Mud and Mangroves

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Written by: Eleanor West

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The Coromandel Peninsula boasts some of the most idyllic beaches in New Zealand; golden sand, pōhutukawa trees and sparkling blue water. Yet despite being a coastal town on this peninsula, the beaches of Thames disappoint. ‘Thames Coastal Walkway’ doesn’t even make top ten for suggested Thames activities on tripadvisor 1, and it is the only suggestion that involves the coast at all. The reason? The Firth of Thames is chock-a-block full with mud and mangroves. Apparently beach goers find this unpleasant.

 

Fortunately, Thames-Coromandel and Hauraki Districts’ councils have a plan to remedy this issue; remove the mangroves. Thus the Mangrove Management Bill 2017 2 was presented to Parliament. Mangrove removal requires approval under the Resource Management Act3, but this bill will see the councils exempt from such a requirement and will enable them to set about removing mangroves at their leisure2. The only hiccup is wide spread opposition. The Department of Conservation (DOC)3, Forest and Bird 4, and the Environment and Conservation Organizations of NZ Inc. (ECO)5 have all made submissions against this bill.

 

The bill states that “Aerial photography from the 1940s shows minimal mangrove incursions into the districts’ harbours and the Firth of Thames, with white sandy beaches being the norm.” 2 This suggests that the councils­­­­­­­­ are under the impression that the mangroves prevent the return of their beautiful beaches. Given that mangroves are proven to trap and retain sediment6,7, this viewpoint makes sense. However, this bill utterly fails to consider the source of the sediment being trapped.

 

Sediment has been washing into the Firth of Thames at an alarming rate since the 1940s. This is largely due to urbanization and farming practices upstream7,8. The silt and mud accumulates on the tidal flats providing perfect conditions for mangroves to establish and spread rapidly7. Essentially, the beaches were wrecked before mangroves were even on the scene.

 

Admittedly, the mangroves are trapping excess sediments and exacerbating the issue, but they are definitely not causing it. A recent study in nearby Tauranga6 finds that sediment also accumulates on the bare tidal flats directly in front of mangroves. This means they are trapping most of the sediment washing out of rivers, but some still escapes and deposits further down the beach. Clearly, some mud would be on the beach regardless and by removing mangroves the councils would merely be “shooting the messenger”.

 

Despite of their alleged involvement in ruining beaches, mangroves provide invaluable ecosystem services to the Firth of Thames. By trapping sediment, they filter and detoxify water flowing into the ocean9 and raise the surface elevation on the coast6,9,10,11. This provides protection from sea level rise and coastal flooding10,12. The mangroves attenuate waves and wind, protecting property from storm surges and erosion9,10,12,13. They are very effective carbon sinks14 and are relatively resilient to being damaged10,14. This bill claims that they encroach on tidal wetland habitat2, which is true3, but relatively insignificant when compared to the habitat they provide for foraging, escape cover, and breeding3,9. A number of bird species rely on them3.

 

This bill is poorly planned and ill considered. The ECO deems it “ecologically reckless.” 5 It fails to consider the overwhelming benefits of mangrove retention and the exemptions it requests would “override and disregard almost all other law.” 5 They are absolutely right.

  1. (n.d.). Things to Do in Thames. Retrieved March 2018, from tripadvisor: https://www.tripadvisor.co.nz/Attractions-g255369-Activities-Thames_Coromandel_Peninsula_Waikato_Region_North_Island.html
  2. Thames–Coromandel District Council and Hauraki District Council. (2017, July 6). Thames–Coromandel District Council and Hauraki District Council Mangrove Management Bill.Retrieved March 2018, from New Zealand Legislation:http://www.legislation.govt.nz/bill/local/2017/0278/latest/d56e2.html
  3. Department of Conservation. (2018, March 5). Thames–Coromandel District Council and Hauraki District Council Mangrove Management Bill – Department of Conservation.Retrieved March 2018, from New Zealand Parliament: https://www.parliament.nz/en/pb/sc/submissions-and-advice/document/52SCGA_EVI_74548_752/department-of-conservation
  4. Forest and Bird Upper Coromandel Branch. (2018, 5 March). Thames–Coromandel District Council and Hauraki District Council Mangrove Management Bill – Forest and Bird – Upper Coromandel Branch.Retrieved March 2018, from New Zealand Parliament: https://www.parliament.nz/en/pb/sc/submissions-and-advice/document/52SCGA_EVI_74548_629/forest-and-bird-upper-coromadel-branch
  5. Environment and Conservation Organisations of NZ Inc. (2018, March 5). Thames–Coromandel District Council and Hauraki District Council Mangrove Management Bill – Environment and Conservation Organisations of NZ.Retrieved March 2018, from Parliament: https://www.parliament.nz/en/pb/sc/submissions-and-advice/document/52SCGA_EVI_74548_646/environment-and-conservation-organisations-of-nz
  6. Stokes, D. J., Healy, T. R., & Cooke, P. J. (2010). Expansion Dynamics of Monospecific, Temperate Mangroves and Sedimentation in Two Embayments of a Barrier-Enclosed Lagoon, Tauranga Harbour, New Zealand. Journal of Coastal Research, 26(1), 113-122.
  7. Swales, A., Bentley, S. J., Lovelock, C., & Bell, R. G. (2007). Sediment Processes and Mangrove-Habitat Expansion on a Rapidly-Prograding Muddy Coast, New Zealand. Sixth International Symposium on Coastal Engineering and Science of Coastal Sediment Process Cite this publication, (pp. 1441-1454).
  8. Hayward, B. W., Grenfell, H. R., Sabaa, A. T., Morley, M. S., & Harrocks, M. (2006). Effect and timing of increased freshwater runoff into sheltered harbour environments around Auckland City, New Zealand. Estuaries and Coasts, 29(2), 165-182.
  9. Barbier, E. B., Hacker, S. D., Kennedy, C., Koch, E. W., Stier, A. C., & Silliman, B. R. (2011). The value of estuarine and coastal ecosystem services. Ecological Monographs, 81(2), 169-193
  10. Alongi, D. M. (2008). Mangrove forests: Resilience, protection from tsunamis, and responses to global climate change. Estuarine, Coastal, and Shelf Science, 76(1), 1-13
  11. Krauss, K. W., McKee, K. L., Lovelock, C. E., Cahoon, D. R., Saintilan, N., Reef, R., & Chen, L. (2013). How mangrove forests adjust to rising sea level. New Phycologist, 202(1), 19-34.
  12. Swales, A. (2007). Mangrove-habitat expansion in the southern Firth of Thames sedimentation processes and coastal-hazards mitigation.Hamilton: National Institute of Water & Atmospheric Research
  13. Das, S., & Crepin, A. (2013). Mangroves can provide protection against wind damage during storms. Estuarine, Coastal and Shelf Science, 134(1), 98-107.
  14. Lovelock, C. E., Sorrel, B. K., Hancock, N., Hua, Q., & Swales, A. (2010). Mangrove Forest and Soil Development on a Rapidly Accreting Shore in New Zealand. Ecosystems, 13(3), 437-451.