The biological effects of nitrate fertilization and water replacement in an oligotrophic cold water pond

Plastic enclosures (limnocorrals) were used to assess the impact of adding NO₃^- to an oligotrophic pond. The pond, Mare's Egg Spring, is spring fed at a constant temperature of 4.5 °C (the main portion of the pool never exceeded 6 °C) and a molar ratio of N:P of 0.5. The pool is dominated by the cyanobacterium (blue green alga), Nostoc pruniforme, which is capable of nitrogen fixation. This, in combination with the low N:P ratio, suggested that non-nitrogen fixing primary producers were limited by nitrogen. Over a period of one month, growth and nitrogen fixation of Nostoc, suspended chlorophyll, sediment denitrification, and benthic diatom diversity were not dependent upon nitrate concentration. However, sedimentary chlorophyll levels increased slightly when NO_inf3^sup-levels were increased from 0.71 µM (ambient) to 214 µM. Limnocorrals with slit sides gave water replacement rates between those for complete enclosure and those in open water. In the open channel, Nostoc growth was highest, and suspended chlorophyll the lowest. In the closed corrals, Nostoc growth was the lowest and suspended chlorophyll the highest, with intermediate values in the slit corrals. Therefore, short term increases of NO_inf3^sup-levels in nitrogen poor aquatic systems do not necessarily affect the biological community, but slowing water replacement in small ponds may have significant effects.