|Subsystem||Estuarine Open Water
|Tidal Zone||Estuarine Open Water Subtidal
|Definition||The Estuarine Open Water Subsystem includes all waters of the Estuarine System with a total depth greater than 4 meters, exclusive
of those waters designated Tidal Riverine Open Water.
The Open Water Subsystem is subject to a number of physical factors that make it distinct from the Coastal Subsystem, including reduced air-water exchange, potentially reduced light at depth, reduced physical impact from waves and surface currents and reduced interaction between the water column and the bottom. Moreover, because of the formation of stratified layers in the Estuarine System, the Open Water Subsystem is often "capped" by a relatively strong density or stability gradient that distinctly separates the lower water column from the upper water column, separated by a zone of transition (such as a pycnocline, halocline, or thermocline).
The Open Water Subsystem may be heterotrophic, because it often acts as a receiving basin for organic material settling from the shallower, better lit surface waters, especially when the waters are stratified and form shallow flanks of the water body. At times, this role as a heterotrophic zone may support high rates of respiration (relative to production) and therefore consume much of the available oxygen and lead to the formation of hypoxic or anoxic zones, generally in the deeper parts of these waters. Additionally, stratification and the mechanics of estuarine circulation often promote the formation of a salt wedge intrusion (from the marine environment) that renders the bottom waters more saline than waters in the surface layer above.