Association Between the Environmental Parameters and Biological Indicators in Reservoirs with Intensive Net-Cage Aquaculture: A Case Study in Kardzhali Reservoir, Bulgaria

The present study analyzed the interactions between the environmental parameters, microbiological water quality indicators, and phytoplankton in Kardzhali reservoir, which is one of the largest and economically significant water bodies in Bulgaria, with highly developed cage- aquaculture. Data sets of eighteen parameters from 5 monitoring sites during 2016–2018 were used for analysis. We have applied multivariate methods, aiming to identify the key parameters affecting the communities, including the impact of the net-cage farms. The ANOSIM (analysis of similarities), showed significant differences in the values of physicochemical factors between the control site and the area for aquaculture, with higher nitrate, total nitrogen, and COD (chemical oxygen demand) content near the net-cages. The results were confirmed by the high R-value (R=0.87; p<0.01). The conducted PCA (principal component analysis) showed that only three principal components (PCs) are need to group the physicochemical parameters, explaining 90.5% of the total data variation. PC1 was formed by nitrogen forms and COD, PC2 represents the physical source of the variability (pH and dissolved oxygen) and PC3 was loaded with total phosphorus (0.537) and ammonium nitrogen (0.764) concertation. The parameters with the highest impact on the abundance of heterotrophic bacteria (TVC) include temperature, TN, and COD, while the phytoplankton community was negatively correlated with Secchi depth and COD. The redundancy analysis confirmed that the location of the sampling station significantly affects the studied variables and that net-cage aquaculture is a major anthropogenic factor in Kardzhali reservoir.