Effects of Changing Environmental Conditions on Lagoon Ecology - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 211

the bottom, with associated fish kills and possible viral infections from consumption
of contaminated shellfish. 88
Fish yield can increase to a certain extent due to an increase of nutrient load,
but if the conditions are too extreme, massive fish mortalities can result from
oxygen depletion and H 2 S release. These effects will also be reflected in the total
catches in the surrounding coastal sea, as most of the species are migratory. 63 A
reduction in total catches is then expected, which can also be a consequence of
over-fishing. For instance, Parsons 98 has questioned if phytoplankton blooms may
be driven not only by bottom-up processes of nutrient enrichment, but also as a
result of overfishing of commercial fish stocks and removal of top predator control.
These predators may be replaced by another predator, such as the jellyfish, which
feeds on the same food organisms ( see Chapter 9, Section 9.3, Mar Menor Lagoon
case study).
The birds feed essentially on the intertidal areas or in very shallow waters. In
moderately eutrophic lagoons, the amphipods and crabs proliferate in the algal mats,
as long as important algal-free areas exist that ensure a healthy population. Many
species of birds, which feed on these preys, can be enhanced. 99 However, if macro-
algae mats continued to proliferate and spread over larger areas, there will be a
much-reduced overall area for bird feeding 100 and, as a consequence, there will also
be a general decrease in bird populations.

5.3 WATER RENEWAL RATES

Two components underlie the concept of water renewal, and both are important for
understanding aquatic ecosystem function: changes in the vertical mixing and
exchange with adjacent systems. As stated above, shallowness is one of the main
features characterizing coastal lagoons, ensuring that effects of wind usually act over
the entire water column and vertical mixing of waters occurs. Only in shelter areas
and in areas covered by dense meadows (e.g., of Caulerpa prolifera or Ruppia cir-
rhosa ) does water remain stagnant at the bottom level, leading to anoxic environments
and consequently to fauna impoverishment ( see Chapter 9, Section 9.5, Koycegiz-
Dalyan Lagoon case study).
The main aspects of the renewal concept applied to lagoons therefore relate to
their exchange with the adjacent aquatic systems and its magnitude reflects the time-
scale and the range of hydrological variability. 101-103 The renewal rate affects, in the
first place, the physico-chemical characteristics of water mass, mainly salinity. This
parameter has been frequently used to classify the type of lagoons (for example, the
Venice system; 104 see also Petit 105 and Por 106 ), explaining composition of their com-
munities on the basis of autoecological criteria and indicator species. 107 At this point
it is important to differentiate between salinity and ionic composition. Salinity is
the total amount of dissolved salts in the water and depends mainly on the hydro-
logical balance in the lagoon (rain and other inputs of fresh water and evaporation
processes). Ionic composition refers to the qualitative composition of the salts.
Thalasogenic waters are waters of marine origin that maintain constant proportion
of constituents. Limnogenic waters have variable composition, depending on the
nature of the geologic materials in the watercourses and drainage area.