Eight experimental ponds containing submersed vascular plants (predominantly Potamogeton perfoliatus and Ruppja maritirna) were subjected in duplicate to 4 levels (including controls) of fertilization from June to August 1981. Seston and phytoplankton chlorophyll a increased with fertilization, and pronounced algal blooms were evident under high dosage. Of the total seston. phytoplankton exerted the greatest influence on attenuation of photosynthetically active radiation (PAR), such that there was insufficient light for submersed vascular plant growth at the sediment surface during bloonls. An extensive epiphytic community developed on plants in all nutrient-treated ponds at densities similar to those observed in nature on senescent plants. At high nutrient treatments the accumulation of epiphytic material resulted in > 80 % attenuation of the incident radiation at the leaf surface. Biomass of submersed macrophytes decreased significantly under high and medium nutrient treatments compared to control and low treatments within 60 d following initial fertilization. Apparent production of vascular plants (based on oxygen production and I4C-bicarbonate uptake) was reduced at the higher nutrient treatments for both R. maritirna and P. perfoliatus. Most of this reduction in macrophyte photosynthesis could be explained by attenuation of PAR associated with epiphytic material. However, without PAR attenuance in the overlying water, observed levels of epiphytic growth would be insufficient to reduce light below compensation levels needed to sustain vascular plant growth. At the high fertilization rates, integrated primary production of pond communities was significantly reduced with the loss of the vascular plants, even though phytoplankton and epiphytic growth were enhanced.

Rates of direct denitrification were measured using I5N isotope techniques in intact sediment cores from fringe and basin mangroves in Terminos Lagoon, Mexico. Sediments were injected with 15N0, and the distribution of I5N was measured over time in the head space, overlying water, and sediments. Experiments included an investigation of spatial variation in denitrification rates by comparing results from the fringe and basin mangroves in the rainy season (July 1991); a second experiment was to determine the effect of 3 diffel-ent NO.; concentrations (25, 100, 200 pm01 core-' "N-KN03-) on denitrificatlon rates and was performed in the fringe mangrove during the 'Norte' season (January 1992). Highest I5N-N2 fluxes were measured in the fringe mangrove at 9.4 pm01 m-* h-', while denitrification rates in the basin mangrove ranged from 1.9 to 4.5 pm01 m-' h-'. I5N-Nz fluxes in sediment cores from the fringe mangrove were significantly higher (4.5 to 7.7 pm01 m-2 h-') in cores enriched with 200 pm01 core-' 1 S~-~~0 3 -c o m p a r e d to cores enriched with 25 and 100 pm01 core-' I5N-K N 0 3 -( < l pm01 m-2 h''). Most of the applied I5N was recovered as particulate nitrogen in the sediment and a small fraction reduced to NH,' in both experiments. The low denitrification rates observed In the fringe and basin mangroves indicate that the capacity for sediment denitrification is limited by low NO3-availability. Previous nutrient exchange studies concluded that the fringe mangrove was a 'sink' of NO3-since sediment uptake of NO3-was assumed lost through denitrification. Results from this study show ~1 0 % of sediment N O 3 uptake in fringe mangroves may be lost to denitrification; the remainder being immobilized in the sediment.

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.