Seasonal dynamics of phytoplankton in two Amazon flood plain lakes of varying hydrologic connectivity to the main river channel

Abstract

Seasonal dynamics of phytoplankton were evaluated in Lakes Amapa and Pirapora, small oxbows in the floodplain of Rio Acre, a tributary of the Amazon River in western Brazil. The two lakes differ primarily in their degree of connectivity to the main river channel, with the younger Lake Pirapora maintaining a connection year-round and displaying physical and chemical characteristics responding to rising river water levels, versus the older Lake Amapa connecting to the river only during a 2-3 month flooding phase and having a distinct period of isolation at the end of the dry season. The phytoplankton of Lake Pirapora has a low numeric density year-round and is dominated by small C-strategists (cryptomonads). The phytoplankton of Lake Amapa has a similar low density when the lake is connected to the river. However, once that lake is isolated and turbidity declines, density increases an order of magnitude and the assemblage becomes dominated by filamentous N-fixing Aphanizomenon tropicalis. Coincidentally concentrations of dissolved oxygen increase and levels of TN decline, similar to changes during autogenic succession in temperate shallow lakes. The seasonal changes in phytoplankton of Lake Amapa, from the time it is connected to the time when it is isolated from the river, generally conform to the C-S-R conceptual model of succession. This is the first study to directly compare lakes of different developmental stages, in terms of connectivity with the river channel. There remains a need for more comprehensive long-term comparisons of these unique lake ecosystems.