Abstract
The light compensation irradiance for planktonic metabolic balance, defined as the irradiance where gross planktonic primary production equals community respiration, is an important property describing ecosystem dynamics. Planktonic communities receiving irradiances above the compensation irradiance or compensation depth (i.e., the depth at which the compensation irradiance is received) are autotrophic and act as CO2 sinks, whereas those at lower irradiances or located deeper in the water column act as CO2 sources. However, this property is undefined for heterotrophic communities in which metabolic balance is not set by light availability. The compensation irradiance for planktonic metabolism in the ocean was quantified experimentally and calculated using data available in the literature to assess its variability and possible controls. Gross primary production by the oceanic planktonic communities examined here meet their respiratory requirements at irradiances of about 1.1 +/- 0.4 mol quanta m(-2) d(-1) and tend to be autotrophic above a depth of 36 +/- 9 m, on average. The depth of nitracline is closely correlated with the compensation depth for community metabolism across the studied areas, but the compensation depth tends to be located above the depth of the nitracline. This is expected from the facts that the underlying, net heterotrophic communities should act as sources of inorganic nutrients and that the nitracline cannot develop within the mixed layer where the compensation depth is often located. These results imply that the planktonic communities examined extending from 36 m depth, on average, to the bottom of the euphotic layer tend to be heterotrophic, acting as CO2 and inorganic nutrient sources.