Seasonal variations in chlorophyll-a, temperature and major water chemistry close to an open pen fish farm in Lake Fyresvatn - 2019
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In 2019, seasonal variations in water chemistry, including total phosphorus, total-nitrogen, and chlorophyll-a were investigated in Lake Fyresvatn, Vestfold and Telemark county, to reveal potential local and whole lake effects of open pen fish farming in the lake. The investigation was performed monthly from May to September 2019 at six local stations, 10 m, 100 m, and 1000 m north and south from the fish farm. Integrated water samples were taken in epilimnion (0-6 m), and at one defined hypolimnion depth (20 or 40 m) depending on the station depths. Early July, a bottom fauna investigation was implemented with four transect, 0 m, 10 m and 30 m out from the fish farm. In addition, integrated epilimnion samples (0-12 m) were sampled at the same dates as the local water sampling, near the lake inlet in north and near the lake outlet in south. These data were also compared with corresponding data from 2016, 2017 and 2018. Different statistical analyses were used to reveal possible local and whole lake effects of the fish farm activity. The concentrations of nutrients (Tot-P: total phosphorous and Tot-N: total nitrogen), in Lake Fyresvatn were low, with subsequent low concentrations of chlorophyll-a (Chl-a) during the investigation period. No significant differences were found between the stations, though one station close to the fish farm (N-10), had somewhat higher phosphorus concentration, and thus reduced ecological status according to EQR values (Ecological Quality Ratio) set by the Norwegian classification system, in accordance with the European water framework directives. Despite significant differences in nutrients and Chl-a between months, it did not alter the lake classification status. Only small effects of the fish farm on the bottom fauna was documented at > 10 m from the pens. The two most common bottom fauna organisms found belongs to the family Chironomidae and the subclass Oligochaeta, the latter primarily within the genus Tubifex. Of the two predominant bottom fauna groups, the Chironomidae individuals are generally far more sensitive to high loads of organic matter and nutrients compared with Oligochaeta. Accordingly, while increasing numbers of Chironomidae individuals were found by increasing distance from the fish farm, the opposite gradient was revealed for Oligochaeta. However, at the edge of the pens (0 m), a decreased number of Oligochaeta was observed compared with at 10 m, where very high numbers of individuals were observed, i.e. > 2000 ind. m-2. This may indicate suboptimal conditions, even for the low oxygen adapted Tubifex individuals, very close to the fish farm. The EQR values for sight depth, Chl-a, Tot-P and Tot-N, both near the inlet and outlet of Lake Fyresvatn, corresponded to very good to good ecological status in 2019, similar to 2016, 2017, 2018 investigations. The reduction in Tot-N in the lake in 2019 caused a change in the EQR status for this parameter, from good to very good compared to the previous investigated years. Overall, there is so far minor indications of fish farm effects on the water chemistry and Chl-a in Lake Fyresvatn, despite the production volume has increased significantly the last two years. However, as the lakes residence time is ≈ 8 years, it takes time to document the whole lake effects of at date production. Accordingly, further monitoring is needed.