Whanganui Inlet

Whanganui Inlet has largely avoided major human impacts due to much of the catchment being protected. Approximately 90% of the catchment is native forest/scrub, with just 2% exotic forest, 2% high producing grassland, and 6% low producing grassland. Consequently, the estuary is perceived to be near pristine.

Click on the legend in the lower left corner of the map to identify different land uses within the catchment.

The largely unmodified catchment means the risk of direct human stressors on the estuary is low. The largest stressor is expected to be mud inputs from intensive land use or forest harvesting if run-off is poorly managed. Climate change related increases in storm frequency or intensity are expected to exacerbate this risk in future.

Seagrass beds are a significant feature of Whanganui Inlet. Seagrass grows in soft sediments in most New Zealand estuaries and provides enhanced primary production and nutrient cycling, stabilisation of sediments, increased biodiversity, sequestration of carbon, and nursery and feeding grounds for a range of invertebrates and fish. The photo shows a patch of dense and healthy seagrass supporting cockles and marine snails.

Although tolerant of a wide range of conditions, seagrass is vulnerable to fine sediments in the water column (reducing light), sediment smothering (burial), and impacts related to excessive nutrients (e.g., algal blooms) leading to degraded sediment quality (e.g., increased organic enrichment and depleted oxygen levels from decaying algae).

Any decline in seagrass extent is of concern due to the associated loss of the important ecosystem services provided. If there are no obvious direct impacts linked to seagrass decline (i.e., changes in local catchment land use), then other drivers of change external to the catchments of the estuary (e.g., marine heat waves, marine sediment or nutrient inputs) may be present. 

In March 2024, seagrass in the estuary was mapped by Tasman District Council as part of their regional state of the environment monitoring programme (such mapping was also carried out in 2013, 2016, and 2021 - see next sections).

Seagrass was present across 165ha (8.5% of the 1954ha intertidal area), with high to complete (50-100%) cover over 33.1ha (1.7%) of the intertidal flats. Most seagrass (88%) was growing in sand substrate (<10% mud content), with a further 11% in muddy sand (10-25% mud content).

Click on the map to the left to explore the 2024 seagrass extent in the estuary. Click the lower left button to show a legend of seagrass percent cover.

The 2024 results reveal recent seagrass losses have continued at an alarming rate.

Scroll down to use a slide bar to interactively compare changes in seagrass cover between 2013 and 2024. More detail on changes between survey years is presented below the slide bar. 

To assess changes over time, seagrass cover has been mapped from aerial photography flown between 1945-1948, and compared to ground-truthed mapping of the estuary undertaken in 1990, 2013, 2016, 2021, and 2024.

The results are summarised below, and in the following maps (click on each title to show the relevant map), with temporal changes assessed based on the following rating table:

Baseline seagrass cover in 1948 was digitised from historic black and white aerial imagery, with only clearly obvious seagrass beds (e.g., >50% cover) able to be mapped. Results showed seagrass covered 902ha (46.1% of the intertidal area), with near-continuous beds covering most of the eastern arm. In the western arm, where sediments appear to be more mobile, seagrass was present in smaller beds primarily on the intertidal flats near the south-eastern shoreline. 

The first comprehensive ground-truthed mapping of the estuary was reported by Davidson (1990). Seagrass beds with >50% cover were present over 814ha (41.7% of the intertidal area), an 87ha (9.7%) reduction since 1948. The losses were primarily in the north of the east arm and, to a greater extent, in the west arm along the south-eastern shoreline. The percentage decrease in seagrass between 1948 and 1990 was rated as "Good".

2013 results showed seagrass beds with >50% cover were present over 714ha (36.5% of the intertidal area), a 101ha (12.4%) reduction since 1990, and a 188ha (20.8%) reduction since 1948. Most losses were in the western arm and lower eastern arm. The eastern arm also had widespread dieback of seagrass within existing beds, with large areas changing from complete (>90%) cover to dense (70-90%) cover. The percentage decrease in seagrass over the 23 years between 1990 and 2013 was rated as "Fair".

2016 results showed seagrass beds with >50% cover were present over 329ha (16.8% of the intertidal area), with a very rapid and extensive loss of 385ha (54%) over the 3 years since 2013, and a 573ha (64%) reduction since the 1948 baseline. The vast majority of the losses were from the eastern arm. The decrease in seagrass extent over the 3 years between 2013 and 2016 was rated as "Poor".

2021 results showed seagrass beds with >50% cover were present over 183ha (9.3% of the intertidal area). From 2016 to 2021, there was a loss of 146ha of high cover (>50%) seagrass, a 44% reduction over the 5-year period, and a 719ha (80%) reduction compared to the 1948 baseline. Most of the losses occurred in the eastern arm where dead or dying seagrass fronds, or rotting root masses, were the only remaining evidence of previously extensive seagrass beds. The decrease in seagrass extent over the 5 years between 2016 and 2021 was rated as "Poor".

2024 results showed seagrass beds with >50% cover present over 31.2ha (1.6% of the intertidal area). There was a large decline in extent between 2021 to 2024, with the loss of 151ha of high cover (>50%) seagrass, an 83% reduction over the 3-year period, rated as "Poor".

The vast majority of the losses were in the northeast, with most remaining seagrass beds now fragmented and small in size. In some areas where extensive seagrass beds have been lost, there has been a visually evident decrease in the extent of muddy sediment, with soft muds eroding to re-expose previously sand-dominated substrate.

Since 1948, 870ha of >50% cover seagrass has been lost, (with just 3% of the 1948 cover remaining. Most of the losses (683ha, 78%) occurred in the 11 years between 2013 and 2024. This likely represents one of the largest recent losses of intertidal seagrass recorded in New Zealand.

In a regional context, there is only 21.6ha of >50% seagrass cover in Waimea Inlet, 3.1ha in Moutere Inlet, and 14.6ha in Ruataniwha Inlet. The 683ha loss in Whanganui Inlet between 2013 and 2024 is therefore more than 17 times the combined area of seagrass present in all the other large estuaries in the Tasman region.

Healthy seagrass beds in the estuary had dark green and luxuriant growth, in contrast to unhealthy beds which were stunted with a sparse cover of brown fronds, or had completely died leaving only residual root masses or raised mounds where sediments previously trapped by seagrass roots remained (see photo to the left).

The images below show very large dead seagrass beds in March 2021, and highlight raised beds where where fine muddy sediment was previously trapped by healthy seagrass. Erosion of unstabilised sediment from within parts of the raised beds is becoming evident.

Early signs of seagrass dieback were present in December 2015, with the browning of seagrass leaves indicating plant stress, and extensive dead fronds being washed up on channel margins.

Further information on the seagrass changes is contained in associated reports, maps and data which can be downloaded through the following links.