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Oceanology, Vol. 41, No. 6, 2001, pp. 823832. Translated from Okeanologiya, Vol. 41, No. 6, 2001, pp. 860869. Original Russian Text Copyright © 2001 by Antal, Venediktov, Matorin, Wozniak, Rubin. English Translation Copyright © 2001 by MAIK "Nauka / Interperiodica" (Russia).

MARINE BIOLOGY

A Study of the Variability of the Parameters for the Model of the Calculation of the Rate of Phytoplankton Photosynthesis by the Fluorescent Method from the Example of the Baltic Sea
T. K. Antal1, P. S. Venediktov1, D. N. Matorin1, B. Wozniak2, and A. B. Rubin1
2Institute

Moscow State University, Moscow, Russia of Oceanology, Polish Academy of Sciences, Sopot, Poland

1

Received March 14, 2000; in final form, December 13, 2000

Abstract--In this study, we examined the possibility to determine the rate of phytoplankton photosynthesis in situ with a submersible pump-and-probe fluorometer. A biophysical model was suggested to describe the relationships between the photosynthesis, the underwater irradiation, and the intensity of phytoplankton fluorescence induced by an artificial light source. The fluorescence parameters Fo and Fv/Fm were used to determine the coefficient of light absorption and the efficiency of the primary reactions of phytoplankton photosynthesis. Two parameters of the model which cannot be measured by this method, namely, the coefficient of proportionality (k) between the fluorescence yield Fo and the light absorption phytoplankton capacity, as well as the light intensity semisaturating photosynthesis (I1/2), were found by means of calibrating the fluorometer in terms of the primary production values. The latter were measured by the radiocarbon modification of the flask method at 23 stations in different areas of the Baltic Sea in the period from May to September inclusively. It was shown that the standard deviations of these parameters in situ did not exceed 20%, and the values of the phytoplankton photosynthesis rate determined by the fluorescent method with the use of the averages of these parameters correlated well with the measured values (r = 0.89). The accuracy of determination of the primary production values decreased, for the most part, because of the decrease in the fluorescence quantum yield under the conditions of a hyperoptimal irradiance and variations of the I1/2 parameter related mainly to the phytoplankton concentration.

INTRODUCTION The contribution of the gross primary production of phytoplankton is about 95% of the total production in the oceans and 40% of the global carbon assimilation. It should be noted that the microalgae biomass is about 2% of the total mass of plants. This fact points to the high efficiency of light energy conversion by microalgae cells [14]. The measurements of the rate of phytoplankton photosynthesis give us the possibility to estimate the aquatic biosystem productivity on a local and global scale, as well as to study the mechanisms of the influence of environmental factors, including those of anthropogenic origin, on it. Photosynthesis of microalgae is estimated by the rate of radiocarbon assimilation by cells [37] or by the changes in the water-soluble oxygen concentration [26]. These methods are reasonably labor consuming, and, when used, artifacts associated with the long-term isolation of phytoplankton cells in flasks [9], with the difference between net and gross photosynthesis [6], as well as with metal toxicity [15], arise. Therefore, at present, the development of methods that enable one to avoid these problems and to rapidly and continuously estimate the rate of phytoplankton photosynthesis, without affecting their physiological condition, is an urgent task. To some extent, the methods for chloro-

phyll a fluorescence registration are helpful in the solution of these problems [16, 18]. The relationship between the chlorophyll a fluorescence and photosynthesis values is described by a number of biophysical models of the primary processes of photosynthesis [22, 39]. In this study, we consider the possibility for determination of the rate of carbon assimilation by phytoplankton Vc (µmole C m3 s1) in terms of fluorescence measured by the pump-and-probe method [25, 29] and excited by an artificial source of light. The basis for this model was the lightphotosynthesis dependence [19], which was described through the coefficient of sunlight absorption by microalgae suspension and the efficiency of the light energy conversion absorbed during photosynthesis. In terms of the fluorescence parameters Fo and Fv/Fm, the coefficient of light absorption by microalgae (the light absorption capacity) and the efficiency of the primary photosynthetic reactions were determined. In the model, there are two parameters that cannot be measured by the method suggested. The first is the coefficient of proportionality k between the fluorescence yield Fo and the phytoplankton light absorption capacity, as well as the light intensity semisaturating photosynthesis I1/2. The latter depends on the dark reactions limiting the rate of pho-

823

824

ANTAL et al.

