Influence of sublittoral microphytobenthos on the oxygen and nutrient flux between sediment and water. A laboratory continuous-flow study

Abstract

Influence of sublittoral microphytobenthos on the flux of oxygen and inorganic nutrients ( N , P, Si) at the sediment-water interface was studied using undisturbed cores of sandy and muddy sediment incubated in a laboratory continuous-flow system, either in darkness or with a 16/8 h L/D cycle at in situ light level during summer. Sediment was collected In July at 15 m depth in a non-tidal, stratified bay in SE Kattegat. To test whether the higher content of inorganic nutrients below the halocline, compared with surface waters, could stimulate microphytobenthic growth, 2 levels of nutrient concentrations were used. Diel variations were found In L/D cores, but not in darkened cores, for oxygen, dissolved inorganic nitrogen and phosphorus content in the water overlying the sediment. The flux of NH4+, NO3and ~ 0 ~ ~ out of the sediment decreased during light periods and occasionally a net uptake was recorded. Light-induced O2 production, and correlations between A fluxes (differences between day and night fluxes of 0, and nutrients), chlorophyll a content and algal cell numbers in the sediment, indicate that the decreased outflux of IN and POA3was mediated by photosynthetic organisms. Diel variations were not studied for silicon, but a significantly lower outflux, or even an uptake, of Si(OH), from L/D cores supports this conclusion. This suggests that diatoms play a major role in the nutrient flux between sediment and water. Also, the differences in pore-water nutrient gradients between LID and dark cores point to the importance of sediment-associated organisms. Daily (24 h) net fluxes of nutrients were primarily out of the sediment, but the magnitude depended on both light conditions and sediment type. Daily net outflux was significantly lower in U D cores than in darkened cores for all nutrients except NO3in muddy cores and NO2in sandy cores. Net uptake in L/D cores was recorded for Si(OH)4 and NO3In sandy sediment. Outflux of nutrients was significantly higher from muddy sediments in comparison w ~ t h sandy sediments (except NO3-), especially in permanent darkness. No significant effect of nutrient enrichment on the abundance of sublittoral benthic microalgae could be shown. Results suggest that m~crophytobenthos can influence sediment-water exchange of inorganic nutrients even at sublittoral depths, and when measuring nutrient flux in permanently darkened cores from depths around 15 m in the Kattegat, summer flux rates will be overestimated by a factor varying between 2 and 6, depending on sediment type.