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Sedimentation rates remain elevated in Te Awarua-o-Porirua Harbour. Between 2022 and 2023, high net accretion of sediment with fine mud was recorded at Onepoto intertidal sites O2 and O3, and at subtidal site OS6. Accretion recorded at the Pāuatahanui intertidal sites was attributable to the movement of marine sands and was of no ecological concern, but subtidal sites PS2, PS3 and PS4 all showed increases in mud elevated sediments (Stevens et al. 2023). High accretion is commonly associated with high mud contents (>25% mud) and poor sediment oxygenation (<10mm) and is likely causing adverse ecological effects in the Harbour. The longer term 5-year and 10-year mean annual sedimentation rate results show ongoing high deposition in the Pāuatahanui and Onepoto subtidal zones, and moderate to high increases in the intertidal zones. However, following the widespread intertidal deposition of soft muds in Pāuatahanui Inlet following a flood event in December 2020, there has been an improvement in intertidal sediment condition, and a reduction in the spatial extent of mud-elevated sediments (>25% mud). Nonetheless, the monitored changes over the past decade, indicate an overall decline in estuary quality with a general trend of increasing or elevated mud content, and high rates of deposition, indicating excessive sediment inputs to the Harbour.
Our annual intertidal and subtidal sediment rate survey (18 sites) and an intertidal fine scale environmental health survey (four sites) were both conducted in the Te Awarua-o-Porirua Harbour in January 2022.
Sediment accrual rates in Te Awarua-o-Porirua Harbour remain elevated, particularly in the Pāuatahanui Inlet. Between December 2020 and January 2022 there was high accretion associated with a ‘poor’ condition rating at two Onepoto subtidal sites, and two intertidal and three subtidal sites in Pāuatahanui. Accretion is commonly associated with elevated mud content and poor sediment oxygenation, which makes sediment unsuitable for many sediment dwelling organisms. There has been some recovery from the widespread intertidal deposition of soft muds recorded in January 2020 in Pāuatahanui Inlet near Kakaho and Ration Point, but increased deposition was evident at Horokiri, along with a trend of increasing intertidal sediment mud content. Greater sedimentation is still occurring at subtidal sites relative to intertidal sites (Stevens et al. 2022).
Despite intertidal sedimentation rates exceeding the 2 mm/year national guideline value at three of the four state of the environment (SoE) monitoring sites, almost all sediment quality indicators except sediment oxygenation and mud content were consistent with ‘good’ or ‘very good’ condition (Forrest et al. 2022). Mud and sediment oxygenation showed an improvement since 2020 despite a weak overall trend of increasing mud content at Onepoto B and Pāuatahanui B in the inner Harbour.
Concentrations of trace metal contaminants remained low and semi-volatile organic compound screening for contaminants such as the pesticide DDT returned nondetectable results. Overall, fine scale monitoring shows that Porirua SoE sites remain in a relatively healthy condition and are in a better state than SoE sites monitored in other estuaries regionally in terms of mud, trophic state and macrofaunal indicators.
Reasons for the apparent improvement of most environmental health indicators in Te Awarua-o-Porirua Harbour in 2022, including the macrofaunal community, are unclear and there appear to be drivers of spatial and temporal change that are not reflected in any of the sediment constituents measured. It is likely that these temporal changes reflect a combination of drivers including local natural variability and catchment wide external environmental factors. An upcoming review of the SoE monitoring programme will inform selection of future monitoring sites and variables to ensure improved quantification of ecological condition in estuary locations that are most vulnerable.
As part of ongoing monitoring in Te Awarua-o-Porirua Harbour, annual sediment plate measurements were recorded in December 2020. While there was some intertidal recovery from the widespread deposition of soft muds recorded in January 2020, there was also degradation observed in new areas. Of particular concern was a thick slurry of deposited fine sediment covering previously sandy intertidal flats at Kakaho (Site P7) with 21.5 mm of intertidal deposition over the previous eleven months, the largest mean annual increase recorded for that site since monitoring began in 2007 (Roberts et al. 2021). Mud content increased from an already very high 63.5% to 67.3% and sediment oxygenation was ‘poor’. Decreases in the spatial extent of intertidal mud near Horokiri and Pāuatahanui streams coincided with significant increases in subtidal deposition - Kakaho (Site PS1; 41 mm), Horokiri (Site PS2; 30 mm) and Duck Creek (Site PS3; 13 mm). Mud mobilised from the intertidal zone was likely deposited in nearby subtidal areas, which is consistent with trends in the estuary-wide bathymetric surveys of predominantly subtidal areas and with NIWA’s estuary sediment load estimator.
Results from our four-yearly subtidal sediment survey of Porirua Harbour (samples collected in November 2020) showed that bottom sediments at all sampling sites were mostly mud (66-97% mud), except for the site located off Camborne, which was mostly fine sand. Organic matter content was very similar across sites, ranging from 4.6-8.0% (Cummings et al. 2022). Any contaminants that may enter the Harbour via rivers, stormwater, or road runoff, are likely to collect in the deeper muddy areas in the Onepoto Inlet and at the southern end of the Pāuatahanui Inlet. None of the sites exceeded guideline ‘safe’ concentrations for the heavy metals arsenic, cadmium, chromium, nickel or mercury; although in the Onepoto Inlet guidelines were exceeded for lead, zinc and copper, while mercury concentrations were close to exceedance. Overall, the Pāuatahanui Inlet was found to be less contaminated than the Onepoto Inlet, likely due to its distance from the Porirua City Centre.
