MEPS 258:275-281 (2003)  -  doi:10.3354/meps258275

ABSTRACT: Problems associated with increasing eutrophication of estuaries have stimulated many investigators to examine primary productivity and nutrient dynamics and transformations within and between the water column and sediments. Surprisingly, rocky surfaces, both natural and man-made, while comprising significant areas of many estuaries, have received virtually no investigation. In the Douro River estuary (Portugal), a flooded granitic valley, human intervention has progressively replaced intertidal soft sediments with hard surface areas. In this study we examine the role of the inorganic nutrient and metabolic dynamics of intertidal rocky biofilms in the lower estuary. Hourly net primary production (389 ± 168 mg O₂ m^-2 h^-1) greatly exceeded hourly net respiration rates (50 ± 18 mg O₂ m^-2 h^-1), results comparable to those of adjacent muddy and sandy sediments. These rocky biofilms consistently removed NO₃^- and SiO₄^4- from the water column (-595.4 ± 236.7 and -305.7 ± 209.6 µmoles m^-2 h^-1, respectively), whereas the net flux of NH₄⁺ depended on light conditions (-123.15 ± 22.45, 144.54 ± 151.90 µmoles m^-2 h^-1, for light and dark treatments respectively). NO₂^- and PO₄^3- net fluxes were low, with no clear trends in flux direction (-0.57 ± 9.1 µmoles NO₂^- m^-2 h^-1 and 1.4 ± 10.5 µmoles PO₄^3- m^-2 h^-1). Although rocky substrata constitute only 21% of the total planar intertidal zone of Douro estuary, they were responsible for 43% of the nitrate and 45% of the silicate uptake when compared to sandy and muddy sediment hourly rates within the total intertidal area. Thus intertidal rocky biofilms can represent important sites for biogeochemical activity.

KEY WORDS: Douro River estuary · Intertidal rocky biofilms · Nutrient flux · Oxygen flux · Biogeochemistry