Abstract
Abstract. The Arctic Ocean and its marginal seas are among the marine regions most affected by climate change. Here we present the results of a diagnostic model used to assess the primary production (PP) trends over the 1998–2010 period at pan-Arctic, regional and local (i.e. 9.28 km resolution) scales. Photosynthetically active radiation (PAR) above and below the sea surface was estimated using precomputed look-up tables of spectral irradiance, taking as input satellite-derived cloud optical thickness and cloud fraction parameters from the International Satellite Cloud Climatology Project (ISCCP) and sea ice concentration from passive microwaves data. A spectrally resolved PP model, designed for optically complex waters, was then used to assess the PP trends at high spatial resolution. Results show that PP is rising at a rate of +2.8 TgC yr−1 (or +14% decade−1) in the circum-Arctic and +5.1 TgC yr−1 when sub-Arctic seas are considered. In contrast, incident PAR above the sea surface (PAR(0+)) has significantly decreased over the whole Arctic and sub-Arctic Seas, except over the perennially sea-ice covered waters of the Central Arctic Ocean. This fading of PAR(0+) (−8% decade−1) was caused by increasing cloudiness during summer. Meanwhile, PAR penetrating the ocean (PAR(0−)) increased only along the sea ice margin over the large Arctic continental shelf where sea ice concentration declined sharply since 1998. Overall, PAR(0−) slightly increased in the circum-Arctic (+3.4% decade−1), while it decreased when considering both Arctic and sub-Arctic Seas (−3% decade−1). We showed that rising phytoplankton biomass (i.e. chlorophyll a) normalized by the diffuse attenuation of photosynthetically usable radiation (PUR), accounted for a larger proportion of the rise in PP than did the increase in light availability due to sea-ice loss in several sectors, and particularly in perennially and seasonally open waters. Against a general backdrop of rising productivity over Arctic shelves, significant negative PP trends and the timing of phytoplankton spring-summer bloom were observed in regions known for their great biological importance such as the coastal polynyas of northern Greenland.
Highlights
The impacts of environmental changes on Arctic and Sub-Arctic marine ecosystems are already detectable from field-(Grebmeier et al, 2006; Li eSt aol.l,id200E9a) arnthd satellite-based measurements (Arrigo and van Dijken, 2011; Arrigo et al., 2008; Kahru et al, 2011)
The last three parameters were derived from satellite data following the method developed by Zhang et al (2004) and were obtained from the International Satellite Cloud Climatology Project (ISCCP) web site
From 1998 to 2009, which corresponds approximately to the SeaWiFS era, photosynthetically active radiation (PAR)(0+) generally decreased at a rate ranging from −100 to −50 mol photon m−2 yr−1 over seasonally and permanently open water (Fig. 1b), while it increased (∼ +50 to +100 mol photon m−2 yr−1) in over the permanently ice-covered Central Arctic waters
Summary
The impacts of environmental changes on Arctic and Sub-Arctic marine ecosystems are already detectable from field-(Grebmeier et al, 2006; Li eSt aol.l,id200E9a) arnthd satellite-based measurements (Arrigo and van Dijken, 2011; Arrigo et al., 2008; Kahru et al, 2011). The shortwave radiation reaching the sea surface during summer months dropped at a mean annual rate of 0.66 W m−2 yr−1 between 1982 and 1999 due to increasing cloudiness (Wang and Key, 2005). Climate models predict both a reduction in sea ice and an increase in cloud cover for the 21st century as the Arctic warms (Vavrus et al, 2010). Previous studies suggested that the Arctic region will become more productive overall due to a decline in the duration and extent of sea ice (Arrigo and van Dijken, 2011; Arrigo et al, 2008), the net effect of the opposing trends in the evolution of sea ice and cloud cover on PAR, and on the marine primary productivity, has never been assessed
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