Elsevier

Journal of Marine Systems

Volume 147, July 2015, Pages 76-84
Journal of Marine Systems

Characterizing the sea ice algae chlorophyll a–snow depth relationship over Arctic spring melt using transmitted irradiance

https://doi.org/10.1016/j.jmarsys.2014.01.008Get rights and content

Highlights

  • Sea ice chl a was measured using the normalized difference index method.

  • Chl a-snow relationships were assessed at time series, transects, and snow-free sites.

  • Timing of bloom peak was dependent on snow depth, timing of bloom termination was not.

  • Four phases of chl a–snow depth association were measured over the spring.

  • Rapid snow removal decreased bloom length and magnitude of chl a accumulation

Abstract

The bottom ice algae chlorophyll a (chl a)–snow depth (HS) relationship was investigated for first-year sea ice in Allen Bay, Nunavut, from 27 April to 13 June 2011. A transmitted irradiance technique was used to estimate ice algae chl a throughout the period at time series locations covered and cleared of snow. Furthermore, chl a was estimated along transects perpendicular to dominant snowdrift orientation, and at short-term snow clear experimental sites. The association between chl a and most snow depths was characterized by four phases over the spring; light limitation (negative relationship), a transitional period (no relationship), chl a decline associated with higher transmitted irradiance (positive relationship), and a final phase of chl a decline independent from HS (no relationship). Algal chl a under areas cleared of snow was lower, reached zero chl a earlier and declined faster than snow-covered control sites. Results indicated that snow removal caused these chl a responses through photoinhibition, as well as ice melt later in the spring. Based on this research we propose that weather events that can rapidly melt the snowpack could significantly deplete bottom ice chl a and cause early termination of the bloom if they occur late in the spring.

Introduction

Accessibility of light to sea ice primary producers during the spring bloom is largely dependent on radiative transfer through the snow and sea ice cover. Absorption and scattering of photons within the sea ice matrix, composed of ice, brine, air, and sometimes salts, efficiently attenuate light, resulting in an exponential decline of photosynthetically active radiation (PAR; 400–700 nm) transmittance with ice thickness (Ehn et al., 2008, Perovich, 1996). Despite the attenuation properties and thickness of sea ice, it is actually the much thinner layer of overlying snow that is often present, which primarily controls the magnitude of bottom ice PAR because of its high albedo and greater capacity to scatter light. In addition, snow also affects bottom ice temperature, and consequently bottom ice ablation, insulating it from the warming atmosphere during the spring (Sturm and Massom, 2010). As a result of these characteristics, the uneven distribution of snow on sea ice from wind forced displacement of drifts creates a non-uniform light and thermal environment at the ice bottom that translates to a spatially variable distribution of ice algae on the order of 100 m (Gosselin et al., 1986). Algae further vary at the microscale because of ice substructure and brine hole spacing (Mundy et al., 2007a), as well as at much larger scales (kilometers) due to the influences of ocean water salinity, ice thickness, and nutrient availability (Gosselin et al., 1986, Granskog et al., 2005, Robineau et al., 1997).

The manner in which snow depth (HS) influences algal chl a can change as the growth season evolves. At the beginning of the growing season, when the ice surface is dominated by a relatively thick snow cover (i.e. in late winter or early spring), ice algae are often negatively associated with HS as the energy required for photosynthesis is greatly restricted by decreased transmitted radiation below thicker snowdrifts. During this period, algae acclimate to the low irradiance by modifying their photosynthetic apparatus (Barlow et al., 1988, Robinson et al., 1995) and by producing accessory pigments to enhance light harvesting (Arrigo et al., 2010), but positive net photosynthesis cannot begin until a minimum level of irradiance, on the order of 7.6 μmol photons m 2 s 1, is reached (Gosselin et al., 1985).

