Moutere Inlet
Estuary Monitoring Programme
Overview
Moutere Inlet is a medium-size (764 ha), shallow, well-flushed tidal lagoon that discharges to Tasman Bay via tidal entrances at Port Motueka and Kina Peninsula (at either end of Jackett Island). The estuary is made up of one main basin and several embayments, most of which are enclosed by causeways. The estuary almost completely drains at low tide and has a residence time of less than 1 day.
It has high human use and moderately high ecological values, is a valuable nursery area for marine and freshwater fish, has large shellfish beds, and is important for birdlife.
Tasman District Council (TDC) has been monitoring Moutere Inlet as part of its State of the Environment (SOE) programme since 2006. The programme is designed to detect and understand key changes in estuaries over time, and determine catchment influences, especially those due to the input of nutrients and muddy sediments.
Catchment
The Moutere Estuary catchment is highly modified. Land use within the catchment is dominated by pasture (53%), horticulture (15%), exotic forestry (12%) and built up areas (2%), including the commercial port and marina located at Port Motueka. Native forest cover is low (2%).
Click on the legend in the lower left corner of the map to identify different land uses within the catchment.
Pasture on reclaimed land next to the estuary - the muddy area floods on spring tides
Channelised upper estuary margins
Terrestrial Margin
A densely vegetated terrestrial margin helps trap and assimilate nutrient and sediment inputs, protects the estuary from terrestrial weed invasions, provides food, shelter and habitat for a variety of species, reduces the risk of shoreline erosion, and removes carbon from the atmosphere and stores it, not only in the margin, but also in the estuary and seabed to mitigate climate change.
The land directly adjacent to the estuary has been highly modified by residential/industrial development (22%), pasture (29%) and horticulture (20%). Only 16% was densely vegetated and this was predominantly located on Jackett Island and Kina Peninsula.
The buffering vegetation on land surrounding the estuary, and associated habitat diversity, has been significantly reduced as a consequence of extensive historical drainage of wetland and salt marsh areas, which is ongoing in some areas.
Armoring of the Estuary Margin
The estuary margin is highly modified and dominated by a mix of steep-faced concrete seawalls, earth bunds and rock reinforcing. This severely restricts the area available for salt marsh growth and disrupts the natural connectivity between the land and the estuary, preventing the migration of estuarine species in response to predicted sea level rise.
Rock walls protect the reclaimed margin along most of the western side of the estuary
TDC has developed a coastal hazards map viewer which illustrates the extent of low lying coastal land in Tasman Bay and Golden Bay that may be affected by a range of sea level elevations.
The coastal hazards map shows a range of sea level rise scenarios, potential storm tide impacts and the locations of coastal protection (armouring).
Substrate
Most estuaries have a range of substrate types including rock, cobble, gravel, shell, sand and mud, and are very good at trapping sediment - 96% of the sediment entering Moutere Inlet is predicted to be retained within it.
Clean sands near the estuary entrance, April 2020
Fine (muddy) sediments in particular cause problems as they are often poorly oxygenated, nutrient rich, contribute to low water clarity, and generally support a relatively tolerant, low-diversity animal community. Heavy metal contaminants are not generally an issue in Tasman estuaries.
The presence of large areas of soft mud are likely to lead to major and detrimental ecological changes that are generally very difficult and slow to reverse. Currently the amount of mud-dominated sediment in Moutere Inlet is very high - 222ha (29%) and is the main ecological pressure on the estuary.
Widespread muds in the central basin, April 2020
Problematic Muddy Sediments
The amount of mud in the estuary has changed over time. Historically, clean sands would have been widespread throughout the estuary.
Catchment development through human history has increased fine sediment inputs, making sediments muddier. This change can be gradual, but usually occurs as pulses of deposition after heavy rain. While some sediment is re-suspended and washed from the estuary, much of it remains trapped and has resulted in widespread infilling of the upper estuary.
Large areas of soft muds are present in the upper estuary, April 2020
Most of the mud is located in the central basin, north of Wharf Road, to the southeast near Tasman and, to a lesser extent, in the sheltered embayments on the western side of the estuary. These are ideal settling areas for fine sediments (mud). Forensic methods show approximately 90% of the recent sediment deposited in the estuary is considered to be of pine forest origin, from either forest harvesting or clearance for rural residential subdivision in the decade or so prior to 2017. Other sources also contribute but in much smaller amounts.
