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Sedimentation in Te Awa Kairangi (Hutt River) estuary has been variable since monitoring began, with large deposition and erosion events recorded between 2011 and 2023 (Stevens 2023). Over the past 10 years there has been an overall trend of moderate deposition, with net erosion recorded over the past five years. The sediment mud content has also been variable but has been trending downwards since 2019, while the moderately shallow and variable aRPD depth has been improving at the same time. This tentatively suggests an improvement in sediment condition, although because of the variable sediment deposition and erosion, only tolerant species of infauna are likely to be able to survive on the intertidal flats. It is also noted that while recent years indicate a period of erosion and an improvement in sediment condition, most fine sediment from catchment sources is likely to deposit in subtidal basin areas which are not monitored.
Mean sedimentation rates in Te Awa Kairangi (Hutt River) estuary over the past five- and 10-years corresponded to a condition rating of ‘poor’. The long-term calculation showed sedimentation of 2.6 mm/year, while that of the five-year mean was 3.5 mm/year. Sediment oxygenation reduced from ‘good’ to ‘fair’ since the previous year, while mean sediment mud content remained ‘fair’ but increased from 12.3% to 17.4% (Stevens 2022). These results indicate the intertidal estuary flats remain under pressure from sediment deposition. They reinforce previous recommendations to assess and manage fine sediment inputs to the estuary, noting that most fine sediment is likely to deposit in subtidal basin areas which are not currently monitored.
Mean sedimentation rates over the past five and 10 years corresponded to a condition rating of ‘poor’. The long-term calculation showed sedimentation of 3.1 mm/year, while the five-year mean increased to 9.3 mm/year due to consistent sediment accrual since 2016. Sediment oxygenation was ‘good’ and mean sediment mud content was ‘fair’ (12.3%), almost the lowest value recorded during the monitoring programme (Roberts 2021).
The sedimentation rate over the past 10 years shows an overall trend of deposition, which has increased over the last five years. Most recent sediment accrual is sand dominated with a relatively low mud content, comparable to previous years. Sediment movement and deposition is variable due primarily to the influence of stream inputs and flows but also due to dredging in the lower Te Awa Kairangi (Hutt River).
Some of the ways that contaminants may enter our harbours is via point source discharges such as stormwater, river systems, or in water runoff during rainfall events. In Te Whanganui-a-Tara (Wellington) Harbour our monitoring sites are strategically placed in areas of high activity such as operational quays, and near rivers that discharge large volumes of potentially sediment and nutrient laden water into our coastal environment. We also have monitoring sites towards the centre of the Harbour, which act as control or less impacted sites as they are located in deeper water away from major sources of contaminants.
The November 2020 sediment survey of Te Whanganui-a-Tara Harbour showed that bottom sediments were very muddy (69-96% mud) at 14 of the 15 subtidal sites sampled. Only southern Evans Bay was found to be mostly sandy. Most heavy metals measured (arsenic, cadmium, chromium, nickel) and total PAHs were below safe guidelines at all sites, while the site near the entrance of the Te Awa Kairangi (Hutt River) as well as the one near Matiu (Somes) Island did not exceed guidelines for any of the chemical contaminants measured. Lead and mercury, two of the most toxic heavy metals, concerningly exceeded guidelines at all other sites. Zinc and copper were also above guideline concentrations in eastern Evans Bay, and in and around Lambton Basin, while sites near Aotea Quay were approaching exceedance concentrations for copper. Should these sediments be disturbed, toxic contaminants may be released from the mud, possibly killing marine life. The benthic communities at each site were generally diverse with reasonable abundances of bivalves, polychaete worms and crustaceans, mixtures of functional types (e.g., suspension feeders, deposit feeders), and an assortment of disturbance tolerant and sensitive species. This shows that a healthy infaunal community is managing to persist despite pollution and sedimentation pressures. Southern Evans Bay was again clearly different with an abundance of seafloor animals (average 38 taxa and 315 individuals per sample) as opposed to other areas of the Harbour which hosted a smaller variety of species and fewer animals (between 14 and 22 taxa and 40-124 individuals per sample). The Hutt River site had the lowest diversity and abundance.
We do not yet have sufficient data to accurately identify trends over time; however, it is apparent that contamination within Wellington Harbour sediments remains of high concern.
The sedimentation rate in the Te Awa Kairangi (Hutt River) Estuary over the past 10 years shows an overall trend of deposition and a relatively consistent elevated mud content. There has been an overall mean sedimentation rate of +1.9mm/year across the 10 years of monitoring, with a rolling mean over the past five years of 7.4mm/year, a condition rating of ‘poor’ (Stevens 2020).
Sediment oxygenation is ‘fair’, mostly as a result of bioturbation (crabs, cockles and worms in surface sediments creating voids that allow air and water to transfer oxygen to underlying sediments). The estuary flats remain under pressure from sediment impacts related to poor water clarity and the muddy intertidal substrates support mostly mud tolerant species.
A ‘moderate’ macroalgal Ecological Quality Rating reflects the widespread presence but generally low biomass and absence of entrainment of intertidal macroalgae in the estuary, with growths not causing significantly degraded intertidal sediment conditions. However, the artificial rock walls that replace natural intertidal estuarine habitat tend to result in an overestimate of habitat health. Regular flushing of the estuary is also likely to remove macroalgae from intertidal areas and limit the development of rotting macroalgae and poorly oxygenated sediments to very localised areas on intertidal flats. Indeed, there is ongoing evidence of significant subtidal impacts from excessive nutrient-driven macroalgae growth. The consistent widespread cover of opportunistic green macroalgae throughout the intertidal estuary strongly suggests that elevated catchment nutrient inputs from water column, sediment and groundwater sources are driving the observed growths (Stevens & Forrest 2020).