Benthic primary production and nutrient cycling in sediments with benthic microalgae and transient accumulation of macroalgae

Annual rates of sediment denitrification and sediment–water fluxes of oxygen and nutrients were quantified at two shallow locations, Virksund and Ulbjerg, in the Limfjorden, Denmark. The sediment was sandy and colonized mainly by bivalves with a wet weight of 2,508 g m22 at Virksund and 572 g m22 at Ulbjerg. A benthic microalgal community was present throughout the year, and for 1–2 months in the summer, floating macroalgae partly covered the sediment. Annual budgets for the sediment both including and excluding the activity of macroalgae were calculated. In the absence of macroalgae, the benthic primary production was highest at Ulbjerg, which was autotrophic on an annual basis, whereas Virksund was heterotrophic. When macroalgae were included, both sites were strongly autotrophic on an annual basis. From 13% to 58% of the NH produced by mineralization was retained 4 in the sediment in the absence of macroalgae, primarily because of the assimilation of NH by the microphytobenthic 4 community. Only 25% and 38% of the total NO uptake at Ulbjerg and Virksund, respectively, was denitrified in 3 the absence of macroalgae, whereas in the presence of macroalgae, 12% and 39% of the NO uptake was denitrified 3 at those sites. Nitrate uptake associated with benthic primary production limited denitrification through competition for NO . The release of NH from the sediment at Virksund was reduced more than 50%, and at Ulbjerg, release 2 1 3 4 of NH changed to uptake when the macroalgae were included in the annual budget. Nutrient uptake by macroalgae 4 competed with all other nutrient-consuming processes, and the transient occurrence of macroalgae totally changed both the primary productivity and the nutrient budgets for the two sites. In the marine environment, primary production is remineralized both in the water column and in the sediment, and the relative importance of these two compartments depends very much on the water depth. In Chesapeake Bay, for instance, it was found that at water depths of ,5 m, the benthic respiration exceeded the planktonic respiration (Rowe et al. 1975; Kemp et al. 1992). The nutrients that are produced by degradation of organic matter in the sediment can be transported back to the photic zone where it can fuel new primary production (Rowe et al. 1975). The relative importance of this supply of nutrients, the so-called internal loading, compared to the external loading has been found to vary substantially. In a study in the Chesapeake Bay, release of remineralized ammonia from the sediment could account for between 13% and 40% of the photosynthetic nitrogen demand in the water column (Boynton and Kemp 1985), and in the Neuse River estuary, the internal loading of dissolved inorganic nitrogen (DIN) contributed 13% to 21% of the total DIN input to the water column. A compilation of data from 10 different estuaries demonstrated an even more var1 Corresponding author ([email protected]).