The effects of irradiance and nutrient supply on the productivity of Arctic waters: a perspective on climate change

A previous analysis of published data suggested that annual, pelagic primary production in the Arctic Ocean is related linearly to the duration of the ice-free period, presumably through cumulative exposure to solar irradiance. However, the regions with the longest ice-free periods are located in peripheral seas and polynyas where nutrient supply by advection or the vertical mixing induced by winds and convection can be extensive. The ensuing replenishment of nutrients drives primary production to levels unattained in the strongly stratified interior (e.g. the Beaufort Sea), with the exception of upwelling areas. A reanalysis of published data showed no relation between cumulative production and incident solar radiation during the growth season. We propose that changes in annual primary production per unit area in seasonally ice-free waters are controlled primarily by the environmental forcing of nitrogen supply. Incidental changes in light regime should mostly affect the timing and, possibly, the species composition of the main production pulse(s) in the upper mixed layer and, underneath, the ability of phytoplankton to exploit nutrients in the lower euphotic zone. While the ongoing rise in the supply of heat and freshwater to the Arctic Ocean should bolster vertical stratification and further impede the mean upward supply of nutrients, episodic yet direct atmospheric forcing of the upper ocean may act in synergy with a prolonged exposure to light and greatly augment pelagic productivity.