Physical Processes - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 75

not pass through the 2- m filter, but individual particles cannot be identified. In

favorable conditions colloidal substances can become solid. This can happen at the
entrance of estuaries with very low salinity values. Colloids are very important for
coastal water chemistry, but their contribution to the overall balance of solid material
is small and usually negligible.
Particles are also generated by biological activity. Primary producers are
“machines” converting dissolved matter (inorganic nutrients, CO 2 , and water) into
particulate material. Secondary producers feed on those particles and generate even
larger particles (their own bodies and faecal pellets). Biological particles do have
adhesive properties and act as gluing nodes favoring the formation of flocks formed
by several small particles. These flocks, which are larger than individual particles,
have a higher volume/surface ratio and settle more quickly than individual particles.
Ionic attraction between inorganic particles is another flocculation mechanism and
is enhanced by free ions into salt water. This is the dominant flocculation process
at the entrance of the coastal lagoons at salinities higher than 2 psu. Details of these
processes are presented in Chapter 4.
Particles are usually called sediment because they can settle and deposit on the
bottom. While in suspension they are called suspended sediment or detritus. Sedi-
ment dynamics depends on transport by the currents, flocculation, sinking, and
formation of new particles due to biological activity or originated by colloidal
substances. The way particles are transported depends on their settling velocity and
on the interaction between the water column and the bottom, described by the bottom
shear stress.
Two classes of sediment, cohesive and noncohesive particles, and two regimes
of transport can be considered for both types of particles, according to the concen-
tration close to the bottom. In the case of cohesive sediment the flocks modify the
settling velocity. In the case of very high concentrations close to the bottom, sedi-
ments modify the properties of the flow itself. Cohesive sediment dynamics differs
from noncohesive sediment dynamics in terms of settling velocity and bottom
exchange parameterization. Flocculation is specific to cohesive sediment, but the
water-bottom dynamics is also different for noncohesive sediment. The freedom of
movement of individual particles and their rapid sinking make the bed-load transport
the most effective mechanism of noncohesive sediment dynamics.
The detailed description of cohesive and noncohesive sediment mechanisms and
modeling of sediment dynamics is beyond the scope of this topic.

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