Distribution of Epilithic Diatoms in Response to Environmental Conditions in an Urban Tropical Stream, Central Kenya

摘要

Use of diatoms in monitoring water quality is well acknowledged in developed countries, but only recently has the assessment started gaining importance in developing countries. Diatoms can be obtained from natural and artificial substrates. Appreciating the differences and similarities of diatom assemblages on both substrates may contribute to a better understanding and standardization particularly during monitoring of water quality. During this study we assessed diatom assemblages, biodiversity andtrophic indices in relation to water quality along the Nairobi River. Fifteen sites were sampled in September 2000 during the dry season. Diatoms were collected from natural substrates (stones, pebbles) and artificial substrates (100% acrylic wool). On artificial and natural substrates, a total of 190 and 151 taxa were found, respectively, the majority of these taxa (80%) have cosmopolitan distribution and are also widespread throughout tropical African. Species composition changed downstream, five taxadominated upper and mid stream sites whereas lower stream sites were dominated by one or two taxa. Species richness, diversity, dominance and evenness were positively correlated with NO_3, O_2 and altitude but decreased markedly downstream with a simultaneous increase in total dissolved solids, alkalinity, chemical oxygen demand and PO4. Ordination and classification (CANOCO and TWINSPAN) showed that diatom assemblages in the Nairobi River responded strongly to water quality changes with respect to concentrations of NO_3, NO_2, total dissolved solids and temperature. Taxa common at less impacted upstream sites included Gomphonema gracilis, Anomoeoneis brachysira and Fragilaria biceps; while common taxa at midstream sites with agricultural catchments were Gomphonema parvulum, Navicula cryptocephala, N. schroeteri, N. bryophila, N. halophila, Nitzschia linearis var. linearis and Cymbella silesica. Achnanthes minutissima var. saprophila, Gomphonema angustum, Navicula subminuscula, N. arvensis, Nitzschiapalea and N. umbonata were most common at urban sites, which were polluted by residential and industrial effluents. Trophic diatom indices suggested that water quality was poor at most sites in the Nairobi River. Most sites along the river had low Generic Diatom Index values, GDI (<12) and high Trophic Diatom Index values, TDI 73–78 (median = 76) and 75–84 (median = 77) for artificial and natural substrates, respectively. This study showed that diatoms' response on natural and artificial substrates were similar and reflected environmental conditions correctly.