Developing a diatom monitoring network in an urban river-basin: initial assessment and site selection

There is increasing interest in the restoration of urban river systems because they are affected by multiple point- and diffuse-source impairments. In South Wales, these issues are exemplified by the rivers Taff and Ely, which have a well-documented history of pollution. We developed a network of river-monitoring sites to capture as many sources of impairment as possible and to provide an integrated assessment of basin-wide conditions using established, recently revised and new methods to assess ecological status as required by the Water Framework Directive (WFD). Most of the 34 river sites, except those in upstream areas, had moderate or poor ecological status and low species richness. There were significant correlations between the UK Trophic Diatom Index (TDI), phosphate and nitrate concentrations, but the strongest correlation was with sodium concentrations. Sodium also differentiated best between ecological status classes. The UK TDI, Ecological Quality Ratios and the Indice de Polluosensibilité Spécifique (IPS) correlated strongly, but the IPS reflected chemical conditions less well. There were also possible metal impacts including some upstream sites despite apparently good or high ecological status. These data reveal strong capacity in diatoms for reflecting stressors that affect urban river networks. We recommend (i) further assessments including geographically comprehensive and repeated surveys to underpin local decision-making and (ii) further refinement of the tools currently in use for WFD purposes through the development of specific diagnostic indices or multi-metric methods for a comprehensive assessment of complex catchments affected by multiple impairments

Ecology and biogeography of Himalayan diatoms: distribution along gradients of altitude, stream habitat and water chemistry

Himalayan streams, an important natural resource, are subject to numerous threats such as pollution and hydrological changes with deteriorating water quality in some areas. In order to develop diatom-based monitoring tools, we investigated diatom species composition at 206 sites in relation to variation in hydrochemistry, habitat characteristics (land use, shading, substrate), altitude and geographical position across a ca. 900 km east-west distance and a 4300 m altitudinal range in Nepal and north-west India. A total of 261 species were found, of which naviculoid (55), monoraphid (47), Gomphonema and fragilarioid (30), cymbelloid (26) taxa and Nitzschia (22) were most abundant. Many species were cosmopolitan, but some had restricted distributions. Of these, most were confined to the eastern Himalaya. More than one third of the species could not be identified. Richness and diversity increased in streams with higher Si concentrations and smaller substratum size, but were not correlated with altitude as a single factor. Canonical correspondence analysis and non-metric multidimensional scaling ordination analyses showed that different regions had distinct diatom assemblages. Important environmental gradients included basestatus, reflecting differences in geology, concentrations of Si (0.5-11.5 mg/L) and Na (0.2-9.0 mg/L), reflecting impacts from agriculture, and habitat, such as substratum size, land use and stream size. The importance of these factors varied in different climatic zones. Across the east-west range and altitudinal gradient hydrochemistry and habitat characteristics explained more of the variation in species composition than altitude and geographical position, but these factors were inter-correlated and their effects difficult to disentangle. Some species had restricted distributions along the altitudinal gradient, and particularly in the eastern Himalaya a change in species composition occurred most clearly at 2000 m a.s.l. Consequently, areas above and below this altitude should be considered separately when developing diatom-based monitoring methods

Diatoms as indicators of stream quality in the Kathmandu Valley and Middle Hills of Nepal and India

1. Diatoms are recognised as indicators in temperate streams, but only recently have assessments begun of their value in indicating stream quality in the tropics and sub-tropics. Here, we extend previous studies by assessing stream diatom assemblages in relation to water quality and habitat character in the Kathmandu Valley, and in the Middle Hills of Nepal and northern India. We also assessed whether the U.K. Trophic Diatom Index (TDI) was sufficiently portable to reveal pollution in Himalayan rivers. In the more urbanised and highly agricultural Kathmandu Valley, we compared diatom response to water quality classes indicated by a local invertebrate index, the Nepalese Biotic Score (NEPBIOS). 2. Thirty and 53 streams in the Kathmandu Valley (2000) and Middle Hills (1994–96), respectively, were sampled in October and November during stable flows following the monsoon. Diatoms were collected in riffles, water samples taken for chemical analysis, and habitat character of the stream channel, bank and catchment assessed using river habitat surveys. In the Kathmandu Valley, macroinvertebrates were collected by kick-sampling. 3. In total, 113 diatom taxa were found in the Kathmandu Valley streams and 106 in the Middle Hills. Of 168 taxa recorded, 62 occurred only in the Kathmandu Valley, 56 only in the Middle Hills and 50 were common to both areas. Most taxa found only in the Kathmandu Valley belonged to the genus Navicula while most taxa confined to the Middle Hills were Achnanthes, Fragilaria and Gomphonema. 4. In the Kathmandu Valley, richness and diversity increased significantly with K, Cl, SO4 and NO3, but declined significantly with Al, Fe, surfactants and phenols. Richness here also varied with habitat structure, being lowest in fast flowing, shaded streams with coarse substrata in forested catchments. In all streams combined, richness increased significantly with Si, Na and PO4, but declined significantly with increasing pH, Ca and Mg. 5. Diatom assemblage composition in the Kathmandu Valley strongly reflected water chemistry as revealed by cations (K, Na, Mg, Ca), anions (Cl, SO4), nutrients (NO3, PO4, Si), and also substratum composition, flow character and catchment land use. The commonest taxa in base-poor forested catchments were Achnanthes siamlinearis, A. subhudsonis, A. undata and an unidentified Gomphonema species; Cocconeis placentula and Navicula minima in agricultural catchments; and Mayamaea atomus var. alcimonica, M. atomus var. permitis, and Nitzschia palea at polluted sites near settlements. Diatom assemblages in none-agricultural catchments of the Kathmandu Valley and Middle Hills were similar, but they contrasted strongly between urban or agricultural catchments of the Kathmandu Valley and the less intensively farmed catchments of the Middle Hills. 6. In keeping with variations in assemblage composition, most streams in the Kathmandu Valley had higher TDI values (33–87, median = 64) and more pollution tolerant taxa (0–78%, median = 16) than streams in the Middle Hills (25–82, median 45, 0–26%, median = 2). TDI values correlated significantly with measured PO4, Si, and Na concentrations in the Kathmandu Valley, and with Si and Na concentrations in the Middle Hills. There was some consistency between water quality classes revealed by NEPBIOS and diatoms, but also some contrast. Water quality class I–II sites had lower TDI values and were less species rich than water quality II sites, however, there were no significant differences in detrended correspondence analysis (DCA) assemblage scores and relative abundances of pollution tolerant taxa between NEPBIOS classes. 7. While diatoms in the Middle Hills indicate unpolluted or only mildly enriched conditions, they reveal pronounced eutrophication and organic pollution in the densely populated Kathmandu Valley. In addition, diatoms appear to respond to altered habitats in rural agricultural and urban areas. As demands on water resources in this region are likely to increase, we advocate the continued development of diatoms as indicators using methods based on what appear to be consistent responses in the TDI between Europe and the Himalaya