Composition, Buoyancy Regulation and Fate of Ice Algal Aggregates in the Central Arctic Ocean

Sea-ice diatoms are known to accumulate in large aggregates in and under sea ice and in melt ponds. There is recent evidence from the Arctic that such aggregates can contribute substantially to particle export when sinking from the ice. The role and regulation of microbial aggregation in the highly seasonal, nutrient- and light-limited Arctic sea-ice ecosystem is not well understood. To elucidate the mechanisms controlling the formation and export of algal aggregates from sea ice, we investigated samples taken in late summer 2011 and 2012, during two cruises to the Eurasian Basin of the Central Arctic Ocean. Spherical aggregates densely packed with pennate diatoms, as well as filamentous aggregates formed by Melosira arctica showed sign of different stages of degradation and physiological stoichiometries, with carbon to chlorophyll a ratios ranging from 110 to 66700, and carbon to nitrogen molar ratios of 8–35 and 9–40, respectively. Sub-ice algal aggregate densities ranged between 1 and 17 aggregates m−2, maintaining an estimated net primary production of 0.4–40 mg C m−2 d−1, and accounted for 3–80% of total phototrophic biomass and up to 94% of local net primary production. A potential factor controlling the buoyancy of the aggregates was light intensity, regulating photosynthetic oxygen production and the amount of gas bubbles trapped within the mucous matrix, even at low ambient nutrient concentrations. Our data-set was used to evaluate the distribution and importance of Arctic algal aggregates as carbon source for pelagic and benthic communities.海冰硅藻（Sea-ice diatoms）已知会在海冰内部、海冰下方以及融水池中大量聚集形成聚集体。近期来自北极的研究证据表明，这类聚集体从海冰沉降时，可对颗粒输出产生显著贡献。在高度季节性且营养盐与光照均受限的北极海冰生态系统中，微生物聚集体的作用与调控机制目前仍未得到充分阐明。为阐明调控海冰来源藻类聚集体形成与输出的机制，我们于2011年与2012年夏末，通过两次前往中央北冰洋欧亚海盆的科考航次采集了样本。由羽纹硅藻（pennate diatoms）紧密填充形成的球形聚集体，以及由北极海链藻（Melosira arctica）形成的丝状聚集体，均呈现出不同降解阶段与生理化学计量特征：其碳与叶绿素a的比值范围为110至66700，碳氮摩尔比则分别为8~35与9~40。冰下藻类聚集体的密度介于1至17个·m⁻²之间，其估算的净初级生产力为0.4~40 mg C·m⁻²·d⁻¹，占总光合生物量的3%~80%，并可达当地净初级生产力的94%之高。调控聚集体浮力的潜在因素之一为光照强度：其可调控光合产氧量以及黏液基质内捕获的气泡量，即便在环境营养盐浓度较低的情况下亦是如此。本数据集被用于评估北极藻类聚集体的分布特征及其作为远洋与底栖群落碳源的重要性。