Seasonal mechanisms driving phytoplankton size structure dynamics in a tropical deep lake
(Dom Helvécio Lake, South-East Brazil)
BARROS1, C.F.A, SOUZA1, M.B.G. & BARBOSA1, F.A.R.
1 Laboratório de Limnologia - Instituto de Ciências Biológicas/ Departamento de Biologia Geral/ Universidade Federal de Minas Gerais - Av. Antônio Carlos, 6627, 30161-970. Belo Horizonte – MG, Brasil
ABSTRACT: Dom Helvécio Lake, located inside Rio Doce State Park (south-east Brazil) is the largest and deepest lake of the middle Rio Doce lacustrine system. Although data on physical, chemical and biological features of some of those lakes have been known since the 70’s, the biovolume of phytoplankton species have not yet been studied. In this paper we characterize, for the first time, algae size structure in Dom Helvécio Lake and assess seasonal variations to establish possible associations between algae size structure and the environment, particularly water column stability along stratification changes. Samples were collected in three months during each dry and rainy season from June 2001 to February 2004. Air and water temperature, precipitation, relative humidity and stability of thermal stratification all significantly increased during the rainy (stratified) seasons, while total-P, total-N, DIN and SRSi concentrations were higher during the dry (mixing conditions) seasons. Results suggest a persistent phosphorus limitation to phytoplankton primary production and a variable amount of nitrogen was detected. No significant seasonal variation was verified for total phytoplankton biovolume. Despite this, the distribution of size classes in different stability conditions was distinct: a predominance of nano- and ultraplankton (< 20 mm), mainly due to Zygnemaphyceae, during stratification periods and, of micro- and netplankton (> 64 mm), specially Cyanophyceae in mixing ones. This result indicate that the constancy of algae biovolume in Dom Helvécio Lake along 2002 was not a result of a stable phytoplankton community but it occurs as a consequence of significant changes in size structure of phytoplankton.
Key-words: Phytoplankton, biovolume, mixing conditions, tropical lake.