Wriggle coastalmanagement Porirua Harbour Intertidal Fine Scale Monitoring 2009/10 Prepared for Greater Wellington Regional Council July 2010 Harvesting shellfish near the entrance to the Onepoto Arm, Porirua Harbour. Porirua Harbour Intertidal Fine Scale Monitoring 2009/10 Prepared for Greater Wellington Regional Council By Barry Robertson and Leigh Stevens Wriggle Limited, PO Box 1622, Nelson 7040, Ph 0275 417 935, 021 417 936, www.wriggle.co.nz Wriggle coastalmanagement iii Contents Porirua Harbour - Executive Summary . vii 1. Introduction . 1 2. Methods . 3 3. Results and Discussion . 7 4. Conclusions . 19 5. Monitoring . 20 6. Management . 20 7. Acknowledgements ������������������������������������������������������������������������������������������������������������������������������������������������������������������������ 21 8. References ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 21 Appendix 1. Details on Analytical Methods ��������������������������������������������������������������������������������������������������������������������������������22 Appendix 2. 2010 Detailed Results . 22 Appendix 3. Infauna Characteristics . 32 List of Figures Figure 1. Location of sedimentation and fine scale monitoring sites in Porirua Harbour. 4 Figure 2. Grain size, Porirua Harbour, Jan 2008, 2009 and 2010. 8 Figure 3. Percent mud content at fine scale monitoring sites, Greater Wellington and Southland estuaries. .8 Figure 4. Porirua Harbour sedimentation rate (plate data 2007-2010, and hydrographic surveys 1974-2009). 9 Figure 5. Mean number of infauna species, Porirua Harbour compared with other NZ estuaries. 10 Figure 6. Mean total abundance of macrofauna, Porirua Harbour compared with other NZ estuaries. 10 Figure 7. NMDS plot showing the relationship in macro-invertebrate community composition . 11 Figure 8. Mud tolerance macro-invertebrate rating. 11 Figure 9. Porirua Harbour 2008-2010 - mud sensitivity of macro-invertebrates at four sites . 13 Figure 10. RPD depth (mean and range), Porirua Harbour, Jan 2008, 2009 and 2010. 14 Figure 11. Sediment profiles, depths of RPD and predicted benthic community type. �������������������������������������������� 14 Figure 12. Total organic carbon (mean and range) at 4 intertidal sites, Jan 2008, 2009 and 2010. 15 Figure 13. Percentage macroalgal cover at 4 intertidal sites, Jan 2008, 2009 and 2010. �������������������������������������������� 15 Figure 14. Total phosphorus (mean and range) at 4 intertidal sites, Jan 2008, 2009 and 2010. 15 Figure 15. Total nitrogen (mean and range) at 4 intertidal sites, Jan 2008, 2009 and 2010. 15 Figure 16. Benthic invertebrate organic enrichment rating, Porirua Harbour, Jan 2008, 2009 and 2010. 16 Figure 17. Porirua Harbour 2008-2010 - macroinvertebrate organic enrichment sensitivity . 17 Figure 18. Mean abundance of epifauna per quadrat - Porirua Harbour and other NZ estuaries. 18 Figure 19. Mean number of epifauna species per quadrat - Porirua Harbour and other NZ estuaries . 18 Figure 20. Total recoverable metals (mean and range) at 4 intertidal sites, Porirua Harbour . 19 List of Tables Table 1. Summary of the major issues affecting most NZ estuaries. 2 Table 2. Summary of the broad and fine scale EMP indicators. 2 Table 3. Physical, chemical and macrofauna results (means) for Porirua Harbour (2008-2010). 7 Wriggle coastalmanagement v PORIRUA HARBOUR - EXECUTIVE SUMMARY This report summarises the results of the first three years of fine scale monitoring of four intertidal sites Porirua Harbour Estuary (2008-2010) within Porirua Harbour, an 807ha tidal lagoon estuary. It is one of the key estuaries in Greater Wellington Regional Council’s (GWRC’s) long-term coastal monitoring programme. An outline of the proc- ess used for estuary monitoring and management in GWRC is outlined in the margin flow diagram, and the following table summarises fine scale monitoring results, condition ratings, overall estuary condition, and Vulnerability Assessment monitoring and management recommendations. Identifies issues and recommends monitoring and management. Fine Scale Monitoring Results Completed in 2007 (Robertson and Stevens 2007) • Sediment Oxygenation: Redox Potential Discontinuity was 1-2cm deep indicating poor oxygenation. • The benthic invertebrate community condition rating indicated a slightly polluted or “good” condition. • The indicator of organic enrichment (Total Organic Carbon) was at low concentrations in all years. Porirua Estuary Issues • Nutrient enrichment indicators (total nitrogen and phosphorus) were at low-moderate concentrations in Moderate eutrophication all years. Excessive sedimentation • Sediment plates indicate low-moderate sedimentation