Total organic carbon (TOC), sediment phosphorus concentrations and sediment nitrogen concentrations are all measures of nutrient content and are used as indicators of water or sediment quality - high values may suggest that sediments are anoxic or unsuitable for most living organisms. TOC concentrations varied around the Harbour, with the highest levels recorded in the Onepoto Inlet. The site near Camborne had the lowest percentage TOC and was rated as ‘good’, while Browns Bay, Duck Creek and one site in the Onepoto Inlet were rated ‘fair’, and the site closest to Porirua City rated ‘poor’. Sediment phosphorus levels were ‘poor’ in the Onepoto Inlet and at Browns Bay, but ‘fair’ at Duck Creek and Camborne, while sediment nitrogen levels were ‘fair’ at all five sites. This indicated that the Onepoto Inlet is receiving high nutrient inputs, likely from the Porirua Stream and stormwater outlets around the Harbour edge.
Again the Onepoto Inlet scored lower than the more rural Pāuatahanui Inlet, fewer species of sediment dwelling invertebrates were found closer to Porirua City. On average, 16 different species were found within each sample collected, with worms and bivalve shellfish most common (Cummings et al. 2022). Each of the sites had distinct communities but there were double the number of animals in the Onepoto Inlet compared to the Pāuatahanui Inlet (average of 268 and 145 individuals per sample respectively). Although the two sites in the Onepoto Inlet were in poorer health than those in the Pāuatahanui Inlet as indicated by the concentrations of contaminants and nutrients measured and the very high proportion of muddy sediments, the benthic communities at all sites contained a mix of organisms, from species sensitive to mud and organic enrichment to mud-loving species.
Our long-term monitoring shows that pressures on the Harbour do not appear to be localised in a particular area of the estuary. Encouragingly, the proportion of fine sediments has decreased over time at sites near Porirua City as well as close to Camborne. Average concentrations of lead have also declined at these sites, as have copper in sediments at some sites within both sections of the Harbour. While there is difficulty with statistically comparing changes over time with the small dataset available, these slight improvements in the subtidal are encouraging.
Between January 2019 and January 2020, the Pāuatahanui Inlet experienced a doubling of the amount of mud settling on the estuary floor and the largest annual increase of intertidal sediment deposition since monitoring began in 2008. Intertidal sites in the Onepoto Inlet also experienced an increase in sediment deposition but to a lesser extent. Average annual sedimentation rate across all intertidal sites and all monitoring years shows a sediment increase of 1.2 and 3.2 mm per year respectively. Increased deposition appears to be associated with occasional inputs of disturbed sediment from the catchment above the Estuary (Stevens & Forrest 2020a - Sediment plate monitoring). Most subtidal sites within the Harbour experienced an increase in sediment deposition, as sediments are washed into deeper areas where they settle. These results are consistent with those of the bathymetry survey carried out in June 2019 (Waller & Stubbing 2019 - Bathymetry), which indicated moderate to high sediment deposition rates in both inlets of the Harbour, particularly in Pāuatahanui, warning us of the increased likelihood of significant environmental damage.
Sediment quality is generally related to mud content as muddy sediments have less oxygen, so don’t support healthy communities, and contaminants easily attach to the greater surface area provided by fine particles. Despite the increase in sediment mud content, intertidal sediment quality assessed in 2020 was mostly good with low levels of metal contaminants and little evidence of nutrient enrichment (Forrest et al. 2020 - Intertidal monitoring). Sediments were found to be poorly oxygenated, largely due to the high mud content, which excludes species that aerate the sediments and fills air spaces that would usually be present between coarser sand particles.
The habitat survey, undertaken every five years, not only maps the spatial extent of mud but also macroalgae, seagrass, and salt marsh (Stevens & Forrest 2020b - habitat mapping). During the January 2020 survey nuisance seaweeds were uncommon, however, over the last year there has been an apparent ‘bloom’ of a green mat-forming species near the two sites closest to the Paremata Bridge. Despite massive historical losses of seagrass (Zostera muelleri), densities have changed little in extent since 2008. In contrast, a 43% decline in salt marsh extent between 2013 and 2020 was recorded, with the decline primarily located in the eastern Pāuatahanui Inlet where this habitat is artificially drained.
Looking at a finer scale, the invertebrate animals living within the sediments are experiencing a gradual decline in diversity and abundance, which again appears to be partially due to increased sediment mud content. At the eastern end of the Pāuatahanui Inlet, several previously common species intolerant of mud were no longer present in 2020 (Forrest et al. 2020 - Intertidal monitoring). These results are inconsistent with the findings of the most recent volunteer cockle survey coordinated by the Guardians of Pāuatahanui Inlet which found that cockle (Austrovenus stutchburyi) counts increased by 40.9% between 2016 and 2019, with densities being the highest recorded since 1992 (Michael & Lyon 2020 - Cockle report). In 2019, population size estimates were the highest since 1976, while the percentage of juvenile cockles declined. The increase in terrestrial sediments considered harmful to cockles do not appear to have affected the overall intertidal cockle population, although the more sensitive juvenile cockles may have been impacted. Results suggest that other influences may be causing shifts in invertebrate communities and might be worth investigating in future studies.
Overall, the health of the Harbour is in gradual decline, largely due to the increased pressure of sedimentation. Our monitoring reveals a long-term harbour-wide increase in the extent of mud-dominated sediments indicating that targeted investigations and remedial action is urgently required. Whaitua objectives were set in 2019 (Te Awarua-o-Porirua Whaitua Committee 2019) to address the most pertinent problems facing Te Awarua-o-Porirua Harbour and implementation is ongoing through numerous community projects.