Resumen
Biogeochemical cycling in the marine coastal zone regulates the availability of nitrogen and carbon within soft sediment habitats. However, these pathways are being fundamentally altered by anthropogenic increases in nutrient delivery. Few studies have incorporated long-term enrichment and ecological complexity (in situ experiments), restricting our ability to manage effectively and prevent ecological shifts. This study investigates the influence of sediment nutrient availability (at 3 levels, across 2 seasons) on biogeochemical cycling over a 20-month period in 4 estuaries. Overall, net denitrification rates were highly variable, ranging between 4 and 208 µmol N m-2 h-1. However, no increases were observed with increasing enrichment highlighting the limited capacity for nitrogen removal in response to large increases in bioavailable nitrogen. Additionally, macrofaunal communities and sediment trophic status were shown to have important influences on nitrogen processing. Overall, alterations to ecosystem relationships and the appearance of non-linear responses to increasing nutrient enrichment reveal the vulnerability of estuaries to increasing stressor loads owing to the increased likelihood of reaching a tipping point.