Aw. Morris et al., THE ESTUARY PLUME ZONE - SOURCE OR SINK FOR LAND-DERIVED NUTRIENT DISCHARGES, Estuarine, coastal and shelf science, 40(4), 1995, pp. 387-402
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.