Production of phytoplankton in the Arctic Seas and its response on recent warming

Alexander A. Vetrov and Evgeny A. Romankevich

P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia, [email protected], [email protected]

Abstract

New maps of the mean monthly distribution of the primary production in the Arctic Seas of Russia (Barents, Kara, Laptev, East Siberian, and Chukci Seas) were compiled using joint processing of CZCS (1978-1986), SeaWiFS (1998-2007), MODIS (2002-2007) satellite data, and field measurements. The annual production of phytoplankton is estimated at 163 • 106 t of C per year. Flux of organic carbon to seafloor is estimated at 68 • 106 t of C per year. The trends of the production changes within 1998-2007 were considered in the Greenland, Norwegian, Barents, Kara, Laptev, East Siberian, and Chukchi seas using satellite and field data. In these seas positive trends of summary production of phytoplankton were revealed, which range from 3.7% to 18% per year with respect to the values averaged over the period of the observations.

Introduction

The transformation of the mineral form of carbon into the organic form by algae in the process of photosynthesis represents one of the principal elements of the carbon cycling in the ocean. The primary production that is formed provides energy for all the subsequent heterotrophic levels of life. Carbon cycle controls many parameters of the biosphere functioning: fluxes and geochemistry of almost all chemical elements; fluxes of greenhouse gases N2O, CH4, CO2; bioproductivity, biomass and in many cases biodiversity of marine communities; weathering, halmirolysis, authigenic mineral formation; microbiological activity. Organic matter (OM) serves as a source of energy for all lithogeochemical processes of sedimentation and diagenesis. Unclosed (unbalanced in system: synthesis-destruction-burial) carbon cycle realizes accumulation in bottom sediments organic matter initial for oil and gas formation and has the potential to influence the climate system through feedback pathways involving responses in terrestrial and marine systems.

The most direct method for primary production measurements is the radiocarbon method based on the consideration of the increments of the phytoplankton biomass in the course of the CO2 fixing. These kinds of measurements are very laborious and therefore not numerous. Because of the obvious lack of expeditionary data, in order to quantitatively estimate the primary production in the Arctic seas, one has to apply indirect estimation methods using empirical relations between the primary production and chlorophyll content in the water. The measurements of the latter are more abundant; meanwhile, in the Arctic seas, the season of direct measurements is usually restricted to August-September.

Additional information about the intensity of photosynthesis in seawater may be obtained from the satellite scanning of the ocean color. Owing to the significant distance of satellites from the surface of the sea, it is feasible to cover the entire World Ocean with continuous measurements. The disadvantages of the satellite method are also related to the great distance of satellites from the sea, since the solar irradiance that is reflected by seawater and reaches the satellite comprises only a few percent of the incident irradiance. Special algorithms were elaborated that help to separate the signal corresponding to the chlorophyll content in the water from the solar irradiance reflected by the sea surface and attenuated by the atmospheric aerosols. In the Arctic seas, the separation of this kind of signal is related to additional difficulties caused by the low sun standing, enhanced cloudiness, and intensive runoff of organic and particulate matter to the sea by Siberian Rivers (Artem'ev et al. 2003; Burenkov et al. 2001; Kopelevich et al. 2003). Nevertheless, despite the numerous assumptions accepted for conversion of the data on the surface water color to chlorophyll concentrations and the insufficient development of special algorithms for Arctic coastal regions, satellite data verified with respect to direct chlorophyll measurements should be used in order to cover large areas and to increase the observation frequency.

Materials and methods

The maps of the chlorophyll distribution in the Arctic Seas of Russia based on the processing of field data and those obtained using a CZCS satellite radiometer (Vetrov and Romankevich 2004), regardless of the long observation period (19781986), have significant gaps (about half of the sea area) caused by the permanent cloudiness partly covering the sea. In this paper, in order to complement the maps of the chlorophyll distribution in the Arctic Seas and to estimate its primary production, we analyzed the satellite data of third level with chlorophyll concentrations averaged over 8-day-long intervals over 0.7° x 0.7° areas obtained with the help of SeaWiFS (1998-2007) and MODIS (2002-2007) radiometers together with the data of field measurements (Fig. 1).

The determinations of the chlorophyll contents in the surface waters were performed following the standard scheme (Vedernikov et al. 1990; Vetrov 2008). It included separation of particulate matter from the seawater (1-10 l) over GF/F glass-fiber filters with a pore size of about 0.7 |im, extraction of chlorophyll from the particulate matter, and measurements of the concentration of chlorophyll a in the eluent from the absorption and fluorescence spectra obtained with a Fluorat-02 Panorama spectral fluorometer. A comparison of the satellite data with the results of field measurements allowed us to insert a correction into satellite data that has a circumcontinental character.

Fig. 1. Location of field measurements of chlorophyll used for verification of satellite data. 1 - our data; 2 - Juterzenka and Knickmeier 1999; 3 - Heiskanen and Keck 1996; 4 - Gleitz and Grossmann 1997; 5 - Tuschling 2000; 6 - Bidigare et al. 1992; 7 - Hameedi 1978.

Primary production

The maps of the production of phytoplankton in the Arctic Seas (Fig. 2) were calculated using the new maps of the chlorophyll distribution and the empiric relations between the chlorophyll concentration in the surface waters and the primary production in the water column (Table 1) established on the basis of direct parallel measurements of the primary production and chlorophyll content in the Arctic Seas (Vinogradov et al. 2000; Vetrov et al. 2008).

30 E 60 90 120 150
0 100 200 500 1000 Fig. 2. Distribution of production of phytoplankton in the Arctic Seas, mg C m-2 per day.
Table 1. Mean values of primary production in water column (mg/m2 per day) for zones with different chlorophyll "a" concentration in surface water (mg/m3).

