Pan‐Arctic sea ice‐algal chl a biomass and suitable habitat are largely underestimated for multiyear ice

Abstract

There is mounting evidence that multiyear ice (MYI) is a unique component of the Arctic Ocean and may play a more important ecological role than previously assumed. This study improves our understanding of the potential of MYI as a suitable habitat for sea ice algae on a pan-Arctic scale. We sampled sea ice cores from MYI and first-year sea ice (FYI) within the Lincoln Sea during four consecutive spring seasons. This included four MYI hummocks with a mean chl a biomass of 2.0 mg/m 2 , a value significantly higher than FYI and MYI refrozen ponds. Our results support the hypothesis that MYI hummocks can host substantial ice-algal biomass and represent a reliable ice-algal habitat due to the (quasi-) permanent lowsnow surface of these features. We identified an ice-algal habitat threshold value for calculated light transmittance of 0.014%. Ice classes and coverage of suitable ice-algal habitat were determined from snow and ice surveys. These ice classes and associated coverage of suitable habitat were applied to pan-Arctic CryoSat-2 snow and ice thickness data products. This habitat classification accounted for the variability of the snow and ice properties and showed an areal coverage of suitable icealgal habitat within the MYI-covered region of 0.54 million km 2 (8.5% of total ice area). This is 27 times greater than the areal coverage of 0.02 million km 2 (0.3% of total ice area) determined using the conventional block-model classification, which assigns single-parameter values to each grid cell and does not account for subgrid cell variability. This emphasizes the importance of accounting for variable snow and ice conditions in all sea ice studies. Furthermore, our results indicate the loss of MYI will also mean the loss of reliable ice-algal habitat during spring when food is sparse and many organisms depend on ice-algae. K E Y W O R D SCryoSat-2, habitat classification, hockey stick regression, light transmittance, piecewise regression, sea ice algae, sea ice bio-optics, the Last Ice AreaThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

Ice and Snow Thickness Variability and Change in the High Arctic Ocean Observed by In Situ Measurements

“…The gathered colocated, high‐resolution snow and ice thickness data are also required for assessing the differences between first‐year and multiyear ice with regard to their suitability as habitat for sea ice microorganisms and support of high algal biomasses. Our results confirm that even with a thick snow cover on average on multiyear ice, there are abundant locations with shallow or high hummocks with negligible snow cover where light levels underneath can be high enough to support algal growth (Lange et al, ). Our measurements have the potential to better assess the conditions of snow variability in support of deriving light level parameterizations and biophysical modeling.…”

Contrasting Ice Algae and Snow‐Dependent Irradiance Relationships Between First‐Year and Multiyear Sea Ice

“…Nevertheless, a few recent studies suggest that the transition from MYI to FYI may have implications beyond our current understanding of Arctic sea ice ecology. For example, Lange et al, , Lange, Flores, et al, showed that pan‐Arctic estimates of MYI algal habitat and chl a may be underestimated by over an order of magnitude due to an underestimation of hummock habitat. Furthermore, thicker sea ice and sea ice features such as hummocks and ridges have been identified by observations and modeling studies as potential sea ice algal hot spots (Castellani et al, ; Fernández‐Méndez et al, ; Lange, Flores, et al, ; Lange, Katlein, et al, ).…”

Algal Hot Spots in a Changing Arctic Ocean: Sea-Ice Ridges and the Snow-Ice Interface

“…Ridges are complex ice structures that are challenging to sample using the regular ice-core drilling techniques (Timco and Burden, 1997;Gradinger et al, 2010;Lange et al, 2017a). In our study, the ridge algal community was very loosely attached to the surfaces of the ledge and is therefore, partially lost when sampling the entire core by drilling.…”

“…Bright areas were present especially in ridges associated with leads and thin ice (Figure 2 and Videos in the Supplementary Material). Furthermore, cracks at the sides of ridges (Katlein et al, 2015a), as well as often snow-free portions of high points in a ridge due to wind erosion (Sturm and Massom, 2010) have been suggested to transmit more light than adjacent level thick ice (Lange et al, 2017a). In addition, the side with less snow and close to the thin ice received more light and could therefore support higher algal growth rates, assuming light limitation at the thick-ice side ( Table 1).…”

“…In contrast, ice algal areal production is probably decreasing on a pan-Arctic scale due to the loss of sea-ice habitat (Dupont, 2012). In addition, biomass standing stocks are low in young ice compared to the disappearing older ice, probably limited by recruitment, adding to the reduction in sea-ice algal areal production (Lange et al, 2017a;Olsen et al, 2017). As the ice edge retreats further north each summer, ice algae will be limited to the stratified deep basins of the Central Arctic with more oligotrophic conditions compared to the more productive shelves (Barber et al, 2015).…”