tosynthesis. These parameters were determined by means of calibrating the fluorometer with respect to the rate of carbon assimilation by alga cells measured by the radiocarbon method. In this study, the variations of the k and I1/2 parameters were examined in the different areas of the Baltic Sea in the warm period of the year (May to September inclusively), depending on the accuracy of the measurements, abiotic factors, and phytoplankton characteristics. In addition, the possibility of using these parameters as constant values while estimating the primary production of phytoplankton in the test area during the season was examined. METHODS Abbreviations: RC--reaction center; PhS--photosystem; Fo--constant of chlorophyll fluorescence, relative units; Fv/Fm--relative fluorescence variable, relative units; Chl--chlorophyll a concentration, mg Chl m3; CPP and PP--primary production values of phytoplankton calculated in terms of fluorescence and measured by the direct method, respectively, mgC m3 per unit time; aPSP--coefficient of light absorption by the photosynthetic pigments of photosystem II of microalgae suspension (the light absorption capacity), m1; afl--coefficient of light absorption by algal suspension cells exciting Fo, m1; as--coefficient of light absorption by algal suspension cells, m1; k--coefficient of proportionality between Fo and the coefficient of light absorption by phytoplankton, namely, k = aPSP/Fo, relative units, km--average value over the water column; E--factor determined by the difference between the coefficients of absorption of the underwater irradiance by phytoplankton and the light exciting Fo, namely, E = aPSP/(aPSP)fl; Fo--quantum yield of Fo; I1/2--intensity of the light semisaturating the rate of photosynthesis, µE m2 s1, I1/2m--average value over the water column; z--depth, m. Model structure for the PP calculation in terms of the phytoplankton fluorescence parameters. The basis for this model is the photosynthesislight relationship [19] V c( I ) = a
PSP

photosystem II in a microalgae suspension (PSP--photosynthetic pigments [8]); (µmol C µE1) is the efficiency of the light energy absorbed by these pigments; and I is the underwater irradiance in the range of photosynthetically active radiation (µE m2 s1). The value is proportional to the relative amount of the functional (f) and open (qP) reaction centers of photosystem II in algal cells, to the efficiency of the photochemical light energy conversion in an open reaction center (RC, µmol electrons µE1), and to the efficiency of the electron transfer from H2O to CO2 (e, µmol C (µmol electrons)1): V c( I ) = a
PSP

fq P ( I ) RC e I .

(2)

Determination of the aPSP Values and the Product ( fRC) in Terms of Phytoplankton Fluorescence The fluorescence signal Fo excited by an artificial light source in open microalgae reaction centers can be found from the following equation: F 0 = GI fl ( a
PSP ) fl Fo

,

(3)

where Ifl is the intensity of the exciting light (in our fluorometer, Ifl() is fairly evenly distributed over the spectral band 400550 nm), a constant; (aPSP)fl is the coefficient of absorption of the light exciting fluorescence by photosynthetic pigments of photosystem II in the algal suspension, averaged over the range of 400-550 nm; Fo is the fluorescence quantum yield in the cells with open reaction centers of photosystem II; and G is the coefficient determined by the geometric characteristics and sensitivity of the signal sensor, a constant. Taking into account that (GIfl)1 = const, the coefficient of absorption of the underwater irradiance by the pigments of photosystem II in the microalgal suspension is related to the fluorescence in the following manner: a
PSP

= const Fo EF 0 = k ( Fo, E ) F 0 ,
1

(4)

( I ) I ,

(1)

where aPSP (m1) is the coefficient of absorption of the underwater irradiance by photosynthetic pigments of

where E = aPSP/(aPSP)fl and k is the coefficient of proportionality that depends on Fo and E. The efficiency of the photochemical light energy conversion in an open reaction center of photosystem II is expressed through the ratio of fluorescence parameters, namely, RC = (Fm F0)/Fm = Fv /Fm [23]. It was also shown that the decrease in the Fv/Fm value corresponds to a reduction of the fraction of the functional reaction centers of photosystem II (f) in phytoplankton cells [24, 25]. Such a reduction can be observed due to the destruction processes in photosystem II with a deficiency of nutrients [12, 17] and/or hyperoptimal irradiance (photoinhibition) [28, 38]. Thus, the product of the parameters RC and f is proportional to the relative yield
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A STUDY OF THE VARIABILITY OF THE PARAMETERS FOR THE MODEL

825

of the variable fluorescence of microalgae adapted to the natural irradiance: f
RC

= Fv / Fm .

