Abstract
The sea ice cover of the Arctic Ocean has changed dramatically in the last decades, and the resulting consequences for the sea-ice-associated ecosystem remain difficult to assess. Algal aggregates underneath sea ice are of great importance for the ice-associated ecosystem and the pelagic-benthic coupling. However, the frequency and distribution of their occurrence is not well quantified. During the IceArc expedition (ARK-27/3) of RV Polarstern in late summer 2012, we observed different types of algal aggregates floating underneath various ice types in the Central Arctic basins. We investigated the spatial distribution of ice algal aggregates and quantified their biomass, using under-ice image surveys obtained by an upward-looking camera on a remotely operated vehicle. On basin scale, filamentous aggregates of Melosira arctica are more frequently found in the inner part of the Central Arctic pack ice, while rounded aggregates mainly formed by pennate diatoms are found closer to the ice edge, under melting sea ice. On the scale of an ice floe, the distribution of algal aggregates in late summer is mainly regulated by the topography of the ice underside, with aggregates accumulating in dome-shaped structures and at the edges of pressure ridges. The average biomass of the aggregates from our sites and season was 0.1–6.0 mg C m−2. However, depending on the approach used, differences in orders of magnitude for biomass estimates may occur. This highlights the difficulties of upscaling observations and comparing results from surveys conducted using different methods or on different spatial scales.Electronic supplementary materialThe online version of this article (doi:10.1007/s00300-014-1634-3) contains supplementary material, which is available to authorized users.
Highlights
The Arctic Ocean has changed dramatically in recent decades
We investigated the spatial distribution of ice algal aggregates and quantified their biomass, using under-ice image surveys obtained by an upward-looking camera on a remotely operated vehicle
Filamentous aggregates of Melosira arctica are more frequently found in the inner part of the Central Arctic pack ice, while rounded aggregates mainly formed by pennate diatoms are found closer to the ice edge, under melting sea ice
Summary
The Arctic Ocean has changed dramatically in recent decades. Changes of physical processes in the climate system—such as decreased sea ice extent (Serreze et al 2007) and thickness (Haas et al 2008; Kwok and Rothrock 2009), the trend from multi-year to younger first-year sea ice (Maslanik et al 2007), a longer melt season (Markus et al 2009), increased melt-pond coverage (Roesel and Kaleschke 2012), and increased light transmittance through the ice (Nicolaus et al 2012)—are affecting the sea ice ecosystem (Arrigo et al 2008; Lee et al 2011; Arrigo 2014). Changed melting conditions and increased light availability are expected to affect the life conditions of sea ice algae (Lee et al 2011; Leu et al 2010). The term ‘‘algal aggregates’’ refers to macroscopic ([1 cm) mostly free-floating aggregations mainly formed by typical seaice-associated algae such as those described by FernandezMendez et al (2014). These aggregates have previously been described in the literature using various names such as sub-ice assemblages, algal filaments or aggregations (Nansen 1906; Melnikov and Bondarchuk 1987; Horner et al 1992; Gutt 1995). Despite the long history of observations of algal aggregates under Arctic sea ice, little is known about the factors controlling their spatial distribution on both floe and basin scales
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