Range Cchl (mg/m3)

Kara, Pechora and East Siberian2

Barents and Chukchi a

Laptev

<0.1

40

20

20

0.1-0.3

50

60

60

0.3-0.5

60

140

90

0.5-1.0

70

350

120

1-3

120

900

150

3-6

350

1,300

350

>6

1,000

1,800

1,000

^Vinogradov et al. 2000

In the Barents which is underwent to influence of atlantics water active vegetation of phytoplankton starts in March (Fig. 3). The maximums of both summary and specific primary production are observed in May-June during the period of high insolation and ice melting. In July, their values drastically decrease, nevertheless keeping on rather high level, which seems to be related to reduce of nutrients and eating away of phytoplankton by herbivorous grazers. High value of primary production remains also in autumn that is related to the decrease in the rate of the zooplankton metabolism, whereas for phytoplankton, the life conditions remain favorable due to mixing of water. In the Kara, Laptev and East Siberian seas, characterized by shorter vegetation period, which starts in April, single maximum of primary production is observed. In the Chukchi Sea, the phytoplankton bloom commences in May, and keeps high level to the end of September due to the penetration of the comparatively warm waters of the Anadyr Current enriched in nutrients into this area.

Annual production of phytoplankton (Fig. 4) is estimated taking into account all months of the year including nonproductive ones. The annual production of phytoplankton in the Russian Arctic Seas estimated using the maps compiled comprises 163 • 106 t of C per year, which is 1.3 times higher than the previous estimate (126 • 106 t of C per year) based on the CZCS data (Vetrov and Romankevich 2004). The most total production of phytoplankton per year is estimated in the Barents Sea, but most annual production per day in the Chukchi Sea (Table 2). These seas are classified as mesothrophic (500 > PP > 100 mg/m2 per day). Kara, Laptev and East Siberian seas are oligothrophic (PP < 100 mg/m2 per day).

Barents mm

700 600 500 400 300 200 100 0

Mar Apr May Jun Jul Aug Sep Oct

Kara nM-

Mar Apr May Jun Jul Aug Sep Oct

Laptev -

-

II

-

Mar Apr May Jun Jul Aug Sep Oct

East Siberian

Mar Apr May Jun Jul Aug Sep Oct

Mar Apr May Jun Jul Aug Sep Oct

300 2 O

200 a E

Mar Apr May Jun Jul Aug Sep Oct

Fig. 3. Season variation in the primary production; light - mg C m 2 per day, dark - 106 t C per month.

Fig. 3. Season variation in the primary production; light - mg C m 2 per day, dark - 106 t C per month.

0 100 200 500 1000

Fig. 4. Annual production of phytoplankton in the Arctic Seas, mg C m 2 per day.

Table 2. Production of phytoplankton of the Arctic Seas of Russia (ASR).

The sea

Annual production (106 t C year-1)

Mean annual production

(mg C/m2 per day)

Flux of organic carbon to seafloor (106 t C year-1)

Barents

78.5

152

22

White

1.5

46

0.9

Kara

22.3

70

11.6

Laptev

11

41

5.2

East Siberian

8.2

25

4.3

Chukchi

42

200

24

ASR as a whole

163.5

68

Based on the new map of the mean annual primary production (Fig. 3), we compiled a map of the mean annual Corg flux toward the bottom (Fig. 5). The calculations were performed with the use of the empiric relation Fc = 33-PP/Z between the Corg flux to the sea bottom Fc, the value of the primary production in the water column PP, and the sea depth Z (Tseitlin 1993). At sea depths less than 50 m, the Z value was accepted to be 50 m. Main part of OM is dissolved and decomposes by bacteria during sinking. Nevertheless rather considerable part of OM reaches seafloor due to predomination of shallow waters in the Arctic Seas (Table 2).

0 5 25 50 100 150

Fig. 5. Flux of organic carbon to seafloor in the Arctic Seas, mg C m 2 per day.

0 5 25 50 100 150

Fig. 5. Flux of organic carbon to seafloor in the Arctic Seas, mg C m 2 per day.

Response on recent warming

To estimate response of production of phytoplankton on climate warming in the Arctic Seas we have considered their trends for 1998-2007 using satellite data SeaWiFS and MODIS on chlorophyll value averaged over 8 days.

The interannual variations in the primary production and the trend of its changes may be characterized by its average value over the test areas (mg C/m2 per day) and by the total production for each of the test areas (t C/day). The average value of the primary production for the test areas was calculated with the formula PP = S(PPo.7 x S).7)/?So.7, where PP0.7 is the primary production in a 0.7° x 0.7° cell, and S0.7 is the cell area with account for the latitude. We considered only the cells for which the data on chlorophyll were obtained. Thus, the influence of the clouds that covered the water surface on the final result was partially eliminated, as well as the decrease in the test area caused by ice.

The total production over the test areas was calculated with the formula PP =

S(PPq

S0.7). In this case, the decrease in the area due to the ice cover is included automatically, but the result is more distorted because of the clouds. This kind of distortion is somewhat decreased because the data on chlorophyll we used were averaged over 8 days of surveys. Thus, if a 0.7° x 0.7° sea area was partially opened for surveying during at least one satellite path, the value of the concentration measured is ascribed to the entire 8-day time interval in this area. The number of sites scanned this way over the test area in 8-day intervals (Fig. 6) tends to a significant increase, which points to the increase in the open water space in these tests areas and to the increase in the number of clear days.

2000 1000 0

4000

3000

0 0

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