The necessity of management in a lake of the Atlantic Forest biodiversity hotspot: nitrogen levels connected to a persistent bloom of Cylindrospermopsis raciborskii

Abstract

Conservational studies of the threatened Atlantic Forest biome are frequently restricted to terrestrial ecosystems. We know little about the water bodies, specially considering that this biome covers the third largest system of lakes in Brazil. Some of these lakes are located inside the protected "Rio Doce State Park", but many others are found outside this reserve. Th ese external lakes are seldom studied, but understanding their response to human activities is essential for the conservation and the protection of the lakes inside the Park. We evaluated the eff ects of degradation in a lake outside the Park, which shows a constant bloom of the toxic invasive cyanobacteria Cylindrospermopsis raciborskii. Phytoplankton, climate and physico-chemical variables were assessed from 2011 to 2013 to evaluate which were the major determinants of the lake dynamics. Despite the seasonal changes, the lake was always eutrophic, and cyanobacteria, transparency and nutrients were the major indicators of water characteristics. Th e lake seems to be nitrogen-limited and cyanobacteria were negatively correlated with nitrogen levels, since the constantly dominant C. raciborskii is a superior competitor for N. We suggest that the monitoring of nitrogen levels is fundamental to establish management strategies to avoid harmful algae blooms in this Atlantic Forest lake.

Environmental factors and thresholds for nitrogen fixation by phytoplankton in tropical reservoirs

“…Reservoirs are multipurpose systems built worldwide for buffering floods, generating electricity, supplying water for different uses, and thus significantly changing hydrodynamics, water chemistry and physical characteristics, biological communities, and other features of the modified aquatic system. Blooms of N 2 ‐fixing cyanobacteria have been reported in eutrophic reservoirs (Bouvy et al, 2000; Chellappa et al, 2008; Figueredo et al, 2016; Wood et al, 2010) and are primarily controlled by the high and disproportionate availability of P (Gonzáles & Quirós, 2011). As eutrophic reservoirs commonly have a greater relative abundance of cyanobacteria, they can support N 2 fixation rates up to 10‐fold higher when compared with oligomesotrophic reservoirs (Howarth et al, 1988, Scott et al, 2009).…”