The addition or removal of dissolved constituents at mid-ocean ridge hydrothermal vents can significantly alter seawater chemistry. Oxygen and carbon dioxide levels vary in seawater not only because of changes in temperature, salinity, and partial pressure, but also because of biotic activity. Nitrogen is usually the major limiting nutrient element to algal and vascular plant growth in marine waters. In estuarine and coastal marine waters, various anthropogenic activities contribute additional quantities of organic matter. Some trace elements are critically important to the life processes of marine organisms, despite their low concentrations. Cobalt, copper, manganese, molybdenum, vanadium, and zinc are essential elements for functioning of marine flora. Cobalt, copper, and manganese are also vital for other metabolic functions. Most of these trace metals are likewise essential for life processes of marine fauna. The accumulation of trace metals in estuarine and marine organisms depends on several factors, most notably the chemical behavior of the element, composition of food ingested, and habitat occupied.