The nitrogen cycle (Figure 1.0.1) is complex and involves chemical, geological and biological processes. Flora, fauna and various bacteria are involved. Air is approximately 79% nitrogen but nitrogen gas cannot be used by most living organisms. Instead, plants must obtain their nitrogen from the soil as nitrate and animals obtain their nitrogen from plants or other animals. Nitrogen is an essential component of proteins and nucleic acids, so all organisms require a supply of it.
Humans cannot fix nitrogen biologically, but industrially humans make 450 million metric tonnes of fixed nitrogen each year. Majority of this fixed nitrogen is used to make fertilizer for agriculture. When artificial fertilizers containing nitrogen are used in agriculture, the excess fertiliser may be washed into bodies of water by surface runoff. Nitrogen itself is not hazardous when present in water, but excess nitrogen in the environment from human activities can lead to freshwater and saltwater eutrophication. Eutrophication is the process by which nutrient runoff causes overgrowth of algae or other microorganisms.
If eutrophication causes overgrowth of algae and microorganisms, the research question posed is, ‘How is marine biodiversity affected as a result of nitrate levels?’ It is hypothesised, that since eutrophication causes overgrowth of algae, this organism will overly deplete the dissolved oxygen and thus biodiversity will reduce with higher nitrate levels.
How is marine biodiversity affected as a result of nitrate levels?
The original experiment investigated the effect of one abiotic factor in the field on another.
This quantitative investigation uses a dissolved oxygen probe and a nitrate probe to demonstrate the relationship between variable abiotic factors and make inferences as to causes for the relationships.
The nitrate probe and the dissolved oxygen probe were attached to a data logger and inserted into the water (Figure 3.0.1) at 10m intervals along a transect. The data was recorded for each location. It is noted that the general trend showed a strong negative relationship between nitrate levels and dissolved oxygen levels; where the nitrate levels were higher, the dissolved oxygen levels were lower.
In the original experiment, the independent variable was nitrate levels and the dependent variable the amount of dissolved oxygen present. The controlled variables were the probes and data logger used, the time of day, and the ecosystem. The original experiment was redirected to investigate the effect that nitrate levels had on biodiversity. In the modified experiment, the independent variable was nitrate levels and the dependent variable the biodiversity, determined using Simpson’s Diversity Index.
Simpson’s Diversity Index was used because it was determined to be the most reliable indicator of biodiversity as it considers both species richness and evenness of abundance of species present. A low Simpson’s diversity index indicates: relatively few successful species in the habitat; the environment is quite stressful and has relatively few ecological niches; only a few organisms are really well adapted to the environment; relatively simple food webs; changes in the environment would probably result in serious effects. A high Simpson’s diversity index suggests: more stable ecosystem with greater successful species; less likely that the environment is hostile with more ecological niches; complex food webs; environmental change less damaging to ecosystem.
To calculate Simpson’s Diversity Index accurately in each of the intervals along the transect, 1m quadrats were placed over the area where nitrate and dissolved oxygen levels were measured. Relative abundance and name of each species were recorded, and the interval along the transect where the data was taken. Data was collected at approximately 10m intervals along the transect to ensure accurate representation.
It is noted that the highest Simpson’s Diversity Index value (0.738) occurred at the point where the lowest nitrate concentration (5.5mg/L) was recorded, Quadrat 4. Furthermore, the quadrat with the lowest SDI of 0.4372 also had the highest nitrate concentration (32.1mg/L).
Figure 6.0.1 displays the relationship between Simpson’s Diversity Index and nitrate concentration. There is a strongly negative linear trendline between the points that indicates that biodiversity does decrease with increased nitrate concentrations. However, the data points do not fall exactly on the line. While differences in environment between quadrats would account for some discrepancy, this potentially suggests low precision during the collection of data or unreliability of the equipment used.
Furthermore, the data on the algae amount as a percentage of the quadrat and the diffused oxygen levels support the hypothesis. The data on nitrate levels in water states that excess nitrates in the water, usually as a result of fertiliser runoff is algae blooms – where the algae is not limited in its growth by the availability of nitrogen. This leads to reduced dissolved oxygen and thus reduced biodiversity. This trend is shown quite clearly in the data where the nitrate concentration is highest at 32.1mg/L, the algae occupies 30% of the quadrat, the greatest in any quadrat. The oxygen concentration is also lowest at 4.5mg/L and the SDI is lowest at 0.4372.
