Environmental Engineering Reference
In-Depth Information
water systems is related to a number of factors. Perhaps the most important factor is water
residence time. Systems with long residence times, such as lakes, bury a higher percent of
the P that enters the system and export a lower fraction of the P that enters.
Much of the P is delivered to sediments as sinking particles. These particles include
organic matter originating from phytoplankton production or from plant production in the
watershed, and inorganic particles (e.g., iron oxides). The size and density (i.e., specific
gravity) of particles strongly influence sinking rates and hence the likelihood of particles
reaching sediments. Phytoplankton, which can be an important component of P delivered
to sediments, vary substantially in both size and density and also vary in mobility, adding
to great variation in sinking rate. Diatoms are often dense (due to a silica shell) and rela-
tively large and sink rapidly compared to low-density cyanobacteria with gas vacuoles or
small and mobile plankton. The delivery of P in phytoplankton or other particles can be
enhanced by zooplankton grazing, some of which can package small particles into larger
relatively rapid-sinking fecal pellets. While pelagic grazers can alter the capacity of parti-
cles to sink to sediments, benthic grazers such as bivalves can increase P delivery more
directly. These organisms filter and remove particles from the water column; some of the
P is incorporated into their biomass and some is egested onto the sediment surface.
Particles that reach sediments can return to the water column, either in dissolved form
or through resuspension. Resuspension is likely in high-energy environments, and in run-
ning waters can be a major fate of P previously deposited during floods. In lakes, wave
action generated by winds, boating, or dredging can be a major cause of resuspension.
Benthic organisms can either stabilize sediments and reduce resuspension or increase
resuspension. Microbial and algal films can act as stabilizing elements as can macrophytes
and bivalves that increase structural resistance at the benthic boundary and decrease
hydrologic flow. On the other hand, benthic-feeding fish such as carp can greatly increase
resuspension. Humans can also increase resuspension in shallows directly through the use
of motorized vehicles and indirectly by eliminating benthic plants and animals that previ-
ously stabilized sediments.
The particulate P that is not resuspended is subject to decomposition and chemical reac-
tions associated with pH and oxidation-reduction differences within sediments. These
reactions change the form of P but only sometimes result in release of P to overlying
waters. A classic example of these interactions takes P from particulate organic P to
dissolved inorganic P in porewater to P bound to particulate iron oxides at the sediment
surface, and finally to dissolved P released to bottom waters.
The link between oxygen concentrations in bottom waters and P release from sediments
of many inland systems has the potential to generate a positive feedback between P loads
and production. This positive feedback has been termed accelerated eutrophication and
occurs in both lakes and estuaries. Briefly, P loads lead to greater primary production,
which leads to greater sedimentation of organic matter. Decomposition of this material
results in lower oxygen in bottom waters. This lower oxygen in turn can lead to higher
release of P from sediments. In softwater lakes (low alkalinity), iron tends to control inter-
nal P dynamics. Iron binds with P, giving sediments a capacity to tightly bind P.
However, in the low-oxygen conditions common in eutrophic lakes, iron in the sediments
releases P that it had previously bound, meaning that eutrophic lakes that become anaero-
bic can actually release more P into the water column. The link between P release and
