Matching indicators to issues for stressors in water and in sediment
We provide guidance on the types of stressors that are commonly associated with different issues, so as to aid in the selection of indicators.
The selection of appropriate indicators relies on a good understanding of the issue or issues potentially impacting on one or more community values. These connections are made while examining current understanding at Step 1 of the Water Quality Management Framework, and when defining relevant indicators at Step 3.
Issues can potentially involve all the pressure–stressor–ecosystem receptor causal pathway elements (pressures, stressors and ecosystem receptors). Based on these causal pathway elements relevant indicators can be determined. Measuring indicators across multiple lines of evidence using a weight-of-evidence process can provide a better assessment of water quality than using a single indicator.
Physical and chemical stressors in water
Types of PC stressors
PC stressors can be separated into two categories depending on whether they have direct or indirect effects on the ecosystem.
Two types of PC stressors that directly affect aquatic ecosystems can be distinguished:
- directly toxic to biota (includes toxicants)
- not directly toxic, but can result in adverse changes to the ecosystem (e.g. to its biological diversity or its usefulness to humans).
Excessive amounts of direct-effect stressors cause problems, but some of the elements and compounds/salts covered here are essential at low concentrations for the effective functioning of the biota (e.g. nutrients such as phosphorus and nitrogen, and some metals such as copper and zinc).
The guideline values for toxic stressors can be determined from laboratory ecotoxicity tests conducted on a range of sensitive aquatic plant and animal species.
Salinity, pH and temperature are 3 direct-effect stressors that are naturally variable among and within ecosystem types and seasonally. Natural biological assemblages are usually able to adjust to such site-specific variability. This suggests that guideline values for these 3 stressors need to be based on the site-specific water composition or temperature regime, including their variability, and biological effects data.
Non-toxic direct-effect stressors include, as examples:
- nutrients — can result in excessive plant growth, including algal and cyanobacterial blooms
- suspended particulate matter (SPM) — can reduce light penetration into a waterbody and result in reduced primary production, reduced growth or loss of phytoplankton, macrophytes and seagrasses, inhibited feeding of visual predators, inhibited breeding displays and hence reduced reproduction or smothered benthic organisms and their (spawning) habitats
- organic matter decay processes — can significantly reduce the dissolved oxygen (DO) concentration and cause death of aquatic organisms, particularly fish
- water flow — can significantly affect the amount and type of habitats present in a river or stream.
Indirect stressors are those that, while not directly affecting the biota, can affect other stressors making them more or less toxic. For example:
- DO can influence redox conditions and affect the uptake or release of nutrients by sediments
- pH, dissolved organic carbon (DOC) and SPM, at lower levels than would cause direct effects, can have a major effect on the bioavailable concentrations of most metals.
Using a conceptual model, managers can consider these indirect stressors, with ecosystem-specific modifying factors, during the assessment of each issue. Although many effects of these modifying factors are reasonably well known from a theoretical viewpoint, there are few quantitative relationships (or models) that allow them to be used to develop more ecosystem-specific guideline values (Schnoor 1996).
Biotic ligand models, for a number of metals, over the past decade have undergone significant development enabling us to better quantify the relationships between bioavailability-modifying factors and toxicity to organisms (Paquin et al. 2002, Schlekat et al. 2010, Erickson 2013).
Issues affecting aquatic ecosystems that are controlled by PC stressors
Many aquatic ecosystems experience a range of issues that affect biodiversity or ecological health. These are mostly a result of human activities.
We examine issues that can result from different PC stressors (see Table 1).
Guideline packages from ANZECC & ARMCANZ (2000) guidelines provide additional information on both direct and indirect PC stressors.
The issues explored in the guideline packages can occur in water or in sediments, although the discussion is mostly focused on effects in the water column.
Toxicants in water
A toxicant is a substance capable of producing an adverse response (effect) in a biological system, which may seriously injure structure or function or produce death at sufficiently high concentration. The Australian Inventory of Chemical Substances lists over 40,000 chemicals available for industrial use in Australia, while over 28,000 are listed on the New Zealand Inventory of Chemicals search. There are additional natural substances and newly-developed substances that are not included in these inventories, with more being developed each year. Many of the substances have the potential to reach aquatic environments.
The Water Quality Guidelines have over 250 DGVs for toxicants.
You can search for a toxicant DGV in a searchable database. The DGVs are divided into chemical categories that are primarily related to the chemical characteristics of the substances, rather than their uses.
Table 2 provides additional context for the uses and sources of toxicant DGV categories.
Toxicant DGV technical briefs are provided for each of the Water Quality Guidelines toxicant DGVs, and include additional details on the use and occurrence of the toxicants.
PC stressors and toxicants in sediments
Sediments are principally derived from weathering processes, with major transportation from terrestrial sources under high runoff during storms and floods (Simpson et al. 2013). Discharges from urban, industrial and mining activities are potential sources of particulates.
Anthropogenic contaminants, including metals, organics and nutrients are associated with particulate and dissolved inputs to natural waters. It is important to distinguish between point source and diffuse inputs. The former includes effluent streams, drains or licensed discharges, which can be the target of management actions.
Diffuse inputs generally lead to a gradual build-up of contaminants in sediments, especially in coastal lakes, estuaries and marine waters. Diffuse sources include aerial deposition and land runoff of stormwater. The consideration of ongoing inputs from diffuse sources may be necessary for many assessments.
Particulate matter can act as binding sites for contaminants in soluble forms. Biological processes add particulate matter in the form of algal mats, dead cells, degradation and excretion products of animals, and living and dead plant biomass. Suspended particles gradually settle and accumulate as part of the bottom sediments. A change in salinity from fresh to saline waters can induce the precipitation of iron and manganese oxyhydroxides from both soluble ions and colloids, carrying with them other metals and organics.
Sediment-associated contaminants partition in dissolved forms into associated sediment pore waters, and in this phase are readily bioavailable to benthic biota. This partitioning will be dependent on the redox status of the sediments (oxic vs anoxic). Depending on the organisms, toxic effects can also result from the direct ingestion of sediment particles by biota. Whole sediment bioassays detect the combined effects of these exposure routes (Simpson et al. 2013, Simpson & Batley 2016).
Erickson RJ 2013, The biotic ligand model approach for addressing effects of exposure water chemistry on aquatic toxicity of metals: genesis and challenges, Environmental Toxicology and Chemistry 32:1212–1214.
Paquin PR, Gorsuch JW, Apte SC, Batley GE, Bowles KC, Campbell PGC &Delos CG 2002, The biotic ligand model: a historical overview, Comparative Biochemistry and Physiology Part C: Toxicology & Pharmacology 133: 3–35.
Schlekat CE., Van Genderen E, De Schamphelaere KAC, Antunes PMC, Rogevich EC & Stubblefield W.A. 2010, Cross-species extrapolation of chronic nickel biotic ligand models, Science of the Total Environment 408, 6148–57.
Schnoor JL 1996, Environmental modelling: Fate and transport of pollutants in water, air and soil, John Wiley & Sons, New Jersey, USA.
Simpson SL & Batley GE 2016, Sediment quality assessment; a practical handbook, CSIRO Publishing, Clayton, Vic., 346 pages.
Simpson SL, Batley GE & Chariton AA 2013, Revision of the ANZECC/ARMCANZ sediment quality guidelines, CSIRO Land and Water Report 08/07, 132 pages.