Material exchange and food web of seagrass beds in the Sylt-Romo Bight: how significant are community changes at the ecosystem level?

Material exchange, biodiversity and trophic transfer within the food web we re investigated in two different types of intertidal seagrass beds: a shelt ered, dense Zostera marina bed and a more exposed, sparse Z. noltii bed, in the Northern Wadden Sea. Both types of Zostera beds show a seasonal develo pment of aboveground biomass, and therefore measurements were carried out d uring the vegetation period in summer. The exchange of particles and nutrie nts between seagrass beds and the overlying water was measured directly usi ng an in situ flume. Particle sedimentation [carbon (C), nitrogen (N) and p hosphorus (P) constituents] from the water column prevailed in dense seagra ss beds. In the sheltered, dense seagrass bed, a net particle uptake was fo und even on windy days (7-8 Beaufort). Dissolved inorganic N and orthophosp hate were mainly taken up by the dense seagrass bed. At times of strong win ds, nutrients were released from the benthic community to tidal waters. In a budget calculation of total N and total P, the dense seagrass beds were c haracterised as a material sink. The seagrass beds with sparse Z. noltii we re a source of particles even during calm weather. The uptake of dissolved inorganic N in the sparse seagrass bed was low but significant, while the u ptake of inorganic phosphate and silicate by seagrasses and their epiphytes was exceeded by release processes from the sediment into the overlying wat er. Estimates at the ecosystem level showed that material fluxes of seagras s bells in the Sylt-Romo Eight are dominated by the dense type of Zostera b eds. Therefore, seagrass beds act as a sink for particles and for dissolved inorganic nutrients. During storms, seagrass beds are distinct sources for inorganic nutrients. The total intertidal area of the Sylt-Romo Eight coul d be described as a sink for particles and a source for dissolved nutrients . This balance of the material budget was estimated by either including or excluding seagrass beds. Including the subtidal part, the function of the e cosystem as a source for particles increased, supposing that all seagrass b eds were lost from the area. During the vegetation period, seagrass beds ac t as a storage compartment for material accumulated in the living biomass o f the community. There was great biodiversity among the plant and animal gr oups found in intertidal seagrass beds of the Sylt-Romo Bay, representing 5 0-86% of the total number of species investigated, depending on the particu lar group. Since most species are not exclusively seagrass residents, the l oss of intertidal seagrass beds would be of minor importance for biodiversi ty at the ecosystem level. Food web structure in seagrass beds is different from other intertidal communities. Primary production and detritus input i s high, but secondary production is similar to that of unvegetated areas, a lthough the relative importance of the trophic guilds is different. The los s of seagrass beds leads to profound alterations in the food web of the tot al ecosystem. Historical as well as recent changes in material fluxes and e nergy flow due to manmade alterations to the ecosystem are discussed.