This puts limits on how much oxygen is available for the plants and animals to use. Even in the winter, shallow tidal pools can get quite hot. The tidal waters bring cooler water loaded with oxygen. This variability is difficult to live with. Plants and animals must be adapted to it to survive.
Periwinkles are vegetarians.
Many small shrimp, fishes, crabs, and other animals, often plankton themselves, feed on the phytoplankton. These small animal plankton, zooplankton, are preyed upon by larger estuarine creatures. These are preyed upon by animals larger still or scavenged by a variety of creatures after their death. The basis is this food web is the phytoplankton. These small plants grow just like their very visible counterparts.