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

Abstract

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 a n 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 b e n t h~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.

Effect of aquacultured oyster biodeposition on sediment N<sub>2 production in Chesapeake Bay

“…Water was filtered through Whatman GF/F filters (25 mm diameter, 0.7 µm nominal pore size), and the filtrate was analyzed with a Lachat Quick-Chem 8000 automated ion analyzer for NO 3 and NH 4 . Benthic fluxes were reported as mmol N m −2 d −1 and were calculated using the equation (C out − C in ) × F/A, where C represents the concentration of any analyte, C out and C in are the outflow and inflow concentrations (µM), respectively, F is the peristaltic pump flow rate (l h −1 ), and A is the surface area of the core (m 2 ) (Miller-Way & Twilley 1996, Ensign et al 2008. Although some previous studies have conducted light/dark experiments and analyzed the samples using MIMS (Ferguson & Eyre 2007), we chose to not use light incubations because this approach often results in the formation of gas bubbles, which significantly and selectively affect gas concentrations in water (Reeburgh 1969), an effect reflected by our experience.…”

“…All direct measures of N 2 production have the potential to bias rates because they are impacted by several microbe-mediated processes such as oxygen availability, substrate diffusion to reaction sites, light conditions, and variability due to benthic microalgal activity (Miller-Way & Twilley 1996, Groffman et al 2006. A possible limitation of the study is that the MIMS N 2 flux rates could be interpreted as underestimates due to potential impacts on the microbial community during transport and that MIMS core incubations were conducted in the dark.…”

“…Impacts on the microbial community during transport were minimized by icing the cores and maintaining overlying ambient water in uncapped cores. To minimize problems associated with maintaining steady-state conditions in light−dark incubations using the flowthrough core setup (a critical assumption of this method, Miller-Way & Twilley 1996), MIMS N 2 flux rates were based on dark incubations and extrapolated based on 12 h d −1…”

Phytoplankton response to high salinity and nutrient limitation in the eastern Adriatic marine lakes

“…growth and grazing rates) that affect phytoplankton abundance (Smayda, 1980;Reynolds, 1997;Longhurst, 1998). Pelagic production in these systems is stimulated by nutrient inputs from the watershed (Kemp and Boynton, 1984) and coupled to benthic production (Johnson and Wiederholm, 1992;Miller-Way and Twilley, 1996). Regardless of their proximate source, nutrients that support phytoplankton growth may be characterized broadly in terms of the atomic ratios of inorganic nitrogen, phosphorus and, in the case of diatoms, silicon (N:P:Si) (Redfield et al, 1963;Brzezinski, 1985).…”

Dynamics of nutrient cycling and related benthic nutrient and oxygen fluxes during a spring phytoplankton bloom in South San Francisco Bay (USA)

“…Pelagic production in these systems is stimulated by nutrient inputs from the watershed (Kemp & Boynton 1984) or upwelling of offshore waters (Taylor 1992) and coupled to benthic production in both marine and freshwater ecosystems (Johnson & Wiederholm 1992, Goedkoop & Johnson 1996, Miller-Way & Twilley 1996. Benthic-pelagic coupling works in both directions: sediment nutrient fluxes fuel primary production in the water column (Nixon et al 1976, Callender & Hammond 1982, and cycles of production in the water column provide pulsed inputs of labile organic substrate for regeneration in sediments.…”