Background

Eutrophication, followed by re-oligotrophication during lake restoration, in many peri-alpine lakes has caused important changes to the functioning and biodiversity of freshwater ecosystems. In Lake Geneva, total phosphorus (TP) concentration has been reduced since the eutrophic years, and is now close to the upper value of the target range of 10-15 µg/L. For over 60 years the lake has been monitored at SHL2, the central and deepest point in the Eastern basin, complemented by data from GE3 in the Western basin. Selection of these reference points was based on a lake-wide analysis of TP in the 1950s. Fishers have for some time expressed concerns that further reductions in TP could damage the sustainability of their livelihoods. They have called for a re-evaluation of the historical sampling points to determine whether SHL2 and GE3 can still be considered representative in terms of nutrient concentration of the lake.

Results

Here, we present the scientific and societal impacts of CoFish, a co-designed research project between scientists and fishers of Lake Geneva. To reassess the spatial variability of TP we applied a low-cost method to collect integrated water samples across the lake, using stoppered hosepipes as a collection instrument. In this article, we present four key messages:

i)     There is spatial variation in phosphorous levels, and in most cases the two points fall within an acceptable range of variability;

ii)    The concentrations of phosphorus are generally low across the lake in ranges that could have an impact for plankton development.

iii) Citizen science can complement long-term monitoring, allowing for instance for better spatial coverage of environmental data;

iv) The co-design process resulted in community empowerment, a willingness to further collaborate.

Conclusions

The management implications of this work are that using a single reference station as being representative of the lake as a whole is not straightforward. In the discussion, we advocate for a re-evaluation of the TP targets, given radical changes in the lake`s physical structure and food web. We further highlight the important role of engaging fishers in citizen science, which resulted in bridging existing gaps between lake management, science, and fisheries, providing a broader basis for lake conservation.

Sample collection

The data were collected by citizen scientists. To collect samples, we constructed an 18m long integrated water sampler out of a garden house, fitted with a cork stopper at one end and a 5kg weight at the other following a design used in the European Multi Lake  Survey protocol (Mantzouki et al., 2018). While not the central objective of the study, we also measured lake transparency and trained fishers to use Secchi disks. Average Secchi depths were calculated as the mean of the depth at which the Secchi disk was no longer visible and the depth that it starts to be visible again, once bringing it up to the surface.

Lab analyses

Samples were analyzed for TP and P-PO₄ using a Seal Analytical AQ2, following protocols EPA 119-A-Rev. 7 and EPA 155-A, which is based on a reaction of acidic molybdate in the presence of antimony. The limits of quantification were 3μg/L for both with a range of 3-100 μg/L for TP and 3-300μg/L for P-PO₄. Methods were chosen to correspond to those used by the CIPEL for their long-term monitoring of the lake`s two reference points. All 36 stations were sampled in all four campaigns except CRG10 that was not sampled in the first campaign due to windy conditions. Orthophosphate measurements were conducted within 48 hours of sample collection. The second bottle was sent to Eawag, in a frozen state, where the measurements for total P and orthophosphate were repeated, initially to serve as an inter-laboratory comparison.