Threats to the Condition of Tasman’s Estuaries

Estuaries are a sink for contaminants that run off the land. If we look after the land well the estuary with be in good health. 

Estuaries such as Whanganui Inlet are generally in very good condition, probably similar to that in pre-human times. Over the last century most of our estuaries with greatly modified catchments and intensive land use near to or around them have become highly modified, particularly from fine sediment discharges and habitat loss. In some cases, these activities have compromised the health of the ecosystem. Additionally pests such as stoats and rats, as well as cats and dogs have reduced the number of many bird species.

Sedimentation

Sediment discharges from subdivision developments, farming, forestry and roading activities is an on-going issue. The area of soft mud in some estuaries, particularly Waimea and Upper part of Motupipi was elevated (a “poor” rating). It was also elevated in parts of the Ruataniwha, Moutere, and Motueka Delta estuaries and rated “fair”. Low sediment oxygen and lower biodiversity was also apparent in these areas. Sediments can also carry toxins such as heavy metals. Zinc is an example of a heavy metal which is elevated in sediment near urban or industrial areas.

Habitat Loss

Threat to saltmarsh habitat due to reclamation has reduced in the last two decades. There has been a significant loss of saltmarsh in all the estuaries but particularly the Waimea and Motueka Delta. The rate of loss is much lower than two decades or more ago. Seagrass areas are low in most estuaries except Wanganui estuary. The area of seagrass in the Waimea has declined.

Disease

Disease-causing organisms discharged as run-off from intensive agricultural or urban land or sewage discharges can affect the safety of swimming or in-water recreation as well as shellfish food safety. Water quality monitoring shows the Motupipi and Ruataniwha estuaries to have the highest concentrations of enterococci, an indicator disease-causing organism. Moutere and Waimea estuaries are only occasionally unsafe for swimming. All these monitored estuaries are unlikely to regularly contain shellfish that are safe to eat.

Nutrients

Generally less of an issue in Tasman’s estuaries. However parts of the Moutere and Motupipi are nutrient enriched but nuisance macro-algal cover is only occasionally a nuisance issue.

Toxins

Generally less of an issue in Tasman’s estuaries. Higher concentrations of toxins in estuarine sediment are only found in very isolated areas of Waimea estuary associated with the urbanised streams.

Discharges to Estuaries from Sediments

Sediments around slipways used for cleaning of boats in the coastal area are often contaminated with anti-fouling paints containing heavy metals and other chemicals such as try-butyl tin. Further information and the report describing heavy metal contamination of sediments slipways in estuaries in Tasman is available: