Influence of river water expansion on the spatial distribution of chlorophyll-a in the Kara Sea

In August-September 2013, the thermohaline characteristics of water masses and the concentration of chlorophyll-a were studied in the Kara Sea. Expansion of surface desalinated water to the north, according to the central type, influenced the spatial distribution of chlorophyll. Beyond the boundary of the Stock Front, the average pigment concentration in the surface layer was 0.45±0.16 mg/m3. In the area affected by the surface desalinated layer, most of the chlorophyll (0 m – 1.04±0.27 mg/m3, 5 m – 0.97±0.27 mg/m3, 10 m – 0.69±0.36 mg/m3) was localized within this layer (10-17 m). High concentrations were observed in the bays of the Kara Sea. The maximum concentration of chlorophyll-a in the surface layer of the Ob Bay was 4.22 mg/m3, in the Yenisei Bay – 3.22 mg/m3.


Introduction
The key characteristics of the Kara Sea are the predominance of shallow depths combined with a large volume of incoming river runoff. The continental runoff into the Kara Sea is more than 40 % of the total river runoff into the seas of the Russian Arctic. River runoff has a significant impact on the hydrological regime of the sea. Under its influence, a system of currents is formed, a pronounced seasonality of the salinity regime is established, and ice processes proceed in a special way. The main part of fresh water comes from the waters of the Ob and Yenisei rivers, the total river runoff of which is more than 1000 km 3 per year, and from the waters of the Pur, Taz and Pyasina rivers.
One of the most important hydrological structures of the Kara Sea is the surface desalinated layer (SDL) [1]. It is formed on the Ob-Yenisei coastal area as a result of the mixing of sea and river waters. The SDL spreads over the Kara Sea by a system of surface geostrophic currents. Depending on the hydrological and meteorological, processes prevailing in a certain period of time, the spread of desalinated waters can occur according to three different scenarios: western, central or eastern. In the case of the western type of distribution, the layer of desalinated waters reaches the eastern coast of arch. Novaya Zemlya; with the central scenario -desalinated waters penetrate far to the north; at the east scenario -river waters along the coast are transferred to the east, reaching arch. Severnaya Zemlya, and are carried out into the Laptev Sea [2]. The thickness of the desalinated layer varies from 5 to 20 m [3].
The study of the production potential and concentration of chlorophyll-a in the Kara Sea has been carried out since the 1980s [4,5]. In the last decade, the focus of research has shifted from field measurements [6,7,8] to their combination with satellite data [9,10]. A major work that takes into account the influence of abiotic and biotic factors on the primary production of the Kara Sea is the  [11]. However, the effect exerted on the phytoplankton community by possible SDL distribution scenarios (western, central, eastern) has not been sufficiently studied.
The aim of our study was to determine the direction of the distribution of SDL in the Kara Sea during the survey period and the degree of its impact on the spatial distribution of chlorophyll-a.

Materials and methods
The studies in 2013 in the Kara Sea were carried out from August 28 to September 23 during the expedition "Yamal-

