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 exa mine ways of quantifying the influence of estuarine plume zones on the transfer of land-derived material from rivers and estuaries to coasta l 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 b ehaviour 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. Th e Wash area affected the plume distribution of phosphate in winter but not at other times. A second method, mass balancing based on estimate s of the rates of all nutrient-controlling processes within the plume was considered impractical using presently available data due to the h igh spatial heterogeneity of significant nutrient-controlling processe s, particularly primary production (as evidenced by chlorophyll distri bution) and chemical fluxes across the sediment-water interface (as ev idenced by shipboard, box-care 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 acro ss the plume boundary based on computer modelling of residual (non-tid al) water transport within the plume. Without unlimited resources, thi s procedure is the most practical method of obtaining quantitative flu x estimates for assessing the influence of estuarine plume zones on ri ver/estuary discharges. Using this latter procedure, it is shown that, in winter, fiver-derived nutrients are conservatively exported throug h the plume to the coastal sea. In spring, the plume zone acts as a ne t sink for nutrients derived both from the estuary and from the adjace nt coastal sea. That is, nutrients are being consumed at a faster rate than they are supplied by the river/estuary system to the plume regio n. In summer, the plume is a net sink for nitrate and phosphate and a net supplier of silicate.