Indicators and how to select them for water/sediment quality management

Pressure lines of evidence are the drivers of change to the stressors affecting a water or sediment system. Pressures on aquatic ecosystems can come from many sources, such as:

• agriculture and its associated stressors (e.g. pesticide effects or eutrophication from fertiliser use)
• biota
• climate change
• land management and its numerous pressures and stressors (e.g. land clearing, stocking rates; irrigated area, acid sulfate soils, salinisation)
• maritime activities and industries (e.g. anchoring, dredging, fishing, shipping)
• mining
• renewable energy developments
• urban and industrial activities
• water use.

Indicators for pressures should be quantifiable to enable comparisons with un-pressured and less-pressured systems. This will help you to make linkages between putative effects and likely causes through a weight-of-evidence evaluation. Those linkages, and the likely nature and extent of change in the indicator as a result of the expected level or intensity of the pressure, should be established via the current understanding (Step 1 of the Water Quality Management Framework).

This is how the sensitivity of the indicator can be adjudged as appropriate to the potential level of impact, and the statistical power of the monitoring program can be designed to sufficiently assess achievement of management goals.

Suitable measurements might be of loads or areal disturbance units, or even activity count data for a location. Examples of pressure indicators include:

• annual loads of discharged salts or brines
• areas of urban or mining development
• hectares of clearing
• human population measures
• pesticide application rates (tonnes per km2)
• pesticide sales
• ship movement counts.

Chemical and physical line of evidence

Physical and chemical (PC) stressor and toxicant indicator types comprise concentration or strength measures that have both direct and indirect effects on ecosystems.

Direct PC stressors and toxicants have non-toxic and toxic direct effects on biota and ecosystems, and examples include:

• non-toxic: nutrients (total N, inorganic N, organic N, total P, filterable reactive P, ammonia), chlorophyll a, temperature and turbidity
• toxic: heavy metals, organics, ammonia, salinity, pH and dissolved oxygen (DO).

Indirect PC stressors and toxicants can modify the effects or toxicity of PC stressors or toxicants, examples and the effect of the indirect PC stressors and toxicants include:

• pH changes can release metals
• presence of dissolved organic carbon and suspended particulate matter can complex metals and reduce toxicity
• temperature changes can alter physiological rates
• DO changes can alter redox conditions

For some toxic stressors (e.g. metals), more detailed measurements might be required that target total vs dissolved forms, together with speciation and the bioavailable toxicant fraction.

See also:

• Evaluating monitoring data against default guideline values

Measurement of indicators from the chemical and physical line of evidence follow standard analytical procedures that specify the requirements for in situ vs laboratory measurements, sample collection, preparation and storage protocols, and analytical methods, including appropriate quality assurance/ quality control (QA/QC). All measurements must have detection limits that permit reliable comparisons of data with default guideline values (DGVs) for water quality.

A similar set of indicators apply to sediment samples. Additional co-stressor indicators include grain size and total organic carbon. Smaller grain size sediments have more binding sites, while organic carbon (OC) provides binding sites for organic contaminants. Normalisation to OC or grain size can account for differences in bioavailability when comparing to guideline values.

Non–water quality line of evidence

Indicator types within the non–water quality line of evidence that are not directly related to water quality might include invasive species, riparian condition, sedimentation and environmental flows. Some of these are only marginally removed from being considered as pressures. Some form of quantification will be required in the measurement programs for these indicators to ensure changes to biodiversity are not confounded by changes associated with these non–water quality stressors.

See also:

• Other non–water quality related stressors

A set of ecosystem receptor indicators is available for each line of evidence within ecosystem receptors.

Biodiversity line of evidence

Management goals are typically linked to detecting changes at the population, community or ecosystem level of organisation. Indicators used for this purpose are classified together as ‘biodiversity indicators’. Such indicators are used to detect an ecosystem-level response to an effect, or to assess changes to biodiversity or conservation status. Biodiversity indicators may directly represent, or be surrogates for, these requirements.

Typical biodiversity indicators are:

• species of high conservation value or species important to the integrity of ecosystems
• communities of organisms, such as phytoplankton, macrophytes (also constituting important habitat), zooplankton, macroinvertebrates and fish, or
• important ecosystem processes (e.g. measures of gross primary production and respiration).

Changes to biodiversity (populations and communities of organisms) may be manifested in both the short and longer term, depending on the biotic assemblage examined and the intensity of the stressor.

Some taxa and assemblages may be diagnostic for particular chemical and physical indicator types. For species and communities, typically changes to population or community structure are assessed at sites by way of comparison to a reference condition (baseline data and data from contemporaneously sampled control or reference sites).