Sources of Error
This experiment was conducted in the field, which accounts for much of the error in this experiment. The field trip this experiment was conducted on was only two days long, which meant limited time to conduct the experiment. The Rocky Shore ecosystem at Hastings Point is only accessible when the tide is less than 0.5m, and on its way out due to safety reasons. The field trip arrived at Hasting’s Point at 09:30 on the 27th of February and left Hasting’s Point at 13:30 on the 28th of February. During this time period there were only two times at which the tide met the safety specifications: between 16:00 and 18:30 on the 27th and between 04:00 and 06:30 on the 28th. This second period occurred before the sun rose and thus it did not meet the safety guidelines. Therefore, there was only two and a half hours during which to conduct the experiment in an unfamiliar area. Due to this only four samples were able to be taken, with only one trial. This reduced the reliability and validity of the data.
The fact that the experiment was conducted in the field also meant that not all the other variables could be controlled properly, reducing the reliability of the data. The quadrats were at different locations on the Rocky Shore ecosystem along a transect line that ran from the shore to the sea. Thus, other factors could have affected the biodiversity, including but not limited to: distance from the sea; salinity; presence of nutrients other than nitrogen and oxygen; amount of water located in each quadrat; and, amount of flow from the sea it received.
The equipment used to test the nitrate concentration and amount of dissolved oxygen was not entirely reliable as one of the data loggers malfunctioned and had to be replaced. Furthermore, the readings shown for nitrate concentration and dissolved oxygen were in constant flux before eventually settling on one value.
Humans always err, as they are not perfect beings and human error is an inevitable part of any experiment. Human error was present in this experiment particularly through the identification of species and counting of relative abundances within a quadrat. Consequently, the experiment is limited in its ability to draw conclusions from the findings about how nitrate concentration effects the biodiversity of marine life.
Improvements and Extensions
Reducing the amount of error in the experimental process would improve its reliability and validity. In this experiment, the reliability could be improved by increasing the frequency along the transect line that tests were performed. It could also be improved through increasing the number of trials. For this improvement to occur, the field trip would need to be at least 3-4 days long so that the experiment could be run at the same tidal point at roughly the same time of day to reduce inaccuracies. If the number of trials was increased, standard error, standard deviation and percentage error could all be calculated resulting in greater analysis and ensuring the reliability and validity of the data.
It could be improved by ensuring that there is the same percentage of water present in each of the quadrats and ensuring that that isn’t a factor affecting the reliability or validity of the data. Other abiotic factors could also be measured (i.e. salinity, temperature), to ensure that they aren’t affecting the reliability or validity of the results.
A more accurate data logger and nitrate and oxygen probes could be obtained to improve the reliability of the equipment used to obtain results.
The experimenters could be trained in how to properly identify a species and the differences between species with similar appearances in order to improve the accuracy of species identification.
The experiment could be extended by simulating it in controlled conditions. In a series of environments that mimic nature, scientists could test the effects of different nitrate concentrations on the biodiversity of the organisms living in these environments. Furthermore, scientists could adjust the nitrate concentration in serial dilutions, which would result in a geometric progression of the concentration in a logarithmic fashion. If this extension was performed, the relationship between nitrates and biodiversity be able to be properly assessed without any uncontrolled variables. The effect of nitrate concentration on biodiversity could be assessed over time to determine how long it takes to adversely effect biodiversity, but the nitrate concentration could also be gradually lowered to determine approximately how long it would take marine ecosystems to recover if humans began reducing the amount of nitrogen pollution.
In conclusion, the evidence suggests that nitrate concentration effects biodiversity in marine environments negatively, due to the processes of eutrophication. However, there are significant limitations to the experimental design and further statistical analysis would be required to support this conclusion.
- Khan Academy. (2020). The nitrogen cycle. Retrieved from Khan Academy: https://www.khanacademy.org/science/biology/ecology/biogeochemical-cycles/a/the-nitrogen-cycle
- Pearson Australia. (2020). Pearson Biology Queensland: Student Book Year 12. Melbourne: Alicia Brown.
- Willy Weather. (2020, February 27 and 28). Hastings Point Tide Times and Heights. Retrieved from Tides: WillyWeather: https://tides.willyweather.com.au/nsw/far-north-coast/hastings-point.html