Tham khảo tài liệu 'waste water evaluation and management part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Effects of Waste Water on Freshwaters in Semiarid Regions 229 but this event has not been explored further. Clearly this pattern can increase the spatial heterogeneity of ecological effects of waste water. All in all it is clear that waste water dynamics in freshwater environments also relies on the quantity of water available. The interplay between water amount and the nature and concentration of waste water compounds is still poorly known. For example the length of water renewal time is certainly related to biogeochemical dynamics of waste water including its effects on changes of biological populations but to our knowledge no study has been undertaken to support this. Retention time is related to water availability and the discharge ecosystem volume ratio but these key factors are seldom measured in semiarid environments because 1st it is not a simple task to measure discharge accurately in semiarid streams 2nd it is not easy to estimate ecosystem volume in semiarid freshwater environments which are very fluctuating and 3rd funding is often lacking. Until now we have dealt with chronic inputs of waste water which are mostly of urban origin. Another entirely different topic is that of infrequent but sudden inputs of waste water which are the case for accidental pollution events. Obviously they can hardly be predictable and hence it is impossible to implement sampling procedures and designs for them to study their magnitude and ecological effects. It is likely that these impacts are very different depending upon magnitude and stoichiometry but we still have no data to support this. 5. Biological effects Inputs of waste water to freshwater ecosystems usually result in changing species composition and this is the more studied feature of this topic Liebmann 1960-1962 Fjerdingstad 1965 Sladecek 1973 . Diatoms and chrysophytes are usually substituted by green algae and cyanobacteria most submerged macrophytes decline many invertebrate groups molluscs copepods .
