Comparison of Nutrient Limitation in Freshwater and Estuarine Reservoirs in Tropical Urban Singapore

Nutrient limitation was examined for two contrasting tropical reservoirs in Singapore: the semiurban, freshwater Kranji Reservoir and the highly urbanized, estuarine Marina Reservoir. Nutrient enrichment experiments in the laboratory showed that the Kranji Reservoir was phosphorus limited, whereas the Marina Reservoir was nitrogen limited. This was supported by field measurements of nitrogen (N) to phosphorus (P) ratios that showed average values of 16 and 8.3 for the Kranji Reservoir and Marina Reservoir, respectively. In addition to nutrient concentrations, total suspended solids measurements (TSS) showed high levels of turbidity in the Marina Reservoir (average TSS=74.2 mg/L) compared to the Kranji Reservoir (average TSS=3.9 mg/L). Thus, light could also be a significant limiting factor for algal growth in the Marina Reservoir. To estimate whether the estuarine system was a source or sink for nutrients, budget models were used to determine fluxes of nitrogen, phosphorus, and carbon. The calculations showed that there was a net loss of nitrogen from the estuarine Marina Reservoir through denitrification and a net loss of carbon. Phosphorus, in contrast, was estimated to be released from the sediments. These results support the earlier findings that the Marina Reservoir is currently nitrogen-limited. The next logical step is to trace the main sources of nutrients into the reservoirs and to deal with nutrient load reduction, bearing in mind that one reservoir is phosphorus limited and the other is nitrogen limited. The implication is that different remediation methods may need to be employed to tackle the eutrophication problems in the two contrasting reservoirs.