Relationships between environmental factors, periphyton biomass and nutrient content in Garças Reservoir, a hypereutrophic tropical reservoir in southeastern Brazil

Abstract

This study focused on evaluating the factors related to seasonal variations of periphyton biomass and nutrient content (N, P) in Garcas Reservoir, a tropical, shallow hypereutrophic tropical reservoir in southeastern Brazil that exhibits a permanent cyanobacteria bloom. Artificial substrata were placed in the subsurface water (20 cm) of the lake on a monthly basis (incubation time of 28 days) over a 1-year period. Two limnological periods were characterized, including: (i) a period of decreased water clarity, higher levels of soluble reactive phosphorus and higher phytoplankton biomass; and (ii) a period of relatively higher water clarity (as measured by Secchi depth), higher nitrate concentration and lower phytoplankton biomass. The periphyton chlorophyll-a levels were lower during the first period, being negatively correlated with phytoplankton biomass. The results of this study suggest that during the period of decreased water clarity, periphyton was primarily light-limited. In contrast, the periphyton biomass was higher during the second period, regardless of P limitation of periphyton growth. Rehabilitation of this highly degraded tropical reservoir must consider the light regime, which is controlled by phytoplankton abundance. Thus, a reduction in the P loading to the lake should be considered to suppress its cyanobacterial blooms, thereby improving conditions for submerged macrophytes and the re-establishment of periphyton.