Programme Poster session 2
On the origin of cyanobacteria blooms in the Enxoé reser
Author(s): Henrique Coelho (1), Adélio Silva (1),Pedro Chambel Leitão (2), Matthias Obermann(3)
(1) Hidromod, Ltd
(2) Instituto Superior Técnico
Keyword(s): Water Quality, Cyanobacteria Blooms,
Session: Poster session 2
In the framework of AquaStress (an EU funded integrated project), a small reservoir (Enxoé) located in the
South of Portugal was studied. The reservoir was built to supply water for human consumption, but it shows several
water quality problems namely cyanobacteria blooms. In the last years and as a consequence of these problems, it
was not possible to use it to supply water during the summer. In order to try to understand the possible causes of the
problems and, hopefully, find proper solutions to solve them, an integrated study involving the catchment, the data
available concerning the nutrient loads, the meteorology and the water quality and mathematical models was set up.
This approach allowed the suggestion of some possible actions that may lead to improve the conditions within the
Some major conclusions of data analysis and first model implementation put in evidence that there was a
sudden change after the winter 2000/2001 floods, namely a rapid phosphorous enrichment and a rapid decrease of
TN:TP ratio, bottom anoxia and presumably phosphorous release from sediments and permanent cyanobacteria
dominance since the 2001/2002 floods.
The first modelling results also put in evidence that the model was not
reproducing very well the behaviour of the reservoir. The main cause for this is probably due to errors in the methods
of the loads quantification, although a standard approach based on OSPAR guidelines that showed to be successful
in other applications made in the north of Portugal is being used. The cyanobacteria dominance started only after the
winter of 2000/2001, indicating that the big floods that occurred on this year and associated erosion, may be
responsible for a major source of P for the reservoir. This source is possibly misrepresented by any estimate of
In order to try to clarify these aspects it was decided to find similar data from other reservoirs in the
neighbourhood (e.g. Monte Novo and Roxo) to validate the thesis that floods might be a triggering mechanism for
cyanobacteria dominance. Also to solve the problem of boundary conditions for the reservoir model, a simple
inverse model that computes the loads as a function of measurements of Phosphorus in the reservoir and exchanges
between the water column and the sediments was used.
In this paper a discussion of the influence of the floods in
the nutrient loads that are transported to reservoirs in such semi-arid regions will be made based on the data
available for Enxoé and Monte Novo reservoirs and an improved erosion/retention model for the catchment able to
reproduce the nutrient loads in a more proper way will be proposed.