Primary Productivity and Nutrient Variability
K.E. Frey1, K.R. Arrigo2, W.J. Williams3
1Graduate School of Geography, Clark University, Worcester, MA, USA
2Department of Environmental Earth System Science, Stanford University, Stanford, CA, USA
3Institute of Ocean Sciences, Fisheries and Oceans Canada, Sidney, BC, Canada
November 14, 2012
- Massive phytoplankton blooms beneath a 0.8-1.3 m thick, fully consolidated (yet melt-ponded) sea ice pack were observed in the north-central Chukchi Sea in July 2011. The blooms extended >100 km into the ice pack and biomass was greatest (>1000 mg C m-3) near the ice-seawater interface, with nutrient depletion to depths of 20-30 m.
- New satellite remote sensing observations show (a) the near ubiquity of ice-edge blooms across the Arctic and the importance of seasonal sea ice variability in regulating primary production, and (b) a reduction in the size structure of phytoplankton communities across the northern Bering and Chukchi seas during 2003-2010.
- A unique marine habitat containing abundant algal species in so-called "melt holes" was observed for the first time in perennial sea ice in the central Arctic Ocean.
- During the last decade, the intensification of the Beaufort Gyre has pushed the nutricline deeper, and the subsurface chlorophyll maximum that was at 45 m in 2002 has deepened to 60-65 m in 2008-2012.
Sea ice melt and breakup during spring strongly drive primary production in the Arctic Ocean and its adjacent shelf seas by enhancing light availability as well as increasing stratification and stabilization of the water column. Previous large-scale, synoptic estimates of primary production in the Arctic Ocean typically assume that phytoplankton in the water column beneath the sea ice pack is negligible. However, massive phytoplankton blooms beneath a 0.8-1.3 m thick, fully consolidated, yet melt-ponded, sea ice pack were observed in the north-central Chukchi Sea in July 2011 (Arrigo et al., 2012) (Fig. 3.1). These blooms, primarily consisting of pelagic diatoms of the genera Chaetoceros, Thalassiosira, and Fragilariopsis, indicating this was not a remnant sea ice algal bloom, extended from the ice edge to >100 km northward into the pack ice. Biomass was greatest (>1000 mg C m-3) near the ice/seawater interface and was associated with large nutrient deficits in the upper 25-30 m of the water column beneath the ice (Fig. 3.2). Given these new observations, previous estimates of annual primary production in waters where these under-ice blooms develop may be ~10-times too low (Arrigo et al., 2012).
Although it is not clear whether these under-ice phytoplankton blooms are a new phenomenon, a shift away from snow-covered multi-year ice (typical of these areas in the 1980s) towards a thinner, more melt-ponded sea ice cover (typical of current conditions) will enhance the light transmittance (Frey et al., 2011) necessary for primary production, given the presence of sufficient nutrients. During the early 1980s, the location of these under-ice phytoplankton blooms was covered throughout the summer by thick (~3 m) multiyear ice with a deeper snow cover (0.4 m) and fewer melt ponds than the first year ice observed in July 2011. As such, the amount of light transmitted through snow-covered multi-year ice in the 1980s (<0.1% of surface light) would have been far less than that transmitted through melt-ponded first year ice during July 2011 (13-59% of surface light) and inadequate to support the large under-ice phytoplankton blooms observed that year (Arrigo et al., 2012).
Additional insights into Arctic Ocean primary production have been derived from satellite remote sensing observations, including information related to ice-edge blooms and size structure of phytoplankton communities. Perrete et al. (2011) show strong connections between seasonal sea ice retreat and primary production, where these near-ubiquitous ice-edge blooms across the pan-Arctic are observed in 77-89% of locations where adequate data exist. These bloom are typically observed to peak within 20 days of ice retreat and average >1 mg m-3 (with major blooms >10 mg m-3). Ice-edge blooms are less common in areas of early sea ice melt (at lower latitudes) and their contributions to annual primary productivity rates are reduced owing to the long periods available for open-water blooms. In contrast, at higher latitudes, it is shown that primary productivity rates at the ice-edge may be 1.5-2 times greater than those in open-water conditions. In addition, Fujiwara et al. (2011) have developed a new satellite-based algorithm for deriving the size structure of phytoplankton communities across the northern Bering and Chukchi seas. Through these analyses, it is suggested that phytoplankton size is inversely related to variability in sea surface temperature. Furthermore, during the period investigated (2003-2010), the derived phytoplankton size index was shown to significantly decrease (likely related to limitations in nutrients), which is an important finding in the context of carbon turnover rates and the vertical pathways of carbon flow. For example, picoplankton-based systems, such as those observed in the Canada Basin, typically do not support large exports of biogenic carbon, neither through extraction via heterotrophic activity nor sequestration in benthic environments (Li et al., 2009).
