Effects of laundry wastewater on benthic algae in ditches in the Netherlands

Summary A modified, artificial substrate method was used to study the effects of laundry wastewater on benthic algae in a polder in the western part of the Netherlands. In a polluted ditch a clear chemical gradient was found with regard to the parameters conductivity, B.O.D., C.O.D., NH₄, ortho-P, total-P and Cl, showing decreasing values with increasing distance from the polluting source. pH and oxygen values showed the opposite. Algae, such asEuglena viridis, Nitzschia palea andOscillatoria spp. were positively correlated with the amount of pollution, while others, such asChrysochaete brittannica, Chrysophaera gallica, Cocconeis placentula andSynedra ulna showed a negative correlation with the amount of pollution. The results of algal counts are evaluated by means of diffenent methods,e.g. similarity coefficients, species abundance curves, diversity and saprobity indices. In different seasons benthic algal biomass proved to be a good indicator of pollution, except very close to the polluting source. The role of algal periodicity, influencing species composition, diversity and saprobity values, and biomass growth, has been discussed. The need of revising the saprobity system for stagnant ditches in the Netherlands, using artificial substrates, has been stressed.