Extraction of macrophyte community spatial variation allows to adapt the macrophyte biological index for rivers to the conditions of the middle Dnipro river

Abstract

The aquatic macrophytes are widely used to indicate the trophic status of water bodies. For Ukrainian water bodies, which represent a wide ecological diversity, the indicator scales of the trophicity regime have not been developed. The purpose of the research is to develop an approach to adapt the Macrophyte Biological Index for Rivers (MBIR) for the conditions of the middle course of the Dnipro River. The difficulty in solving the problem is that the effect of eutrophication occurs against the variability of a complex of environmental factors and processes of natural and anthropogenic origin. The extraction of the eutrophication gradient is a prerequisite for assessing the response of plants to this factor and for constructing adequate indicator scales. The influence of natural factors on aquatic macrophyte communities is assumed to be spatially structured due to the long-term coevolution of the environment and biota. The pure plant response to the effects of eutrophication was assessed after extracting the spatial component of plant community variation. The pure response of aquatic macrophytes to the eutrophication factor provided an opportunity to construct an adequate scale for indicating the trophic status of water bodies, which correlates strongly with the scores of the MBIR. The studies were carried out within the Dnipro-Orilskiy Nature Reserve in the waters of the Dnipro reservoir, which was created on the Dnipro River. The aquatic macrophyte surveys were conducted at 76 locations. Studies were conducted in 50–100 m long plots along the shoreline. The remote sensing data from the Sentinel-2 satellite was used to estimate the indexes of the eutrophication, turbidity, and transparency of the water. The distance-based Moran's eigenvector maps were produced from the geographic coordinates of the sampling points to be used as the spatial variables. Three versions of the Canonical Correspondence Analysis (CCA) were performed: 1) the CCA with the environmental factors and spatial variables as constrained predictors; 2) the CCA with the spatial variables as constrained predictors; 3) the CCA with the environmental factors as constrained predictors and the spatial variables as conditional predictors. The species scores on the corresponding ordinate axes were considered to estimate the species optimum. The variations of the species scores on the corresponding ordinate axes were considered to estimate the species tolerance. Species scores were converted into the CSd indicator values, and tolerance was converted into the Ed coefficients of ecological amplitude. The indicator values and environmental amplitude coefficients obtained for the ordination accounting for spatial effects had no statistically significant correlation coefficients with other indicator scales. The CSd indicator values that were obtained for community ordination accounting for pure environmental effects were statistically significantly correlated with the MBIR and other of the indicator scales. The coefficient of environmental amplitude Ed was negatively correlated with the E. The MBIR revealed that the level of eutrophication in 102 locations (67.1%) was estimated as Bad, in 37 locations (24.3%) it was estimated as Poor, and in 13 locations (8.6%) it was estimated as Moderate. The extraction of the spatial component of macrophyte community variation resulted in the evaluation of indicator scales that correlated statistically significantly with the original MBIR scale.