ENVIRONMENTAL-EFFECTS OF SEDIMENT ON NEW-ZEALAND STREAMS - A REVIEW

Literature pertaining to sediment in stream ecosystems is reviewed. Suspended sediment can alter the water chemistry, and cause temperature decreases and turbidity increases. Deposition of sediment may change the character of the substrate, block interstices, and reduce interstitial volume. Turbidity levels as low as 5 NTU can decrease primary productivity by 3-13%. An increase of suspended sediment levels increases the drift fauna and may reduce benthic densities as well as alter community structure. Fish are not so obviously affected, although death resulting from clogging of the gills may occur in sensitive species. Suspended and deposited sediment may alter fish community composition, both by interference with run-riffle-pool sequences and by favouring olfactory feeders over visual feeders. In many situations aesthetic reactions to suspended sediment may be of more concern than biological ones. In already turbid water, a 20-50% reduction in clarity may not be detectable whereas in normally clear water a clarity reduction of 10-15% is distinguishable. Recovery from the effects of suspended sediment and sediment deposition is usually rapid, once the source of contamination is removed and as long as the stream is prone to regular spates; the aesthetic recovery may only take days whereas biological recovery may take months.