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Methods - Sedimentation, sediment quality and biota indicators
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Measuring sediment plates in the lower estuary
Collecting a fine-scale sediment core
Substrate oxygenation (aRPD) at Site B
| Indicator | General rationale | Sampling method |
|---|---|---|
| Physical and chemical | ||
| Sedimentation rate | Increases in fine sediment (i.e., mud <63μm) deposition can lead to the smothering of coarser sediments and biotic habitats (e.g., cockles, seagrass). Fine sediments also concentrate contaminants, nutrients and organic matter, and lead to degradation of ecological communities by displacing sensitive species including shellfish. | The sediment plate method monitors temporal change in the depth of sediment that is deposited over buried concrete pavers. The change in depth indicates whether sediment is accumulating (sediment depth increases) or eroding (sediment depth decreases). |
| Sediment grain size | Indicates the relative proportion of fine-grained muddy sediments that have accumulated. | Composited surface scrape to 20mm sediment depth. |
| Nutrients (nitrogen), organic matter | Reflects the enrichment status of the estuary and potential for algal blooms and other symptoms of enrichment. | Surface scrape to 20mm sediment depth. Organic matter measured as Total Organic Carbon (TOC). |
| Trace elements (arsenic copper, chromium, cadmium, lead, mercury, nickel, zinc) | Common toxic contaminants generally associated with human activities. High concentrations may indicate a need to investigate other anthropogenic inputs such as pesticides or hydrocarbons. | Surface scrape to 20mm sediment depth. |
| Substrate oxygenation (apparent Redox Potential Discontinuity depth; aRPD) | Measures the enrichment/trophic state of sediments according to the depth of the aRPD. The aRPD can occur closer to the sediment surface as organic matter loading or sediment mud content increase. | Sediment core, split vertically, with average depth of aRPD recorded in the field where visible. The aRPD depth represents the visual transition between brown oxygenated surface sediments and deeper less oxygenated black sediments. |
| Biological | ||
| Macrofauna | Abundance, composition and diversity of infauna living within the sediment are commonly-used indicators of estuarine health. | 130mm diameter sediment core to 150mm depth, sieved to 0.5mm to retain ‘macrofauna’. |
| Indicator | Unit | Very good | Good | Fair | Poor |
|---|---|---|---|---|---|
| Sediment quality | |||||
| Mud content1 | % | <5 | 5 to <10 | 10 to <25 | ≥25 |
| aRPD depth2 | mm | ≥50 | 20 to <50 | 10 to <20 | <10 |
| TN1 | mg/kg | <250 | 250 to <1000 | 1000 to <2000 | ≥2000 |
| TOC1 | % | <0.5 | 0.5 to <1 | 1 to <2 | ≥2 |
| Macrofauna | |||||
| AMBI1 | na | 0 to 1.2 | >1.2 to 3.3 | >3.3 to 4.3 | ≥4.3 |
| Sediment trace contaminants3 | |||||
| As | mg/kg | <10 | 10 to <20 | 20 to <70 | ≥70 |
| Cd | mg/kg | <0.75 | 0.75 to <1.5 | 1.5 to <10 | ≥10 |
| Cr | mg/kg | <40 | 40 to <80 | 80 to <370 | ≥370 |
| Cu | mg/kg | <32.5 | 32.5 to <65 | 65 to <270 | ≥270 |
| Hg | mg/kg | <0.075 | 0.075 to <0.15 | 0.15 to <1 | ≥1 |
| Ni | mg/kg | <10.5 | 10.5 to <21 | 21 to <52 | ≥52 |
| Pb | mg/kg | <25 | 25 to <50 | 50 to <220 | ≥220 |
| Zn | mg/kg | <100 | 100 to <200 | 200 to <410 | ≥410 |
| Sedimentation | |||||
| Sedimentation rate4 | mm/yr | <0.5 | ≥0.5 to <1 | ≥1 to <2 | ≥2 |
- Ratings from Robertson et al. (2016).
- aRPD based on FGDC (2012).
- Trace element thresholds scaled in relation to ANZG (2018) as follows: Very good <0.5 x DGV; Good 0.5 x DGV to <DGV; Fair DGV to <GV-high; Poor >GV-high. DGV = Default Guideline Value, GV-high = Guideline Value-high.
- Sedimentation rate adapted from Townsend and Lohrer (2015).
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Page last updated 7 April 2025
Page last updated 7 April 2025
