Nutritive and photosynthetic ecology of subsurface chlorophyll maxima in Canadian Arctic waters

Abstract. Assessments of carbon and nitrogen (N) assimilation in Canadian Arctic waters confirmed the large contribution of subsurface chlorophyll maxima (SCM) to total water-column production from spring to late fall. Although SCM communities showed acclimation to low irradiance and greater nitrate (NO₃⁻) availability, their productivity was generally constrained by light and temperature. During spring–early summer, most of the primary production at the SCM was sustained by NO₃⁻, with an average f-ratio (i.e., relative contribution of NO₃⁻ uptake to total N uptake) of 0.74 ± 0.26. The seasonal decrease in NO₃⁻ availability and irradiance, coupled to the build up of ammonium (NH₄⁺), favoured a transition toward a predominantly regenerative system (f-ratio = 0.37 ± 0.20) during late summer and fall. Results emphasize the need to adequately consider SCM when estimating primary production and to revisit ecosystem model parameters in highly stratified Arctic waters.