Geology Reference
In-Depth Information
fluctuations in the concentration of the component of interest occur on
the same relative time scales as estuarine flushing.
A component can undergo considerable physico-chemical speciation
alterations in an estuary. With respect to dissolved constituents, the
composition and concentration of available ligands changes. Depending
upon the initial pH of the riverine water, OH
may become markedly
more important down the estuary. Similarly, chlorocomplexes for met-
als such as Cd, Hg and Zn become more prevalent as the salinity
increases. Conversely, the competitive influence of seawater derived Ca
and Mg for organic material decreases the relative importance of humic
complexation for Mn and Zn.
Estuaries are particularly well known for dissolved-particulate interac-
tions. Such changes in phase come about via several mechanisms. Firstly,
dissolution-precipitation processes may occur. This is especially impor-
tant for the authigenic precipitation of Fe and Mn oxyhydroxides.
Secondly, components may experience adsorption-desorption reactions.
Desorption can occur in the initial mixing zone, partly in response to the
pHminimum. Adsorption, particularly in association with the Fe and Mn
phases, can accumulate material within the suspended sediments. Thirdly,
flocculation and aggregation processes can remove material from solu-
tion. This occurs as particulate material with negatively charged surfaces
adsorb cations in the estuary. The surface charge diminishes and as the
ionic strength increases, the particles experience less electrostatic repul-
sion. Eventually, the situation arises whereby particle collisions lead to
aggregation due to weak bonding. This process can be facilitated if
particle surfaces are coated with organic material (then known as
flocculation rather than aggregation). The three types of processes out-
lined above often happen simultaneously in the estuarine environment.
Dissolved Fe, as shown in Figure 14, typifies such non-conservative
behaviour. The transformation of dissolved into particulate phases can
then be followed by deposition to the estuarine sediments. Thus, the flux
of material to the ocean can be considerably modified, particularly as such
sediments may be transported landward rather than seaward.
Biological activity in the estuarine environment can also influence the
speciation of constituents, notably dissolved-particulate partitioning. A
complex regeneration cycle determines distributions and modifies the
riverine flux. This is especially the case for nutrients, and estuaries are
often termed a nutrient trap. Estuaries tend to be regions of high biological
productivity as rivers have elevated nutrient concentrations. Moreover,
several freshwater organisms die upon encountering brackish water with
consequent cell rupture and the release of contents into solution. Regard-
less of the source, the nutrients stimulate phytoplankton productivity.
