Effect of fish and mesozooplankton manipulation on the phytoplankton community in the Patos Lagoon estuary,Southern Brazil

An enclosure experiment in the Patos Lagoon estuary, southern Brazil (32°S, 52°W) investigated the response of phytoplankton to the enhancement of mesozooplankton and fish abundance. Addition of nutrients (NO₃ ^? and PO₄ ^?) stimulated the growth of 3–20 μm diameter phytoplankton, especially the diatomCylindrotheca closterium, which, in turn, was heavily grazed by the dominant mesozooplankter, the copepodAcartia tonsa. Acartia did not consume small (2–3 μm) autotrophic flagellates and the cyanobacteriaAnabaena sp., despite their high cell number. Largest grazing ofC. closterium by the copepod occurred only after a decrease of ciliate abundance. The addition of mesozooplankton did not change the levels of primary production, but it significantly increased the phytoplankton assimilation number. Highest chlorophylla concentrations were measured in enclosures stocked with juveniles of the fishXenomelaniris brasiliensis. The large phytoplankton biomass and low mesozooplankton abundance found in this treatment indicates an, effective predator-mediated action on the phytoplankton community.     

Spatial and temporal characteristics of rainfall in a data-scarce region: case of Kinshasa and Bas-Congo in Democratic Republic of Congo

ABSTRACT

The aim of the present paper was to improve understanding of the rainfall dynamics in Bas-Congo and Kinshasa provinces, in Democratic Republic of Congo (DRC). The first objective of the study was achieved by analysing the spatial correlations of monthly, seasonal, annual and individual monthly rainfall amounts of Kinshasa and Bas-Congo. The second objective was achieved through investigating and quantifying the temporal trends and their spatial variations. The results demonstrated notably high average inter-station correlation of +0.63 for dry season series, followed by monthly rainfall series with an average inter-station correlation of +0.58. However, there was no station with a stable monthly rainfall regime, i.e. with mean precipitation concentration index lower than 10% (it varies between 14.2 and 21.9%). Moreover, Kinshasa experienced an increase of rainfall with an average annual rate of change of +4.59 mm/year for the period 1961–2006. The results will be helpful for efficient water resources management and for mitigating the adverse impacts of future extreme drought or flood occurrences.

Editor M.C. Acreman Associate editor N. Verhoest