Estuarine Systems

Estuarine systems are transitional between open marine and fully freshwater environments, semi-enclosed water bodies with a free connection to the open sea. The variability of their physical, chemical and biological properties has profound effects upon the nature of the estuarine biota.
In these systems, phytoplankton growth is limited due to the high sediment suspension, which blocks or absorbs light; biofilms composed of diatoms and sugar polymers bind the sediment, and this is an excellent food source. Whilst an estuary is a heterogenous system, it is a stressful habitat that limits biodiversity they do support micro-environments however, with their own salinities and nutrient loads.

Types of estuary

1. Coastal plain estuaries ; formed at the tail-end of the Ice Age with rising seawater level. They are the commonest type of estuary, such as Chesapeake Bay. 
2. Bar-built estuaries; these are semi-enclosed lagoonal, shallow estuaries principally in low-lying regions where shingle or sand bars partially isolates river water from the sea e.g. Dutch Waddenzee.
3. Tectonic estuaries; the sea has invaded the land by tectonic processes resulting from land subsidence, particularly along fault lines e.g. San Francisco Bay. 
4. Fjords; drowned glacial valleys whose estuaries are deep and narrow with a submerged sill, restricted water circulation at lower levels, and are prominent in Norway, Chile and Columbia.

The Salt Wedge

The shape, width and length of the estuary determines how well-mixed the system is: geology determines geomorphology determines ecology. The salt wedge profile is indicated by isohalines, which are distorted due to boundary layer effects, and a reflected wave from an incoming tide (a standing wave) aids in the mixing of fresh and saline water. 

In the highly stratified estuary, sea water invades the river bed, with the less dense fresh water flowing over the top; frictional forces at the interface cause entrainment of some sea water upwards in to the freshwater layer, meaning there is a continual slow upstream flow within the salt wedge. A halocline exists between the two masses as mizing is marked discontinuous. Superimposed on such mixing circulation patters is is the Coriolis effect, which deflects water to one side of the estuary. 

Water mixing

Energy inputs in to the system control water mixing - wind (turbulence); heating & cooling (stratification & turnover); tide (flood speed) and gravitational circulation (salinity gradient). The Reynolds ratio number divides the power to mix by the power available: RR > 0.8 is a stratified system, if RR < 0.08 it is fully mixed.

Sedimentation

Fine particulate material flocculates when it encounters salt water, and larger flocculates sink to form a liquid mud on the surface of the submerged mud-flats. In vigorous vertical mixing some particles will be resuspended, deflocculate and repeat the process. Sediments may be derived from the freshwater, some are carried in by tidal flow. Sedimentation is largely related to seasonal fresh water input, storms and floods. Sediment sources may be autochthonous (originate from within the estuary) or allochthonous (imported in to the estuary from other sources). 

Muddy cohesive clays, mixed and sandy sediments are all found; clay is composed of lamellae plates which are all negatively charged, meaning they cannot combine. The addition of water, with positive hydrogen iions, binds to the particles as the Van der Waals forces attract. Interstitial water with positive ions from the water column and biological polymers reduce the clay repulsion and improve cohesion.

    

Estuary productivity

50% of productivity is benthic, so the estuarine environment is based on bottom-up control - nutrients are often a severe limiting factor. Phosphorous binds well with iron compounds in fresh water, but less well in saline water; it is released by diffusion in to the water column. Nitrogen is usually used up before it reaches an estuary and is generally limiting, its availability controlled by bacterial mineralisation. Muddy sediments are unsuitable for macroalgae attachment, although saline-tolerent species such as Enteromorpha spp., or Ulva spp., are seasonally abundant. 

Adaptations

Microcycling describes the behaviour of diatoms according to light variations; when light intensity is too bright, the diatoms bury down in tot he sediment, but dig upwards when there is a defecit - this migration is not understood. They are grazed directly by Hydrobia snails and shrimp.

Osmoconformer species regulate neither cell volume nor ionic composition - they are tolerant to a large variation of ionic concentration in their body fluids. Internal ospmotic pressure changes depend on the extreme osmotic potential, and body tissues must be tolerant e.g. Nereis diversicolour. They are much more restricted in habitat range.

Most marine species exhibit varying degrees of osmoregulation. Some organisms even alter the concentration of disslved free amino acids in response to changes in external salinity. They maintain a constant osmotic potential irrespective of the external changes, but this does have metabolic costs - they are able to colonise a wider range of habitats however e.g. Upogebia spp. - mud shrimp.

Food web 

Estuaries are rich in detrital feeds, which may be deposit and suspension feeders - two means of feeding not mutually exclusive. Susepension feeders are rarer because such delicate filters may become easily clogged - despite this, there are several locally abundant, commercially exploited species such as Crassostrea virginica. Detritus feeders ensure continual bioturbation.