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Matthew Hawes

The effect of fertilisers...

16/09/2013

The effect of fertilisers and other nitrogen sources on the aquatic environment

Those of you involved in the transport of emergency response or the transport of dangerous goods will be well aware of substances and mixtures which are classified as hazardous to the environment, these are labelled with the “dead tree and fish” symbol. However chemicals which could be thought of innocuous can also have a devastating effect.

It is easy to think of fertilisers, as a group, as chemicals which would not only not be hazardous, but beneficial, as they encourage the vigorous growth of plants. Conversely it is this very effect which is one of the reasons they can have such detrimental consequences when they are released.

But let us start first with any potential acute toxicity. A number of fertilisers will release ammonia when dissolved in water. The exact form of the ammonia in the water is dependent upon pH of the water; it will exist as either the ionised (NH4+) or unionised form (NH3). As the pH decreases a greater proportion of ammonia exists as the ionised form. This is caused by a greater number of hydrogen ions being present in acidic conditions.

H3O+    +    NH3                 H2O    +    NH4+

Hydrogenated Water    +    Ammonia             Water     +    Ammonium

Because the ionised form is less toxic to aquatic life than the unionised form the effects of a spill in aquatic environment can be less severe in bodies of water with a low pH. The USEPA   1999 Update of Ambient Water Quality Criteria for Ammonia (1999 Ammonia Update) presented that ammonia was approximately 30 times more toxic at a pH 9 compared to pH 6.

Once spilled in the aquatic environment the ammonia is changed to sequentially less toxic forms by naturally occurring denitrifying bacteria. These bacteria use oxygen to convert ammonia into nitrite, which is then converted to nitrate. The nitrate is then either used by algae and plants as a nitrate source or further mineralised into atmospheric nitrogen. The decrease in toxicity can be demonstrated by examining the lethal concentration effecting 50% of a population (LC50) for ammonia and nitrate as determined for the fat head minnow. The quoted ammonia LC50 ranges between 2.7 and 8.2 mg/l whilst the nitrate LC50 ranges between 1000 and 1600 mg/l, which clearly reveals the reduction in toxicity.

Though the conversion could be seen as a good thing in a standing, and/or small, body of water it can result in hypoxia, the depletion of aquatic oxygen, which in itself can be hazardous to aquatic life. This is because oxygen is required in each of the biological reactions.

2NH3    +    3O2        →    2NO2-    +    2H2O    +    2H+
Ammonia    +    Oxygen    →    Nitrite    +    Water    +    Hydrogen
2NO2-    +    O2        →    2NO3-
Nitrite    +    Oxygen    →    Nitrate

The above reactions display that twice as much oxygen as ammonia is required to convert the ammonia into the nitrate. Additionally the reactions can proceed at aquatic oxygen concentrations much lower than those required for most fish species, the exception being species that are able to use atmospheric oxygen, such as those belonging to Anabantoidei. It is not just ammonia that will proceed along this reaction pathway but any chemical with biologically available nitrogen such as urea, which is another common component of fertilisers.

As stated at the beginning the encouragement of the vigorous growth of plants caused by the fertilisers can be hazardous to aquatic life. This is because eutrophication, a rapid increase in the nutrients required for plant growth, results in a hurried increase of plant life. Unfortunately this process favours the simpler plants, such as algae, bacteria and phytoplankton, rather than the higher plants, for the reason that the simpler plants are more capable of swift reproduction. This is due to their simpler structure and that they are most often found at the water/atmosphere interface, so not limited by the concentration of aquatic carbon dioxide. The rapid increase is often termed as an algal bloom or a red tide, in marine environments.

Once the available nutrients have been consumed the algae die and sink to the lake/river/sea bed before decomposing. The decomposition converts the organic matter into inorganic chemicals and, due to the requirement of oxygen for this, can result in hypoxia. Because the blooms are located at the water/atmosphere interface they often stop the higher plants, which are more often on the bed, from receiving sufficient oxygen to survive.

It is not just the decomposition of the blooms that can have detrimental effects on the aquatic environment. A number of the organisms which thrive where eutrophication occurs produce toxins which can cause symptoms in humans either directly, through bathing, or indirectly. The indirect symptoms are the result of bioconcentration of the toxin in the food chain. For example a common cause of shell fish poisoning is due to a toxin produced by blue-green algae, which is actually a bacteria.

Interestingly the source of nitrogen can affect the amount of toxin produced by the bloom, at least in some instances. Research funded by the National Oceanic and Atmospheric Administration discovered that the phytoplankton responsible for a red tide in Monterey Bay, California produced more toxin when urea was the nitrogen source compared to other sources.

Of course it is not just nitrogen sources which can be hazardous to the environment but not marked as such. Any substance which results in hypoxia either directly through decomposition, such as milk, or indirectly through eutrophication, such as phosphates, can be harmful to aquatic life. So the next time you have a spill of a substance which is not classified as hazardous to the aquatic environment think before you wash it away, are you sure it won’t have any damaging consequences?

 

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