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The water-mixing regime, light availability, nutrient limitation and trophic state were evaluated for the Duas Bocas Reservoir (DBR; Espírito Santo State, southeastern Brazil), a small (0.51 km 2 ), shallow (z max < 10 m) water supply reservoir located in a conservation area. Monthly water sampling was conducted for the lacustrine zone (z max = 10 m) next to the water withdrawal tower, during one hydrological cycle (October 2002 to September 2003. Water samples were taken at four depths in this area, including subsurface, Secchi depth, euphotic ⁄ aphotic boundary and near the bottom of the lake. The variability of the reservoir's limnological features defined two periods. Period 1, comprised of the summer months (October to April), was characterized by strong thermal stratification, nutrient limitation in the mixing layer, a metalimnetic biomass maximum and anoxia and high nutrient concentrations in the hypolimnion. Period 2, comprised of the winter months (May to September), was characterized by overturn events during which the phytoplankton population experienced light limitation. The observed water-mixing regime was characteristic of warm monomictic system, presenting atelomixis, the incomplete vertical mixing of stratified water masses. The average total phosphorus and chlorophyll-a concentrations suggested a meso-eutrophic water system. The nutrient concentrations exhibited nitrogen and phosphorus limitation, suggesting that nutrients, rather than light, limit or regulate the phytoplankton biomass in the DBR for most of the year.

Tailings from bauxite mining in Porto Trombetas (Pará state, Central Amazonia, Brazil) was discharged (1979–1989) into Batata Lake affecting about 30% of its area. The lake belongs to a clear-water flood-plain system along the Trombetas River, a tributary of the Amazon River. Siltation is the main perceived factor impacting aquatic and flooded communities. Besides natural regeneration, a program to restore a section of igapó forest in the impacted area (IA) has been conducted since 1991. Decreased light is the main factor reducing total phytoplankton biomass (PhyBM) in IA. We hypothesized that PhyBM in IA increases over time because of the improvement of the underwater light conditions due to the natural regeneration and restoration. We sampled quarterly PhyBM and limnological variables (depth, transparency, temperature, pH, conductivity, dissolved oxygen, turbidity, suspended solids, total Kjeldahl nitrogen, and total phosphorus), over 15 years (2005–2019) at eight sampling sites in the two areas (N = 349). We also obtained daily climatic and hydrologic data. PhyBM was higher in NIA than in IA. The temporal trend in the annual mean of PhyBM increased significantly over time only in the IA, approximating the NIA values, confirming our general hypothesis. The increase of PhyBM in the IA was negatively related to the residual light attenuation caused by non-phytoplankton turbidity and to total phosphorus, and positively to air temperature and site depth (p < 0.05; Marginal r2 = 0.18; Conditional r2 = 0.29). Instead, in NIA, PhyBM was explained only by the increase in air temperature (p < 0.05; Marginal r2 = 0.15; Conditional r2 = 0.34). We concluded that the PhyBM in the IA positively responds to the synergy between increasing light availability, air temperature, and site depth, and decreasing total phosphorus concentrations, regardless of hydrologic phase.

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