Control of suspension feeding bivalve production by current speed

Abstract

Growth experiments confirm that production by a bed of blue mussels (Mytilus edulis) may be controlled by tidal current speed through its effect on seston supply. The mechanism of this effect involves a downstream seston depletion and is thus applicable only to populations of mussels. Individual physiological responses by mussels, such as increased filtration, ingestion and assimilation rates at higher current speeds, are not involved in the enhanced bivalve production observed.

Does flow speed also have a direct effect on growth of active suspension‐feeders: An experimental test on oysters

“…2). This result is consistent with those of Wildish and Kristmanson (1985) for blue mussels and of Wildish et al (1987), Eckman et al (1989), and Wildish and Saulnier (1993) for two species of scallops. Among these studies, direct effects of flow were detectable, although the pattern of response of growth to increasing flow differed.…”

“…This confounding of direct and indirect effects of flow on suspension-feeder growth in nature and in flumes motivated us to establish our test of oyster feeding response inside pipes, where despite complex flow regimes steady flows can be maintained and flow character is generally predictable. Furthermore, the absence of a vertical dimension to the water column in pipes prevents establishment of vertical concentration gradients (Kirby-Smith 1972;Wildish and Kristmanson 1985;Eckman et al 1989). Our intent was not to duplicate natural boundary-layer flows, but rather to artificially isolate the direct effects of flow on feeding and growth from the indirect effects that are typically confounded in nature so that food depletion would not interfere with a test of this one component of response to flow.…”

“…Growth of active (and also passive) suspension-feeders can be enhanced at higher flow speeds because of enhancement of turbulent vertical mixing. In faster flows, greater vertical mixing reduces the intensity of vertical gradients in food concentration created by feeding of benthic suspension-feeders and minimizes the resultant depletion of foods near the sea floor (Wildish and Kristmanson 1979;Frechette et al 1989;Skilleter and Peterson 1994). This enhancement of vertical mixing by flow increases represents an indirect effect of flow velocity on growth, acting through the mechanism of enhancing food concentrations around feeding animals.…”

“…This process is most clearly suggested in studies of scallops, especially studies demonstrating that the influence of external flow varies with the orientation of the feeding animal to the direction of flow. When the inhalant opening to the mantle cavity is directed into the flow, growth rates of scallops are enhanced with increasing flows over a range of relatively slow current velocities, whereas if the exhalant opening is pointed into the flow, the unfavorable pressure gradient inhibits feeding and reduces growth (Wildish et al 1987;Eckman et al 1989;Wildish and Saulnier 1993). Despite this evidence of direct effects of flow on feeding and growth of scallops, general models of feeding by active suspension-feeders do not incorporate any direct effect of flow speed on feeding rate (Frechette et al 1989;Monismith et al 1990;O'Riordan et al 1993), andFrechette et al (1994) have argued that the apparent relationship of growth in active suspension-feeders to flow and food flux is illusory (an "epiphenomenon"), driven only by the indirect effects of flow on food concentration.…”

“…To test whether oysters within a pipe were exposed to the same food conditions or whether, alternatively, foods were depleted by upstream oysters (cf. Kirby-Smith 1972;Wildish and Kristmanson 1985), we performed separate linear regressions of individual oyster growth on position (distance from the beginning of each pipe). Subsequently, a two-way, crossed model 1 ANOVA with five levels of current speed and two levels of food concentration was conducted on untransformed growth data to test whether average growth in shell width varied with average external current speed, food concentration, or their interaction.…”