Fertilization of Aquarium Ecosystems with Phosphate

Paige Roberts

CU Boulder, Fall 2008


Phosphate is a limiting nutrient for many life forms in aquatic ecosystems, including algae, cyanobacteria, and fungi.  This experiment asked how these environments would respond to increased concentrations of phosphate, hypothesizing that phosphate fertilization would cause an increase in the systems productivity.  Increasing concentrations of phospate (in the form of dipotassium hydrogen phosphate, K2HPO4) were added to three beakers containing aquarium water and equal biomasses of algae.  (One beaker was not given phosphates, thus acting as a control).  The beakers were then allowed to incubate for six days.  The hypothesis predicted an increase in biomass and an increase in the count and diversity of cyanobacteria.  The systems indeed saw an increase in biomass across the beakers, mainly in the form of a white-grey, cotton-like fungus, with a high statistical significance of R2= 0.934.  Furthermore, algae were observed to be generally healthier with greater amounts of chlorophyll and thicker blades.  While the systems did show an increase in the diversity of cyanobacteria, they also showed a dramatic decrease in the population count of cyanobacteria.  A t-test was run in order to test the relationship between the increase in phophate concentrations and the decrease in the cyanobacteria count; the test returned a value of p=0.128, meaning that these samples did not have a significant relationship.  Previous studies suggest a negative correlation between the growth of algae and bacteria populations due to food competition; thus a second t-test was run between the systemÕs increase in biomass and decrease in cyanobacteria count.  This test returned a p-value of 0.129, again showing no significant correlation.

The observed increase in fungal biomass supports the hypothesis.  However, the results in the decrease in cyanobacteria suggest new interpretations and new hypotheses to explain this observation. This experiment returned insignificant results regarding any relationship between the phosphate concentrations increase and the decrease in cyanobacteria, but perhaps the relationship is indirect; if the increase in phosphate concentration increased the biomass, the increased biomass could affect the cyanobacteria populations.  While this experiment found no such significant correlation, further trials are needed to support this contention that no relationship exists.  Furthermore, more precise techniques for population and diversity counting would improve the experiments validity.