprising pebbles and sand with varying mussel densities. At these sites, where mussels experience high current velocities on spring tides (up to 0.9 m s -1 ), there was no difference between the erodability of pebble/sand substrate with 0% and 100% mussel cover. The sediment erosion was also lower than the 100% mussel cover on the sandy substrate, particularly at currents >0.4 m s -1 . Sampling of different parts of the mussel bed at Exmouth showed mussels at low densities were made up of smaller clumps with a lower mass ratio of mussels to attached substrate (pebbles/sand), thus providing a greater degree of anchorage.

Scale‐Dependent Feedback and Regular Spatial Patterns in Young Mussel Beds

“…This hypothesis concurs with an extensive body of literature reporting that depletion of the algae in lower water layers limits the growth of mussels in real (Frechette and Bourget 1985;Newell 1990;Muschenheim and Newell 1992;Frechette and Despland 1999;Dolmer 2000) and artificial conditions (Wildish and Kristmanson 1984;Widdows et al 2002). Moreover, mussels were found to generate local heterogeneity in the concentration of algae in the lower water layers, even on spatial scales of !10 m (Asmus et al 1992;L.…”

“…A significant body of literature exists that shows that, in soft-sediment or subtidal mussel beds, algal food availability limits intake by mussels (Wildish and Kristmanson 1984;Dolmer 2000;Widdows et al 2002) and mussel growth (Frechette and Bourget 1985;Page and Hubbard 1987;Newell 1990;Muschenheim and Newell 1992;Frechette and Despland 1999;Oie et al 2002). Dislodgment and predation depend strongly on local mussel density (Bertness and Grosholz 1985;Okamura 1986;Cote and Jelnikar 1999;Hunt andScheibling 2001, 2002) because nearby conspecifics are the main substrate for attachment on soft-bottom sediment.…”

Long-term changes of intertidal and subtidal sediment compositions in a tidal basin in the northern Wadden Sea (SE North Sea)

“…In particular subtle changes in speed and force of the tidal currents leave traces in the granulometry of associated sediments. In addition, biological factors like seagrass coverage (Ward et al 1984;Fonseca and Fisher 1986;Heiss et al 2000), mussel beds (Widdows et al 2002) or the presence of the lugworm Arenicola marina (Wohlenberg 1937;Volkenborn and Reise 2006) can have a signiWcant impact on the sediment composition. Changes in the grain size composition of Wadden Sea sediments can also be man-made.…”

Production efficiency of mussel bottom culture

“…Suitable substrates with high stability might consist of solid material such as rocks, or artificial structures (such as floating spat mussel collectors), but can also be sedimentary, such as macrophytes, shell material, or pebbles (Seed 1976, Witman & Suchanek 1984, Dankers & Zuidema 1995, Widdows et al 2002, Kangeri et al 2014. Several studies have suggested that beds with older mussels are a preferred settlement substrate, adding to the rejuvenation of existing mussel beds (Herlyn & Millat 2000, Dare et al 2004, Dolmer & Stenalt 2010, Commito et al 2014).…”

“…Mussels can thereby maintain filtration at current velocities up to 0.8 m s-1 (Widdows et al 2002). Furthermore, vertical mixing is enhanced by the roughness of the sea floor and thereby enhanced by the structural complexity of the mussel bed, thus exerting a positive feedback loop of mussels on their food transport (Fréchette et al 1989, Wiles et al 2006, Saurel et al 2013, Folmer et al 2014.…”