| Summary: | Phytoplankton and zooplankton occupy pivotal positions within marine ecosystems. These small organisms fuel and support the foodwebs upon which almost all higher organisms depend. Fisheries and related economic activities are highly dependent on the production, size, and composition of zooplankton which, in turn, rely on primary production by phytoplankton. In addition to their role as prey for herbivorous zooplankton, phytoplankton absorb enormous quantities of dissolved CO2 via photosynthesis. Zooplankton then plays an essential role in the biological pump by consuming phytoplankton and transporting carbon from the upper ocean to the deep ocean, where it is sequestered for hundreds to thousands of years. Given the ecological and economic importance of phyto‐ and zooplankton, it is essential to understand and predict how they are likely to respond to climate change. Climate‐related hydrographic changes may also directly affect the abundance and composition of zooplankton, shifting the distribution of dominant species, changing the structure of the zooplankton community, and altering the timing, duration, and efficiency of zooplankton reproductive cycles. Ocean acidification through increased carbon dioxide dissolution in the upper ocean is lowering the pH in surface waters. A lower pH could impair the physiology and ultimately the abundance of many phytoplankton and zooplankton species. It is important to understand how phytoplankton and zooplankton are likely to respond to climate‐induced changes in the ocean. This section explores what is known about the sensitivity of phytoplankton and zooplankton to climate change and summarizes the trends that are evident in plankton communities within the ICES Area.
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