The mean spring Calanus helgolandicus index was 5.1 times as large in 1989–2019 as in 1958–1988 across 22 central North Sea cells
We tested whether changes in two Calanus species reduced their summed spring surface count index in the central North Sea. We used the open Continuous Plankton Recorder Survey v1.3. A fixed set of 22 one-degree cells supplied 25,104 samples with usable counts in 1958–2019. We gave equal weight to cells, calendar months and sampled years within cells. We compared 1958–1988 with 1989–2019. The mean March–May Calanus helgolandicus index increased from 1.58 to 8.10, a ratio of 5.14 (95% five-year block interval 2.67 to 10.57). Its spring occurrence frequency doubled. The spring finmarchicus ratio was 0.96 (0.51 to 1.73). The two-species spring sum had a ratio of 1.45 (0.85 to 2.53), which did not establish a decrease or an increase. With two-degree cells that covered more of the region, the helgolandicus ratio was 5.51 (3.13 to 10.03), but the summed spring ratio was 0.93 (0.66 to 1.31). A capped-count check still gave a helgolandicus ratio of 3.01 (1.76 to 5.63). Thus, the helgolandicus increase included spring. The evidence does not support an assumption that its gain occurred only in autumn. The summed spring result depends on spatial support and large counts. These indices describe recorded late copepod stages near the surface. They do not measure larval fish food, biomass, the full water column, or a cause of change. The package has source rows, provenance, code, all estimates, resampling draws, figures and offline repeat instructions.
Calanus · North Sea · Continuous Plankton Recorder · seasonal abundance · species replacement · sampling coverage · reproducible data
References
7- usesdoi:10.17031/1629
The CPR Survey v1.3 supplies the source sample and species records. Attribution: Johns D, Broughton D, The Marine Biological Association of the United Kingdom. CC BY 4.0.
- usesNorth Atlantic CPR plankton temperature indices followed year-to-year SST in 1958–2019, but three copepod taxa caused all of the long-term copepod change sci:sha256:c29a4bae0ad65df70e4cf9662f04d1bc69cf64884b54191b26a61c834c7e2afa
We reuse its frozen OBIS source pages and check our source extract against its derived tables. We test a seasonal two-species question rather than its community temperature index.
- Public CPR data replicate the 1958–2002 dinoflagellate and meroplankton timing advances in the central North Sea, which stayed to 2019 as an SST model predicted, but not the copepod advance sci:sha256:a7d8c84d8314f88cd2d6fc75a2e72bba1a38ce77dd1d05fdff7c8382ef5bb6e9
This prior study tests broad plankton timing shifts in the same central North Sea box. It helps define the scope of the present seasonal comparison.
- doi:10.3354/meps134101
Planque and Fromentin (1996) show distinct seasonal and spatial patterns of the two species in 1962–1992. They report spring and autumn helgolandicus peaks over the larger study area.
- doi:10.3354/meps06775
Helaouet and Beaugrand (2007) discuss a loss of spring Calanus prey after the North Sea shift. Their methods also describe CPR identification of stages V and VI. Our longer-period, fixed-cell test gives a narrower measure than fish food supply.
- doi:10.1038/nature02164
Beaugrand and colleagues (2003) relate plankton change to North Sea cod recruitment. This gives ecological context; our study does not test recruitment.
- doi:10.1038/nature02808
Edwards and Richardson (2004) show why different seasonal responses can affect food-web timing.
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