Low disturbances favor steady state: case of cyanobacterial monodominance in a Brazilian coastal lagoon

Abstract

Steady state occurs mainly under less pronounced abiotic variation, leading to low diversity and nonsuccessional change, opposite from the nonequilibrium concept. Permanent circulation of a waterbody creates a low disturbance status but does not represent a disturbance because it does not change the diversity and dominance of species. Phytoplankton composition was analyzed in a shallow Brazilian coastal lagoon, Peri (Santa Catarina State, Brazil), in the cold-dry and hot-rainy periods during 2009–2010. Uninterrupted steady state conditions occurred along with the mono-dominance of the cyanobacteria Cylindrospermopsis raciborskii, which dominated under a nonstratification regime and low availability of dissolved nutrients. Primary production measured as resource-use efficiency indicated a capacity for this species to convert phosphorus into biomass as well as its ability to effective exploit resources. Differences in temperature and nitrogen availability seemed to be the driving parameters for interseasonal changes in the phytoplankton community in Peri Lagoon. Models of competition that predict greater diversity, which lead to greater levels of primary productivity and more complete utilization of limiting resources, were not confirmed in this study. The presence of the dinoflagellate Peridinium umbonatum and C. raciborskii together is suggested to represent shallow, mixed, turbid, and nutrient-deficient ecosystems. Although competitive exclusion would be the primary mechanism that selects for species equilibrium, higher adaptability by a few species under selective conditions seems to be the most important factor in the occurrence of steady states.