Introduction
Freshwater habitats, such as rivers, lakes, and wetlands, host approximately one-third of all vertebrate species while covering less than 1% of the Earth’s surface. With such high biodiversity, freshwater ecosystems, compared to terrestrial and marine environments, are more vulnerable and endangered to biodiversity losses, declining ecological function, decreasing natural benefits, and climate change. The relationship between freshwater aquaculture and biodiversity is complex, presenting opportunities and challenges. When managed sustainably, freshwater aquaculture can contribute to reduced pressure on wild fish stocks, habitat restoration (regenerative aquaculture), conservation of endangered species, and support ecosystem services, thus enhancing global and local biodiversity.
During the common carp (Cyprinus carpio) production season, earthen ponds accumulate more nutrients than are discharged to the river system. However, seasonal draining of the ponds requires the transfer or harvest of fish (Oct–Dec), which mobilises marked amounts of sediment settled in the bottom of the pond. This often leads to adverse effects on the water quality and biodiversity of the downstream environment. The study aimed to evaluate the impact of common carp farming on the biodiversity of macroinvertebrates and benthic diatoms . The study also aimed to assess the survival of bioindicator species installed downstream of the farm while implementing SAFE filtering technology.
Materials and Methods
The study was conducted at the ICR carp farm in NW Poland, located on the Piaskowa river, one of the tributaries of the Rega river that opens to the Baltic Sea at Mrzeżyno. The biodiversity assessment involved a comparative evaluation of the species diversity of benthic macroinvertebrates across different seasons (winter, spring, and summer of 2023 and 2024). The overall impact of the common carp farm on diatom communities was assessed above and below the farm. Finally, three bioindicator species (noble crayfish, Astacus astacus ; thick-shelled mussel, Unio crassus ; and river water-crowfoot, Ranunculus fluitans) were installed below the discharge channel at the time of pond drainage and sediment sequestration with the SAFE filtering system.
Benthic samples were collected using kick sampling from a 1 m² cross-section area of the river at sampling points. Next, samples were sieved through a 0.5 mm mesh, stored , and delivered to the laboratory, where benthic organisms were separated from the sediment, identified, and preserved in 70% alcohol.
Diatom samples were taken with a toothbrush from the upper surfaces of gobble -sized stones collected from the stream bottom. The sample material from the stones (n = 10) was pooled into one sample. The samples were stored in alcohol and then sent to the laboratory of the Finnish Environment Institute for further processing . In the laboratory, the samples were cleaned of organic material with a strong acid solution. Permanent slides were made to count and identify the diatom individuals.
The primary biodiversity metric employed in the comparative analysis was the Shannon-Wiener diversity index (H’). The method developed in Zar (2010) was applied to assess the significance of differences. The analysis was conducted using the sample-size-based rarefaction and extrapolation (R/E) sampling curve, which presents diversity estimates with confidence intervals as a function of sample size.
Results and Discussion
The b iodiversity level downstream on the farm was significantly higher after implementing the SAFE filtering technology, which significantly reduced the emission of sediments from the ICR carp farm . Comparative analysis using sample-size-based R/E sampling curves showed that after the discharge process ceased in early winter 2024 (SAFE filtering) , interpolated species diversity indicated a significant increase (p = 0.007) in benthic organisms’ diversity downstream of the farm in 2024, when compared to 2023. Extrapolated predictions further strengthen this observation . Similar significant trends were observed for comparisons obtained for spring (p = 0.041) and summer (p < 0.000) seasons . These results indicate that minimising sediment release by implementing SAFE filtering technology can contribute to better regeneration of the river ecosystem in early spring, thus helping maintain biodiversity in the freshwater environment.
Taxon richness and H’ diversity indices for benthic diatoms were lower in the inflow channel above the farm, compared to the discharge channel of the ICR carp farm. Surprisingly, the proportion of abnormal forms was higher in the inflow than in the outflow channel. This indicates that the carp farm does not increase the proportion of abnormal diatom forms during the production season. The 10 most common diatom taxa varied between the inflow and outflow channels. In winter, A. minutissimum s.l. was the most common taxon in the inflow channel, closely followed by P . lanceolatum (prefers eutrophic, alkaline conditions) , A. minutissimum, and P. frequentissimum – together they formed 79% of the diatoms present in the inflow channel. In the same period, the five most common taxa in the discharge channel were planktic taxa from the Stephanodiscus , Cyclostephanos and Aulacoseira genera. Overall, the inflow channel was generally more dominated by one or two taxa, contrary to the discharge channel.
Even with SAFE filtering technology, t he discharge of post-production waters into the Piaskowa River caused adverse effects. For instance , some experimental cages were damaged or buried in sediments. The tearing and perforation of the cages by objects carried by the river influenced the distortion of the noted bioindicators’ survival rate. In particular, noble crayfish might have escaped from damaged cages. However, in one of the undamaged cages, a 100% survival rate of noble crayfish was observed, indicating that such a sensitive organism could survive when SAFE filtering technology was implemented. Thick-shelled mussels have limited mobility on the riverbed, making it unlikely that they could escape from damaged experimental cages. The main cause that reduced the survival rates of these animals was the organic and inorganic material that filled the cages . The mortality of thick-shelled mussels downstream of the discharge channel was significantly higher (33.4%) than the control group of mussels located upstream (0%; p = 0.037). The selection of the plant bioindicator, river water-crowfoot, proved to be too sensitive to habitat conditions in the Piaskowa River, resulting in 100% mortality regardless of nutrient discharge. SAFE filtering technology allowed the survival of two bioindicator species that had not been observed before in the Piaskowa River, in the most challenging period of the year, i.e., water discharge during fish harvesting.
These findings highlight the potential of SAFE filtering technology to mitigate the negative impacts of sediment discharge during aquaculture operations, supporting macroinvertebrate and diatom biodiversity. While challenges remain in fully safeguarding sensitive species, the technology represents a promising step toward more biodiversity-friendly freshwater aquaculture practices.
References
Zar, J. H. (2010). Biostatistical analysis (5th ed.). Prentice Hall.
Funding
This work was supported by the funding from the European Union’s Horizon Europe programme under grant agreement no. 101084549.