Phosphorus and sediment – two of the major pollutants of freshwater stream ecosystems – impact upon epilithon

Ecological degradation of rivers and streams resulting from multiple stressors is a big concern in the UK and other countries all over the world. The two largest stressors introduced by agriculture are phosphorus and fine sediment. The combined impacts of the multiple stressor and relative strength of each individual stressor needs to be understand. A Number of ecological response variables were tested through a field mesocosm experiment, including algal and ecosystem variables: (1) The subsidy-stress for phosphorus and sediment (where at first, an ecological variable increases positively with the increased level of phosphorus and sediment until very high levels are reached, when negative effects would be expected); (2) Whether the stressors work individually or as multiple stressors and whether they interact; (2.a) Three ecological guilds of algae (‘low profile’ growth form, ‘high profile’ growth form, ‘motile’ growth form) were used in order to test whether the high profile growth form decreases and motile growth form increases with increase of sediment deposition, or whether (2.b) Both high profile growth form and motile growth form increase with increase concentration of phosphorus. Most species showed subsidy stress responses for the gradient of phosphorus, but for the gradient of sediment the response was negative. Phosphorus and sediment together generally acted as multiple stressors and usually in a simple additive way, but complex interactions were also found. The algal community was impacted synergetically by phosphorus and sediment, as shown by the field study. The combined results from the field study and the mesocosm experiment indicate that phosphorus and sediment should be managed together in view of their acting most of the time as multiple stressors in their impacts on epilithic algae. Finally, in order to have a better evaluation for the possible reasons of a stream health decline, it is strongly recommended to measure routinely both fine sediment and phosphorus in the future.