MEPS 140:257-269 (1996)  -  doi:10.3354/meps140257

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

Miller-Way T, Twilley RR

Continuous flow systems have not been widely used in the study of benthic-pelagic coupling in marine systems. This paper discusses the theoretical and practical use of continuous flow systems for the study of benthic exchange processes and presents the results of experiments which compared continuous flow (open) and closed (batch) systems and investigated the sensitivity of exchange rates to residence times 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 incubation. However, the selection of a supply rate (i.e. residence time) is critical in their operation. Variable residence times delineate 3 patterns of sediment-water exchange: a linear 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 NH4, NO2, PO4 (1 station), and Si(OH)4 (both stations) were significantly greater when measured using continuous flow methodology than when using closed (batch) incubations. Sediment oxygen consumption as well as fluxes of NO3+NO2 varied directly with supply rate, while fluxes of NH4 were not significantly different over the range of supply rates tested. Examinations of overlying water nutrient concentrations indicated 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 residence time of the overlying water in the core. Rates were similar when both techniques resulted in linear responses. Differences between techniques resulted when either feedback or washout responses were observed. The observed variable response among nutrients indicated that the kinetics of benthic regeneration differ among nutrients and implies that operation of continuous flow systems (i.e. residence time) should be optimized for the analyte under study. Open (continuous flow) systems 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 increased statistical rigor and permits the direct study of cause-effect relationships through the use of experimental manipulations. The primary disadvantage of continuous flow systems lies in their labor-intensive operation.

Benthic-pelagic coupling · Methodology · Experimental manipulation · Chemostat · Nutrient flux · Sediment oxygen consumption · Continuous flow · Open system

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