Changes in Mud Deposition
There was a significant increase in fine sediment deposition in the central basin of the estuary from 2004 to 2013, followed by a reduction from 2013 to 2019. Much of the increase is likely due to extensive conversion of pine forest to rural residential land in various parts of the catchment in 2007-2008 to beat the government’s 2008 Climate Change (Forestry Sector) Regulations. The more recent reductions appear to reflect relatively large-scale erosion from tidal flats within the estuary.
While the estuary is ~10% muddier in 2019 than it was in 2004, the vast majority of the muddy sediment was already present in the estuary by 2004. Current land management practices are significantly improved on what they historically used to be, and while there is obviously room for improvement, if catchment inputs are well managed, the estuary has the capacity to naturally flush some fine sediment out to sea.
Eroding muds in the central basin, April 2020
Mud in a larger context
The Moutere Inlet is muddy in both a regional and national context. It is well above the threshold used in national estuary assessment criteria to indicate 'poor' conditions. The following graph compares the percent of intertidal area that is muddy for selected estuaries. The dotted red line indicates the threshold below which conditions are considered 'poor'.
In the meantime, mud can be fun. The annual Muddy Buddy Adventure Fun Run is a fund-raising community event used to raise awareness of the ecological value of the estuary. The route is entirely within the mudflat and avoids the most sensitive estuary areas. Monitoring found that impacts were relatively minor and reversible (densities of crabs and snails returned to previous levels within 6 weeks to 6 months).
Salt Marsh
Salt marsh is vegetation able to tolerate saline conditions where terrestrial plants are unable to survive and is important as it is highly productive, naturally filters and assimilates sediment and nutrients, acts as a buffer that protects against introduced grasses and weeds, and provides an important habitat for a variety of species including fish and birds.
Salt marsh is a prominent and highly valued feature within Moutere Inlet. It covers 83ha (10.8% of the estuary), and is comprised mainly of rushland (55%) and low growing salt and desiccation-tolerant herbfield species (40%).
Coastal rushland was dominated by searush (Juncus kraussii) with smaller areas of jointed wirerush (Apodasmia similis) and sedgeland (three square, Schoenoplectus pungens).
Herbfield was dominated by glasswort (Sarcocornia quinqueflora) and sea blite (Suaeda novaezelandiae). Salt marsh ribbonwood (Plagianthus divaricatus) was the dominant estuarine shrub, often forming a narrow boundary to the upper estuary margins.
Changes in salt marsh
The natural (pre-human) extent of salt marsh has not been assessed and the earliest mapped baseline data are from 1947. Widespread reductions in salt marsh had already occurred by that time, and since then there has been a further reduction of 54ha (39%). This has mainly been the result of reclamation and drainage, in particular the construction of SH60, and expansion of Motueka township, port and marina.
As much of the estuary margin has been filled-in and hardened, the ability for salt marsh to expand and migrate in response to future sea level rise is severely limited.
In this image, herbfield can be seen growing in cobbles at the top of the tide range.
Without changes in management approaches, the likely result is a progressive reduction of salt marsh habitat over time. The cost of this reduction is high. The value of ecosystem services provided by healthy salt marsh has been estimated as ~NZ$368,220 per ha per year (Costanza et al. 2014). These values include: habitat and ecological community services, food and water provisioning, filtering of contaminants, erosion control, carbon sequestration, buffering of floods and coastal storm surges, and cultural and recreational services. In virtually all cases, the cost of salt marsh loss greatly exceeds that of retaining existing salt marsh and allowing its natural expansion.
Eroding earth bund margin, April 2020
Seagrass
Seagrass (Zostera muelleri) beds are important ecologically because they enhance primary production and nutrient cycling, stabilise sediments, elevate biodiversity, and provide nursery and feeding grounds for a range of invertebrates and fish. Though tolerant of a wide range of conditions, seagrass is vulnerable to excessive nutrients, fine sediments in the water column, and sediment quality (particularly if there is a lack of oxygen and production of sulphide).
Seagrass beds are currently restricted largely to the central basin near the Kina entrance. Seagrass appears unable to establish on the perched intertidal flats of the estuary, most likely due to a combination of desiccation (long periods of exposure between tides), excessive muddiness, and poor water clarity.
As a percentage of the estuary, seagrass in the Moutere is similar to other estuaries in the Tasman region, (e.g. Waimea, Motupipi, Ruataniwha and Motueka), but is significantly less than the similarly sized, but less muddy, Nelson Haven (135Ha, 15%).
Seagrass Change
The natural (pre-human) extent of seagrass has not been assessed and the earliest mapped baseline data are from 2004. These show seagrass beds in the estuary in similar locations to where they were recorded in 2019. The larger area recorded in 2019 is attributed to improved mapping.
