Effects of Changing Environmental Conditions on Lagoon Ecology - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 194

relationships, and the scale of the scenarios to be modeled should be previously
defined ( see Chapter 2 for details).
In coastal lagoons, the relevant spatial and temporal scales for management
purposes mainly affect the overall lagoon ecosystem and its successional stages. At
these scales, heterogeneity can be explained mainly by variations in two principal
factors: water renewal rate related to isolation degree, and trophic status related to
availability of nutrients.
The focus of this chapter is to outline changes in the main biological features
and processes in lagoons under different eutrophication states and water renewal
rates that must be considered when implementing ecological modeling as a decision
support tool for sustainable use and development.

5.2 EUTROPHICATION PROCESS

As explained in Chapter 2, human activity is responsible for extensive modifications
of many of the global element cycles, to the extent that more elements/nutrients are
fixed annually by human-driven activities than by natural processes.

Coastal lagoons
may receive nutrients from a wide range of sources such as domestic sewage,
agricultural activities, industrial wastewater, and atmospheric fall-out. The process
in which there is an increase in the rate of addition of nitrogen and phosphorus,
considered as the two main limiting factors for primary production to a natural
system, usually aquatic, is called eutrophication.
Eutrophication is a process,

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not a trophic state, meaning “an increase in the
rate of supply of organic matter to an ecosystem.”

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It is mainly identified with an
increase in the input of inorganic nutrients in the ecosystem. It must be taken into
account, however, that if the level of primary production, even though it is high,
remains constant over time, it does not imply that eutrophication will occur because
there will not be any change in the carbon supply rate.
It is well known that small amounts of nutrients usually stimulate primary
production. This does not automatically imply a linear increase of the whole pro-
duction of the ecosystem, but it frequently produces changes in the biological
structure and functioning of the whole ecosystem. This leads to the progressive
replacement of seagrasses and slow-growing macroalgae by fast-growing macroal-
gae and phytoplankton, with a final dominance of phytoplankton at high nutrient
loads.

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Competition of primary producers for nutrients is one of the responsible
processes, but not the only one. Alteration in water turbidity, changes in the hydraulic
conditions resulting in modifications of water residence time and a decline of grazing
pressure, are also factors that promote shifts in the dominant plant communities. A
comprehensive sequence of changes in major plant groups following nutrient enrich-
ment in a wide range of ecosystems has been given by Harlin.

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These changes in
submerged vegetation during eutrophication appear to occur as a step process, with
sudden shifts in submerged vegetation, not directly coupled to increased nutrient
loading alone, but occurring due to many indirect and feedback mechanisms.

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Changes in the primary producers' structure affect secondary producers, as they
are the basis of the trophic food web. The trophic status of a coastal lagoon, however,
does not depend exclusively on the nutrient load but on the hydrodynamics, which, in