2019/20 Annual report summary
Between January 2019 and January 2020, the Pāuatahanui Arm 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 Arm 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 net 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 arms 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 trace metal contaminants and little evidence of nutrient enrichment (Forrest, Stevens & Rabel 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 Arm 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 Arm, several previously common species intolerant of mud were no longer present in 2020 (Forrest, Stevens & Rabel 2020 - Intertidal monitoring). These results are inconsistent with the findings of the most recent volunteer cockle survey coordinated by the Guardians of Pāuatahanui Arm 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.
