Nutrient fluxes in intertidal communities of a South European lagoon (Ria Formosa) – similarities and differences with a northern Wadden Sea bay (Sylt-Rømø Bay)

Abstract

During an annual cycle, flux rates of oxygen, nitrate, nitrite, ammonium, phosphate and silicate were measured in light and dark bell jars at three sites in Ria Formosa (Algarve, Portugal) enclosing either a natural macrophytic community (macroalgae on sand or mud, a seagrass bed of Zostera noltii) or bare sediments. The results are compared with a preceeding study in which the same bell jar technique has been applied in the Sylt-Romo Bay of the northern Wadden Sea. Nitrate flux was mainly directed from the water column to the benthic communities in Ria Formosa, as well as in the Sylt-Romo Bay. However, nitrate uptake was higher in the northern, more eutrophic study area. In Ria Formosa, nutrient concentrations were lower than in the Sylt-Romo Bay possibly due to strong water exchange with Atlantic waters. High temperatures and strong insolation had a greater impact on nitrate fluxes in Ria Formosa than in the Sylt-Romo Bay. Bioturbating macrofauna increased ammonium efflux in the Sylt- Romo Bay while this effect was not as pronounced in the Ria Formosa study sites. Benthic phosphate uptake dominated in the Ria Formosa and was correlated to initial phosphate concentrations in incoming waters. At both study sites, oxygen and nutrient fluxes were correlated with temperature. Additionally, flux rates were strongly influenced by biotic components and levels of eutrophication. A literature survey showed that mainly in temperate regions, material fluxes increase with temperature, whereas in warmer areas, ammonium and phosphate fluxes between sediment and water were generally lower.