Significant Change in Marine Plankton Structure and Carbon Production After the Addition of River Water in a Mesocosm Experiment.

Rivers are known to be major contributors to eutrophication in marine coastal waters, but little is known on the short-term impact of freshwater surges on the structure and functioning of the marine plankton community. The effect of adding river water, reducing the salinity by 15 and 30%, on an autumn plankton community in a Mediterranean coastal lagoon (Thau Lagoon, France) was determined during a 6-day mesocosm experiment. Adding river water brought not only nutrients but also chlorophyceans that did not survive in the brackish mesocosm waters. The addition of water led to initial increases (days 1-2) in bacterial production as well as increases in the abundances of bacterioplankton and picoeukaryotes. After day 3, the increases were more significant for diatoms and dinoflagellates that were already present in the Thau Lagoon water (mainly Pseudo-nitzschia spp. group delicatissima and Prorocentrum triestinum) and other larger organisms (tintinnids, rotifers). At the same time, the abundances of bacterioplankton, cyanobacteria, and picoeukaryote fell, some nutrients (NH4+, SiO43-) returned to pre-input levels, and the plankton structure moved from a trophic food web based on secondary production to the accumulation of primary producers in the mesocosms with added river water. Our results also show that, after freshwater inputs, there is rapid emergence of plankton species that are potentially harmful to living organisms. This suggests that flash flood events may lead to sanitary issues, other than pathogens, in exploited marine areas.

Recurrent blooms of Heterosigma akashiwo (Raphidophyceae) in the Piraquê Channel, Rodrigo de Freitas Lagoon, southeast Brazil.

Six blooms of Heterosigma akashiwo (Raphidophyceae) were observed from March 2007 through March 2008 in the Rodrigo de Freitas Lagoon, a semi-confined eutrophic system located in Rio de Janeiro state, southeast Brazil. Vegetative cells of H. akashiwo analysed by optical and electron microscopy showed morphology as described in the literature. The blooms (2.8 × 10(4) to 4 × 10(8) cell.L(-1)) were restricted to the middle section of the Piraquê Channel, which is situated in the northeastern part of the lagoon and receives freshwater inflow. The salinity of subsurface water and the channel depth showed significant negative correlations with H. akashiwo abundances, and appeared to restrict the blooms to this compartment of the lagoon. No fish mortality was associated with the H. akashiwo blooms, nor were brevetoxins detected in a cell extract obtained from the bloom observed on 19 March 2007.

Recurrent blooms of Heterosigma akashiwo (Raphidophyceae) in the Piraquê Channel, Rodrigo de Freitas Lagoon, southeast Brazil / Florações recorrentes de Heterosigma akashiwo (Raphydophyceae) no Canal do Piraquê, Lagoa Rodrigo de Freitas, sudeste do Brasil

Six blooms of Heterosigma akashiwo (Raphidophyceae) were observed from March 2007 through March 2008 in the Rodrigo de Freitas Lagoon, a semi-confined eutrophic system located in Rio de Janeiro state, southeast Brazil. Vegetative cells of H. akashiwo analysed by optical and electron microscopy showed morphology as described in the literature. The blooms (2.8 × 104 to 4 × 108 cell.L–1) were restricted to the middle section of the Piraquê Channel, which is situated in the northeastern part of the lagoon and receives freshwater inflow. The salinity of subsurface water and the channel depth showed significant negative correlations with H. akashiwo abundances, and appeared to restrict the blooms to this compartment of the lagoon. No fish mortality was associated with the H. akashiwo blooms, nor were brevetoxins detected in a cell extract obtained from the bloom observed on 19 March 2007.

Seis florações de Heterosigma akashiwo (Raphidophyceae) foram observadas em março de 2007 a março de 2008 na Lagoa Rodrigo de Freitas, um sistema semi-confinado eutrófico localizado no Rio de Janeiro (Sudeste do Brasil). As células vegetativas de H. akashiwo analisadas por microscopia óptica e eletrônica mostraram morfologia como descrito em literatura. As florações (2.8 × 104 a 4 × 108 cel.L–1) foram restritas à zona intermédia do canal Piraquê, que se situa na parte nordeste da lagoa e recebe aporte de água doce. A salinidade da sub-superfície da água e a profundidade do canal apresentaram correlação negativa significativa com a abundância de H. akashiwo e parecem determinar a formação de florações restritas a este compartimento da lagoa. Não houve mortalidade de peixes durante as florações de H. akashiwo e não foi detectada a presença de brevetoxinas em um extrato celular obtido a partir da floração observada em 19 de março de 2007.