Temporal dynamics of phytoplankton biomass and chlorophyll a concentration exhibit variable and often contrasting patterns in different spatial scales during the past several decades. At least two major trends are currently affecting phytoplankton dynamics in the North Sea: warming and the decline in eutrophication as a result of measures to reduce riverine nutrient inputs that started in the 1980s. Both these trends affect primary production through altered water column stratification and the corresponding effects on the physiology of phytoplankton species.

Primary production is generally highest in the coastal regions due to nutrient inputs from the rivers and turbulent mixing in the water column. Net primary productivity has generally been lower and below average since 2015 compared to the previous years but has been gradually increasing since 2018 (Figure 13).

Continuous plankton recorder (CPR) and coastal station records have shown a decreasing trend in dinoflagellate abundance over time, whereas the total abundance of diatoms has remained unchanged. This has resulted in the dominance of diatoms. Among the dinoflagellate species, Tripos furca, Protoperidinium spp., and to a lesser extent Prorocentrum spp., have shown a substantial reduction in summer since the beginning of the 2000s.

​Zooplankton

Based on the CPR data, zooplankton communities in the northern North Sea are generally composed of offshore cold water species (such as Calanus finmarchicus and Metridia lucens) owing to the stratification of the water column during the summer months. The zooplankton community of the southern North Sea primarily consists of neritic and coastal species (the copepods Centropages hamatus and Calanus helgolandicus and decapod larvae) which are adapted to the mixed warmer waters of this region.

There has been a clear trend for a poleward distributional shift in the northeast Atlantic zooplankton community, progressing at a rate of around 200–250 km per decade. The consequence of this shift has been to increase the diversity of calanoid copepods in the North Sea due to an influx of southern warmer‑water species.

Species with warmer-water affinities, e.g. C. helgolandicus, continue to move northward in the ecoregion. However, C. helgolandicus never reaches high population densities and the species usually occurs later in the season. Population abundance of the previously dominant copepod C. finmarchicus has declined in biomass by 70% between 1960s-2010s. A redistribution of C. finmarchicus relative to C. helgolandicus will result in lower total zooplankton biomass available for higher trophic levels with consequences for the fisheries targeting them.

Small copepods have decreased by about 50% during the last three decades, particularly in the central and southern areas of the North Sea. The declining trend in small copepods has been attributed to a combination of earlier spring blooms and lower summer food quantity and quality, suggesting an overall bottom-up control of the foodweb structure in the North Sea. Also, the abundance of Pseudocalanus/Paracalanus spp. has decreased across the North Sea, the change being linked to the decrease in dinoflagellates.

Zooplankton size decreased and total abundance increased in the English Channel in winter during 1991–2013. Zooplankton abundance was influenced by temperature, chlorophyll a concentration, and North Atlantic Oscillation index, whereas zooplankton size was influenced by depth and Atlantic water inflow.

The observed changes in zooplankton composition and distribution might have a cascading effect on their predators and even the human activities targeting these predators. For example the decline in C. finmarchicus has been linked to the reduced survival of fish larvae (e.g. cod) and the growth of lesser sandeel in the North Sea. The northward shift in the distribution of C. finmarchicus may have caused a northward shift in the feeding migration of North Sea herring resulting in different spatial distributions of the pelagic fisheries effort targeting them.

​The gelatinous macro- and megazooplankton community in the North Sea can be quite diverse, especially in terms of smaller‑sized hydromedusae. Due to the very patchy spatial distribution, strong seasonal abundance signals and accumulation in shallow coastal areas, it is difficult to reliably assess their population size and role in the ecosystem. For the North Sea, the highest abundances are reached for the large sized scyphozoan jellyfish species Cyanea capillata, C. lamarckii, Chrysaora hysoscella, Aurelia aurita, and Rhizostoma pulmo as well as the hydrozoans Aequorea vitrina, Aglantha digitale, and Tima bairdii and the ctenophores Pleurobrachia pileus, Beroe sp., and Bolinopsis infundibulum.