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This report describes a
numerical study of the trophic processes in the Tagus, Sado
and Mondego Estuaries (Portugal), in order to evaluate their
trophic levels and to identify the factors determining them.
The study includes (i) a description of the hydrodynamics of
each estuary and of the residence time of the water inside
the estuary and inside each part of the estuary, (ii) a
simulation of the estuary in the present conditions and
(iii) simulations of several scenarios of reduction of
nutrients, in order to evaluate the implications of
different management scenarios.
The study puts into
evidence the importance of the residence time for the
ecological characteristics of the estuaries and shows that
three ranges of residence time are important. If the
residence time is smaller than the time required for
developing a bloom, the nutrients discharged in the estuary
will be exported without being consumed (this is the case of
the Mondego estuary). The Sado estuary in on the other
extreme, being the residence time long enough to allow the
formation of a bloom and the regeneration of nutrients by
mineralization of the organic matter generated. The Tagus
Estuary is between this two ranges: the residence time is
enough to generate a bloom, but their products are exported
to the ocean as they are produced.
The study shows that in
the Tagus estuary the trophic level is limited by light
penetration due to the turbidity in the water column, which
is associated to the resuspension of the fine sediments
deposited in the intertidal areas by tidal currents and
surface waves generated by the long fetch of local wind. As
a consequence, a reduction of the nutrient loads discharged
by the rivers or by the Urban Waste Water Treatment Plants
has no consequences for the trophic activity in the estuary.
In the Sado estuary
primary production is limited by nutrients and by the
interaction between the phytoplankton and zooplankton and is
modulated by the residence time inside the estuary and by
the strong mixing between the different zones of the
estuary. Although the residence time of the estuary is
longer than one month, the residence time of the water
inside each part of the estuary is of the order of one week,
resulting into a strong mixing between zones of the estuary,
with deposition and mineralization of the particulate
organic matter in the shallow intertidal areas. The
reduction of the nutrients load in the estuary does not
modify the general behaviour of the estuary, maintaining the
structure of a stronger bloom in spring followed by several
smaller blooms. The amplitude of the peaks following the
first one are however smaller, especially for the
zooplankton.
In the Mondego estuary,
the limiting factor of phytoplankton production is the
residence time, which is not long enough to allow the
growing of a bloom. In this estuary the concentration of
nutrients is higher in the northern channel; however
eutrophication symptoms are detected in the southern channel
(growth of macro-algae). It was shown that this is a
consequence of the hydrodynamical properties of this
channel. In fact this channel is a net importer of
particulate organic matter from the northern channel, which
enriches the sediments creating conditions for macro-algae
growth, which is also stimulated by the small values of the
transient and residual velocities. The settling
characteristics of the southern channel have been stimulated
by the artificial closing of the upper connection between
the two channels. This study shows that the modification of
the trophic characteristics of the southern channel requires
the reopening of the communication between the channels and
can not be achieved by a realistic reduction of nutrients
discharged by the rivers. |
