Phytoplankton as a monitoring tool in a tropical urban shallow reservoir (Garcas Pond): the assemblage index application

This study aimed at evaluating phytoplankton as a monitoring tool for water quality assessment in an urban shallow eutrophic reservoir considering temporal and vertical scales. Garcas Reservoir is located in the Parque Estadual das Fontes do Ipiranga Biological Reserve (23 degrees 38 ' 08 '' S and 23 degrees 40 ' 18 '' S; 46 degrees 36 ' 48 '' W and 46 degrees 38 ' 00 '' W) that lies in the southeastern part of the Municipality of Sao Paulo, southeast Brazil. Samplings were carried out monthly during 8 consecutive years (1997-2004) following the water column vertical profile (5 depths: subsurface, 1, 2, 3 m and similar to 20 cm from the bottom). Abiotic variables analyzed were: water temperature, electric conductivity, DO, pH, total alkalinity, free CO2, dissolved inorganic carbon, N series, P series and SiO4H4. Biological variables studied were: total density, total biomass and chlorophyll a, which were integrated arithmetically. At the beginning of the 8 year series, Garcas Reservoir was an eutrophic ecosystem with 20% of its surface covered by Eichhornia crassipes (phase I: January 1997-March 1998). Water hyacinth reached 70% of pond surface coverage (phase II: April 1998-August 1999), and then it was mechanically removed (phase III: September 1999-December 2004). After this intervention, drastic alteration in the limnological features was detected, leading to the conclusion that removal of the aquatic macrophyte modified nutrient dynamics drastically reduced water transparency and led to photosynthetic productivity and phytoplankton biomass increase, the latter becoming a physical barrier to light penetration. Twenty one functional groups 'sensu' Reynolds were identified. Cyanobacteria contribution played the main role during the drastic alterations that occurred after water hyacinth removal. Results of ecological status of reservoir using Q index showed statistical difference among the 3 limnological phases (one way ANOVA; F = 119.4; P = 0.000). Regarding Q index classification, Garcas Reservoir limnological phases were characterized as follows: (1) phase I: 0 >= Q <= 2.9, medium to bad; (2) phase II: 1.4 >= Q <= 3, tolerable to medium; and (3) phase III: 0 >= Q <= 1.5, bad to tolerable ecological states.