Material exchange
The role of intertidal benthic communities for the exchange of nutrients and particulate matter
The benthic-pelagic material exchange is of crucial importance for the functioning of coastal ecosystems. In the Sylt-Rømø Bay, a part of the Northern Wadden Sea in the North Sea, flux rates of particles (POC, PN), total nitrogen and phosphorus and dissolved inorganic nutrients are measured directly with an in situ Sylt flume in dominant benthic communities e.g. mussel beds, seagrass beds and Arenicola flats in a nearly natural current regime.
Particle uptake from the water column prevails in macrofaunal and -floral communities. Detritus and phytoplankton imported from coastal waters contribute to the primary food supply of tidal flat communities. Flume measurements reveal an accumulation of detrital material in mussel beds (Mb), Arenicola flats (Ar) and Zostera marina beds (Zm) while most other communities (Zn: Zostera noltii beds) release particulate nitrogen (PN).
Communities dominated by macrofauna release remineralised nutrients. Mussel beds release dissolved inorganic nitrogen (DIN) in high rates. Arenicola flats show a distinct but smaller release. The higher the macrozoobenthic biomass the more DIN is released. The reason is presumably that macrofauna promotes remineralisation processes by enriching sediments with organic matter. Macrofauna contributes further to the DIN release by excretion and bioturbation. Communities covered by macroflora act as sinks for nutrients. Dense Zostera marina beds take up DIN, while sparse Zostera noltii meadows react more similar to muddy and sandy sediments low in macrobenthic biomass. In all communities investigated, flux rates depend on wind induced currents and turbulence. In situ flux rates, including macrofauna and -flora as well as the natural physical regime, surpassed by far those measured in vitro which were mainly based on microbial and diffusional processes. When using the Sylt flume for measurements, most of the important environmental conditions are considered. Thus the balance of release and uptake rates is estimated considering the area cover of the communities in the whole Sylt-Rømø Bay. The intertidal area of this ecosystem can be described as a sink for particles and a source for dissolved nutrients.
Conclusions
- Intertidal benthic communities act as particle sinks and as nutrient sources
- Macrofauna and macrophytobenthos determine the direction and the amount of material fluxes
- Hydrodynamics are of crucial importance for the material balance of a tidal area
Selected Publications:
Asmus H, Asmus R (2000) Material exchange and food web of seagrass beds of the Sylt-Rømø Bight - how significant are community changes on the ecosystem level? Helgoland Mar. Res. 54:137-150
Asmus H, Asmus R, Francés Zubillaga G (1995) Do mussel beds intensify the phosphorus exchange between sediment and tidal waters? Ophelia 41:37-55
Asmus H, Asmus RM, Prins TC, Dankers N, Francés G, Maaß B, Reise K (1992) Benthic-pelagic flux rates on mussel beds: tunnel and tidal flume methodology compared. Helgoländer Meeresunters. 46:341-361
Asmus RM, Asmus H (1991) Mussel beds: limiting or promoting phytoplankton? J. expl. mar. Biol. Ecol.148:215-232
Asmus, RM, Asmus, H, Wille, A, Francés Zubillaga, G, and Reise, K (1994) Complementay oxygen and nutrient fluxes in seagrass beds and mussel banks? Dyer, K. R. and Orth, R. J. Changes in Fluxes in Estuaries: Implicatons from science to Management. ECSA 22. Fredensborg, Olsen & Olsen. p 227-237
Asmus R, Sprung M, Asmus H (2000) Nutrient fluxes in intertidal communities of a southern European lagoon (Ria Formosa) - similarities and differences with a northern Wadden Sea Bay. Hydrobiologia 436:217-235
