Marine ecosystems include oceans, which cover 70% of Earth's surface and are interconnected by currents, dominated by waves, influenced by tides, and characterized by saline waters. There are two main types of open sea ecosystems: pelagic ecosystems in the water column and benthic ecosystems on or in the ocean bottom. Major groups in open sea ecosystems include phytoplankton as primary producers in surface waters, zooplankton that graze on phytoplankton, nekton like whales that can actively swim, and benthos like corals that live on or in ocean sediments. Rocky shores are alternately exposed and submerged by tides.