Traditionally, taxa have been identified and enumerated by eye or using optical microscopy. Increasingly and into the future, effects on populations and communities will be more readily and efficiently assessed using improved ecogenomic techniques. Ecogenomics examines a broader range of taxa comprising micro-, meio and macro-organisms to identify operational taxonomic units (OTUs) that can be compared between sites.

Toxicity line of evidence

Toxicity determination reflects the direct effects of contaminant concentrations on one or more sensitive test organisms and is an important part of the toxicity line of evidence. Tests may be:

• acute toxicity testing (a lethal or adverse sublethal effect that occurs after exposure to a chemical for a short period relative to the organism’s life span), or
• chronic toxicity testing (lethal or sublethal effects over a substantial portion of the organism’s life span or an adverse effect on a sensitive early life stage).

Chronic effects are preferred for ecosystem protection in most circumstances.

Measurements of toxicity are compared to those on a matched water or sediment control from a toxicant-free reference site. The organisms’ response is the summation of the effects of all toxicants in the system, not only the target toxicant. The use of toxicity identification evaluation (TIE) procedures permit identification of particular classes of contaminants in the toxicant mix (Burkhard & Ankley 1989, USEPA 1992, 1993, Burgess et al. 2013).

Indicator responses will be the results of a number of acute or chronic bioassays using the test waters or sediments. For most assessments, chronic endpoints are preferred.

From the dose–response curves, the effective concentration (EC) or inhibitory concentration of 10% (IC10) values, or if possible no effect concentrations (NOECs), are determined. Although NOECs are no longer recommended (Warne et al. 2018).

Tests should compare test samples with a reference sample across a suitable dilution series. Appropriate QA/QC should involve the use of a reference toxicant, and measured toxicant concentrations during the tests.

Most toxicity testing is laboratory based in practice but field testing using caged organisms can be undertaken. The use of mesocosms, a semi-field approach, can assess toxicity to assemblages of species under controlled conditions.

For all selected toxicity testing indicators, you should choose the test endpoint — the specified measure of organism response — to be relevant and sensitive to the stressor(s) of concern identified in the conceptual model.

Biomarker line of evidence

Bioaccumulation indicator type

Bioaccumulation indicator types are measured concentrations of contaminants accumulated by aquatic biota from waters or sediments. These concentrations are compared with those in specimens from the same species from reference sites. The measurements indicate whether the contaminants are in forms that are bioavailable, and so complement measurements of chemical concentrations in waters and sediments.

Bioaccumulation can be measured in:

• organisms collected from field sites where they are naturally present (passive biomonitoring)
• caged organisms following field transplantation (active biomonitoring) (Maher et al. 2016)
• caged organisms in the laboratory using microcosms.

Biomimetic devices can be used to estimate total body residues after exposure to complex mixtures of chemicals in waters or sediments (e.g. Simpson et al. 2012, Ghosh et al. 2014, Huynh et al. 2015) and may be preferable where the organisms of interest are highly mobile, or variable in their propensity to accumulate or excrete the contaminants of concern.

You may measure contaminant concentrations in whole organisms, or in specific organs or tissues of the sampled species. If organisms are small, then you may pool samples of whole organisms (or tissues) to reduce variability and analytical costs, but modern instrumentation can achieve acceptable limits of resolution for small tissue samples (< 1 g) in many instances.

We provide advice on procedures for sample collection, preparation and storage and of appropriate QA/QC. Results are reported as either wet weight or dry weight concentrations and it is important to also report moisture content.

Find out about:

• Field sampling program
• QA/QC in laboratory analyses

Biomarkers of exposure and effect indicator type

A biomarker is a measurable indicator of a suborganism biological effect, condition or response. Biomarkers can be indicators of exposure to, or effects due to, contaminants in waters and sediments. They can be measured in the laboratory, or in organisms that have been exposed to contaminants in the field or laboratory.

The biological endpoints comprise molecular, biochemical, physiological or histopathological markers, as summarised by Hook et al. (2014) and Kroon et al. (2017). When viewed in conjunction with other indicator types within different lines of evidence, biomarkers of exposure and effect can provide useful confirmation of potential effects (Hook et al. 2014).

Biomarkers provide an early warning of impending environmental problems that may later manifest as toxicity. Results can be obtained rapidly, typically within 24 to 48 hours of collection of organisms. As such, biomarker responses are typically seen at concentrations below those for toxicity.

Although they may be indicative of some type of physiological impairment as a consequence of exposure to one or more contaminants or to some other non-chemical stressors, it is often difficult to demonstrate the significance of this effect on the health of the organism and extrapolation to longer‐term effects on ecosystem health.