Estuarine benthic habitats provide an important ecosystem service regulating the nitrogen cycle

Abstract

Globally, a key ecosystem service provided by sedimentary estuarine habitats is the regulation of nutrient cycles. The nitrogen (N) cycle is driven by complex biogeochemical transformations within these sediments—including nitrogen fixation, denitrification, assimilation and anaerobic ammonia oxidation—mediated by microorganisms. Evaluating ecosystem processes and their functional value is a knowledge gap for the wet-dry tropics and even more limited for macrotidal estuaries. The capacity of these important environments to withstand and assimilate increasing nitrogenous loads as a consequence of accelerating development pressures in tropical Australia is largely unknown. Because of the critical role nitrogen cycling plays in estuarine ecosystems, identifying important habitats that underpin N cycling, particularly denitrification known to mitigate anthropogenic N inputs, is important. Detailed benthic habitat mapping of the Darwin-Bynoe region of northern Australia has provided a rare opportunity to demarcate its key habitats, such as intertidal mudflats, seagrass, mangroves, reef and saltmarsh. Combined with new measurements of benthic nitrogen fluxes, it has been possible for the first time to map these processes and develop a simple integrated functional value for N cycling across key benthic habitats of a tropical macrotidal estuary. Maps generated in this process have provided broadscale identification of the functional importance of habitats with relevance to N removal processes. The role of intertidal sediments in denitrification has been highlighted. Furthermore, the study emphasises connectivity across benthic seascapes, where multiple services are likely to interact, in supporting overall function and ecosystem health. The distillation of composite processes in this mapping format allows resource managers and scientists to communicate outputs visually with a simple classification scheme which could be superimposed with additional data to support environmental assessment and management.