-a review of Andrew Swales’ presentation at the Trust’s AGM.
Before the 2009 AGM in May, Andrew Swales of NIWA, as our guest speaker, gave us a fascinating presentation on the work he and his colleagues have been undertaking in the mangrove forest area in the southern end of the Firth. Comparison of information from sources such as aerial photography, historical records, weather records, recent mangrove growth and sediment records (gained from 2m sediment core samples undertaken within the forest and on the mudflats beyond) and lead isotope analysis gives an intriguing account to the life cycle of the mangrove forest in the Firth.
A published report on the Firth of Thames mangroves (‘Mangrove Habitat Expansion in the Southern First of Thames: Sedimentation Processes and Coastal Hazards Mitigation’, Report No. TR2008/13) by NIWA for Environment Waikato is available here on the Environment Waikato website and is well worth a read no matter whether you’re a lover or hater of these plants.
As a taste of the presentation Andrew gave us here’s 10 interesting things to know about mangroves in the Firth and in general:
1. Mangrove forest establishment and advance is related to sediment available from the main rivers feeding into the Firth;
2. The main surge in growth correlates to the main period of deforestation in the catchments of the Waihou and Piako rivers. In this period of deforestation sediment loads into the Firth were several times that of previous and current sediment loads;
3. Aerial photos show clearly the gradual advance of mangroves from the Waihou & Piako river mouths to the rest of the coast;
4. For mangroves to establish there must first be sediment (read mud!) in which the propagules (seeds) can germinate and to germinate they need two months of calm weather for good plant recruitment. They tend to be successful when they germinate on top of mud ridges formed by wave action, however these are constantly being reworked;
5. Mangrove propagules are generally only viable for a week;
6. Mangroves need a certain amount of regular exposure to sea water to allow growth;
7. Mangroves are part of a negative feedback system; the sediment allows the mangrove seedling to establish, the mangroves then promote sedimentation at a faster rate, the sediment level builds up so that gradually the mangroves get less and less exposure to sea water until eventually they don’t get enough and start to die back;
8. Mangroves are generally not found further south than Ohiwa on the east coast and Kawhia harbour to the west as they are reasonably sensitive to frost and prolonged low temperatures;
9. Gaps in the lead isotope profiles recorded from retrieved core samples correlate to unconformities in the mud (where erosion and material removal has taken place) which have occurred as a result of a storm. Storms have severely damaged the mangrove forests here on several occasions;
10. Records show that roughly every 10 years there is a jump mangrove advance.