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Trophic status of a
Brazilian urban reservoir and prognosis about the recovery of water
quality
HENRY1,
R., CARMO2, C.F. do & BICUDO2, D.C.
1
Dept. Zoology,
Institute of Biosciences, UNESP, Botucatu, São Paulo, Brazil. e-mail:
rhenry@ibb.unesp.br
2
Section of Ecology, Institute of Botany, São Paulo, Brazil.
ABSTRACT:
Garças Lake is a small tropical urban reservoir, with 68 days hydraulic
residence time (annual theoretical mean) located in São Paulo city
(Brazil). In order to develop management strategies for
the recovery of its water quality, a diagnosis of the trophic state was
performed. Phosphorus and nitrogen mass balances and the development of
a model to evaluate the nutritive condition of the lake were included.
Incoming annual phosphorus and nitrogen loads by the seven tributaries
were, respectively, 6,519 kgP and 37,157 kgN. From the total phosphorus
load, 45% proceeded from the creek of Zoological Garden Foundation of
São Paulo (the most important point-source), while 32% were from
the Agricultural and Food Supply Department (the
second most important point-source). From the nitrogen
total incoming load, 62.8% corresponded to biological fixation; 1.4% to
non-point sources; 1% to wet precipitation and 34.7% to point-sources.
Mass balances revealed high nitrogen (85% of
total input) and phosphorus (61%
of total input)
retention in the reservoir. The annual observed mean of total phosphorus
in the water surface was 132 mg.l-1, indicating a
hypereutrophic condition. A hypothetical scenario of 90% reduction in
the load from the seven tributaries could change the trophic status of
the reservoir to an oligotrophic condition (17
mgTPl-1). Reservoir management strategies for
the improvement of water quality are discussed.
Key-words:
phosphorus load, mass balance, eutrophication model, reservoir
management. |
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