Source: Baeta, A., L.R. Vieira, A.V. Lírio, C. Canhoto, J.C. Marques, and L. Guilhermino. 2017. Use of stable isotope ratios of fish larvae as indicators to assess diets and patterns of anthropogenic nitrogen pollution in estuarine ecosystems. Ecological Indicators 83: 112–121. http://dx.doi.org/10.1016/j.ecolind.2017.07.062
Nitrogen is required for life. It is found in DNA and we use it to fertilize soil in order to grow food. However, when it comes to nitrogen, there can be too much of a good thing as nitrogen that is released to coastal waterbodies can have negative consequences. Just as on farms, in coastal waterbodies, nitrogen can result in the growth of a lot of plant material such as algae. Eventually the algae will die and be decomposed by bacteria that use up oxygen in water causing fish to suffocate and die. There are many human activities that can release nitrogen into coastal waterbodies including the application of fertilizers to farms and lawns, the discharge of human and industrial waste, animal production, and fossil fuel combustion. It is important to determine the impact of this human derived (anthropogenic) nitrogen in coastal environments.
Fish larvae as biological indicators of nitrogen pollution
In this study, researchers set out to determine if the larvae (immature form) of the common goby (Pomatoschistus microps, Figure 1) could be used as a biological indicator for nitrogen pollution. A biological indicator is a species that can be used to determine the effect of a stressor (in this case nitrogen) on an ecosystem. The larvae of the common goby were chosen in this study because goby are among the most abundant species in estuaries (a type of coastal environment).
Tale of two estuaries
For this study, the researchers collected samples from two estuaries in northwest Portugal one with low nitrogen inputs (Minho) and one with high nitrogen inputs (Lima). At each estuary samples of water, common goby larvae, and potential prey (food items) of common goby larvae were collected.
Each of the samples were analyzed to determine their nitrogen fingerprint. Nitrogen from human sources typically has a different chemical fingerprint than nitrogen from natural sources. The researchers used these fingerprints to determine what the common goby larvae were eating. They also compared the nitrogen fingerprint in the fish to that in their prey and environment to determine if the fingerprints included human sources of nitrogen.
The researchers determined that during its larval stage the common goby fed on the same prey in both estuaries, copepods (Figure 2). They also found that the nitrogen fingerprint in fish larvae from Lima (the estuary with high human nitrogen inputs) had a larger human influence than the fish from Minho (the estuary with low human nitrogen inputs). This led the researchers to conclude that the nitrogen fingerprints in the fish larvae reflect those of the water, and therefore the human input of nitrogen into the estuary.
Now that the researchers know that the nitrogen fingerprint in common goby larvae is directly related to the amount of human derived nitrogen in the water, the next step is to determine the exact sources of the nitrogen. Remember that anthropogenic nitrogen inputs to waterbodies can come from many sources including fertilizers and human waste. If future studies can determine the exact source of the nitrogen, then we can begin to reduce the amount of nitrogen released to coastal waterbodies.