Artificial reefs: A habitat improvement tool in lowland rivers?

The underlying geology of river catchments and the hydrological processes occurring within them determine the type of inorganic substrata present at any particular point, these in turn affect invertebrate communities. Heterogeneous substrata tend to support richer and often more abundant invertebrate fauna. The actions of man on river systems, such as dredging to prevent flooding, leads to an increase in substrata homogeneity, usually with small particles such as sand and silt dominant. This has detrimental effects on invertebrate communities. Steps have been taken to rectify such negative effects by introduction of structures that increase physical diversity of the river environment. The use of such structures has led to colonisation of previously damaged aquatic habitats by invertebrates. The river Bure is an intensively managed system and typical of many of the lowland rivers in East Anglia. These are of minimal gradient with the channel often above land level and embanked. The tidal section is uniform in character, 2 - 3m deep, approximately 25m wide, devoid of instream features and the riverbed is composed almost entirely of silt. In the middle reaches the majority of the channel is also uniform, approximately 10m wide and 1 -2m deep with a silty substratum. Macroinvertebrate abundance and richness is uniformly low. Reefs, made of precast hollow concrete blocks, were constructed at four sites along the river. Two blocks were recovered monthly from each reef by divers; riverbeds were sampled using an Ekman grab and a Surber sampler. Invertebrate communities on reef substratum were compared with those of riverbeds. Numerical abundance and species richness were upto ten times greater on reef substratum with some taxa were unique to reefs at every site.