In addition to phytoplankton primary production, sea ice algal production is also important to consider in the overall Arctic Ocean system. Sea ice primary production was recently modeled to account for up to ~40% of total primary production (depending on location) and up to ~7.5% when considering the entire Arctic region (Dupont, 2012). Furthermore, a unique marine habitat for sea ice algae in so-called "melt holes" was observed for the first time in perennial sea ice in the central Arctic Ocean (Lee et al., 2011; Lee et al., 2012). The melt holes are formed in thinning sea ice where surface melt ponds completely penetrate the ice and connect to the underlying seawater, and rarely have been investigated for their role in ecosystem productivity. These open ponds have higher nutrient concentrations than closed surface melt ponds owing to the connection with the seawater; consequently, this newly observed marine habitat contains abundant algal species (mainly Melosira arctica, constituting >95% of the biomass) known to be important for zooplankton consumption. Furthermore, the accumulation of these algal masses attached to refreezing ice in late summer may provide an important food supplement for higher trophic levels as the ecosystem enters winter. Lee et al. (2012) estimate that the total carbon production in melt ponds in Arctic sea ice amounts to ~2.6 Tg C yr-1, which constitutes <1% of recent total production in the whole Arctic Ocean. However, this fraction may be significantly higher when only considering ice-covered portion of the Arctic Ocean. Lee et al. (2011) suggest that continued warming and decreases in sea ice extent and thickness may result in a northward extension of these open pond areas (enhancing overall primary production in these habitats), but even their ultimate potential disappearance as perennial sea ice cover becomes more scarce across the Arctic Ocean.
Important shifts in nutrient availability in recent years have also driven significant changes in primary production of Arctic Ocean waters. A subsurface chlorophyll maximum stretches across the Beaufort Gyre region of the Canada Basin in summer. It occurs at the top of the nutricline, a location that is as close to sunlight as possible while nutrients are still present (McLaughlin and Carmack, 2010). The Beaufort Gyre has intensified dramatically since 2002 owing to Ekman convergence, with a particularly large jump in the winter of 2007/2008 (see the Ocean essay for more information about the intensification of the Beaufort Gyre and consequences for heat and freshwater content). This intensification has pushed the halocline down, as indicated by the deepening of the 33.1 psu isohaline (Fig. 3.3a). Additional melting of sea ice during this time has also resulted in lower surface salinities (Fig. 3.3b) and increased upper halocline stratification (Fig. 3.3c). The combination of a deeper halocline and stronger stratification has pushed the top of the nutricline farther away from sunlight and reduced nutrient availability, thus stressing phytoplankton at the subsurface chlorophyll maximum (McLaughlin and Carmack, 2010). The subsurface chlorophyll maximum is correspondingly deeper (Fig. 3.3d). Data from 2012 are similar to 2008-2010, and consistent with a slight relaxation of the Beaufort Gyre.
Arrigo, K. R., D. K. Perovich, R. S. Pickart, Z. W. Brown, G. L. van Dijken, K. E. Lowry, M. M. Mills, M. A. Palmer, W. M. Balch, F. Bahr, N. R. Bates, C. Benitez-Nelson, B. Bowler, E. Brownlee, J. K. Ehn, K. E. Frey, R. Garley, S. R. Laney, L. Lubelczyk, J. Mathis, A. Matsuoka, B. G. Mitchell, G. W. K. Moore, E. Ortega-Retuerta, S. Pal, C. M. Polashenski, R. A. Reynolds, B. Scheiber, H. M. Sosik, M. Stephens and J. H. Swift. 2012. Massive phytoplankton blooms under Arctic sea ice. Science, 336, 1408, doi:10.1126/science.1215065.
Dupont, F. 2012. Impact of sea-ice biology on overall primary production in a biophysical model of the pan-Arctic Ocean. J. Geophys. Res., 117, C00D17, doi:10.1029/2011JC006983.
Fujiwara, A., T. Hirawake, K. Suzuki and S. I. Saitoh. 2011. Remote sensing of size structure of phytoplankton communities using optical properties of the Chukchi and Bering Sea shelf region. Biogeosci., 8, 3567-3580.
Frey, K. E., D. K. Perovich and B. Light. 2011. The spatial distribution of solar radiation under a melting Arctic sea ice cover. Geophys. Res. Lett., 38, L22501, doi:10.1029/2011GL049421.
Lee, S. H., C. P. McRoy, H. M. Joo, R. Gradinger, X. Cui, M. S. Yun, K.H. Chung, S. H. Kang, C. K. Kang, E. J. Choy, S. Son, E. Carmack and T. E. Whitledge. 2011. Holes in progressively thinning Arctic sea ice lead to new ice algae habitat. Oceanography, 24(3), 302-308, http://dx.doi.org/10.5670/oceanog.2011.81.
Lee, S. H., D. A. Stockwell, H. M. Joo, Y. B. Son, C. K. Kang and T. E. Whitledge. 2012. Phytoplankton production from melting ponds on Arctic sea ice. J. Geophys. Res., 117, C04030, doi:10.1029/2011JC007717.
Li, W. K. W., F. A. McLaughlin, C. Lovejoy and E. C. Carmack. 2009. Smallest algae thrive as the Arctic Ocean freshens. Science, 326, 539.
McLaughlin, F. A. and E. C. Carmack. 2010. Deepening of the nutricline and chlorophyll maximum in the Canada Basin interior, 2003-2009. Geophys. Res. Lett., 37, L24602, doi:10.1029/2010GL045459.
Perrette, M., A. Yool, G. D. Quartly and E. E. Popova. 2011. Near-ubiquity of ice-edge blooms in the Arctic. Biogeosci., 8, 515-524.