at key sites since 2008. Habitat Loss (saltmarsh, dune and • Sand dominated the sediments, but mud contents were relatively high. terrestrial margin) • Heavy metals and DDT were well below the ANZECC (2000) ISQG-Low trigger values (i.e. low toxicity). • Macroalgal cover was elevated at most sites. Baseline est. Fair Good-Very Good Not measured Key To Ratings CONDITION RATINGS High/Poor Good Very good Monitoring Porirua (Onepoto) Arm Pauatahanui Arm Site A Railway Site B Polytech Site A Boatshed Site B Upper Broad Scale Fine Scale Mapping Monitoring 2008 2009 2010 2008 2009 2010 2008 2009 2010 2008 2009 2010 Sediment type Grain size, RPD, Saltmarsh Organic Content Seagrass Nutrients, Metals, Sedimentation Rate Low Moderate Very Low Moderate Macroalgae Invertebrates, Land margin Macroalgae, Invertebrates (Mud Tolerance) Sedimentation, RPD Profile (Sediment Oxygenation) 5 -10 yearly 4yr Baseline then First undertaken 5 yearly TOC (Total Organic Carbon) in 2008. Baseline yet to be completed. Total Nitrogen Next survey 2011. Total Phosphorus Invertebrates (Organic Enrichment) Metals (Cd, Cu, Cr, Ni, Pb, Zn) Condition Ratings Area soft mud, Area saltmarsh, Area DDT seagrass, Area terrestrial margin, RPD depth, Benthic Community, Organic content, N and P, Toxicity, ESTUARY CONDITION and ISSUES Sedimentation rate. Other Information Overall, the first three years of monitoring show the dominant intertidal habitat (i.e. unvegetated tidal-flat) Previous reports, Observations, in the Porirua Harbour is generally in good to moderate condition. Despite the good to moderate condition, Expert opinion the estuary shows signs of increasing mud contents, moderate sedimentation rates, moderate macroalgal growth, declining sediment oxygenation and a benthic invertebrate community dominated by sensitive spe- cies. This suggests that the estuary is on the borderline of a shift towards excessive muddiness and nutrient enrichment or eutrophication. ESTUARY CONDITION Moderate Eutrophication Excessive Sedimentation RECOMMENDED MONITORING AND MANAGEMENT Low Toxicity Habitat Degraded (saltmarsh, ter- Baseline conditions have now nearly been established, with one more year of fine scale monitoring (includ- restrial margin) ing sedimentation rate and macroalgal mapping) recommended for January 2011. It is also recommended that additional sediment plates be deployed in the upper Porirua Arm (Polytech site) in 2011 to better ac- count for the patchiness of sediment deposition at this site. Recommended Management In order to develop sediment and nutrient budgets, nutrient and suspended sediment inputs from major sources during both baseflow and flood conditions should also be monitored. Nutrient and chlorophyll a • Limit intensive landuse. concentrations in the water column of Porirua Harbour should also be monitored in order to assess potential • Set nutrient, sediment guidelines. for nuisance conditions. The following specific management actions are recommended: • Margin vegetation enhancement. • Limit nitrogen inputs to the estuary to levels that will not cause nuisance algal blooms i.e. limit estuary • Manage for sea level rise. areal loading to 50mgN.m-2.d-1 (Heggie 2006) which equates to mean catchment yields of 50 and 100 • Enhance saltmarsh. tonnes N/yr for the Porirua and Pauatahanui Arm catchments respectively (approximately 7-9 kgN/ha/ • Manage weeds and pests. yr). Currently the estuary areal N load is unknown and consequently there is a recommendation above to undertake input monitoring to rectify this. • Limit suspended sediment catchment inputs to the estuary to levels that will not cause excessive estu- ary infilling i.e. limit sedimentation rates to 2 mm/yr (Gibbs and Cox 2009). The catchment SS input load required to meet this limit is currently unknown and therefore sediment budget monitoring/modelling is recommended to rectify this. Wriggle coastalmanagement vii 1. INTRODUCTION OVERView Developing an understanding of the condition and risks to coastal and estuarine habitats is critical to the management of biological resources. In 2007 Greater Wel- lington Regional Council (GWRC) identified a number of estuaries in its region as im- mediate priorities for long term monitoring and in late 2007 began the monitoring programme in a staged manner. The estuaries currently included in the programme are; Porirua Harbour [Onepoto (Porirua) and Pauatahanui Arms], Whareama Estu- ary, Lake Onoke, Hutt Estuary and Waikanae Estuary. Risk assessments have been undertaken for a number of other estuaries in order to establish priorities for their management. Monitoring of Porirua Harbour began in late 2007, but the first year of fine scale baseline monitoring began in January