The Importance of Primary Producers for Benthic Nitrogen and Phosphorus Cycling

In shallow coastal systems where most of the seafloor lies within the photic zone, benthic photoautotrophy plays a key role in regulating nutrient cycling. In these systems, production of submerged vascular plants (seagrasses), macroalgae and benthic microalgae is high and typically exceeds that of phytoplankton (Borum and Sand Jensen 1996). Changes in the patterns of primary production as well as in habitat structure and trophic dynamics (Valiela et al. 1992; Nixon et al. 1996) have been directly related to nutrient over-enrichment; this is also recognized as one of the greatest threats to maintaining marine biodiversity in coastal regions (NRC 2000). The widely-accepted scenario following nutrient enrichment is a shift in the dominance of primary producers, from seagrasses and perennial macroalgae to fastgrowing green macroalgae and phytoplankton (Sand-Jensen and Borum 199 1 ; Duarte 1995; Valiela et al. 1997; see Chapter 3). Interestingly, only one of these models (Sand-Jensen and Borum 199 1) has included explicitly the potential importance of benthic microalgae. Ultimately, one might expect a shift to phytoplankton dominance in the most heavily eutrophied shallow estuaries (Duarte 1995; Valiela et al. 1997), even though total production might not change (Borum and Sand-Jensen 1996). Given this transition, the question then becomes, how might such a shift in primary producer dominance influence nutrient cycling in shallow coastal waters? The influence of primary producers on nutrient transformations has important consequences for the role of shallow estuaries as a buffer zone between land and sea (Fig. 1). Uptake and temporary retention of nutrients in plant biomass, burial of recalcitrant organic matter, and the direct effects of autotrophic metaboiism on bacteriallyand chemically-mediated processes all influence nutrient cycling rates and pathways. Variations in the rates and dominance of these processes as primary producer communities change, will ultimately determine the fate and retention of watershed nutrients on their trajectory to the open ocean.