Implications for climate change
The effect of temperature seems profound when considered across its range, with a 44.7% decrease in copepod length from -1.7 to 30ºC. Bergmann’s Rule thus suggests that as oceans warm under climate change, the size (and mass) of copepods is likely to decline (Walther et al. 2002). Under a high-emissions scenario (RCP8.5), SST is likely to warm by ~2.7°C in 2090–2099 (compared to 1990–1999) based on the mean of the Coupled Model Intercomparison Project 5 (Boppet al. 2013), or warm by ~0.6°C under a low-emissions scenario (RCP2.6) (Bopp et al. 2013). Based on our statistical model, the effect of warming of ~2.7°C under RCP8.5 could equate to a decrease in body mass of copepods globally of ~7%. For RCP2.6, the ~0.6°C warming could equate to a decrease in body mass of copepods globally of ~1.5%. These estimates would translate to a similar decline in copepod biomass globally assuming abundance remains unchanged.
However, most Earth System models also project a decline in primary production and Chl-a (Bopp et al. 2013; Stock et al. 2014; Lefort et al. 2015; Galbraith et al. 2017; Woodworth‐Jefcoats et al. 2017); decreases in primary production of between 2% and 16% by 2100 are predicted under RCP8.5 (Lefortet al. 2015). Using net primary production estimates from Bopp et al. (2013) and the conversion to Chl-a from Marañón et al. (2014), we find that under the RCP8.5 Chl-a is projected to decrease globally by ~0.086 mg m-3, and could lead to an increase in body mass of copepods globally by ~2.5%. Under the RCP2.6, Chl-a is projected to decrease globally by ~0.020 mg m-3, and could lead to an increase in body mass of copepods globally by ~0.6%. Thus, the combined effects of increased temperature and decreased Chl-a are likely to decrease global copepod biomass by ~4.7% under the RCP8.5, or decrease by ~0.9% under a RCP2.6. Current Earth System Models also project a future decline in zooplankton biomass (Woodworth‐Jefcoatset al. 2017) – and copepods dominate zooplankton biomass (Verity & Smetacek 1996; Sommer et al. 2001) – by ~7.9% globally (Stock et al. 2014). This decline in copepod size and mass could negatively impact global fisheries (Sheridan & Bickford 2011). No Earth System Models consider the effect of Bergmann’s Rule on copepod size.
There could be several other important ecosystem consequences of copepod size following Bergmann’s Rule as the climate warms. Because swimming ability and thus the amplitude of their vertical migration is related to their size (Hays et al. 1994; Ohman & Romagnan 2016), a decline in body size with warming implies less extensive vertical migration. Thus, reductions in body size could potentially weaken the biological pump that transfers carbon from surface layers to the deep ocean (Cavanet al. 2019). Further, copepods significantly contribute to carbon exports via their sinking faeces and moults following ecdysis – at rates mostly determined by their body size (Stamieszkin et al.2015). Therefore, reduction in copepod body size with warming could have significant ramifications for deep-ocean systems (Levin & Le Bris 2015; Sweetman et al. 2017) and for feedbacks to the climate system (Portner et al. 2019).