(5)

Parameters qp and e It is well known that the photochemical energy conversion in photosystem II occurs only in open reaction centers. The relative concentration of the open centers qp was derived from the model for the light-dependent transition of the photosystem II reaction centers from the open to the closed condition [20]: q p ( I ) = I 1/2 / ( I + I 1/2 ) , (6)

where I1/2 is the light intensity, at which a half of the reaction centers are in the closed condition. The e value was estimated in the following manner. Reduction of the CO2 molecules requires direct transportation of four electrons from photosystem. Therefore, theoretically, the e value may be as great as 0.25, but the electrons are partially consumed for the reduction of nitrates and sulfates [8, 27], for the cyclic electron flow around photosystem I [31, 36] and photosystem II [11], as well as for reduction of O2 [3]. The correlation of this parameter with the maximum quantum yield of carbon fixation allows us to accept that e is roughly constant [21, 30] and does not exceed 0.16 for phytoplankton under natural conditions [5]. Therefore, we admit that e is equal to 0.16. Substituting (4)(6) into equation (2) and entering the coefficient 6.9 = 12 в 103 (µC µmolC1) в 3600 (s h1), we write the vertical distribution of the rate of phytoplankton photosynthesis (mgC m3 h1) through the fluorescence parameters: I 1/2 V c ( z ) = 6.9 k ( z ) F 0 ( z ) F v / F m ( z ) ---------------------- I ( z ) , (7) I ( z ) + I 1/2 where z is the depth. Determination of k and I1/2. The unknown k and I1/2 parameters were determined by correlating the phytoplankton primary production values (mgC m3 h1) measured by the radiocarbon method with the data on fluorescence and the underwater irradiance according to the formula PP ( z ) = 6.9

i = 1,

n

I 1/2 m k F ( z ) F / F ( z ) ------------------------- I ( z ) t , (8) m0 v m i I ( z ) + I 1/2 m

where n is the number of fluorescence and irradiance profiles observed over the time of the flask exposure at a station; t is the time between these measurements (h); and km and I1/2m are the averages within the water
OCEANOLOGY Vol. 41 No. 6 2001

column in the euphotic zone for the k and I1/2 parameters, respectively. They were calculated by approximation of the relationship between PP and the sea depth using the function with two unknown parameters (km and I1/2m) written in the right-hand part of formula (8). At the first approximation carried out by the least square method with the use of the built-in procedures of the GIM program, rough values of the km and I1/2m parameters were found. After that, through varying these parameters, values were selected at which the PP depth relationship was most exactly described by formula (8) (with the coefficient of correlation not less than 0.95). Data registration. The data on the vertical distribution of fluorescence, irradiance, phytoplankton primary production, and chlorophyll a concentration were obtained during the following cruises to the Baltic Sea (13°1025°15 N, 53°2558°10 E): (1) JuneJuly 1993--cruise of R/V Humboldt by the "Plankton" Program; the data measured at seven stations in the southern and eastern coastal waters of the Baltic Sea are presented; (2), (3), (4), and (5)--May 1993, September 1993, May 1994, and September 1995, respectively. These are cruises of R/V Oceania of the Institute of Oceanology of the Polish Academy of Sciences. The data measured at 16 stations in the central and coastal parts of the Baltic Sea are presented. Fluorescence measurement. The vertical distribution of fluorescence was measured in situ with a submersible pump-and-probe fluorometer developed at Moscow State University, Biological Faculty, Department of Biophysics. The instrument also provides determination of the density of the quanta flux in the range of photosynthetically active radiation (µE m2 s1), temperature(°C), and sea depth (m). The fluorometer produces series of the subsequent pump-and-probe light impulses with a frequency of 2 Hz. The saturating (pump) flash emitting 1 J of light energy over 0.01 ms follows 1 s after the first probing flash (0.01 J 0.01 ms1), and then, after 50 µs, the second probing flash follows. The pulses are produced by a SSh-20 xenon lamp. Flashes are isolated from the sample by a SZS-22 blue green light filter. The excitation spectrum of fluorescence is evenly distributed over the wavelength range from 400 to 550 nm. When submerging the probe, water passively enters into it through a dark chamber, where, over 0.5 s, the fluorescence of the phytoplankton cells adapted to the underwater irradiance is measured. The velocity of submerging is 0.30.5 m s1 allowing us to obtain vertical profiles of the values measured with a high resolution. At illumination by the first probing flash, Fo is registered, i.e., the fluorescence yield at open reaction centers of photosystem II. The saturating light flash takes most of the reaction centers to the closed state, and in

