WaterSubject

Studies on the biodiversity and population dynamics of freshwater planktonic Gastrotricha have been carried out in conjunction with a physical–chemical analysis of the water in the Yaounde Municipal Lake (Cameroon, Central Africa) over a 14 months period (November 1996–December 1997). The results obtained allow to consider the Yaounde Municipal Lake as an eutrophic lake. It harbours eight species of Gastrotricha belonging to four genera (Chaetonotus, Dasydytes, Neogossea and Polymerurus) of the order Chaetonotida. This community was characterized by high abundances of populations, and was dominated by the genus Neogossea and Chaetonotusreaching up to 2000 ind. L^?1. Polymerurus was mostly abundant at the almost anoxic bottom layers. The highest abundances were found mostly during the rainy season, when there is an important sedimentation process of organic matter, and were influenced by several different environmental factors such as dissolved oxygen, temperature and pH of the water.

Finally this community which may play an important role in the water bodies, is a potential water quality indicator.

Gastrotrichs are triploblastic, microscopic, acoelomate metazoans and form a phylum divided into two orders: Macrodasyida and Chaetonotida. They are characterized by worm-like or ten-pin body shape covered by a multilayered cuticle that often forms spine-bearing scales, a muscular pharynx with a triradiate lumen and by the cuticle covering all cilia. Order Chaetonotida contains the greatest number of freshwater species within the Gastrotricha, yet chaetonotidans are among the most poorly known freshwater microinvertebrates.

Historically, Gastrotricha was placed as a class within the Aschelminthes, a now defunct assemblage of various microinvertebrates with not known synapomorphies. Despite advances in knowledge on gastrotrich systematic, much remains to be elucidated about gastrotrich ecology. provided information on several marine species, but freshwater gastrotrichs were mostly neglected until showed importance on structure and functioning of limnic ecosystems. Even today, most hydrobiological research is focused on other microinvertebrates such as Rotifera, Cladocera, Copepoda and the Protozoa (mostly ciliates). The taxonomy and ecology of several microinvertebrate taxa are well documented in Europe and America including various monographs of Gastrotricha. Studies on African microinvertebrates, however, still remain in their infancy, Gastrotricha have seldom been studied. This is curious because reported that gastrotrichs are ubiquitous in the benthos and periphyton of lakes, ponds and wetlands, with densities ranging from 100 000 to 1 000 000 ind. m^?3. Since some species of the Neogosseidae and Dasydytidae are good swimmers, they may be abundant in the plankton, and have been reported in the plankton of some shallow weedy lakes.

In Africa, few known researches concerning freshwater Gastrotricha are the taxonomic works. In Cameroon, no work to our knowledge has been carried out concerning this phylum. The principal objective of this paper is to document the diversity and spatio-temporal distribution of planktonic Gastrotricha in the Municipal Lake of Yaounde (Cameroon), jointly with some physical–chemical factors that may govern gastrotrich distribution.

Sampling

Sampling was carried out on a weekly basis, from November 1996 to December 1997, at three stations (A, B and C) in the pelagic zone, using a 6 l dark Van Dorn sampling bottle mounted horizontally. Water sample (12 l) was concentrated, using a 40 ?m mesh size sieve plankton net. In the littoral zone, sampling was done by agitation, squeezing, scooping of vegetation and concentration using the same technique for qualitative studies. At stations A (entrance of the lake), B (middle of the lake) and C (posterior part of the lake), the samples were taken at five different depths: 0, 0.5, 1.0, 1.5 and 2.5 m).

Abiotic variables

Water temperature, transparency (z_SD) and the pH were measured in situ respectively with a mercury thermometer (1/10 °C), a Secchi disc and a portable pH meter (Schött C.G. 818). The euphotic depth (z_eu) was computered from the water transparency using the relationship z_eu=2.42z_SD, assuming that there is a maximum of 15% transmission of light at the depth of the Secchi disc transparency. The light availability was measured with a luxmeter and the euphotic depth to mixing depth ratio (z_SD/z_mix) was calculated. The attenuation coefficient was calculated according to the law of Beer Lambert (I_z=I_oe^??z). The colour and ammonium nitrogen concentration were measured by spectrophotometry (HACH DR/2000) using Nessler reagent for ammonium nitrogen. The suspended solids (SS) were quantified by filtration on a 0.45 ?m pore size Whatmann GF/C. The dissolved oxygen was determined by the classical method of Winkler.

Biotic variables

Specimens were examined and identified using a binocular stereomicroscope Wild M5 and a Leitz Orthoplan microscope with camera lucida. The described specimens were fixed in 5% formalin and mounted on glass slides in glycerine. Slides were sealed with nail polish and deposited in the Queensland Museum (Australia). Taxonomic identification was carried out with the aid of keys. Quantitative counts were carried out according to in the Dolfuss chamber. For the ecological analysis, biomass was evaluated in an indirect manner by the method of bio-volumes. Some fixed specimens were photographed with an Olympus BH2 microscope with DIC/Nomarski optics and a Sanyo HiRes Color CCD camera at the Queensland Museum (Australia).

To evaluate the relationship between gastrotrich biomass and physical–chemical variables, the rank correlation of Spearman corrected according to Bonferroni approach was performed.

Significant correlations were observed between the biomass of species of Dasydytes and Neogossea and various physical–chemical variables. The significant negative correlation between dissolved oxygen and species biomass indicated that these species can tolerate low oxygen tensions, perhaps avoiding predators more abundant and active in shallow well-oxygenated waters. However, only the species of Polymerurus were consistently found at the nearly anoxic bottom, indicating that most other species may have metabolic constraints that limit their ability to respire in the benthos. Morphologically, the species of Dasydytes and Neogossea seem to be well adapted for a semi-pelagic existence, with short rotund bodies covered with large flexible spines that facilitate floatation, and so perhaps it was not surprising these species were rarely found at the bottom. Species of Polymerurus, however, possessed a general chaetonotidan body plan that was adapted for benthic life: slender body, complete scale covering, and furca with adhesive tubes. Species of Polymerurus are well documented to have a benthic or periphytic existence on submerged vegetation. These results corroborate the conclusions who states that Gastrotricha are among the few animals commonly found in the anaerobic environment. Also, perhaps it was not surprising to find negative correlations between biomass and temperature and pH, since these two physical–chemical variables are linked to water depth and likely influence oxygen tension and therefore invertebrate abundance. These significant correlations suggest these fluctuations in abundance of Dasydytes and Neogossea may be highly influenced by several different environmental factors. These two genera in general and their representative species in particular are adapted to pelagic area and may therefore have potential to be indicators of water quality in future freshwater body assessments.