État écologique de quelques cours d'eau du bassin du Nyong au Cameroun: diversité des macroinvertébrés benthiques et des hyphomycètes aquatiques, décomposition de la litière et physico-chimie des eaux

Abstract

With the aim of contributing to improvement of knowledge on biodiversity and functioning of forest watercourses in Cameroon, a study on the ecology of benthic macroinvertebrates and aquatic hyphomycetes as well as their contribution to the decomposition of dead leaves of Funtumia africana (Benth) Stapf (Apocynaceae) was carried out in seven streams of the Nyong watershed in Cameroon from February 2019 to August 2021.A total of 167 samples of water and benthic macroinvertebrates were taken at 13 stations over 13 months, and experiments on litter decomposition in streams were carried out for 15 days of exposure. Benthic macroinvertebrates were collected using the kick net method following the multihabitat approach and on a monthly sampling frequency, except the month of February 2020 when the Akoumbegue stream dried up (stations CA and C). Benthic macroinvertebrates associated with coarse mesh litter bags were also collected and identified under binocular with appropriated keys. Environmental variables were measured both in the field and in the laboratory per month using devices and methods from literature. Environmental variables were not significantly different between stations, but significant variations were observed temporally. The variables dependent on climatic / meteorological parameters (depth, speed,width, humidity, temperature, etc.) varied significantly from one month to another and especially during the long rainy season. However, the waters were slightly acidic with low levels of nitrogen compounds, orthophosphates, solid particles, low hardness, alkalinity, manganese, potassium and conductivity values. During this study, 13690 benthic macroinvertebrates belonging to 4 phyla, 7 classes, 16 orders and 93 families were collected. Benthic macroinvertebrates were more abundant and richer at stations characterized by moderate current and varied substrate including stations K1 (109 species), K2 (111 species), K3 (129 species), AN1 (93 species), AN2 (96 species), N (87 species) and A (89 species). The diversity varied significantly from 1.33 ± 0.14 bits/ind to 2.00 ± 0.35 bits/ind (P-value = 0.0101) and the high values were found in stations with multiple substrates and well-oxygenated water. In time, the diversity varied considerably from 1.10 ± 0.16 bits/ind in the sampling station NM in February 2020 to 1.87 ± 0.1 bits/ind in the sampling station OB in September 2019 (P-value= 0.0399). In addition, the community was more abundant during the short dry season, especially in August 2019 (1471 individuals) (P-value = 0.0060) but richer during the long dry season in February 2019 (54 families) (P-value = 0.0481). Four typological groups were identified from ascending hierarchical classification, SOM and discriminant factor analyses. The relatively high values of the EPT and EPTB indices testify the good ecological quality of waters, indicating their capacity to host a high diversity of polluo-sensitive organisms. The same applies to the EPTH and EPTO indices in which the affinities between the EPT, the Heteroptera and the Odonata are proven. The trophic groups identified are predators, scrapers, collectors, shredders, herbivores, filter feeders and omnivores, with predators and scrapers being dominant, followed by collectors and shredders. The decomposition process was carried out using the litter bag method with dead leaves of Funtumia africana exposed in seven streams. The installation and removal of the bags were accompanied by the measurement of certain biological and environmental parameters controlling the process of decomposition of the litter. Thus, the decomposition rate varied from 0.035 g.d-1 to 0.056 g.d-1 with an average of 0.042 ± 0.006 g.d-1 in the coarse mesh (Kc) litter bags and from 0.018 g.d-1 to 0.059 g.d-1 with an average of 0.037 ± 0.01 g.d-1 in fine-mesh (Kf) litter bags. No significant difference was observed between stations (P-value = 0.162 for Kc; P-value = 0.045 for Kf) and seasons (Pvalue ˂ 0.043 for Kc; P-value = 0.053 for Kf). Decomposition was mainly due to microbial activities and the contribution of shredders was less. But the rates obtained indicate rapid decomposition (> 0.01 g.d-1). The Kc to Kf ratio and the litter fragmentation rate λF confirmed this weak contribution of the shredding benthic macroinvertebrates in the process. Among the environmental factors considered, only the distance to the source (P-value = 0.0134) and the pH (P-value = 0.063) seem to positively influence the rate of decomposition of the litter of Funtumia africana but not significant.