Introduction
Melt ponds are commonly observed on ice floes in the Arctic Ocean during spring and summer. Previous studies showed that biomass and primary production within the ponds are usually low (0.1 to 2.9 mg Chl a m−3 in the Canada Basin and 0.1 to 0.3 mg Chl a m−3 in the central Arctic Ocean) and contribute less than 1% to the total primary production in the Arctic Ocean (Lee and others Reference Lee, Stockwell and Joo2012: C04030). During the 2014 Chinese Arctic Cruise, the last ice station was in the Canada Basin, Arctic Ocean. Sea ice physical, optical, chemical and biological observations were conducted on a large piece of 1.1 m thickness sea ice, which has 17 cm (average) thickness snow cover and 3 to 4 melt ponds. The researchers landed on the ice by a skiff. During the polar bear defence by telescope, two melt ponds with light brown algal bloom belt were observed and water samples of one of them were collected carefully (149°21.341W, 78°48.191N). In this note, the environmental features and microbial assemblage of this bloom are described, and the reason for, and significance of this melt pond algal bloom, are discussed.
Materials and methods
Sea ice physical, chemical, and biological studies were carried out in Canada Basin during the 6th Chinese Arctic Cruise in summer 2014. A melt pond on an ice station (Fig. 1) was studied on 29 August. Temperature and salinity were measured by a WTW thermosalinograph with a conductivity probe. A total of 4L melt pond water was collected by using an organic glass water sampler. Among them, 500 ml was filtered through 0.7 μM (GF/F) filters. The filter was collected for onboard Chlorophyll a (Chl a) measurement with a Turner Designs 7200 fluorometer according to the method of Parsons (Parsons and others Reference Parsons, Maita and Lalli1984: 101–173). A 100 ml filtrate was examined for nutrient concentration measurement with a Skalar San ++ automatic analyser under the protocol of Grasshoff (Grasshoff and others Reference Grasshoff, Kremling and Ehrhardt1999: 193–198). One liter melt pond water was filtered through 0.2 μM pore size filter and filter was frozen in –80 °C for parallel 454 pyrosequencing of the 18S rRNA gene (Zhang and others Reference Zhang, He and Lin2015). Ice cores from sea ice adjacent to the melt water ponds were collected with a Mark II ice auger (Kovacs). Ice core temperatures were measured at 5 cm intervals. Ice core subsamples for salinity measurements were cut at 10 cm and biological measurements at 20 cm intervals except for the bottom 5 cm. Samples for salinity and chlorophyll measurements were melted at room temperature; then salinity was measured as above. Filtered sea water was added to samples intended for biological analysis at a ratio of 1L : 20 cm (seawater : ice) to reduce osmotic pressure and melting damage in the low-temperature laboratory (4°C). Subsamples of 400 mL pond water and each melted water sample were filtered on a polycarbonate filter, DAPI stained, and frozen for later epifluorescence microscope analysis (He and others Reference He, Zhang and Lin2012: 36–45).
Sampling site in the Canada Basin (ICE07) during the 2014 Chinese Arctic cruise.
Results and discussion
Pond water temperature was –0.1°C and salinity was 0.2 (Table 1). The algal bloom had a chlorophyll a concentration of 15.32 mg m−3, and it was dominated by a chlorophyte (Fig. 2), identified as Carteria lunzensis (Pascher and Jahoda) on the basis of 18S DNA analysis (similarity of 97%), with an abundance of 15.5×106 cells/L. Cell diameters were between 11.9 and 21.6 μm, with an average of 16.2 μm (n = 100). The microbial community associated with the bloom was very simple (Table 2). Analysis of DAPI stained sample showed that the bacterial abundance was lower than that within surface ice and protozoa were almost absent.
The environmental factors and chlorophyll a concentrations in the surface ice (0~20 cm), melt pond, and underlying water.
The microbial assemblages in the melt pond and surface ice (0~20 cm).
VL: very low abundance, almost none in the DAPI stained sample
Photomicrograph of the dominant species: chlorophyte Carteria lunzensis Pascher and Jahoda (x 400).
