Comparison of two methods for mapping freshwater macrophytes in Norway: An analysis of data quality, efficiency and repeatability

Sammendrag

Seven lakes in southeastern Norway were surveyed using two methods for macrophyte sampling. One of the methods was a sector-based approach (NIVA method) while the other was transect-based (UK-lite method). In addition to this, surveying using the Nature in Norway (NiN) system was tested. The aim of this study was to evaluate the difference between the two main methods, discuss their repeatability regardless of surveyor, and if these methods can be used for macrophyte surveying under the NiN framework and the Water Framework Directive (WFD). The NIVA method detected significantly more species than the UK-lite method (p = 0.0099), although both methods appeared to lack sufficient sampling effort to produce complete species inventories. Both methods seemed insufficient for surveying in deeper waters. Despite differences in species richness, the two methods produced broadly similar trophic index (TIc) values (p = 0.77), indicating that both approaches yield comparable WFD ecological status classifications. Overall, the NIVA method was more effective for documenting species richness, whereas the UK-lite method provided more information on macrophyte zonation along depth gradients. The results from this study suggested that both methods may provide acceptable data for WFD ecological status assessment, if sufficient survey effort is fulfilled. Further, this study suggests a more structured transect-based approach, supplemented with snorkelling and underwater camera (capturing max colonisation depth) under the NiN framework, to meet the data quality requirements.