Diurnal and spatial (vertical) dynamics of nutrients (N, P, Si) in four sampling days (summer, fall, winter, and spring) in a tropical shallow reservoir and their relationships with the phytoplankton community.

The vertical and diurnal variation of nitrogen and phosphorus forms, as well as that of soluble reactive silica (SRS), were studied in four sampling days at Gar9as reservoir, a shallow tropical one located in the city of S?o Paulo, in southeastern Brazil. Except for N-NH4, all other inorganic forms of nitrogen (N-NO2, N-NO3, and total N) demonstrated decreased concentrations toward the bottom of reservoir. Similarly, all showed significant diurnal differences on every sampling day, with increased values during the night due to absence of photosynthetic assimilation during that period. In the sampling days, these forms decreased on the spring sampling day due to the bloom of Microcystis registered during this period of the year. All three forms of phosphorus (SRP, particulate P, and total P) showed significant vertical variation, except on the fall sampling day. On the summer sampling day there was an increase of both total P and particulate P, the latter because it constitutes more than 70% of the total P during all sampling days. Hourly phosphorus variation was significant during all sampling days, except for the summer one. The SRS vertical variation was significant during all sampling days, except for that in the spring. It was also different hourly on sampling days.