This report is a contribution to the Strategic Environmental Assessment (SEA3) conducted by the Department of Trade and Industry (now Department of Energy and Climate Change) and has been written as an addendum to the more comprehensive SEA2 document. The two papers give an overview of the phytoplankton and zooplankton community composition in the North Sea and how this has fluctuated through the latter half of the 20th Century in response to environmental change. The study is based on a unique long-term dataset of plankton abundance in the North Atlantic and the North Sea acquired by the Continuous Plankton Recorder (CPR). The dinoflagellate genus Ceratium dominates the phytoplankton community in the North Sea, but diatoms are also important, especially in the southern part. The normal annual blooms of plankton are discussed, as are harmful algal blooms (HABs), which appear to be on the increase, possibly due to a combination of climatic variability and eutrophication. Among the zooplankton, copepods are particularly important and constitute a major food resource for many commercial fish species, such as cod and herring. Calanus is the dominant copepod genus in the North Atlantic. Other important components of the plankton - meroplankton, picoplankton and megaplankton - are also reviewed. Very small picoplankton (~1 micron in diameter) and much larger gelatinous members of the megaplankton (e.g. jellyfish and ctenophores) are poorly sampled by the CPR. Although the picoplankton represents a sizeable fraction of total primary production, its role in the marine ecosystem is poorly understood.