Any species which fall into the categories Vulnerable, Endangered or Critically Endangered are considered to be at risk of extinction. Of the nearly 60,000 species assessed so far, this equates to a staggering 20,222 species, from the majestic tiger to the enormous giant clam.
(Photo: Elkhorn Slough NERRS site). An estuary is a partially enclosed body of water, and its surrounding coastal habitats, where saltwater from the ocean mixes with fresh water from rivers or streams. In fresh water the concentration of salts, or salinity, is nearly zero.The salinity of water in the ocean averages about 35 parts per thousand (ppt).ruly, no two estuaries are the same. Yet they are typically classified based on two characteristics: their geology and how saltwater and fresh water mix in them. However, not all estuaries contain brackish waters. There are a small number of ecosystems classified as freshwater estuaries.
The features of an estuary are determined by a region's geology, and influenced by physical, chemical, and climatic conditions.For example, movements in the Earths crust elevate or lower the coastline, changing the amount of seawater that enters an estuary from the ocean.he amounts of seawater and fresh water flowing into an estuary are never constant. The quantity of seawater in an estuary changes with the changing tides, and the quantity of fresh water flowing into an estuary increases and decreases with rainfall and snowmelt.