Experimental feeding and growth studies on filter-feeding organisms often rely on constant algal concentrations maintained over extended periods of time. Here we present a fluorometer controlled apparatus (FCA) designed for feeding experiments with suspension-feeding mussels at naturally low chlorophyll concentrations above 0.5？μg？L？1. The principle used is feedback regulation of the algal concentration based on continuous monitoring of the fluorescence intensity of chlorophyll in water pumped through the apparatus from an aquarium with mussels. The filtration rate is monitored continuously as the rate of change of measured volume of an algal stock added to the aquarium for keeping a constant algal concentration. As an example, the FCA has been used to study the filtration rates of blue mussels (Mytilus edulis) at algal concentrations both near and above the incipient saturation level for reduced filtration activity. As another example to put the FCA into perspective as a reliable method for environmental effect studies, the apparatus has been used to demonstrate the acute effect of changing salinity on the filtration rate of M. edulis. 1. Introduction Bioenergetic and feeding-behavioural studies on filter-feeding organisms may often rely on supply of feed algae maintained at constant concentrations over extended periods of time. In the case of the blue mussel (Mytilus edulis) algal concentrations below a lower critical level [1–5] and above an upper critical saturation concentration (about 6？μg chlorophyll (chl ) L？1; see minireview by Riisg？rd et al. [6]) lead to partial shell closure and reduced filtration rate. The feeding behaviour of Mytilus edulis was studied by Riisg？rd et al. [6] using the so-called steady-state method (e.g., [7]) at different well-defined but relatively high algal concentrations eventually leading to reduced filtration rate. Thus, the upper algal concentration at which the mussel exploits its filtration capacity over an extended period of time was identified by stepwise raising the steady-state algal concentration, and the threshold concentration for incipient “saturation reduction” of the filtration rate was found to be between 5,000 and 8,000 (Rhodomonas salina) cells？mL？1, equivalent to 6.3 and 10.0？μg chl L？1, respectively [6]. However, the steady-state method can only be used to ensure a constant algal concentration between the lower and upper critical algal concentration where the filtration rate is constant [7]. Therefore, a more sophisticated feedback-controlled apparatus is needed for maintaining a well-defined algal
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