Phytoplankton abundance and diversity in the Congo river at high and low waters

The Congo river is the second largest in the World in terms of freshwater discharge
(1457 km3 y-1) and in terms of drainage basin (375106 km2). In this study we sampled
the Congo river mainstem and major tributaries along the 1700 km stretch from
Kisangani to Kinshasa, during the high water and low water periods. Phytoplankton
was analysed using a combination of microscopy with HPLC analysis of marker
pigments. During high water, phytoplankton biomass in the mainstream was low
(mean Chl a = 0.8 mg m-3). Most tributaries presented lower Chl a, with some
exceptions, as the Oubangui river (3.6 mg m-3). At low water, phytoplankton
development in the R. Congo mainstem was higher (Chla 1.1-7.7 mg m-3).
Phytoplankton was essentially composed of green algae (mostly coccal green algae
and some desmids), diatoms (mostly Aulacoseira spp.) and filamentous
cyanoprokaryotes. In some samples euglenophytes (mostly Strombomonas spp.) and
chrysophytes appeared with greater abundance and diversity. In the low water
period, phytoplankton was, as typical in most large rivers, dominated by diatoms,
whereas the high water phytoplankton was dominated by coccal green algae, both in
the main river and tributaries.