[1] Sea ice in the Canada Basin of the Arctic Ocean has decreased significantly in recent years, and this will likely change the properties of the surface waters. A near-surface temperature maximum (NSTM) at typical depths of 25-35 m has been previously described; however, its formation mechanisms, seasonal evolution, and interannual variability have not been established. Based on summertime conductivity, temperature, and depth surveys and year-round Ice-Tethered Profiler data from 2005 to 2008, we found that the NSTM forms when sufficient solar radiation warms the upper ocean. A seasonal halocline forms in summer once enough sea ice melt has accumulated to separate the surface mixed layer from the NSTM. The NSTM becomes trapped below the summer halocline, thereby storing heat from solar radiation. This heat can be stored year-round in the Canada Basin if the halocline is strong enough to persist through winter. In addition, energy from storm-driven mixing can weaken the summer halocline and entrain the NSTM, thereby melting sea ice in winter. Throughout this cycle, Ekman pumping within the convergent Beaufort Gyre acts to deepen the NSTM. From 1993 through 2007, the NSTM warmed and expanded northward and both the NSTM and the summer halocline formed at successively shallower depths. North of 75°N, the temperature of the NSTM increased from 2004 to 2007 by 0.13°C/yr, and the NSTM and summer halocline shoaled by 2.1 m/yr and 1.7 m/yr, respectively, from 1997 to 2007. The formation and dynamics of the NSTM are manifestations of both the ice-albedo feedback effect and changes to the freshwater cycle in the Canada Basin.
The fate of ice‐bottom algae, before and after release from the first‐year sea ice into the water column, was assessed during the period of ice‐algal growth and decline in Resolute Passage (Canadian Arctic). During spring 1992 (from April to June), algae in the bottom ice layer and those suspended and sinking in the upper water column (top 15 m) were sampled approximately every 4 days. Ice‐bottom chlorophyll a reached a maximum concentration of 160 mg m−2 in mid‐May, after which it decreased to lower values. In the water column, chlorophyll a concentrations were low until the period of ice‐algal decline (∼0.1 mg m−3), with most biomass in the <5‐μm fraction. In both the suspended and sinking material, large increases of algal biomass occurred at the beginning of June, following the release of ice‐algae into the water column. The input of ice‐algal derived carbon to the upper water column and the proportions exported through sinking or remaining in suspension were assessed using a carbon budget for the two periods of ice‐algal growth and decline. For each period the output terms closely balanced the input. The carbon budget showed that most of the biomass introduced into the upper water column remained suspended (>65% of total export) and that ice‐algae were ingested by under‐ice grazers after release from the ice. These results stress the importance of ice algae for pelagic consumers during the early stages of ice melt and show that the transfer of ice algae to higher trophic levels extends beyond the period of maximum algal production in the ice bottom.
Variations of sea-ice microalgae at the ice–water interface (Manitounuk Sound, Hudson Bay, Canada) were studied in relation to various energy inputs (light, tidal mixing, and heat) in April and May 1982. Seasonal photosynthetic activity does not start before the light intensity reaches 7.6 μEinst∙m−2∙s−1. Above this value, the seasonal increase in cell numbers and chlorophyll and in the photoadaptation index (Ik) is related to the increase in underice light intensity. The sea-ice community changes from shade to light adaptation to optimize the use of ambient light energy. Photosynthetic efficiency (αB) is mainly controlled by the fortnightly tidal vertical mixing, which governs the amount of phosphate (or of another nutrient factor) in the upper brackish layer. The ice microflora, which grows at a stable interface, takes advantage of nutrient replenishment during mixed water column conditions. We conclude that production of microalgae depends upon three forms of energy: (1) the flux of solar light, (2) the inputs of auxiliary mechanical energy (here, the fortnightly tides), and (3) the energy exchanges (here, the heat flux) responsible for the maintenance or destruction of energetic interfaces (ergoclines).
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