Nitrogen is a key nutrient for plant and algae growth in estuarine and marine environments. Total nitrogen in sediments is an indicator of their trophic status and potential for algal blooms or other symptoms of excessive enrichment (e.g., low sediment oxygen).

Both of the ‘fine scale’ sites, located in relatively well-flushed parts of the estuary, were rated ‘Good’ for total nitrogen indicating a low level of sediment enrichment.

Within areas of macroalgae in the Catlins Lake and the Ōwaka Arm (see Habitats page), that indicate parts of the estuary are expressing symptoms of nutrient enrichment, additional sampling in 2023 showed sediments were highly enriched with total nitrogen - rated ‘Poor’ (results are presented in Roberts et al. 2024).

Total Organic Carbon (TOC) is a measure of the organic content of sediments, which is associated with their enrichment status. Sediments with a high TOC (>1-2%) often display symptoms that indicate excessive enrichment, including reduced sediment oxygenation.

Similar to total nitrogen, TOC levels at both of the ‘fine scale’ sites, located in unvegetated parts of the estuary, were low (rated ‘Very Good’).

In 2023, sampling within areas of macroalgae in both the Catlins Lake and the Ōwaka Arm (see Habitats page), showed sediments were highly enriched with TOC. These results, rated ‘Poor’, indicate that parts of the estuary with nuisance macroalgal growths are expressing symptoms of nutrient enrichment (results are presented in Roberts et al. 2024).

Sediment oxygenation is visually assessed by the apparent Redox Potential Discontinuity (aRPD) depth, which is where brown oxygenated surface sediments transition to less oxygenated grey/black sediments. The aRPD depth is a useful, but relatively coarse, time-integrated measure of the enrichment state of sediments. The aRPD is usually deepest in un-enriched sandy sediments, and gets closer to the sediment surface as organic matter loading increases, or as sediments become muddier, as mud can limit oxygen penetration into the sediment by filling the small gaps between individual sediment particles.

Sediment oxygenation at both ‘fine scale’ sites has been rated ‘Very Good’ or ‘Good’ in all years, except for Site B in February 2019 where a decrease in sediment oxygenation (rated ‘Fair’) was associated with a peak in mud content (~43% mud).