Stephane Stenuite

Phytoplankton ecology of Lake Kivu (eastern Africa): biomass, production and elemental ratios

Lake Kivu, located between Rwanda and the Kivu Province (Democratic Republic of Congo), is the smallest of the East African Great Lakes. It is a deep (max. 489 m), meromictic lake, with an oxic mixolimnion up to 60 m, and a deep monimolimnion rich in dissolved gases, particularly methane (TIETZE et al. 1980, SCHMID et al. 2005). It is also the sole natural lake in which Limnothrissa miodon Boulenger, an endemic sardine from Lake Tanganyika, was introduced to fill an empty niche (COLLART 1954, SIMBERLOFF 1995). Indeed, prior to the introduction, no planktivorous fish was present in the pelagic waters of Lake Kivu. Recent studies have confirmed the oligotrophic status of Lake Kivu: with an annual average chlorophyll a in the mixed layer of 2.2 mg m–3 and low nutrient levels in the euphotic zone, it combines a relatively shallow euphotic layer (~18 m), usually smaller than its mixed layer (20–60 m), with a weak thermal gradient in the mixolimnion (ISUMBISHO et al. 2006, SARMENTO et al. 2006). The most common phytoplankton species are the pennate diatoms Nitzschia bacata Hust. and Fragilaria danica (Kutz.) Lange-Bert., and the cyanobacteria Planktolyngbya limnetica (Lemm.) Komarkova-Legnerova and Cronberg, and Synechococcus sp. (SARMENTO et al. 2006, 2007, 2008). A survey in Lake Kivu covering the vertical and temporal variations of phytoplankton was carried out during 2003–2005, with a large number of plankton primary production measurements and assessment of phytoplankton nutritional status by seston C:N:P ratios.