Mangroves and mud

Last year, I was lucky enough to spend many months working in the difficult to traverse mangrove forests of South Australia. The importance of mangroves as carbon sinks, as well as the plethora of other ecosystem services they provide, is a topic increasingly covered in recent publications (Kennedy et al. 2010; McLeod et al. 2011). These services include providing nursery habitats for commercially important fish (Nagelkerken et al. 2002), and recreational services such as fishing or tourist attractions.

Despite their importance, mangroves are subjected to high stress, caused by chemical, physical, and biological factors. Chemical factors include changes to the concentration of pollutants within the soil; physical factors include coastal clearing; and biological factors include the introduction of invasive species or pathogens. Human activity can vastly increase the impact that these factors have on the composition and condition of mangroves.

Mangrove condition is usually determined by satellite technology (Carlson and Ripley 1997). However, these methods tend to weight canopy density quite heavily, and are also impeded by cloud cover. Since my project took place during the winter months, ground-based methods were more appropriate. It was with great excitement that I ducked and weaved my way through mangroves dense and sparse, tiptoed between pneumatophores, and braved the notorious smell of mud and densely packed carbon. This was all in the name of gaining thorough measurements to put a number to mangrove health.

Measurements included canopy density, tree density, and tree size. I also measured pneumatophore height and density, and soil temperature and salinity. Since I was out there and had my waders, I sampled fish abundance and species diversity. Using a formula designed by Price et al. (2011) for wetland assessment, I converted the vegetative parameters into a single number, and compared this number to pneumatophore, soil, and fish properties.

Unfortunately, aggressive tides and some stubborn seaweeds meant that fish samples became limited. Instead, I turned my attention to how changes in soil and pneumatophore properties impacted mangrove vegetation condition. It turns out that mangroves are sturdy systems, capable of resisting stress and recovering from degradation (Alongi 2008; Lacambra et al. 2013). I found no correlation between mangrove condition and soil or pneumatophore properties. A strong positive correlation was found between pneumatophore height and fish assemblages (abundance and diversity).

Despite their robust nature, it’s important that mangrove conservation is not treated with complacency. The capacity for mangroves to store carbon over millennia is an essential weapon in the battle against climate change. However, this was seldom studied until recently, and mangroves can appear as uncharismatic to the general public when compared to systems such as coral reefs and kelp forests. These combine to deter attention away from mangrove conservation. I’m hoping that this study, and other “blue carbon” research, raises awareness to their importance, and shows that working in mangroves, whilst difficult and smelly at times, is incredibly rewarding.

References

Alongi, DM (2008) Mangrove forests: resilience, protection from tsunamis, and responses to global climate change. Estuarine, Coastal and Shelf Science 76, 1-13.

Carlson, TN, Ripley, DA (1997) On the relation between NDVI, fractional vegetation cover, and leaf area index. Remote Sensing of Environment 62, 241-252.

Kennedy, H, Beggins, J, Duarte, CM, Fourqurean, JW, Holmer, M, Marbá, N, Middelburg, JJ (2010) Seagrass sediments as a global carbon sink: isotopic constraints. Global Biogeochemical Cycles 24, 1-8.

Lacambra, C, Friess, D, Spencer, T, Moller, I (2013) Resilient natural coastal defences. In 'The Role of Ecosystems in Disaster Risk Reduction.' (Eds R Fabrice, K Sudmeier-Rieux, M Estrella.) pp. 82. (United Nations University Press: Tokyo, Japan)

McLeod, E, Chmura, GL, Bouillon, S, Salm, R, Bjork, M, Duarte, CM, Lovelock, CE, Schlesinger, WH, Silliman, BR (2011) A blueprint for blue carbon: toward an improved understanding of the role of vegetated coastal habitats in sequestering CO2. Frontiers in Ecology and the Environment 9, 552-560.

Nagelkerken, I, Roberts, CM, Van der Velde, G, Dorenbosch, M, Van Riel, MC, Cocheret de la Morinière, E, Nienhuis, PH (2002) How important are mangroves and seagrass beds for coral-reef fish? The nursery hypothesis tested on an island scale. Marine Ecology Progress Series 244, 299-305.

Price, C, Gosling, A, Golus, C, Weslake, M, 2007. Wetland assessment techniques manual for Australian wetlands. WetlandCare Australia, Balina, NSW.