Seagrass is predominantly found in well-flushed channels near the low tide mark
Although there are no data from prior to 2004, 1947 aerial photos indicate seagrass was historically present in many of the same locations that it is currently found in. Therefore, despite major modifications since 1947 (e.g. salt marsh clearance and road construction), seagrass appears to have been relatively stable. Swipe to compare seagrass beds in 1947 and 2019.
Use the slider to compare sea grass meadows mapped in 2019 with aerial imagery from 1947.
Macroalgae
Excessive nutrient inputs can contribute to the rapid growth of seaweed within estuaries. Blooms of nuisance macroalgae are a symptom of nutrient enrichment (eutrophication). These blooms can deprive seagrass beds of light causing their decline, while decaying macroalgae can accumulate subtidally and on shorelines causing oxygen depletion, nuisance odours, and creating adverse conditions for other plants and animals.
Two types of macroalgae are commonly present: the red seaweed Gracilaria that is able to grow within muddy sediments (shown in the large image) and sea lettuce (Ulva) which is a green seaweed that often blooms seasonally between October and April (shown below).
Both these species grow in Moutere Inlet, but there are very few areas with high densities, most probably a result of tidal flats being perched high in the tidal range and being uncovered by water for long periods of time, making it difficult for algae to survive.
Sea lettuce was relatively common in low densities along channel edges near the entrances, while Gracilaria had established smothering beds in several of the western embayments, in muddy areas of the central basin, and especially in the Wharf Road embayment.
In total, 31ha (4%) of the estuary had a macroalgal cover >50% in 2019. Although relatively small in extent, significant adverse ecological effects were being caused by Gracilaria in these parts of the estuary, with sediments being very muddy, anoxic and rich with sulphides (toxic to many marine animals).
Decaying Gracilaria in soft muds in the central basin, April 2020
Changes in Macroalgae
Nuisance macroalgae was much more widespread in 2013 (79ha, 10% of the estuary), particularly within the central basin where recently deposited muds appeared to have created favourable conditions for Gracilaria to establish. These beds had a high biomass, sediments were anoxic and sulphide-rich.
The largest decrease from 2013 to 2019 has been in the central basin. In this area sediment conditions appear to have been degraded (i.e. become anoxic) by macroalgal decay to a point where remaining macroalgae can no longer survive. Local flood events during early 2018 (Cyclones Gita and Fehi) may also have directly scoured macroalgae or aided in flushing already decaying macroalgae out of the estuary.
Extensive nuisance macroalgae near Jackett Island, April 2020
High Enrichment
High Enrichment Conditions (HECs) are defined as the combined presence of high (>50%) macroalgal cover, high sediment mud content >50%, poor sediment oxygenation and high organic content. Persistent HEC areas were present over 31ha (4%) of the estuary in 2019.
This result is of genuine concern given that, outside of very localised patches, HECs should not be present in well flushed estuaries like Moutere Inlet.
Decaying Gracilaria in anoxic muds
Monitoring Summary
The 2019 survey mapped the dominant substrate and vegetation features present including seagrass, salt marsh and macroalgae based on the New Zealand’s National Estuary Monitoring Protocol (NEMP). Changes compared to the most recent 2013 survey are shown in the summary table.
Overall, Moutere Inlet retains many areas of very significant ecological value despite extensive historical habitat modification, particularly around the estuary margin.
Seagrass was scarce, whereas salt marsh was a prominent feature, although roads, seawalls and shoreline armouring restrict the natural connectivity between the land and estuary and restrict opportunities for the migration of estuarine plant species in response to predicted sea level rise.
Mud-dominated sediments were extensive (both regionally and nationally), and persistent nuisance macroalgal beds were causing adverse ecological effects in localised areas. However, improvements in the extent of mud, nuisance macroalgae and enrichment conditions show some improvement in ecological quality from 2013 to 2019.
Without reductions in current nutrient and sediment loads the estuary is likely to remain in a similar state to its present condition, and salt marsh losses are likely to increase in response to sea level rise.
Where to from here...
There are lots of opportunities for the restoration of habitats degraded or lost in the past. Groups such as Keep Motueka Beautiful are doing some great work in the embayment north of Wharf Road.
On-going monitoring the ecological condition of estuarine habitats is critical to their management. The results provide a valuable basis for establishing a benchmark of estuarine health in order to better understand human influences, and against which future comparisons can be made.
Maps and Data
Associated maps and data can be explored and downloaded through the following links.
