The effect of wind and temperature to phytoplankton biomass during blooming season in Barents Sea

Highlights

• Decadal variation of Sea Surface Temperature(SST), Wind Speed, Sea Ice are studied from satellite derived data.

• Phytoplankton (CHL) blooming time and size are studied related to the effect of temperature and wind in post-bloom period.

• Correlations among CHL, SST, Wind Speed, Sea Ice and Mixed layer depth are studied in both southern and northern Barents Sea.

• SST and Wind threshold for the blooming are detailed for both southern and northern regions.

Abstract

The Barents Sea is the most productive sea in the Arctic. The main causes of phytoplankton spring blooms are studied for a decadal time period of 2003–2013 at the region of (70 °N-80 °N, 30 °E-40 °E) in Barents Sea. Due to the rapidly ice melt in the southern region (70 °N-75 °N), almost no ice left after year 2005, sea surface temperature (SST) and wind speed (WIND) are two main dominant factors influencing phytoplankton blooming in the southern region. Ice melt is another important factor of phytoplankton blooming in the northern region (75 °N–80 °N). SST and CHL had positive correlations during blooming season but negative correlations during summer time. The lower SST in spring could result in earlier blooming in the region. Higher SST and higher WIND could result in later blooming. Positive NAO after April 2013 caused higher SST in 2013. Increasing WIND would cause CHL reduced accordingly. Blooming period is from late April to late May in the southern region, and 1–2 weeks later in the northern region. During blooming season, SST was less than 4 °C and WIND was less than 10 m/s. The higher winds (over 15 m/s) in early spring would brought more nutrients from bottom to surface and cause higher blooming (near 10 mg/m³ in year 2010) after WIND is reduced to 5−8 m/s. Higher WIND (around 10 m/s) could generate longer blooming period (more than a week) during late May in the southern region. Decrease of WIND and increase of melting ice, with slightly increase of SST and decrease of mixed layer depth (MLD), are all the factors of phytoplankton blooming in late spring and early summer.

Introduction

The Barents Sea is one of the most studied regions regarding ice-edge phytoplankton blooms, lying between Norway and the Svalbard archipelago. As the most important remote region of Arctic Ocean, Barents Sea plays key role on Arctic ecosystems and climate change.

The climatic variations in the Barents Sea depend mainly on the activity and properties of inflowing Atlantic Water (Ådlandsvik and Loeng, 1991). The Norwegian Atlantic Current brings warm higher salty current into Barents Sea from southwest side and then divides into two main branches with one continues eastwards parallel to the coastal water current system and the other brunch turn north from the left of Bear Island (Loeng, 1991) (Fig. 1). The Arctic Current comes from northeast direction between Frans Josef Land and Novaja Zemlja Island brings colder and fresher water to the Barents Sea (Loeng, 1991; Signorini and McClain, 2009b). The merging of these currents forms the Polar Front identified by the strongest horizontal gradients (Johannessen and Foster, 1978). The variability of Atlantic inflow caused by wind conditions would lead to the strong increase of SST in Barents Sea. The warm periods in the Barents Sea are related to the stable low air pressure, little ice coverage and cyclonic circulation in the atmosphere; while low temperature is linked to high air pressure, anticyclonic air circulation with decreased Atlantic inflow and more severe ice conditions (Ådlandsvik and Loeng, 1991). The transition from one climate state to the other is likely to be enforced by the large scale of oceanic and atmospheric circulation which could be caused by formation of bottom water (Ådlandsvik and Loeng, 1991).

The following environmental factors have significant impact on the phytoplankton biomass in Barents Sea: Strong WIND, convective overturning deep mixed layers in winter, and restratification occurs in summer due to ice melt and increased solar radiation warming the surface waters (Signorini and McClain, 2009a). The highest CHL was observed in the ice edge (Rey et al., 1987; Matrai and Vernet, 1997; Engelsen et al., 2002; Qu et al., 2006). The strong phytoplankton bloom would appear two weeks after the ice edge had retreated column average. ation of China (Engelsen et al., 2002). In the southern region, where water masses are more influenced by Atlantic current, increase of vertical stability of water masses is important for the timing of the spring blooms. Northern region is less productive and influenced by Arctic water masses. Due to more ice gathers up north, ice melting is the major causes of blooming in spring and early summer (Sakshaug and Slagstad, 1992; Sakshaug et al., 1997). Similar like Greenland Sea, the northern region could have higher peak of CHL mainly caused by ice melting during spring time (Qu et al., 2014). After peaks of bloom appear in late April and early May, the blooms stop abruptly when nutrients become depleted, causing the CHL maximum to sink to great depths (Rey and Loeng, 1985). With the increase of solar irradiance and drawdown of nutrients, MLD would decrease to its minimum depth during summer. After September, when solar irradiance is significantly reduced, surface cooling and stronger WIND would deepen the MLD and nutrients levels begin to increase again via vertical mixing in winter months (Sakshaug and Slagstad, 1992; Signorini and McClain, 2009a).

It was found that identified Emiliana huxleyi is the most dominant species of coccolithophores in the Barents Sea (Signorini and McClain, 2009a). Diatom and flagellates are also abundant species in Barents Sea (Rey et al., 1987; Matrai and Vernet, 1997; Signorini and McClain, 2009b). The highest CHL were found near the ice edge (Rey and Loeng, 1985; Matrai and Vernet, 1997; Engelsen et al., 2002). Surface CHL was 60 % higher than the total column average. But the surface CHL is a good predictor for relative levels of total phytoplankton biomass during spring time (Engelsen et al., 2002).

The dynamics of sea ice is capable of influencing primary production and the carbon flux dynamics of the Barents Sea seasonally (Wassmann et al., 1999). When nutrients become depleted, the bloom stops immediately and causes the CHL maximum to sink to greater depths (Signorini and McClain, 2009a). During 1998–2002, the maximum CHL was located in the Marginal Ice Zone (MIZ)(72 °N -73 °N) within 30 °E -35 °E (Qu et al., 2006). However, the location of MIZ can vary by hundreds of kilometers for different years (Matrai and Vernet, 1997).

There are number of factors influence the phytoplankton biomass in Barents Sea. SST, WIND and melting of sea ice are the primary factors. North Atlantic Oscillation (NAO) usually has a dominant influence on wintertime temperatures across much of the North Hemisphere (Hurrell et al., 2003). Mixing in the upper ocean depends strongly on winds and temperature and it controls marine ecosystem dynamics.

Decadal satellite derived data and historical metrological data are used for analysis the relationships of environmental factors with phytoplankton biomass. The correlations between SST, WIND, ICE and CHL are especially analyzed in the blooming season for a better understanding of future phytoplankton trends in Barents Sea.