Geoscience Reference
In-Depth Information
recent history. Adaptation to light intensity mainly consists of varying the amount
of pigments involved in capturing photons, thus varying their carbon to chlorophyll
ratios. More chlorophyll is developed as an adaptation to low light conditions to
improve their ability to collect light.
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A primary consequence of the increased phytoplankton assemblages due to
nutrient enrichment is the increase of the light attenuation coefficient in water. Light
is efficiently trapped by phytoplankton thus decreasing the amount reaching the
bottom for macrophytes growth. In mesotrophic lagoons, phytoplankton growth may
be enhanced seasonally by some environmental variables such as temperature, with
a detrimental effect on macro-algae due to light limitation.
Increasing the trophic state of lagoons not only causes changes in the phytoplank-
ton assemblages structure but also in the whole planktonic food web, eventually
moving from a microbial loop-based web to a herbivorous food web. However,
predicting changes at the whole food web is an intricate task as a result of the
complex interactions among organisms. As supported by theory and frequently
reported, the increased nutrient concentration facilitates growth of larger cells. Fur-
thermore, heavy grazing on small cells, mainly due to microzooplankton (ciliates
and crustaceans larvae stages) and small and medium-sized crustaceans, enhances
large cells to take up nutrients. As a result, increased nutrients and increased large
zooplankton tend to decrease the relative abundance of small phytoplankton and to
increase the average phytoplankton size.
78,79
Increased average size of plankton
assemblages can, in some cases, lead to an increase of gelatinous zooplankton
populations (e.g., ctenophores, jellyfishes),
80,81
and fish larvae because large phy-
toplankton cells and crustaceans are an important component of their diet.
An alternation of domination by either submerged macrophytes or phytoplankton
has also been reported for some lagoons.
15,82
Each of the states remains stable until
a disturbance large enough to override the self-stabilizing capacities, even affecting
only different parts of the ecosystem, causes a shift to the other state.
16
Population
dynamics models have successfully explained such alternation in shallow lakes but
its application to coastal lagoons still needs further development.
It can be said, in short, that the resulting food web structure is a trade-off between
growth rate variation of organisms caused by resource availability (“bottom-up” con-
trols) and loss rate variations caused by predation (“top-down” controls).
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However,
prediction on such trade-offs seems difficult as small changes in the structure of the
primary producers may result in quite unpredictable and large changes at the whole
lagoon level.
5.2.2.3
Benthic Fauna
Mesotrophic lagoons combine phytoplankton, especially at the plankton-benthos
interface, microphytobenthos, and bacteria-enriched detritus, originating from sub-
merged aquatic vegetation, as major food sources for benthic fauna.
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According to the Pearson-Rosenberg model, inputs of organic enrichment usually
involve changes in abundance, biomass, and species richness of macrobenthic assem-
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