An Examination of the Fluxes of Nitrogen and Phosphorus in Temperate and Tropical Estuaries: Current Estimates and Uncertainties

In a number of regions of the world, enhanced flows of nitrogen (N) and phosphorus (P) from land to sea are of major concern because of the observable deterioration in the quality of many nearshore marine waters. Estuaries receive N and P from river and other runoff, from waste discharges, from the atmosphere and ocean and from exchange with coastal groundwaters (which in all likelihood results in a net input to the estuary). For rivers that do not discharge directly onto the continental shelf, seaward fluxes of N and P will be modified by within-estuary transformations of reactive species, the burial of particulate N and P in sediments (sub/intertidal, saltmarsh, mangrove) and the loss of gaseous N and P species by bacterial reduction. Driven by a desire to understand the effects of changing N and P loads on water quality, and to gain insights into the true modification of their fluxes within estuaries, much effort has been expended on providing quantitative estimates of the sources and sinks of these constituents. Yet, accurate and precise estimates on a global scale remain elusive. Riverine inputs of total N and P are calculated to be 35–64 and 22 Mt a 1, respectively. These inputs are dominated by particulate species, and because of this, are likely to be imprecise as overall sediment fluxes are disproportionately influenced by infrequent, poorly sampled, high flow events. Direct aeolian inputs of N to estuaries (P inputs are minor), at a minimum of 1–4 Mt a 1, are small but significant, although again good estimates are hampered by the apparent importance of infrequent, and thus under-sampled, deposition events. Indirect atmospheric inputs via deposition onto and runoff from catchments may be highly significant, at least in environments bounding the North Atlantic Ocean. Groundwater inputs are generally unknown, but, for N, may be 5–10 Mt a 1 (no data on P). Information on the global inputs of N and P from waste discharges and mariculture do not appear to be available. Denitrification, estimated to be ca. 33Mt a 1, may account for 52–94% of the currently estimated total N inputs; in contrast, the loss of P via venting of gaseous phosphine is unknown. The burial of N and P in sediments is about 7% and 30% of their total inputs, respectively. Nevertheless, reliable information on the modifying role of estuarine sediments appears far from complete. Globally, the inputs of N and P to the marine environment from all sources are expected to increase over the next few decades. The resulting effects of these increases on the marine environment, including any influences due to estuarine processing, may be partly assessed through the use of dynamic transport and transformation estuarine models for N and P. A further important development in this respect will be the linking of complementary models (e.g. catchment/river/estuarine/ coastal zone) and their coupling to strategic large scale observations.