Later in the spring, snowmelt from rising air temperatures permits an increase in the amount of radiation entering the sea ice. Ice algae can acclimate to the resultant change in bottom ice irradiance by altering their cellular content of light-harvesting pigments and/or reaction centers (Barlow et al., 1988, Falkowski and Raven, 2007, Michel et al., 1988). Despite the ability of algae to acclimate to increasing light levels, high levels of irradiance can still have negative physiological effects resulting in photoinhibition (Barlow et al., 1988, Michel et al., 1988). For this reason, along with the thermal influence of snow as it buffers the sea ice from warming atmospheric temperatures, thereby reducing bottom ice ablation, the negative association between chlorophyll a (chl a) and HS may switch to a positive relationship (Gosselin et al., 1986, Sturm and Massom, 2010, Welch and Bergmann, 1989). During this latter period of the spring, algae are also limited by depleted nutrient resources because of algal consumption or may experience a form of light limitation as cells higher in the ice column absorb preferred wavelengths. Over the course of the bloom, ice algae populations can thus shift from a phase characterized by light limited growth and accumulation to that of one or a combination of diurnal light limitation, nutrient limitation, self-shading induced light limitation, photoinhibition, and ice ablation (i.e., habitat erosion; Cota and Smith, 1991, Gosselin et al., 1990, Lavoie et al., 2005). Grazing pressure may also influence ice algae biomass throughout the spring although, grazing by ice meiofauna is thought to be minimal (Nozais et al., 2001) and contributions by zooplankton remain largely unknown, with best estimates relying on a set percentage of total biomass (Lavoie et al., 2005). The bloom ends in the late spring with the release of ice algae into the water column following snow and ice melt.

Spring melt in the Arctic is anticipated to occur quicker and earlier due to the warming climate (ACIA, 2005). This may affect the timing and extent of ice algal bloom. Changes in the overall productivity or the timing of bloom could have critical implications for the Arctic ice-covered ecosystem (Leu et al., 2011). It is thus important to understand the potential responses of ice algae to rapid decreases in snow thickness (and associated light conditions) at different periods of the spring.

A study by Juhl and Krembs (2010) concluded that the response of ice algae to rapid light increases depends not only on the magnitude of change in irradiance but also on the photophysiological state of the algae before the change. Furthermore, their observations suggest that ice algae may therefore have the capacity to acclimate to increased light levels given sufficient time. However, algal exposure to sudden increases in irradiance levels without adequate time to acclimate will likely result in a decline in ice algae chl a (Fortier et al., 2002, Juhl and Krembs, 2010).

The objectives of our study were to investigate the relationship between ice algae chl a and HS, and to examine the effects rapidly changing snow depths have on bottom algal chl a across the spring bloom period. The study used a transmitted irradiance technique (Campbell et al., 2014) to estimate bottom ice chl a concentrations at time series sites, along transects, and at locations cleared of snow in the Canadian High Arctic during the spring season. By meeting the objectives, we will provide new information on potential ice algal responses to abrupt warming events expected to occur in the Arctic associated with a warming climate.

Section snippets

Study site

Data were collected between 27 April and 13 June 2011 at a field station located in Allen Bay, Nunavut, Canada (Fig. 1). The region was characterized by smooth landfast first-year sea ice (FYI) approximately 1.3–1.7 m thick overlying a 60 m water depth. For sampling purposes snow depths were categorized as low (< 10 cm), medium (10 to 18 cm), or high (> 18 cm) in the early spring. During advanced melt, areas of low HS are predisposed to form ponds (Iacozza and Barber, 1999); therefore, melt ponds in

Site characteristics

Daily averaged air temperature increased approximately linearly over the sampling period from a minimum of − 21.8 ± 3.3 °C on 27 April to a maximum of 1.4 ± 0.6 °C on 10 June. Ed(0, PAR) also increased during most of this period, until peaking on 2 June and declining slightly thereafter (Fig. 2a, c). During the same period, the snow-covered sea ice transitioned from an average snow depth of about 14 cm to a surface of mixed coverage comprised of melt ponds, snow mounds, and surface drained white ice.

Photoacclimation

Ice algae have the capacity to acclimate to small increases in irradiance levels given enough time (Juhl and Krembs, 2010). Such light acclimation is characterized by a decrease in cellular chl a content, as well as a production of more photoprotective pigments (Barlow et al., 1988, Falkowski and Raven, 2007, Michel et al., 1988). The gradual rise of Ed(0, PAR) over this study (Fig. 2c) would have resulted in algal populations later in the spring exhibiting a higher degree of high light

Conclusions

The association between ice algae chl a and snow depth was investigated in this study. Analysis of time series revealed that the nature of response and total accumulation of chl a during the spring bloom was dependent on the class of overlying snow depth (none, low, medium, or high). For algae under less than a 30 cm of snow cover, the nature of chl a–HS association was summarized by four periods; light limitation in the early spring, a transitional period, losses driven by high levels of

Acknowledgments

The authors would like to recognize the support provided by a Northern Scientific Training Program grant and the Natural Sciences and Engineering Research Council of Canada (NSERC) graduate scholarship to KC, and by the NSERC Discovery grants to CJM, DGB, and MG and the Polar Continental Shelf Program (PCSP) of Natural Resources Canada. This work represents a contribution to the research programs of ArcticNet, the Canada Research Chair to DGB, and the Canada Excellence Research Chair unit at

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