Salinity Intrusion and Seasonal Water Quality Variations in the Tidal ...

eaches 15 g/kg. Work by Boyle, Edmond & Sholkovitz (1977) suggests that for iron
removal the pH change is less important than the mixing of sea water electrolytes with
fresh water. The marine cations Ca++, Mg++ and Na+ neutralize the negative charge of
river borne substances and cause flocculation and coagulation. Sholkovitz (1976) in
laboratory experiments found that the solubility of the inorganic constituents in river
water and their flocculation in sea water are due to their association with organic matter.
The mechanism suggested is the flocculation ofmixed iron oxide -organic matter colloids
due to the neutralization by sea water cations ofthe negative colloid charges.
Several recent review papers (Downing 1997, Smith 1998, Smith et al. 1999, Conley
2000 & the NRC, 2000) concluded that the major nutrients causing over-enrichment
problems in estuaries are N and P. A schematic ofphytoplankton and nutrient circulation
isshown in figure 1.7. P limits primary production in some tropical near shore habitats,
although the study ofthese systems is limited (Howarth et al., 1995). Often the addition
of both N and P will elicit greater phytoplankton biomass stimulation than the sum of
both nutrients added separately (Fisher et al., 1992). There are reported cases where both
Nand P are required to elicit a phytoplankton bio mass production response in estuaries
(Flemer et al., 1998), suggesting that N and P supply rates are equally limiting (Figure
1.8). Tidal fresh and brackish waters in many estuaries are more light limited than higher
saline waters (Flemer, 1970, Sin et al. 1999). As fresh water flows seaward, processes
operate to modify nutrient stoichiometry (e.g., sedimentation of P-absorbed particles,
denitrification, and differential microbial decomposition). Experiments showed that P
addition was not stimulatory, but N or N+P caused large increases in the net primary
production (Oviatt et al. 1995). In a 17-year field experiment carried out in
Himmerfjarden Estuary south of Stockholm, Sweden (Elmgren & Larsson, 1997). The
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