Impact of Ice Algae on Inorganic Nutrients in Seawater and Sea Ice in Barrow Strait, NWT, Canada, During Spring

Abstract

Except in "bottom ice" (lowest few centimetres) and surface waters impacted by autotrophs, the major inorganic nutrients behave conservatively in seawater and sea ice. From mid- to late spring, steep and persistent nutrient gradients were observed in the "well-mixed surface layer" with minima near the ice–water interface. Nitrate, ammonium, and phosphate are highly concentrated in heavily colonized bottom ice relative to seawater and the remainder of the ice sheet; concentrations in darkened, weakly colonized bottom ice are similar to the ice sheet. These nutrients also display strong vertical stratification over millimetre scales. Nitrate and phosphate in the bottom ice layer display strong positive relationships with chlorophyll. The accumulation of these nutrients in bottom ice must be biologically mediated and constitutes a significant sink. In contrast, silicic acid concentrations in bottom ice are close to those expected for sea ice formed from the source seawater, are only weakly related to algal biomass, and vary much less seasonally. Ice algae are apparently shocked osmotically and release their intracellular pools of dissolved nutrients. Intracellular pools of nitrate averaged 1.4–9.5% of total particulate nitrogen. Nitrient stresses, during periods of high biomass and sluggish supply, may be alleviated by pooling.