Continuous Plankton Recorder cnidarian frequency in the central North Sea stayed at its 1958–2000 level in 2001–2007, then rose 3.5-fold with the 2008–2011 change to nematocyst recording

Question: Does the positive winter NAO relation of Continuous Plankton Recorder (CPR) cnidarian ('jellyfish') frequency in the central North Sea (Attrill et al. 2007, fitted to 1958-2000) predict later years, and is the reported post-1997 increase present when the recording rule is constant? Approach: We used all public CPR samples (OBIS copy, 268,536 samples, 1946-2022; replicated with the CC BY GBIF copy to 2020). The CPR taxon notes state that 'Coelenterata tissue' became 'Cnidaria tissue' in 2008, and that nematocysts were recorded from 2008 and routinely from 2011. We fitted frequency on the Hurrell DJFM NAO for 1958-2000 and compared predictions with 2001-2007, 2008-2010 and 2011-2022. We compared the change from 2003-2007 to 2011-2015 in 2x2 degree cells with 38 placebo window pairs in 1958-2007, with 73 other CPR categories and with ERSSTv5 SST change. Results: The 1958-2000 relation replicated (r = 0.50, autocorrelation-adjusted p = 0.018). In 2001-2007 the mean (0.140) was 0.94 times the 1958-2000 mean, near the model mean, but the yearly NAO correlation was -0.26 (n = 7). From 2008 the frequency rose to 0.43-0.60 in 2011-2022 (3.4 times the 1958-2000 mean); the NAO model gave a mean error of +0.35. 39 of 41 cells from the North Sea to the north-west Atlantic increased (median +0.30); placebo medians were -0.11 to +0.10. SST at the samples fell by a median 0.28 C, and cell changes did not follow SST change. In 2011-2022, 79% of presences had a nematocyst plus. Inside 2011-2022 the NAO slope (0.0127) equalled the 1958-2000 slope, with a level 0.35 higher. Conclusion: CPR cnidarian frequencies after 2008 are not comparable with earlier values without a calibration. With a constant recording rule, the central North Sea showed no increase from 1958-2000 to 2001-2007. Limitations: the CPR data cannot separate a real increase in 2008-2011 from the recording effect; the 2001-2007 test has only 7 years; we did not test the pH and inflow relations; the index is presence of mostly small hydrozoan and siphonophore material.

Continuous Plankton Recorder · jellyfish · Cnidaria · North Atlantic Oscillation · North Sea · out-of-sample test · recording change · monitoring artefact · nematocysts · gelatinous zooplankton · time series homogeneity · OBIS · GBIF