The relationship between nitrate and ammonia concentrations in the pelagic zone of lakes

Both nitrate (NO3-N) and ammonia (NH4-N) concentrations are in general highly variable during a lake’s seasonal cycle. During the growing seasonal peak, such concentrations can be very low due mainly to its’ sequestration by phytoplankton. With the aim to study NO3-N and NH4-N mean concentration variation along the increase of the trophic state of lakes, this study simultaneously analyses the information for more than 400 lakes worldwide. Both forms of nitrogen are directly related to the trophic state of a lake when lakes are compared with total phosphorous concentration (TP) ranging across five magnitude levels. However, in lakes a pronounced non-linearity dominates the relationships among the different forms of nitrogen and phosphorous. Our results suggest that the empirical basis for both the limiting resource and inorganic nitrogen hypotheses are related. The NO3-N: NH4-N ratio is directly related to the TN: TP ratio for lakes in general and shallow ones in particular.