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Methods - Broad scale indicators
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Mapping intertidal salt marsh, Catlins Lake
Seagrass at the estuary entrance
Measuring macroalgae cover and biomass
| Indicator | General rationale | Sampling method |
|---|---|---|
| Terrestrial margin vegetation | A densely vegetated terrestrial margin filters and assimilates sediment and nutrients, is a buffer to introduced grasses and weeds, is an important food source and habitat for a variety of species and, in waterway riparian zones, provides shade that moderates stream temperature fluctuations, and improves estuary biodiversity. | Mapped based on areal extent and classified using land cover database (LCDB5) classes. Dominant species are also recorded. Methods described in Roberts et al. (2023). |
| Substrate type | High substrate diversity generally supports high estuary biodiversity. Increases in fine sediment (i.e., mud <63μm) can smother coarser sediments and/or biotic habitats, concentrate contaminants, nutrients and organic matter, and lead to degradation of ecological communities by displacing sensitive species including shellfish. Enrichment of muddy sediments (i.e., high organic carbon and nutrients) can fuel algal growth and deplete sediment oxygen. | Mapped based on areal extent and classified using the method described in Roberts et al. (2023). The substrate type is characterised based on mud content and is supported by grain size samples. Substrate type is also recorded beneath vegetation. |
| Salt marsh | Salt marsh (vegetation able to tolerate saline conditions where terrestrial plants are unable to survive) is important in estuaries as it is highly productive, naturally filters and assimilates sediment and nutrients, mitigates shoreline erosion, and provides an important habitat for a variety of species including insects, fish and birds. | Mapped based on areal extent. Dominant species are recorded and categorised into salt marsh sub-classes (e.g., rushland, herbfield). Pressures on salt marsh (e.g., drainage, grazing, erosion) are also recorded. Methods described in Robertson et al. (2002) and Roberts et al. (2023). |
| Seagrass | Seagrass (Zostera muelleri) beds enhance primary production and nutrient cycling, stabilise sediments, elevate biodiversity, and provide nursery and feeding grounds for invertebrates and fish. Seagrass is vulnerable to muddy sediments in the water column (reducing light), sediment smothering (burial), excessive nutrients (primarily secondary impacts from macroalgal smothering), and sediment quality (e.g., low oxygenation). | Mapped based on areal extent, and percent cover recorded within each seagrass patch. Pressures on seagrass beds (e.g., sediment or macroalgae smothering, leaf discolouration) are also recorded. |
| Opportunistic macroalgae | Opportunistic macroalgae (species of Gracilaria and Ulva) are a symptom of estuary eutrophication (nutrient enrichment). At nuisance levels, these algae can form mats on the estuary surface that can adversely impact underlying sediments and fauna, other algae, fish, birds, seagrass, and salt marsh. The Opportunistic Macroalgal Blooming Tool (OMBT) is a multi-metric index that combines different measures of macroalgae and is calculated as an indicator of ecological condition. | Mapped based on areal extent. Species, percent cover, biomass and level of entrainment are recorded in each macroalgae patch to apply the OMBT (WFD-UKTAG 2014). The application of the OMBT incorporates New Zealand based improvements described in Plew et al. (2020) and Stevens et al. (2022). |
| Indicator | Unit | Very good | Good | Fair | Poor |
|---|---|---|---|---|---|
| Mapped indicators | |||||
| 200m terrestrial margin1 | % densely vegetated | ≥80 to 100 | ≥50 to 80 | ≥25 to 50 | <25 |
| Mud-elevated substrate2,3 | % available intertidal habitat area >25-100% mud | <1 | 1 to 5 | >5 to 15 | >15 |
| Macroalgae (OMBT)2,4 | Ecological Quality Rating | ≥0.8 to 1.0 | ≥0.6 to <0.8 | ≥0.4 to <0.6 | 0.0 to <0.4 |
| Seagrass (≥50% cover)1 | % decrease from baseline | <5 | ≥5 to 10 | ≥10 to 20 | ≥20 |
| Salt marsh extent (current)1 | % of intertidal area | >20 | >10 to 20 | >5 to 10 | 0 to 5 |
| Historical salt marsh extent1,5 | % historical remaining | ≥80 to 100 | ≥60 to 80 | ≥40 to 60 | <40 |
- General guidance from previous broad scale mapping and council SOE reports (e.g., Stevens & Robertson 2014).
- Ratings derived from Robertson et al. (2016).
- Mud-elevated substrate modified from Robertson et al. (2016) to apply to the intertidal area excluding salt marsh, not the total estuary area.
- OMBT = Opportunistic Macroalgal Blooming Tool (WFD-UKTAG 2014).
- Estimated from historic aerial imagery.
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Page last updated 7 April 2025
Page last updated 7 April 2025