Results and discussion
During the study period in 2013, in the Kara Sea, the distribution of SDL along the central type, mainly to the north, was observed. It is considered that the transformation of river waters into sea waters ends . The 25 PSU isohaline was used to draw the SDL boundary on the sea surface (figure 2). A significant intensification of the salinity gradients between stations 8 and 9 (transect II) and stations 18 and 19 (transect III) indicates the passage of the Stock Front in this area. The runoff front has a seasonal nature and it can be traced in the sea area during the warm season. The runoff frontal zone of the Kara Sea is usually marked by salinity gradients. Salinity gradients on the sea surface between the indicated stations reached 0.27 PSU/km. Transect I was performed in the northern part of the Kara Sea at 77 °N latitude. In the western part (stations 77-85), the transect crosses the southern end of St. Anne Trench, and the depth reaches 520 m. The eastern part of the transect passes through a relatively shallow sea area with depths of up to 100 m. Along St. Anna Trench, at depths of more than 150 m to the Kara Sea from the Arctic basin penetrated the transformed Atlantic waters. They had high salinity (34.9 PSU) and temperatures close to 0 °С. The SDL did not reach the latitude of transect I. In the shallow part of the transect, salinity on the sea surface varied from 31 to 33 PSU, and at the bottom, it reached 34 PSU. The temperature, positive at the surface, decreased with depth and reached temperatures close to the freezing point at a given salinity.
In this transect, the concentration of chlorophyll-a in the surface layer did not exceed 0.60 mg/m 3 (average 0.45±0.16 mg/m 3 ) (figure 3). In the 0-50 m layer (table 1), chlorophyll was distributed relatively evenly. At deep-water stations (at depths from 75 m), the concentration of chlorophyll-a decreased. The local maximum of the studied pigment (1.59 mg/m 3 ) was revealed at station 79 at the depth of 50 m. It is known that in the Kara Sea, the subsurface chlorophyll maximum (SCM) is weakly expressed, mainly in waters with a chlorophyll concentration of 0.1-0.5 mg/m 3 ; in more productive regions, the SCM has not been found. Its formation in the Kara Sea is influenced by a whole complex of abiotic factors, of which it is very difficult to distinguish the dominant one. At the same time, SCM does not make a significant contribution to the formation of the productivity of the phytocenosis [11]. The chemical parameters of the water coming from the Gulf of Ob, the waters of the Yenisei, and meltwaters are different. This makes it possible to determine the share of the waters of each of these sources in the formation of the surface desalinated layer. In September 2013, according to A. A. Polukhin [3], the share of Yenisei waters in the formation of the surface layer at transects II and III was extremely small. The waters of the Ob origin predominated (25-55 %). The maximum concentration of the Ob waters was observed directly in the Ob Bay (but even there their share did not reach 100 %), as well as in the area where stations 4-7, 23-29 are located. There was also a great contribution to the formation of the surface layer of meltwater. In the central part of the sea, their share reached 35 %, at the border of the river runoff it decreased to 10 %.
In the gulfs of the Kara Sea, the entire upper part was occupied by a warm, slightly saline layer of waters freshened by river runoff, while the bottom area was occupied by a cold saline wedge.

Conclusions
During the survey period in August-September 2013 in the Kara Sea, desalinated waters spread by the central type in the northern direction. According to the degree of influence of SDL on the concentration and distribution of chlorophyll-a in the studied water area, three different areas have been identified: 1. In the water masses outside the runoff front (34.  [4]. In September 1993, a relationship between surface salinity and chlorophyll concentration was found. The highest values (0.8-22 mg/m 3 ) were measured at S<10 ‰, and the lowest (0.2-0.8 mg/m 3 ) -at S> 22 ‰ [5]. According to the data of long-term satellite and episodical field observations, the concentration of chlorophyll-a in the Kara Sea varied from 0.2 to 6 mg/m 3 . The concentrations which exceed 3 mg/m 3 were confined to the zones of river outflow [9]. In September 2007, chlorophyll-a concentration in the euphotic layer varied from 0.02 to 4.37 mg/m 3 (on average 0.76 mg/m 3 ). The surface waters with low salinity turned out to be more productive and had higher chlorophyll-a concentration than waters with high salinity [6]. In August -September 2012, chlorophyll-a concentration in the mixed layer varied from 0.28 mg/m 3 in the area of the Novaya Zemlya trough to 4.5 mg/m 3 in the freshwater area of the Yenisei Bay [7]. Earlier, on the basis of satellite measurements of salinity and chlorophyll-a, the distribution areas of river waters in the Kara Sea were analyzed [10]. The authors determined the western type of distribution of the surface desalinated layer in 2011, the easternin 2012. The regionalization of the water area, in paper [8], was carried out according to the mean long-term position of the 25 PSU isohaline on the sea surface. Based on chlorophyll-a concentration in the surface, in the estuaries of the Ob and Yenisei, and in the area of river outflow in autumn, these waters are referred to as eutrophic (> 1 mg/m 3 ). At the same time, the southwestern region and St. Anne Trench area are classified as typically mesotrophic (0.1-1 mg/m 3 ) waters.
A compilation of literature data demonstrates the relationship between surface salinity and chlorophyll-a concentration. However, it does not allow to fully assess the scale of the influence of various scenarios of the desalinated waters spread on the processes occurring in the Kara Sea pelagic zone. During the period of our research, the expansion of desalinated waters to the north proceeded according to the central type. Probably, with the distribution of SDL in the eastward or westward direction, a different picture of the distribution of chlorophyll-a would be observed. Further studies of this powerful hydrological factor influence on the chlorophyll-a concentration and other components of the pelagic zone will make it possible to compile a more complete picture of the of the Kara Sea pelagic communities functioning.