Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 297

between these two hypotheses. Modelling studies indicate that semi-diurnal
stratification will not trigger phytoplankton blooms (Lucas et al., 1998 ) , though in
shallow waters the changes in light driven by tidal variations in sea level could be
important (Lucas et al., 1999 ) .

9.8.3 Impacts of riverine material in ROFIs

Rivers play a global role in delivering material important to biogeochemical cycling
into the ocean. Silicate is, as we saw in Chapter 5 , a vital nutrient for diatoms, and
the most important source of silicate (as silicic acid) to the ocean is via rivers and
estuaries (Treguer et al., 1995 ). Considerable attention has been focused on the large
fluxes of organic carbon down rivers and through estuaries. If coastal and shelf
primary production simply recycles this carbon, rather than requiring further carbon
from the inorganic pool, then the shallow seas could be net sources of CO 2 back to
the atmosphere. The physics of estuaries and ROFIs controls the residence times of
silicate and organic carbon in the near shore zone, which affects their modification,
utilisation and subsequent dispersion and supply beyond the coasts. Many estuaries
are found to be net sources of CO 2 to the atmosphere, but evidence collected over
wider shelf regions suggests that this effect is constrained close to the coast and
that shelf seas play an important part in the removal of CO 2 from the atmosphere.
We will discuss that further in Chapter 10 , when we deal with carbon export across
the shelf edge.
Estuaries have long been recognised as sources of both industrial and agricultural
pollutants, since they are used either deliberately or accidentally to transport
unwanted substances into the ocean. In the past there was an implicit assumption
that pollutants input to the coastal zone were quickly dispersed and diluted through
the ocean. More recently it has become recognised that the physics and biology of the
coastal seas undermine the concept of the ocean as a limitless diluter; dispersion away
from the coastal zone can be weak, with plenty of opportunity for pollutants to
influence the biochemistry and cause problems.
An example off the coast of NW Europe is the radioactive discharge from the
Sellafield nuclear power plant and waste reprocessing facility on the northwest coast
of England (Prandle, 1984 ) . The radio-isotope caesium-137 ( 137 Cs), discharged by the
Sellafield plant into the Irish Sea, is distributed by a coastal buoyancy current round
the north of the UK and then southward into the North Sea (see Fig. 3.17 ). The
tendency for 137 Cs to attach to fine sediment particles leads to the onshore near-bed
current in this freshwater-influenced coastal region, driving some of the 137 Cs back
into the estuaries (Mudge et al., 1997 ) . The concentrations of 137 Cs are far below
anything likely to be harmful, but the discharges have provided a serendipitous tracer
of coastal oceanography and also a way to estimate recent sedimentation rates in
estuaries (Mamas et al., 1995 ) .
Remember in Chapter 5 that we noted the large amount of atmospheric nitrogen
that is fixed to support agriculture; humanity has doubled the global rate of N 2
fixation. Once fertilisers have been applied to farmland, rain leaches much of it into