Seasonal sea ice cover as principal driver of spatial and temporal variation in depth extension and annual production of kelp in Greenland

Abstract

We studied the depth distribution and production of kelp along the Greenland coast spanning Arctic to sub-Arctic conditions from 78 °N to 64 °N. This covers a wide range of sea ice conditions and water temperatures, with those presently realized in the south likely to move northwards in a warmer future. Kelp forests occurred along the entire latitudinal range, and their depth extension and production increased southwards presumably in response to longer annual ice-free periods and higher water temperature. The depth limit of 10% kelp cover was 9–14 m at the northernmost sites (77–78 °N) with only 94–133 ice-free days per year, but extended to depths of 21–33 m further south (73 °N–64 °N) where >160 days per year were ice-free, and annual production of Saccharina longicruris and S. latissima, measured as the size of the annual blade, ranged up to sevenfold among sites. The duration of the open-water period, which integrates light and temperature conditions on an annual basis, was the best predictor (relative to summer water temperature) of kelp production along the latitude gradient, explaining up to 92% of the variation in depth extension and 80% of the variation in kelp production. In a decadal time series from a high Arctic site (74 °N), inter-annual variation in sea ice cover also explained a major part (up to 47%) of the variation in kelp production. Both spatial and temporal data sets thereby support the prediction that northern kelps will play a larger role in the coastal marine ecosystem in a warmer future as the length of the open-water period increases. As kelps increase carbon-flow and habitat diversity, an expansion of kelp forests may exert cascading effects on the coastal Arctic ecosystem.

Climate-driven regime shifts in Arctic marine benthos

“…Increased nutrient input associated with warming could have been a source of enhanced growth, but a recent nutrient enrichment study shows that macroalgae in Kongsfjord are not N-limited because they take up and store nitrogen compounds during winter (32). Along the rocky coastlines of the Arctic, boreal macroalgae are predicted to expand within the 21st century as a consequence of climate warming (21,33), an expectation that our findings support.…”

“…The observed increase in macroalgal cover is likely representative of a regional trend toward increased macroalgal biomass, as supported by a separate study in Hornsund, in the south of Svalbard, where a threefold increase in biomass was recorded between 1988 and 2008 (20). In addition, a study from West Greenland documented substantial increases in the productivity and depth extension of macroalgae (kelp beds) in relation to the retreat of sea ice and prolonging of the open water period (21). In our study, the character of the observed structural and functional change in the benthos, in both investigated fjords, is indicative of an abrupt ecological regime shift (8).…”

“…The observed local increases in erect macroalgal cover are predicted to occur throughout the Arctic as a response to changing environmental conditions (21,33). Such a structural change is expected to have implications for the functioning of rocky-bottom ecosystems.…”

Expansion of vegetated coastal ecosystems in the future Arctic

“…In addition to favoring the poleward flux of species, arctic warming will likely affect their metabolic rates (Brown et al, 2004), increasing photosynthetic rates and growth and affecting their phenology, with an earlier initiation of growth and a longer season to complete their growth cycle (Kortsch et al, 2012;Krause-Jensen et al, 2012;Clausen et al, 2014;Olesen et al, 2014). Moreover, future increases in atmospheric CO 2 , and airsea supply of CO 2 may further propel macrophyte expansion in arctic waters.…”

“…Hence, the Arctic Ocean contains a vast potential habitat for marine macrophytes. However, much of this potential habitat is unlikely to be occupied at present because it is permanently covered by ice, impacted by ice scouring or because dense ice cover limits the duration of the growth period (Müller et al, 2009;Kortsch et al, 2012;Krause-Jensen et al, 2012).…”