Theory and operation of continuous flow systems for the study of benthic-pelagic coupling

Abstract

Continuous flow systems have not been widely used in the study of benthic-pelagic coupling in marine systems. T h ~ s paper discusses the theoret~cal and practical use of continuous flow systems for the study of benthlc exchange processes and presents the results of experiments w h ~ c h compared continuous flow (open) and closed (batch) systems and investigated the sensitivity of exchange rates to residence t ~ m e s in a continuous flow system. Continuous flow systems minimize the problem of environmental dependency of benthic-pelagic exchange rates by maintaining initial experimental conditions throughout an ~ncubatlon. However, the selection of a supply rate (i .e. residence timc) is cntical in their operation. Variable res~dence times delineate 3 patterns of sediment-water exchange a hnear response which reflects an optimal supply rate, a feedback response which indicates deviation from initial experimental conditions (environmental dependency), and a washout response which results in an erroneous estimate of exchange. For either closed (batch) or continuous flow incubations, only linear responses result In valid estimates of sediment-water column exchange. Sediment oxygen consumption and benthic fluxes of NH,. NO,, PO, (1 station), and Si(OH), (both stations) were significantly greater when measured using continuous flow methodology than when using closed (batch) incubations. Sediment oxygen consumption as well a s fluxes of NO,+NO, varied directly with supply rdte, while fluxes of NH, were not significantly different over the range of supply rates tested. Examinations of overlying water nutrient concentrat~ons ~ndicated that observed differences in benthic fluxes between the 2 methods and the varying sensitivities of nutrient flux to supply rate were a function of the res~dence time of the overlying water In the core. Rates were s ~ m ~ l a r when both techniques resulted In linear rpsponses. Differences between techniques resulted when either feedback or washout responses were observed. The observed variable response among nutnents Indicated that the kinetics of ben th~c regeneration dlffer among nutrients and implies that operation of continuous flow systems (1.e. restdence time) should be opt~mized for the analyte under study. Open (continuous flow) systcms have many advantages for the determination of benthic-pelagic exchange rates. In a continuous flow system, initial experimental conditions are maintained throughout an incubation. This permits experiments of relatively long duration which allows ~ncreased statistical rigor and permits the direct study of causeeffect relationships through the use of experimental manipulations. The primary disadvantage of continuous flow systems lies in their labor-~ntensive operation.