Project name: Research of the interstitial fauna (aquatic interstitial habitat) of the Sava River in the area of the planned DPN (National Spatial Plan) for HPP Suhadol, HPP Trbovlje and HPP Renke
Client: HSE Invest Ltd.
Period: 01/2022 – 04/2023
Location: Croatia
The hyporheic zone (HZ) is a unique, dynamic ecotone at the interface between surface water and groundwater. It plays a multifaceted role in hydrological, chemical, biological and metabolic processes and can immobilise and transform nutrients and pollutants through the process of filtration. This zone also harbours a specific fauna that plays an important role in self-purification processes.
Field studies of the interstitial (hyporheic) zone of the Sava River, along the section from Litija to Zidani Most, were carried out at a total of nine stations. The research was carried out during three full-day field surveys in January 2022 using the traditional Bou-Rouch pumping method, a piston pump that allows semi-quantitative samples to be taken. Although the volume of water brought to the surface by the piston mechanism is known, the volume of the space from which the fauna was sampled under suction is not known. The samples were taken at a depth of 30 to 60 cm. In places where the Bou-Rouch pump could not be used for all three subsamples, the Karaman-Chappuis method was also used. The Karaman-Chappuis method is very effective on riverbanks, especially gravel banks. The principle is to dig a hole in the sediment (gravel or sand) with a shovel. The interstitial water fills the hole and is then filtered through a sieve to collect invertebrates carried by the water current. This is a quick, qualitative method that requires no specialised equipment and allows the collection of all types of organisms from the interstitial space.
Three subsamples of interstitial fauna and water were collected at each site during each field survey. The interstitial water was filtered through a 100 μm sieve during the field work on the Sava River. The collected filtered samples were preserved in 96% ethanol, stored in plastic bottles and sent to the laboratory for further isolation and analysis. The water remaining after filtration of the fauna was used to measure physico-chemical parameters: Water temperature, dissolved oxygen concentration (mg/L), oxygen saturation (%), pH and electrical conductivity (μS/cm). The laboratory analyses included the determination of alkalinity, total water hardness, the concentrations of nitrites, nitrates and orthophosphates as well as the chemical oxygen demand (COD, KMnO4 method).
In the laboratory, individual specimens of aquatic invertebrates were isolated using a binocular microscope (XTL-3400D) and the total number of individuals in each sample was determined. Permanent slides were prepared for microscopy and species identification. Specimens were then identified using specific keys for each group to the lowest taxonomic level required to achieve the research objectives.
Qualitative and quantitative analyses of the interstitial fauna communities were performed using Microsoft Excel 16 and Primer v.6.1 software. Data on the abundance of individual taxa of interstitial fauna at different sites and stations were entered into tables for calculations in Microsoft Excel 16, where the density of individuals per volume of filtered water was calculated. The results were presented in bar charts to compare the abundance and relative proportions of taxa at the different stations. To assess the diversity of the interstitial faunal communities at each station, the following indices were calculated: Margalef’s species richness index (d), Shannon-Wiener (H’), Simpson’s diversity index (λ) and Pielou’s evenness index (J’) using the Primer v.6.1 software package.
A total of 62 taxa of aquatic invertebrates from 17 higher taxonomic groups were identified in the study. Two vulnerable stygobiont snail taxa and a total of four species of Niphargus amphipods were recorded, including Niphargus spoeckeri, a species listed as vulnerable on the Slovenian endangered species list. The analysis of the composition and structure of the interstitial fauna in the studied section of the Sava River shows the presence of rare, endangered, vulnerable and endemic species among snails (Gastropoda) and crustaceans (Copepoda, Ostracoda and Amphipoda). Therefore, the section of the Sava River from Litija to Zidani Most is of exceptional faunistic interest and value from the perspective of interstitial community composition.
