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

Abstract

Worldwide, eutrophication of estuaries and coastal waters constitutes a serious, rapidly increasing problem (e.g.Jickells 1998).While intertidal and subtidal estuarine sediments are recognized as important sites for nutrient transformation and sequestration via biogeochemical cycling (Ogilvie et al. 1997, Cabrita & Brotas 2000, Schulz & Zabel 2000), many estuaries also possess extensive areas of hard surfaces (e.g.rocky coastlines, fjords, drowned sandstone and granitic valleys, carbonate platforms).Further, man-made structures (e.g.breakwaters and jetties, pilings and walls for marinas and port facilities) progressively replace sediments with hard surfaces.These surfaces provide substrate for the proliferation of biofilms with a wide variety of metabolically diverse microorganisms.Biogeochemical benthic-water coupling in rocky rivers and coral reef flats (carbonate platforms) are documented (e.g.Wetzel 1983, D'Elia & Wiebe 1990), and coastal rocky shores have received much attention by community ecologists studying animal, macrophyte and microalgal distribution, controls of trophic structure, and food webs of these communities (Hillebrand & Sommer 2000, Knox 2000 and references therein, Worm et al. 2000).However, the extent of biogeochemical processes within the thin layer of organic matter that characterizes hard surfaces of many marine coastal habitats has scarcely been considered.For example Decho (2000) reviewed microbial biofilms in intertidal systems, and Knox (2000) published a book on the ecology of seashores, yet neither mention this topic.In this study, rates of primary production and respiration, and net fluxes of NH 44between hard surface biofilms and the water column of the intertidal zone of the Douro River estuary were examined and compared with comparable data from adjacent intertidal sandy and muddy sediments. Materials and methods. Site description:The Douro River and its tributaries drain 17% of the Iberian Peninsula (Fig. 1).Confined 21 km upstream by a dam, the estuary averages 8 m depth, has a semidiurnal tidal range of 2 to 3 m, temperatures between 8 and 25C, salinities from 0 to 35 and water residence times between 0.3 and 16.5 d (Vieira & Bordalo 2000).The estuary receives an average of 455 m 3 s -1 of freshwater, con-