I am on a mission to determine what mangrove and mangrove creatures can actually put up with. At what point does environmental stress start to cause degradation. It is an incredibly important question from an environmental management point of view. With that in mind, I revisited the
naturally toxic watercourse to test the acidity of the water. Acidity is highly detrimental to plants and two significant environmental problems in Australia are acid mine drainage and acid sulfate soils. Plants just will not grow on acid soils – a few specially adapted species can, however they are typically present in unusual habitats such as peat bogs which are not relevant to the general environment. Acid affected areas are normally devoid of vegetation or have shallow rooted grasses which can grow in the shallow layer of sand on the surface where the acid is washed away by rainwater.
To test the waterway, I bought an aquarium pH test kit with a wide testing range for $12 and headed to the watercourse. When I arrived, there was a moderate odour of hydrogen sulfide. A sample draining from the toe of the beach via a blackened rivulet was taken an attempt to get a really acid reading. The result was green (pH 7) which is neutral. Seawater is normally alkaline (~pH 8.5) the water is slightly acid in comparison to the nearby sea. Such a slight effect should not impact on many creatures and indeed, there were gobies in blackest, mankiest rivulets. Some mangrove creeks receive acid waters from freshwater swamps which generate tannic acid (as low as pH 5.5) and the mangroves appear to be fine, even after prolonged exposure. In coming to a proper scientific conclusion, all of the confounding factors have to be accounted for or controlled and in the case of tannic acids, these waters are also fresh, which may confound the effect of the acid. No firm conclusion can be draw about the slight acidity of this waterway, which lacks freshwater inputs.
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| Weird orange (iron-stained?) sand suggests that acid-sulfate chemistry is happening under the beach |
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| Even with a stand of dead mangroves in the seepage zone, the water was neutral |
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| Nearby creeks have a pH in the normal range |
Then I found a dead patch of mangroves and had to investigate. Below the patch of mangroves, there was a nasty black seepage zone, with sticks that looked like bones.
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| Recently killed mangroves |
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| A blackened seepage area |
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| A blackened rivulet, with sticks covered by a white bacterial slime |
Behind the patch of dead mangroves was a blocked creek. In recent times, sand washed up by a cyclone had blocked the mouth of the creek and waters now drain through the sand to emerge in the nasty seep. Putting all this together we get a possible scenario. As the tide retreats, it carries leaves to be point where the creek is blocked and they become trapped. In freshwater creeks, thick rafts of leaves build up where a creek flows through a sandbank and the same may be occurring here. When the leaves were buried with sand, they formed a food source for bacteria. Once the oxygen from the incoming creek water has been consumed the water would continue flowing through the dead leaf litter where anaerobic bacteria decompose the vegetation and release H
2S. The flowing water carries the H
2S through the sand and into the mangrove root zone. Normally, mangroves can cope with H
2S but the sand has buried their breathing roots and the crabs which pipe air into the mud have also moved on. This particular combination of a limitless supply of H2S and loss of soil aeration seems to be fatal. A few metres away other mangroves also have buried root systems but are thriving, so sand alone does not hurt mangrove trees. Blockage of the creek and the prolonged flooding also seem to have no effect.
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| View up the blocked creek |
In this post, I have listed several potential environmental stresses to mangrove forests and have found that mangroves can easily cope with prolonged fresh and saltwater flooding (a few months) and can cope with acidic freshwater. Slightly acidified saltwater probably has little effect, however burial of breathing roots in an environment that generates H
2S seems to be rapidly fatal. What little detailed information there is on the net supports the idea presented here that H
2S is one of the key or perhaps the key factor affecting the survival of mangroves. There is even a
possible case of two lovers perishing at the hand of H2S as they enjoyed each-others company beside a mangrove waterway.
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| Tannin stained water in Wyvuri creek when mouth was blocked by sand |
More reading on acid sulfate soils:
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