The Estuary Plume Zone: Source or Sink for Land-derived Nutrient Discharges?

The results from three cruises surveying nutrient distributions in the coastal plume discharge of the Humber Estuary, U.K., were used to examine ways of quantifying the influence of estuarine plume zones on the transfer of land-derived material from rivers and estuaries to coastal seas. The data were examined in three ways. First, the ‘ mixing curve ’ procedure, conventionally applied in estuarine studies was found to be only partially useful. This procedure indicated near-conservative behaviour of nitrate and silicate in winter. For nitrate, silicate and phosphate at other times, highly scattered data from the plume region indicated significant but generally unresolvable sources and sinks. The Wash area affected the plume distribution of phosphate in winter but not at other times. A second method, mass balancing based on estimates of the rates of all nutrient-controlling processes within the plume was considered impractical using presently available data due to the high spatial heterogeneity of significant nutrient-controlling processes, particularly primary production (as evidenced by chlorophyll distribution) and chemical fluxes across the sediment-water interface (as evidenced by shipboard, box-core incubations of sediments from the plume zone). Third, estimates of nutrient fluxes emanating from either the river or the estuary mouth were compared with estimates of fluxes across the plume boundary based on computer modeling of residual (non-tidal) water transport within the plume. Without unlimited resources, this procedure is the most practical method of obtaining quantitative flux estimates for assessing the influence of estuarine plume zones on river/estuary discharges. Using this latter procedure, it is shown that, in winter, river-derived nutrients are conservatively exported through the plume to the coastal sea. In spring, the plume zone acts as a net sink for nutrients derived both from the estuary and from the adjacent coastal sea. That is, nutrients are being consumed at a faster rate than they are supplied by the river/estuary system to the plume region. In summer, the plume is a net sink for nitrate and phosphate and a net supplier of silicate.