Ecosystem function responses to nutrient enrichment mediated by mud content in soft sediment habitats

Estuaries are threatened with increasing sedimentation and nutrients, however research is limited on the interaction of these stressors on ecosystem functions. A field experiment adding nitrogen (N) fertiliser at high (600 g N m−2) and medium (150 g N m−2) concentrations and control plots was established in two sandy and two muddy sites (mud content 0–15%) to investigate context dependency of nitrogen enrichment on nutrient cycling and primary production. There was a significant interaction of mud x treatment on ammonium (NH4+) fluxes. High N-addition plots had 133x greater dark NH4+ efflux than controls in muddy sediments, compared to 17x greater dark NH4+ efflux in sandy sediments. Net primary production increased with mud content, while photosynthetic efficiency of the microphytobenthos (GPPchl-a) decreased above 4% mud. Mud content also affected the bivalve, Austrovenus stutchburyi (>30 mm), macrofaunal abundance and functional groups, and their influence on nutrient cycling. Thus minor increases in mud can alter the ecosystem response to nutrient enrichment directly through changes in sediment properties and indirectly by affecting the bivalves and macrofauna that influence these processes. This study highlights how multiple stressors interact to predispose estuaries to tipping points.