Limnological changes in Dom Helvécio Lake (South-East Brazil): natural and anthropogenic causes.

In order to verify changes in physical, chemical and biological features of Dom Helvécio Lake (South-East Brazil), this study compares previous and recent data obtained from limnological investigations over three decades. Plankton species composition and density together with environmental data from 1978, 1983, and during 1999 to 2006 showed changes driven by seasonal climatic forces or by anthropogenic causes. Changes in diversity are shown as a consequence of alternation of rainy and dry periods and introduction of exotic fish species. Finally, suggestions are made for improving conservation efforts in the area, through planned actions and monitoring programmes.

Spatial and temporal variability of canopy cover and understory light in a Cerrado of Southern Brazil.

Canopy cover has significant effects on the understory environment, including upon light availability for seedling growth. The aim of the present study was to verify spatial heterogeneity and seasonal changes in the canopy cover of a dense Cerrado area, and their relationship to understory photosynthetic active radiation availability. Leaf area index (LAI) values in the rainy season varied from 0.9 to 4.83, with 40% of the values ranging from 4.0 to 5.0, while in the dry season LAI varied from 0.74 to 3.3, with 53% of the values oscillating from 2.0 to 3.0. Understory light (Q(i)) and the Lambert-Beer ratio (Q(i)/Q(o)) were taken around noon on sunny days (between 11:00 AM and 1:00 PM). They were also statistically different (p < 0.01) between the dry and wet seasons, with 72% of sampled points in the rainy season presenting photosynthetic photon flux density (PPFD) values lower than 250 micromol x m(-2)/s around noon, whereas in the dry season, most PPFD values varied from 1500 to 1817 micromol x m(-2)/s, thus providing high light availability for understory plants. In most of the studied sites, understory plants did not even receive enough light for 50% of their photosynthetic capacity in the wet season. In contrast during the dry season, Q(i)/Q(o) values of 0.8 to 1.0 were observed in more than 50% of the points, thereby allowing for photosynthetic light saturation. Thus, light variability around noon was higher during the dry season than in the wet season, its heterogeneity being related to spatial complexity in the canopy cover.