A method to estimate net community metabolism from profiles of dissolved O-2 and N-2

A method to estimate net community metabolism (NCM) in natural waters using vertical profiles of water temperature, salinity, dissolved O-2, gas tension, and calculated dissolved N-2 is presented. The method utilizes the disparate biological activity of dissolved O-2 and N-2 to estimate metabolism at different depths in the water column. For well-mixed surface waters, N-2 saturation levels are assumed to be the result of a quasi steady state balance of net warming or cooling and air-water gas exchange. Dissolved O-2 levels are assumed to maintain a similar balance, subject to net biological activity, and NCM is then calculated based on the difference between N-2 and O-2 saturation levels and the estimated timescale required to equilibrate the layer with the atmosphere. For deeper stratified layers of water that warmed after layer formation in isolation from the atmosphere, the temperature at formation is calculated using the measured N-2 concentration and an assumed N-2 saturation level of 100% at formation. By assuming that initial N-2 and O-2 saturation levels were equal, the initial O-2 concentration is calculated based on solubility relationships. NCM of the deeper waters is then estimated based on this information and knowledge of the general seasonal heating cycle of the waters. Daily mean water temperature and dissolved gas levels are used in the calculations. The method was assessed using profile measurements collected at Long Pond, Plymouth, Massachusetts, USA, on 23 August 2002. Oxygen was supersaturated relative to N-2 by approximately 4% in the 0-6 m deep epilimnion, and undersaturated relative to N-2 by approximately 7% in the stratified water at 9 m depth. The estimated 4-day average NCM for the epilimnion was 140 +/- 70 mgC m(-2) day(-1). For waters at 9 m depth, the temperature at formation was calculated to be 6.58 degrees C, and the estimated 100-day average NCM was -2.5 +/- 0.6 mgC m(-3) day(-1). An independent estimate of -4.6 +/- 0.9 mgC m(-3) day(-1) was derived from the measured O-2 decline at 9 m depth over the same period of 2003.