Synchronized multidecadal trends and regime shifts in North Atlantic plankton populations

Abstract

Recent changes in oceanic plankton are being reported at unprecedented rates. Most changes are related to environmental factors, and many were identified as driven by climate, either through natural cycles or by anthropogenic effects. However, the separation of both effects is difficult because of the short length of most observational series. Moreover, some changes are related to trends and cycles, while others were perceived as system shifts, often synchronized over large spatial scales. Here, studies on observational series of plankton, with the focus in the North Atlantic, are reviewed. Two main periods of shifts in plankton assemblages were identified: one in the late 1980s and a more recent one at the beginning of the new millennium. While the origin and extent of most shifts varied locally, their synchronization seems to confirm the response of plankton to changes in warming and in large-scale climatic factors. Changes in species abundance and distribution patterns were generally related to hydrographic factors, but also to non-linear effects of warming, the latter particularly affecting species in regions near the limits of their thermal niches. Indeed, most of the changes were attributed to trade-offs between different biological strategies. Taken together, the reviewed case studies indicate a lagged biological response to variations in the local environment driven by large-scale climate forcing. The challenges for interpreting future shifts include considering local changes within a larger geographical area, variations in species life traits, and potential top-down effects of plankton predators.

The Western Channel Observatory: a century of physical, chemical and biological data compiled from pelagic and benthic habitats in the western English Channel

“…However, in recent years, numbers in summer have started to decline substantially, making it hard to obtain sufficient individuals for egg production experiments. This sudden shift supports the concept of abrupt step changes that "reorganize" assemblages both at this site (Reygondeau et al, 2015) and more widely (Bode, 2023).…”

“…Figure 2 compares the key WCO sites in relation to the wider summer pattern of stratification (Fig. 2a) and to the longer trend of climatic cycles across the North Atlantic (Bode, 2023), highlighting a recent phase of intense warming over the last 4 decades (Fig. 2b).…”

“…5.6) this is changing. This copepod species, alongside other zooplankton such as appendicularians (López-Urrutia et al, 2003), decapods (Fileman et al, 2014), bivalve larvae (Lindeque et al, 2015) and Oithona similis (Castellani et al, 2016;Cornwell et al, 2018Cornwell et al, , 2020, has been the focus of a series of detailed studies at L4 (Bonnet et al, 2005;Hirst et al, 2007;Irigoien and Harris, 2003).…”

“…Examples include the importance of nutrition in Calanus spp. reproduction (Pond et al, 1996); the role of stratification, egg sinking and mortality (Irigoien and Harris, 2003); and the interaction among temperature, food and predators on population dynamics (Bonnet et al, 2005;Cornwell et al, 2018;Maud et al, 2015Maud et al, , 2018. Long-term changes in the phenology and rates of Calanus helgolandi- cus egg production have not been studied recently, but these show some interesting patterns (Fig.…”