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Influence of the Barents Sea Frontal Zones on Chlorophyll Concentration in Spring

In spring of 2016, 2018 and 2019, chlorophyll a (Chl a) content was studied in the 0-50 m layer on the vast Barents Sea area to the north of 75∘ N. Standard sampling was carried out at 11 oceanographic transects, including 52 stations. Due to the negative ice anomalies and the high-latitude position of the ice edge, original data on Chl a concentration for spring period were obtained in hard-to-reach and previously unexplored areas of the Barents Sea. The investigation area covered the Marginal Ice Arctic zone, as well as the area where the Polar Front was located quasi-stationary. The effect of the Marginal Ice Arctic and Polar frontal zones on the distribution of Chl a concentration was revealed. The strongest factor influencing the distribution of chlorophyll was the Polar Front. It divided mainly Arctic and Atlantic waters. The highest pigment concentrations corresponded to the Arctic water mass and exceeded the content of pigment in water of Atlantic origin by an order of magnitude. The impact of the Marginal Ice Arctic Front was not so obvious the content of Chl a in waters of various genesis differed, but not more than by a factor of 2.


Introduction
Studying the process of spring development of micro-phytoplankton and its pigment composition makes it possible to obtain a more complete picture of seasonal succession and the production cycle of the Barents Sea, since most of the annual biomass of Arctic phytoplankton is produced during the spring succession cycle. An indicator of the total abundance of phytoplankton is the amount of Chl a.
There are several frontal zones within the way of Atlantic water penetration into the Arctic Basin in the Barents Sea [1,2]. The Polar frontal zone has a climatic character and is well traced throughout the year. It separates mainly Atlantic and Arctic waters. Data on hydrological and phytoplankton observations in the Barents Sea for waters beyond 75 ∘ N are extremely limited, as these areas are difficult to access and are covered by ice for most of the year. BRDEM-2019 Phytoplankton investigations in the ice edge zone are of particular interest, for it is a unique biotope where two components of the primary producers are inseparably linked, the pelagic and ice flora that create the primary organic matter of the Barents Sea. The ice edge is a hydrodynamically active zone covered by ice fields that make it difficult or impossible to conduct in situ research in this area. The Marginal Ice Arctic frontal zone is seasonal and occurs in the warm season. Due to ice melting, a layer of cold freshened water forms on the surface near the ice edge, the horizontal distribution boundary of which is expressed by high gradients of hydrological characteristics and forms the Marginal Ice Arctic Front.
The aim of this work was to study the distribution of Chl a concentration in the pelagial of the Barents Sea related to the zone of influence of the ice edge and frontal zones.

Methods and Equipment
Phytoplankton investigations were carried out in spring (April--May) of 2016, 2018 and 2019 during cruises on R/V``Dalnie Zelentsy'' in the Barents Sea ( Figure 1). In the course of many years of research in a difficult ice environment, standard sampling was carried out at 52 stations, combined into 11 oceanographic transects.
Since 1983 to the present, large negative ice cover anomalies have been observed in the Barents Sea [3].
In 2016, the ice edge in the eastern part of the Barents Sea receded north to the latitude 79 ∘ N. According to the AARI review ice maps (http://www.aari.ru/odata/_d0015. php), in April such a northern position of the ice edge in the east of the Barents Sea has not been noted since 1997, earlier data are not available.
West of 40 ∘ E the ice spread much further south, the ice edge was located along the parallel of 76 ∘ N and went on the western border of the sea far to the south, almost reaching the Bear Island. Such ice edge position is not typical for the Barents Sea. Due to the powerful warming effect of Atlantic water in the west, the ice usually recedes north in the western part of the sea faster than in the eastern.
During the research period in 2018, the ice edge was also located much north of its mean annual position at this time of the year. Over the entire water area of the sea, it was located mainly in the latitudinal direction between 77 ∘ and 78 ∘ N and reached 75.5 ∘ N only in the area of the Novaya Zemlya Archipelago.
The year 2019 was more ice-covered than the previous few years, however negative anomalies in ice cover were also noted. The ice edge crossed the parallel of 75 ∘ N in several places, both in the west and in the east of the sea.
Due to the position of the ice edge in the years of research in such high latitudes, original data for the spring time were obtained in hard-to-reach and previously unexplored areas of the Barents Sea.
Seawater sampling in the 0-50 m layer and sample processing were carried out according to standard hydro-biological methods. Chl a concentration was determined by spectrophotometric method [4]. The temperature and salinity of water masses were measured using the CTD-profiler SBE 19 plus. Gradient method and classical TS-analysis were used for water mass identification and determination of their boundaries [5].
Horizontal gradients of temperature and salinity five times higher than climatic gradients for the Barents Sea (0.01 ∘ C/km and 0.001 ‰/km) were taken as a criterion for the presence of a frontal zone on the sea surface [6].

Results
Transect I was completed in the ice edge zone in April 2016 ( Figure 2). According to the data on ice situation and our observations, the transect I was dominated by brash     (Table 1).
This stratification ceased to be traced to transect X and at stations 34-39 the water column was homogeneous from surface to bottom and was occupied by water of Barents Sea origin. Chl a concentration in Barents Sea water was about 0.2 mg/m 3 (Table 1).
A similar structure of the water column was observed at all stations along transect XI: there were surface water of the Arctic Basin to depths of 75-120 m underlain by the Barents Sea water. The average Chl a concentration in Arctic water was low and varied in a small range of 0.2--0.3 mg/m 3 (Table 1).

Discussion
The climatic Polar front had the strongest influence on the distribution of Chl a concentration in the studied water area. Along those transects where the front line passed between Arctic and Atlantic waters, its influence was most obvious. occurs along the ice edge in springtime [9]. "The marginal bloom" as a phenomenon is a very short process. According to data derived from satellite sensing, the active phase at each specific point lasts no more than 5-6 days [11]. The data obtained within one succession cycle may differ significantly and reflect the complex spatiotemporal organization of the marginal bloom. Thus, according to [12], in May 1999 at sites of the surface layer located from each other at a distance of only few kilometers chlorophyll concentrations differed from each other by an order of magnitude and the phytoplankton community was at different stages of the succession cycle. It is known that in the central and northern Barents Sea one spring maximum of phytoplankton development is recorded and most of the annual biomass of Arctic micro-phytoplankton is produced precisely during the spring succession cycle. According to different authors, the maximum level of peak values in springtime can reach about 13 mg/m 3 in the northwestern Barents Sea [13], about 14 mg/m 3 during sub-ice microalgae bloom in the central part of the sea [10], and up to 10.6±1.5 mg/m 3 in the north of the sea in the Marginal Ice Arctic zone and ice fields [14]. During our investigations such high chlorophyll concentrations were not recorded. Probably we observed the beginning of the spring bloom that was only starting its way to its culmination.
The influence of the Marginal Ice Arctic Front on the distribution of Chl a concentration was less pronounced in the studied area. The highest chlorophyll content corresponded BRDEM-2019 to surface Arctic water (0.4--0.6 mg/m 3 ) and was approximately 2 times higher than its values in water of Atlantic origin and Barents water. Barents Sea water along the all transects was characterized by low chlorophyll content of around 0.2 mg/m 3 .

Conclusion
Long