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

Abstract

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 and trophic 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 taxa dominated 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 PO 4 . 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, Nitzschia palea 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.

Halophilic diatom taxa are sensitive indicators of even short term changes in lowland lotic systems

“…These events caused an increased conductivity, and/or Cl -and/or nutrient concentration in almost all sampling sites. The results supported our third hypothesis, that evenness of diatom assemblages decreases with increasing conductivity and/or nutrient content (NH 4 + , PO 4 3-), according to ZALAT (2002), NDIRITU et al (2006) and STENGER-KOVÁCS et al (2013). Confi rming our fi ndings, they observed a decreasing diversity in diatom assemblages parallel to increasing salinity/conductivity, and/or nutrient (especially phosphate) contents.…”

Using benthic diatom assemblages to assess human impacts on streams across a rural to urban gradient

“…The best indicator species for high nutrient levels or pollution were as follows: N. palea, N. trivialis, and L goeppertiana in spring; E. minima and Gomphonema angustum in summer; E. minima and N. frustulum in autumn; and E. minima in winter. These results confirmed that N. palea is cosmopolitan and a pollution-tolerant species, and it was dominant in polluted rivers in Spain (Urrea and Sabater 2009), Kenya (Ndiritu et al 2006), Vietnam (Duong et al 2006), and Brazil (Bere and Tundisi 2011). E. minima, a small cell-sized diatom, can adapt to different conditions (Raven 1998), and it was the dominant taxon at polluted sites after the summer due to harsh conditions such as flooding and excess light.…”

“…It may be related to the hydrological characteristics of the Ganhe River, which experiences a wet season in summer (Yang et al 2009). Biodiversity indices (e.g., diversity, species richness, and evenness) are not always predictable in response to environmental conditions (Ndiritu et al 2006). We found that the diversity was higher in the urbanized area with an intensity of anthropogenic disturbance, with lower values in the upper reaches of the lower trophic level.…”

“…Stephens et al (2012) hypothesized that the biomass will increase as nutrients are added, unless it is limited by light. Like algae biomass, the use of algal biodiversity as an indicator of river pollution gave ambiguous results (Ndiritu et al 2006;Blanco et al 2012).…”

Changes in the epipelic diatom assemblage in nutrient rich streams due to the variations of simultaneous stressors

“…Pan et al, 1999;Soininen et al, 2004;Kelly et al, 1995;Rott et al, 1997Rott et al, , 1999Coring, 1999) and have been widely related to specific environmental conditions in different geographical regions (Soininen, 2002;Martinez de Fabricius et al, 2003;Ndiritu et al, 2006, Gomez & Licursi, 2001, Lobo et al, 20022004a,b). Consequently, several studies have addressed the tolerances and preferences of diatoms along a number of environmental gradients, such as gradients in salinity, pH, trophy, saprobity and current preference (e.g.…”