SUMMARY 1. Macrophyte loss from Sites of Special Scientific Interest in England has become widespread over the last 20 years. One reason for this may be changing trends in angling, a multimillion pound industry that has an enormous impact on aquatic ecosystems. Stocking with cyprinid fish is a common angling management practice but the particular fish species and distribution of their biomass may be crucial to the ecosystem.
2. Carp (Cyprinus carpio), roach (Rutilus rutilus), bream (Abramis brama) and tench (Tinca tinca) at biomasses ranging from 0 to 800 kg ha−1 and at various sizes were placed into experimental mesocosms in Little Mere, a shallow, fertile lake in Cheshire, U.K. The effects these treatments had on the aquatic ecosystem were studied over two summers. Specifically the effects of the treatments on macrophyte growth, benthic and macrophytic macro-invertebrate populations, water chemistry, epiphyton production and plankton survival were investigated.
3. Carp had a greater detrimental effect on the macrophytes than bream, tench and in particular roach. A biomass of fish > 200 kg ha−1 adversely affected the extent of macrophyte growth.
4. The decline in macrophyte growth was most likely as a result of increased epiphyton growth that probably reduced the amount of light and carbon dioxide available to the plant. There were no observed direct fish impacts on macrophytes.
5. The chemical data suggested that inorganic nitrogen levels were low and it is possible that release of nitrogen, from fish excreta, followed by immediate uptake, could have been a major factor stimulating epiphyton growth and subsequently macrophyte loss. Phosphorus concentrations increased even in the controls and substantial amounts were available. Phosphorus stimulation can therefore be discounted. Macrophyte-associated macro-invertebrates were positively correlated with epiphyton load but had no impact on the extent of epiphytic growth. Shading from disturbed sediment or phytoplankton was also unimportant.