Disentangling the mechanisms behind climate effects on a key zooplankton species
A recently paper published in PNAS, members of the CEES Marine Group explore potential climate effects on Calanus finmarchicus, a key zooplankton species in the North Atlantic. The paper shows how the combination of shallow mixed-layer-depth and increased wind apparently increases chlorophyll biomass in spring, and in turn C. finmarchicus biomass in summer. These findings strongly suggest bottom-up effects of food availability on zooplankton, and highlight the need to consider climate effects “beyond temperature” when projecting zooplankton dynamics under climate change.
Figure 1: Schematic presentation of key results: More C. finmarchicus originate from warm than cold areas. The combination of shallow mixed-layer-depth (MLD) and strong winds favours phytoplankton production, which positively influences C. finmarchicus growth and/or egg production. In result, the combination of high temperature, shallow MLD and increased wind in spring leads to high summer biomass on a local scale (solid arrow), while alternative combinations with lower temperature, increased MLD and wind, or reduced MLD and wind, result in medium or low summer biomass (dashed or dotted arrows).
Zooplankton form the link between primary production and higher trophic levels in pelagic ecosystems. In the Atlantic waters of the Norwegian and Barents Seas, the copepod Calanus finmarchicus is an important food source for both pelagic fish and early life stages of demersal fish. Temperatures in the upper oceans are currently increasing, with the strongest warming occurring at high latitudes. Rising temperatures are linked to increased thermal stratification, both factors potentially influencing zooplankton. But investigating climate effects on zooplankton is extremely challenging due to the influence of advection. Observation data typically give a snapshot of the plankton community in a specific location and moment, but do not reveal the observed specimens’ past drift trajectories or the environment experienced during these drift trajectories.
In a recent study, we combine long-term spatiotemporal survey data, state-of-the-art statistical methods and oceanographic particle tracking to quantify the importance of both (1) drift from spring to summer and (2) environmental variation likely experienced in spring on spatial variation in C. finmarchicus summer biomass. We also explore how environmental variation influence (3) year-to-year variation in C. finmarchicus biomass and (4) spatially resolved chlorophyll biomass in spring, a proxy for food availability.
Accounting for drift, we find that spatially resolved biomass in summer relates positively to temperature at back-calculated positions in spring, likely because more biomass originate from warmer, south-western areas closer to core overwintering areas in the Norwegian Sea. On the other hand, while mean C. finmarchicus biomass in spring is positively related to temperature in spring, the mean change in biomass from spring to summer is lower after a warm spring than a cold spring. Increased temperatures, as predicted in future climate scenarios, might thus increase the C. finmarchicus biomass available for predators in spring, but not in summer.
Further, we find that chlorophyll biomass (a proxy for phytoplankton biomass) is positively linked to ambient wind speed when MLD is shallow. Wind-induced mixing might increase phytoplankton biomass after the spring bloom initiation, possibly due to nutrient renewal. C. finmarchicus biomass in summer is in the same manner positively related to the combination of increased wind and shallow MLD at back-calculated positions in spring. These results indicate that bottom-up effects of food availability in spring influence C. finmarchicus biomass in the NS-BS in summer.
Stratification is predicted to increase globally with ocean warming, potentially increasing primary production in high-latitude areas. Our results suggest that the interaction between MLD and wind will be important in projecting both phytoplankton and zooplankton biomass in spring and summer at high latitudes.
Kvile, K., Langangen, Ø., Prokopchuk, I., Stenseth, N.C., & Stige, L.C. (2016). Disentangling the mechanisms behind climate effects on zooplankton Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1525130113