Estuaries

Estuaries are home to a variety of animals and plants. The benthic community is made up of the animals living in the sediments of the estuarine floor. The diversity and abundance of the benthos indicates the overall health of the estuarine ecosystem. This project evaluates whether predictive computer models used in monitoring environmental health of rivers can be developed for Australian estuaries.
To achieve this two questions needed to be answered:
1. Can such models be constructed for Australian estuarine habitats?
2. Would these models be useful for assessing estuarine environmental health?

What is an estuary?

The estuarine habitat is poorly defined. Different scientific disciplines define estuaries by geographical, topographical, chemical, biological and hydrographical terms.

Estuary

Thus people from different disciplines draw different boundaries. Also there is conflict as to whether a body of water is an estuary or not. For example it is frequently argued that only waters with diluted sea water and regular tidal influences can be estuaries. This would exclude many Australian river mouths because they have closed bars, and thus are not subjected to tidal influences, and/or the evaporation rate exceeds fresh water input so they have salinities similar to or above sea water (i.e. they do not have diluted seawater).

Problems with defining the estuarine habitat were highlighted during the early 1990s in the United Kingdom when, due to there being different regulations for the discharge of efluent into estuarine and coastal waters, courts were required to assess whether discharges were estuarine or not. There is no official definition of an estuary in Australia and this is needed before estuarine health monitoring can be undertaken. For the purposes of this study, we assumed estuaries were the coastal plain regions of streams whether they openly discharged into the sea or formed a coastal saline lake that was intermittently open to the ocean.

The Macquarie dictionary provides the following definitions of estuary;

1. The part of the mouth or lower course of a river in which its current meets the sea's tides, and is subject to their effects.
2. An arm or inlet of the sea.

This study focuses on the environment defined by the first meaning of estuary. This is where fresh river water and seawater mix. Throughout these estuaries the environment changes from marine at the mouth (33 ppt salinity), to fresh water at the head (‹2 ppt salinity). Environmental conditions at a site in these estuaries will vary on an hourly time scale with the tides, seasonally with rainfall and long term chaotic changes associated with unusual rainfall patterns. Unique communities that can survive in these highly variable environments have developed. There is no alternative name for this environment.

Marine embayment is probably a better term for the second environment defined by the Macquarie dictionary. In these habitats, environmental conditions are marine and relatively stable. The communities are made up of inshore coastal species.

Delineating estuaries as defined by the first meaning can be difficult because many flow into marine embayments.