Water Quality Guidelines have been published to provide an authoritative guide for setting water quality objectives required to sustain current, or likely future, environmental values [environmental protection and designated uses of water] for natural and semi-natural water bodies [receiving waters] while maintaining economic and social development.

Environmental values are particular values or uses of the environment that are important for a healthy ecosystem or for public benefit, welfare, safety or health and which require protection from the effects of pollution, waste discharges and deposits.
This includes environmental protection (for six ecosystem types) and designated uses of water for various purposes (e.g. irrigation, drinking water, recreation). See Vol 1 2-6.

A water quality guideline ('trigger value') is a recommended numerical concentration level (e.g. of a contaminant) or a descriptive statement (e.g. visual appearance of a water body) that will support and maintain the designated environmental values [environmental protection and designated uses of water] of a particular water.
Water quality guidelines have been published for chemical and physical parameters of water and sediment, as well as biological indicators. They form the basis for determining water quality objectives.
The guidelines are referred to as 'trigger values' because they represent threshold values that if exceeded should trigger a management response. See Vol 1 2-9.

A water quality objective (= target) goes a step further than a water quality guideline.
It is a numerical concentration level or descriptive statement used by managers to measure and report on performance.
Water quality objectives are targets agreed between stakeholders, or set by local authorities that become the indicators, or measures, of success in meeting agreed management goals.
Although objectives are normally based on water quality Guidelines, a water quality objective may be modified by other inputs such as social, cultural, economic or political constraints and this will involve considering costs and benefits.
The community might decide, after such consideration, to allow a longer period to achieve the desired water quality or even accept a lower water quality. See Vol 1 2-11.

It is not possible to develop a universal set of specific guidelines that apply universally and effectively to the very wide range of ecosystem types.
Environmental factors can reduce or increase the effects of physical and chemical parameters at a site.
Local ecosystems or crops may be more or less sensitive to the water quality stressor and the development of local site-specific guidelines may be warranted to reduce the risk of harm or to reduce the cost of applying guidelines that are more stringent than they need to be.
The published guidelines provide risk-based decision frameworks that allow users to move beyond single-number values, to site-specific guidelines modified according to local environmental conditions.
The water quality guidelines become 'trigger values' that, if exceeded, warn of a high risk of damage to the environmental or impairment to the designated use for the water. See Vol 1 2-15 & 3.1-17.

Biological assessment (bioassessment) can measure the desired management goals for an ecosystem (e.g. maintenance of a certain diversity of fish species or certain level of nuisance algae) as might be described in the management goals. Bioassessment provides information on biological or ecological outcomes; these may result from changes in water quality but may also result from changes in the physical habitat (e.g. increased fine sediment deposition, or changes in hydrology) or from changes in biological interactions (e.g. the introduction of exotic species or diseases). Thus, bioassessment should be seen as a vital part of assessing changes in aquatic ecosystems, and as a tool in assessing achievement of environmental values and attainment of the associated water quality objectives.