Figure 1.
Map of the Arctic Eurasian Basin with sampling stations.
Yellow dots represent ice stations where algal aggregates were observed (Expedition PS78/3 took place in summer 2011 and PS80/3 in summer 2012). The purple line corresponds to the September monthly average sea ice extent in 2011 and the pink line in 2012 (Source: http://nsidc.org/data/seaice_index/).
Figure 2.
Distribution of aggregates in melt ponds.
Degraded pennate aggregates in cryoconite holes at the bottom of a partially open melt pond at station PS78/3_212 (A). Melosira filaments hanging from newly formed ice covering an open melt pond at station PS80/3_349 (B). Spherical floating aggregates below sea ice at station PS80/3_237 (Image taken with the ROV Ronja (Courtesy Alfred Wegener Institute Helmholtz Center for Marine and Polar Research (AWI)) (C). Degraded Melosira filaments trapped at the bottom of a partially open melt pond at station PS80/3_224 (D). Scale bar = 20 cm.
Table 1.
Algal aggregate types and their degradation stages.
Table 2.
Local areal estimates of fresh sub-ice algal aggregates per aggregate type compared to sea ice and water column at the same location and other studies.
Figure 3.
Representative types of aggregates observed.
Macroscopic images of the aggregates (A–D) and their microscopic composition (E–H). Fresh spherical floating aggregates below the ice formed by pennate diatoms (A and E). Degraded aggregate trapped in a closed melt pond formed by dead pennate diatoms with empty frustules (B and F). Filaments hanging from newly formed ice over an open melt pond formed by Melosira arctica cells that contain green chloroplasts (C and G). Degraded aggregate formed mainly by dead Melosira arctica cells trapped at the bottom of a partially open melt pond (D and H). Scale bar = 5 cm (A), 2 cm (B), 10 cm (C and D), and 20 µm (E–H).
Figure 4.
Photosynthesis vs Irradiance curve of Melosira arctica algal slurry (M5).
Carbon uptake measured with the 14C radioactive isotope method (green circles) and oxygen production or consumption using optodes (purple diamonds) of the algal aggregate slurry from the Melosira arctica aggregate used for the buoyancy experiment (Fig. 5). The threshold irradiance for oxygen production is around 20 µmol photons m−2 s−1. Oxygen rates have been transformed to carbon equivalents using the photosynthetic quotient of 1.25 to improve the visualization of the two different methods used to measure NPP.
Figure 5.
(A) A piece of ice with several attached filamentous algal aggregates was kept in a glass beaker at simulated in situ conditions. (B) Once the ice disappeared it was observed that air bubbles were trapped in the mucous matrix of the floating aggregates. (C) Despite the salinity gradient, one of the aggregates sank when the air bubbles were removed. (D) 24 h later all algal aggregates had sunk. (E) Only when increasing the light intensity, some of the algal aggregates were able to produce enough oxygen to regain buoyancy. Scale bar = 2 cm.
Figure 6.
Conceptual model of the mechanisms responsible for the formation and fate of the different types of algal aggregates.
(1) In early spring sea-ice algal growth starts before that of phytoplankton. (2) In summer as the sea ice melts and nutrients become limiting, some sea-ice algae are released to the water column or grow into the water column. Due to their stickiness and the under-ice turbulence they form aggregates. (3) In late summer melt ponds grow in depth and the sea-ice algae that are still in the ice are gradually exposed to very low salinities, high irradiances, and nutrient depletion making them accumulate and degrade in the pond. Depending on the environmental conditions, some sub-ice aggregates sink and others remain floating. (4) In early autumn, the melt ponds can either open completely allowing some phytoplankton species to come into the melt pond, or they can refreeze again, becoming second year ice. (5) In autumn those melt ponds that were open to the seawater freeze again, trapping the floating aggregates that were not grazed in the newly formed ice.