Plant-like phytoplankton and animal-like zooplankton are among the main operators of the biological pump, a set of processes including the fixation of atmospheric carbon dioxide in organic material and its transport from the upper, sunlit ocean to depth. Although it has long been recognized that changes in the biological pump are of great significance to the climate system, they are poorly understood. Quantifying such changes is notoriously difficult because it requires spatially and temporally explicit information about biological, chemical, and physical properties of the ocean, where empirical observations are in short supply.
In this work, we focused on marine copepods, a well-studied, ubiquitous, and dominant group of marine zooplankton, and estimated recent changes in their contribution to the biological pump across the northern half of North Atlantic Ocean. Copepods transport carbon passively, by producing sinking fecal pellets while feeding near the sea surface, and actively, by conducting daily and seasonal migrations to deeper, darker waters, where carbon is released through respiration, defecation, and mortality. In addition to information about where, how many, and what types of copepods were present, we also needed to infer their migration behavior.
Figure 1: A grazing copepod of the genus Acartia (image courtesy of Erik Selander)
To do so, we applied available information on copepod body size, their food, and water turbidity to an optimal behavior model. Optimal behavior models are based upon evolutionary reasoning and assume that organisms always act to maximize their chances of survival and reproduction, i.e., their fitness. We processed tens of thousands of nighttime observations of copepod communities from the Continuous Plankton Recorder (CPR) program, and estimated whether they were more likely to keep feeding at the surface during daylight hours, or to descend to deeper layers to hide from visual predators like fish. Based on these estimates, we quantified the amount of carbon that the copepods actively transported by migrating, and how much they passively transported by producing sinking fecal pellets.
Figure 2: Sampling device of the Continuous Plankton Recorder program attached to a commercial ship crossing the North Atlantic (image courtesy of the CPR Survey)
The findings from these analyses were striking: from southern Iceland to the Gulf of Maine, carbon transport mediated by copepods increased significantly during the last six decades, while widespread decreases were apparent across much of the temperate, central northern North Atlantic. This pattern was largely driven by changes in copepod population distributions and community structure. Our results suggest two important conclusions: First, shifts in species distributions driven by a changing global climate are already impacting ecosystem function across the northern North Atlantic Ocean. These shifts are not uniform along latitude and highlight the complexity of marine ecosystems. Second, long-term biological monitoring programs like the CPR survey are absolutely essential to understanding key processes and changes in the Earth system.