News article

In 2019, an Estuarine Vulnerability Assessment (EVA) was carried out by Robertson Environmental to guide the development of a new estuary monitoring programme.
To provide a representative assessment, 20 sites were included. This included the region’s larger estuaries north and south of the ring plain, as well as a number of smaller
stream mouths where tidal intrusion was marginal but may occasionally occur. Three main outputs were provided for each estuary: a habitat map, a vulnerability rating and
recommendations for future monitoring.

Overall vulnerability to the effects of sedimentation and eutrophication was determined to be ‘high’ in five estuaries, ‘moderate to high’ in another five, ‘moderate’ in nine and ‘minimal’ in one. Where the vulnerability of an estuary was ‘high’ or ‘moderate to high’, this was largely due to the effects of sedimentation rather than eutrophication. This was the case for seven of the 20 estuaries assessed, namely. Mōhakatino, Tongaporutu, Urenui, Mimitangiatua, Waitara, Pātea and Waitōtara.

Vulnerability to sedimentation was generally attributed to high sediment loads from the catchment, and the high proportion of soft mud cover in the estuary that was mapped during the condition assessment. Eutrophication was considered less of an issue. What we know in these estuaries due to them being well flushed, with no primary symptoms such as macroalgae and/or phytoplankton blooms identified during the condition assessments.

Vulnerability to eutrophication effects was ‘moderate to high’ for three estuaries, the Whenuakura, Oakura and Katikara. The latter two were the only estuaries where
symptoms of eutrophication, in the form of phytoplankton blooms, were recorded. Other estuaries, such as the Whenuakura, were considered susceptible to eutrophication
due to large areas of intertidal habitat which can support macroalgal blooms, high catchment nutrient loads, and where they were poorly flushed or restricted at the mouth.

To read more about estuaries in general and detail about the results of the survey go here (PDF, 287.5 KB).