Diatoms as indicators of fine sediment stress

Excessive delivery of fine sediments to water bodies has a detrimental impact on the biotic elements used for water body status classification. Although diatoms are typically used to assess stress from eutrophication, as fine sediment has the potential to impact diatoms in many ways, it is not surprising that an index based on benthic diatom assemblages has been proposed: the relative abundance of motile species. This measure is based on the fact that many raphid diatom species are capable of migrating through deposited sediment to avoid negative impacts. However, the use of such an index has yet to be fully tested. Various data analysis techniques were used to explore how indices based on diatom assemblages (related to eutrophication and siltation), diatom species, the traits motility, and nutrient affinity responded to a gradient of percentage cover of fine sediment. Although diatom species showed marked variation in their affinity for percentage cover of fine sediment, the relationship between motility (both percent motile and the trait motility) and deposited fine sediment is not sufficiently strong to be used as a reliable indicator of fine sediment stress. We present an approach, which could potentially be used to develop a new index (diatom indicator of sediment conditions) on the basis of the response of diatoms to fine sediment, but caution that this index requires further development before use. Despite the hydromorphology having considerable potential to affect benthic diatoms, existing indices designed to assess eutrophication were robust to hydromorphological modification, reducing the possibility of false diagnosis of impacts.