The solar zenith distance is defined as the angle between the solar rays and the normal to the Earth's surface at a particular point. See sections 2.1.3.2, 2.1.4 and 5.4.1.
The adjective zonal refers to the east-west direction. For instance, the zonal mean, for any latitude, is the average over all longitudes.
Zooplankton is the type of plankton that consumes (or grazes) phytoplankton.