A new method for monitoring macrophyte communities in small shallow lakes and ponds

Abstract

Macrophytes are key to the functioning of small shallow lakes (SSLs) because they maintain a clear water state through numerous positive feedbacks. The composition of macrophyte communities changes under anthropogenic pressures; as a result, tools designed to easily and rapidly assess their structure and composition are increasingly requested. We tested three sampling methods (the S3m sampling method, a stratified method, and a mapped inventory) to monitor macrophytes in 26 SSLs. The effect of each method was evaluated on seven descriptors of macrophyte communities, including the median conservation value. The results were comparable for the three methods, but the stratified method failed to accurately monitor the median conservation value and the number of species present at a low frequency, including exotic and patrimonial species, hence serious consequences for management decisions. S3m was applied to 262 SSLs ranging from 1 m² to 43 ha in surface area. Generalised additive models were used to investigate the environmental factors correlated with four conservation value or ecosystem functioning descriptors. The S3m method showed that surface area, distance from the source, elevation, and bank verticality were determinants of macrophyte richness. Invasive crayfish impacted the macrophyte richness and the coverage of submerged macrophytes, whereas fish presence increased the macrophyte richness and the percentage of exotic macrophytes and reduced patrimonial interest. S3m was successfully applied to a wide diversity of SSLs in France. It proved to be rapid, reproducible, and representative for monitoring macrophytes in SSLs. Therefore, it should be applied for SSL management.