Effects of Changing Environmental Conditions on Lagoon Ecology - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 216

channels to the confined areas. Meanwhile, benthic biomass and abundance of indi-
viduals exhibit the inverse pattern, with maximum biomass and abundance of individ-
uals in the intermediate areas ( Figure 5.8)
These gradients also correspond to changes in benthic taxonomic composition.
Molluscs usually are the dominant groups close to the sea, where some echinoderms
can also be found (such as Asterina , Holothuria or Paracentrotus ). Polychaetes,
crustaceans, and chironomids successively increase their relative abundance with
confinement.
In zones where the environmental conditions are more stable (near the channels
due to the buffered influence of the open sea environment) the settlement of species
exhibiting K -selection strategies is possible. As stated above, these species are larger,
reproduce later in life, and develop more slowly. They are, however, superior com-
petitors to r -species in such stable environments. The K -strategists can control,
through predator-prey relationships, the r -strategists, which would proliferate expo-
nentially in their absence (top-down control), allowing the establishment of several
species spread by the different trophic levels. The diversity is usually high with the
use of all available resources, and this leads to a highly diverse community.

5.3.3 L EAKY L AGOONS

Leaky lagoons have many wide ocean entrance channels and tidal currents are
sufficiently strong to overcome the trend of being closed by wave action and littoral
drift. They are characterized by unimpaired water exchange with the ocean, strong
tidal currents, and salinity similar to that of the coastal ocean. 119 In these lagoons
biologic assemblages are similar to marine environments in sheltered open coastal
areas, and the horizontal zonation is less evident or completely absent. The relative
level of recruitment into coastal lagoons is determined by the total number of
potential recruits along the coast. 120 Vegetation on sandy or muddy bottoms is
characterized by meadows of Caulerpa prolifera , Zostera spp., Cymodocea nodosa ,
Thalassia spp., or Posidonia oceanica (in the Mediterranean lagoons).
These lagoons offer a protected shallow marine habitat, which can be highly diver-
sified and productive, comparable to other marine shallow water ecosystems. Well-
structured communities, controlled by K -strategists, can develop and settle down in leaky
lagoons. In the absence of exogenous disturbance, biomass can accumulate in large
organisms, while the choked lagoons, characterized by variable or persistent physical
stress, appear to be dominated by communities of small-sized r -strategists organisms.
The restricted lagoons have characteristics of both leaky and choked lagoons. 101

5.3.4 W ATER R ENEWAL R ATE AND E UTROPHICATION

In eutrophic choked lagoons the instability of the environment increases by several
orders of magnitude. Frequent algal blooms can occur, followed by “crashes” or
dystrophic crises, resulting in high ammonia and low oxygen concentrations 123,124 and
subsequent organism mortality. These crises are followed by a rapid development of
the r- strategist species. In eutrophic lagoons with high hydrodynamics the accumulation
of nutrients and subsequent problems of development of phytoplankton and macro-
algae are minimized as the excessive production may be exported out of the lagoon