Macrozoobenthos assemblage patterns in European carp (Cyprinus carpio) ponds - the importance of emersed macrophyte beds

Qualitative and quantitative differences in benthic macrozoobenthos distribution in carp pond littoral zones (macrophyte areas; LM) and pelagic zones (macrophyte-free areas; MF) were assessed in four commercial carp (Cyprinus carpio) grow-out ponds in the Czech Republic (semi-intensive management) and Austria (organic management) monthly over the growing season (June-September) of 2016 and 2017. While differences in environmental parameters and granulometric composition between LM and MF were statistically non-significant (p > 0.05), organic matter content was significantly higher (p < 0.05) in LM. Average macrozoobenthos density and biomass in LM (mean 431 ind.m(-2) and 6.78 g.m(-2)) was usually significantly higher (p > 0.05) than MF (371 ind.m(-2) and 3.17 g.m(-2)). A similar trend was observed for zoobenthos diversity, with LM having a higher diversity (76 taxa) than MF (47 taxa). At the start of the growing season, chironomid density was higher (p < 0.05) in muddy MF zones, regardless of management type, while oligochaete density was higher in muddy LM. The density of both groups later declined, such that density was significantly higher (p > 0.05) in sandy substrates, regardless of habitat or management type. Our data suggest a significant drop in macrozoobenthos density and biomass compared with historical data, mainly due to new management techniques (fertilisation, supplementary feeding) and intensification (higher stock densities), suggesting that pond management is crucial as regards benthic invertebrate development and diversity. Our results indicate that emersed LM beds positively influence macrozoobenthos performance in carp ponds, potentially making them biodiversity hotspots. Further, LM beds can be regarded as invertebrate harbours, and hence should be protected and encouraged.