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

Intertidal biofilms on rocky substratum can play a major role in estuarine carbon and nutrient dynamics

Catarina M. Magalhães1,*, Adriano A. Bordalo1, William J. Wiebe2

1Laboratory of Hydrobiology, Institute of Biomedical Sciences, University of Porto, 4099-003 Porto, Portugal
2Department of Marine Sciences, University of Georgia, Athens, Georgia 30602, USA

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 SiO44- 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 PO43- 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 PO43- 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

Full text in pdf format