Biogeochemical effects of macroalgal decomposition on intertidal microbenthos: a microcosm experiment

The decomposition of macroalgal detritus (tubular and planar Ulva spp.) was studied in a microcosm under a daily light:dark cycle to simulate the decomposition on intertidal sediment. The consequences of bloom decay were evaluated in the bulk water phase and in the sediment. Inorganic nutrients (NO3-, NO2-, NH4+, PO43- and SiO32-), dissolved organic carbon (DOC) and inorganic carbon (DIC) were measured in the inflowing and outflowing seawater. Vertical microprofiles Of 02, pH and H2S at the sediment-water interface, sediment contents of organic matter (OM), inorganic and organic carbon (C,,,g), total nitrogen (N) and inorganic nutrients were measured before and after addition of macroalgal detritus. Changes in the taxonomic composition of microphytobenthos were studied by optical microscopy and by the analysis of photosynthetic pigments. Macroalgal detritus vanished from the sediment surface in 6 d. Macroalgal decomposition shifted the microcosm net balance to higher releases of DOC, DIC and inorganic nutrients, suggesting rapid release from macroalgal biomass. Besides being released to the water column, a fraction of macroalgal carbon and of nitrogen was incorporated into the sediment as indicated by a transient increase in C-org and N. Aerobic mineralization of macroalgal detritus reduced 02 in the water column and the sediment. Microbenthos photosynthetic activity was initially suppressed but recovered from the third day as macroalgal detritus decomposed. Photosynthetic 02 production by microbenthos largely determined the fraction of macroalgal detritus that was aerobically mineralised. Decomposition of macroalgal detritus favoured the dominance of cyanobacteria over diatoms in the microbenthos.