Side view of a Euphausiid (photo K. Young)
A copepod viewed from the top.
Zooplankton samples in a jars. The whiteish stuff at the bottom are dead zooplankton which have settled to the bottom of the jar.

Zooplankton are the critical food web link between microscopic plants (primary producers or phytoplankton) and juvenile fish. Although there are hundreds of different species of zooplankton, these can be organized into a smaller number of groups containing similar species. The large crustacean groups presented here are considered the preferred diet for juvenile salmon in the Strait of Georgia.

Copepods of all sizes typically make up more than 40% of the zooplankton biomass (the smaller copepods are actually the most abundant in terms of number). The large omnivorous Neocalanus plumchrus (3-5 mm in length) are typically dominant in spring, when they migrate to the surface to feed and spawn. The rest of the year they remain dormant at great depths. Some of the other (mostly smaller) copepods are present all summer, but undergo daily vertical migrations (like Metridia spp.), rising to the surface at night to feed and but sinking to depths at night to avoid being eaten themselves. Others spend their whole lifetime near the surface.

The biomass of decapods (primarily shrimp and crab larvae), which range in size from about 4 mm to 1 cm or so in length, also tends to increase in spring, while the biomass of carnivorous amphipods tends to increase in summer. The omnivorous euphausiids (1 cm or larger) are less tied to a seasonal signal; they are also quite good at trying to escape nets as they are towed through the water so estimating their numbers is more difficult. These groups make up about another 35% of the zooplankton biomass.

Note that making quantitative estimates of zooplankton is difficult, so that biomasses associated with individual samples can vary greatly. However, in general, summer biomass is significantly greater than winter biomass, and there appears to be little spatial variation once the data has been averaged over time.