Ecological impact of salt farming in mangroves on the habitat and food sources of Austruca occidentalis and Littoraria subvittata.
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Alex Nehemia, Margaret Chen, Marc Kochzius, Frank Dehairs, Natacha Brion


The impact of salt farming on the habitats and food sources of Austruca occidentalis and Littoraria subvittata was studied in mangroves along the coast of Tanzania using stable isotopes (13C and 15N) and sediment particle size analysis. The 13C and 15N stable isotope composition in mangrove leaves, sediments and invertebrate tissues, were used to evaluate whether there are differences in feeding ecology of the crab Austruca occidentalis and the snail Littoraria subvittata collected from natural mangroves and mangroves around the salt ponds. Organic C, total N content and particle size distribution in sediments were used to assess if there are differences in habitat characteristics of mangroves around the salt ponds. Mangrove leaves and sediments were found to be 13C enriched around salt ponds compared to those from natural mangroves. Likewise the macroinvertebrates collected from mangroves around salt ponds were found be enriched in 13C relative to undisturbed mangroves. In addition, mangrove sediments around salt ponds were poorer in organic carbon and nitrogen and had more sand content compared to sediments from natural mangroves. These results indicate that salt pond activities have contributed to the modification of the habitats of macroinvertebrates by causing δ13C stable isotopes enrichment and alteration of sediment characteristics in the ecosystem.

Main Results and Conclusions:
  1. Salt farming can be identified as one of the largest threats to mangrove forests and a huge factor in their disappearance.
    • “ Brazil it has been estimated that about 50,000 ha of forest have disappeared over the last 25 years and salt farming is one of the factors driving this loss.” (Ferreira and Lacerda 2016) (24)
    • “Moreover, in Kenya, about 10,000 ha of mangroves have been cleared between Ngomeni and Karawa due to salt farming.” (Abuodha and Kairo 2001) (24)
  2. There are many negative side effects of logging and removing mangrove trees for salt farming that are evident throughout the ecosystem, outside the immediate area of removal.
    • “Logging of trees from the mangrove forest can be detrimental to ecosystem functions because it can alter the microbial processes that are highly sensitive to the chemical quality and quantity of litter entering the soil. These processes include non-symbiotic N fixation, denitrification, net N mineralisation and nitrification.” (Perez et. al. 2009) (24)
    • “Logging of mangrove trees during salt production can also cause changes in chemical, physical and biological parameters of the sediments.” (Ellegaard et al. 2014) (24)
    • “The dykes constructed around the salt ponds become obstacles to the free movement of water, which are essential for dispersal of larvae of many invertebrates.” (Mazda et al. 2002; Ocholla et al. 2013) (24)
    • “Salt production in mangroves therefore has a major influence on the distribution and diet of macroinvertebrates. Faunal communities tend to utilise more mangrove carbon as their food source in systems with less input from adjacent waters - typically undisturbed systems.” (Bouillon et al. 2004) (25)
  3. Not only does this type of farming destroy the mangroves but it is also detrimental to all the species dependent upon this habitat for shelter, food, and reproduction.
    • “The crab, Austruca occidentalis is a dominant species in the mangroves along the coast of East Africa.” (Litulo 2004) (25)
    • “The distribution of A. occidentalis is known to be affected by many factors, including sediment grain size composition and oreganic and moisture content, as well as presence or absence of mangrove vegetation.” (Mokhtari et al. 2008, 2015) (25)
    • “The snail, L. subvittata is the dominant arboreal species in mangrove forest along the East African coast.” (Torres et. al. 2008) (25)
    • “Their distribution pattern has been suggested to be controlled by tidal oscillations and be affected by human induced disturbances that influence environmental variables, such as temperature and oxygen concentrations.” (Blanco and Cantera 1999) (25)
  4. The salt from the salt farms affects the stable isotopes found in the mangrove leaves. 
    • “Green mangrove leaves from salt ponds were enriched in 13C by 1.1% compared to those from natural mangrove sites (V = 120, P < .05).” (27)
    • “Brown mangrove leaves at salt ponds had a higher δ13C isotope composition compared to natural mangroves (V = 0, P <.05).” (27) 
    • “For nitrogen, no differences were detected between samples from natural mangroves and mangroves around the salt ponds.” (27)
    • “The difference in δ13C between A. occidentalis and sediments at natural mangroves was 5.5% and around the salt pond sites 7.2%.” (28)
    • “The environmental stresses, such as high temperature caused by canopy openness and increased solar radiation, are known to influence the carboxylation process, which can lead to enrichment of 13C in mangrove and terrestrial plant.” leaves (Crowley et al. 2012; McKee et al. 2002; Wei et al. 2008) (28)
  5. The presence of salt farms can affect intact mangrove stands that are located near the salt farms.
    • “The findings show that A. occidentalis and L. subvittata in mangrove around the salt ponds may be switching to the more available carbon source as their food, once mangroves are removed. This study revealed also that mangrove removal around the salt ponds may be contributing to large particle size distribution in sediments which ultimately lower the amount of organic C and N content in sediments.” (31)
    • “ The existence of salt ponds and selective logging of mangrove trees may be responsible for 13C enrichment in sediments and leaves of mangroves where macroinvertebrates obtain their food.” (31)
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