As noted above, the melt pond habitat on Arctic pack ice floes has previously been thought to be ultra-oligotrophic. A previous study showed that a snow alga, Chlamydomonas nivalis (Bauer) Wille was common and dominant on the surface of Arctic pack ice (Gradinger and Nürnberg Reference Gradinger and Nürnberg1996: 35–43), however, a bloom in an Arctic surface melt pond of the intensity seen here has never previously been reported.
Two parameters are important for supporting this bloom: a suitable algal ‘seed’, and nutrients. Fig. 3 shows the profiles of temperature, salinity, and chlorophyll a concentrations within the ice column. Chlorophyll a concentrations in the surface were comparable to those of bottom ice and much lower than in the meltwater pond. Compared with values in the surface ice, pond nitrate and nitrite were exhausted, probably due to utilisation by algae (Table 1). Silicate was similar to that in the surface ice, but as a chlorophyte dominated the algal assemblage, little silicate would be expected to be consumed during the bloom. Phosphate in pond water was higher than that in surface ice. Comparatively, the nitrogen was much lower, but the silicate was much higher, while phosphate was similar with those in the closed melt pond in 2005 and 2008 examined by Lee and others (Reference Lee, Stockwell and Joo2012: C04030). This suggests that there was a replenishment mechanism or that nutrients were inhomogeneously distributed in the ice or snow and would release during the pond formation process (Tovar-Sanchez and others Reference Tovari-sanchez, Duarte and Alonso2010: C7). The nutrients can be replenished in many ways, such as wind, snowfall, animal faeces, etc. Algae ‘seed’ to the surface ice during autumn and winter. This ice could be the blooming site in the melt pond.
The vertical profiles of temperature (red line), salinity (blue line), and chlorophyll a concentration (green line) in the ice core at site ICE07.
Sea ice is an important habitat. It has been estimated that sea ice biota constitute about 4~25% of the total primary production in seasonal ice-covered waters in the Arctic Ocean (Legendre and others Reference Legendre, Ackley and Dieckmann1992: 429–444) or even more in the central Arctic Ocean (Gosselin and others Reference Gosselin, Levasseur and Wheeler1997: 1623–1644). As physio-chemical factors distributes continuously in snow and ice, the microflora in the ice-associated habitats may be the same anywhere (Bursa Reference Bursa1963: 239–262). Both marine diatom and freshwater green algae, as well as other flagellate has been found in the sea ice, melt pond and water under sea ice (Bursa Reference Bursa1963: 239–262; Mundy and others Reference Mundy, Gosselin and Ehn2011: 1869–1886). The ecosystem of the Arctic ice represents the poorest oligohaline condition during a short duration and is thought to be of small importance as far as general plankton production in the ocean is concerned (Bursa Reference Bursa1963: 239–262). However, under-ice blooms are a widespread phenomenon in polar regions, due to the Arctic's thinning ice cover and subsequent increase in transmitted irradiance to the under-ice environment (Mundy and others Reference Mundy, Gosselin and Ehn2009: L17601). A massive phytoplankton bloom was observed under the Arctic sea ice in the Chukchi Sea (Arrigo and others Reference Arrigo, Perovich and Pichart2012: 1408). Gradinger (Reference Gradinger1996: 301–305) reported a chlorophyte (Pyramimonas sp.) bloom in a melt pond under pack ice with an abundance of 19.1×103 cells ml−1 and a pigment concentration of 29.6 mg m−3. Primary production of the surface melt pond is thought very low (Bursa Reference Bursa1963; Lee and others Reference Lee, McRoy and Joo2011: 302–308). However, the algal bloom reported in this study suggests that phytoplankton community and primary production within surface melt ponds may need more attention with Arctic warming.
Acknowledgement
We thank associate professor Qiang Hao for providing chlorophyll a data. This study was supported by the National Natural Science Foundation of China (41476168, 41206189), Special programme for Antarctic and Arctic environmental investigation and evaluation (CHINARE2011-2015).