826

ANTAL et al.

the time comparable to the turnover time of a reaction center (0.1 ms), the second probing flash is delivered. The fluorescence (I1) corresponding to the maximum level of the fluorescence saturation with a short powerful flash is registered [35]. Fm is calculated according to the formula Fm = 1.4I1, where 1.4 = FmDCMU/I1 is the ratio of the maximum fluorescence yield in the presence of diuron, an inhibitor of the electron transport to photosystem II, to the fluorescence yield measured instrumentally. The fluorescence signal is registered with a FEU-68 detector after passing through a KS-17 boundary light filter with a transmission wavelength of > 680 nm. The fluorescence signals, the underwater irradiance, temperature, and pressure (depth) registered are transmitted through a cable to the onboard controlling computer in the online mode. Phytoplankton primary production. Phytoplankton PP was measured by radiocarbon modification of the flask method at 510 sea depths down to 30 m by the standard procedure [37]. The flasks were exposed in situ within the water column in cruise 1 for six hours; in cruises 2, 3, and 4 for four hours; and in cruise 5 for 2 h. Chlorophyll a concentration. This was measured in the microalgae samples by the spectrophotometric method after filtration of a water sample through a membrane filter with a mesh size of 0.45 µm and extraction of the sediment obtained with acetone [34]. Marine algae such as diatoms Phaeodactylum tricornutum [Bohlin] and Thalassiosira weissflogii [Grunow], chrysophyta Nephlochloris salina [Cart], and green alga Platymonas viridis [Rouch.] were cultivated in bottles at a constant temperature of 20°C and
a(), nm1 0.08 E(0) = as/a 0.06 a 0.04 0.02 0
fl fl

illumination of 10 W m2 in a Goldberg medium, which was prepared with artificial sea water [2]. RESULTS AND DISCUSSION It was experimentally shown [10, 40] that the photosynthetic parameters Fo and I1/2 could vary at a stress impact of abiotic factors. Under natural conditions, the value of the Fo parameter does not depend significantly on the effect of the environmental factors [32, 33], whereas the I1/2 parameter depends mainly on the water temperature [7, 40]. We admit that these parameters are close to constant within the region with similar characteristics of the water temperature during a season. The k and I1/2 values in the euphotic zone averaged over the water column, i.e., km and I1/2m, respectively, were calculated (equation (8)) according to the data from 51 measurements of the vertical fluorescence profiles and the underwater irradiance at 23 stations in the Baltic Sea. The stations were carried out in the central and coastal (from the Gulf of Riga to Pomorskaya Bay) parts of the Baltic Sea and were characterized by an average chlorophyll a concentration within the water column from 0.7 to 10 mg m3. The results obtained are given in the table. Variations in the km values. The average value of this parameter at all the stations was 5.6 в 105 (standard deviation SD = ±17%). The dispersion of the k values can be related to the factor E = aPSP/(aPSP)fl ~ a/afl (see formula (4)), which depends on the attenuation of the underwater light (at a certain depth) and that of the excited phytoplankton fluorescence. The value E depends, for the most part, on the taxonomic composition of microalgae and their physiological condition. Figure 1 shows an example of the calculation of the E value for the phytoplankton exposed to solar light with respect to the light spectrum absorbed by the suspension of microalgal cells. Owing to the averaging of the absorption values over the spectrum in the spectral band 400700 nm, we found the coefficient of solar light absorption (as) and, in the spectral band 400-550 nm, the coefficient of absorption of light exciting fluorescence (afl). For example, the E values calculated in such a way in four species of the sea microalgae varied from 0.6 to 0.75 (the data are not presented). These microalgae refer to three widespread taxonomic groups such as diatoms Ph. tricornutum and Th. weissflogii, chrysophyta N. salina, and green algae P. viridis grown under optimal conditions at low irradiance (<1 W m2), as well as after a 20-h cultivation in a KNO3-free medium. For the samples of natural phytoplankton from the Baltic Sea, the E value is equal to 0.74. The experimental values did not exceed unity due to the fact that the coefficient of absorption of the color blue by the sea algae was usually higher than that of solar light because of the great content of carotinoids absorbing the light in the blue region. Therefore,
OCEANOLOGY Vol. 41 No. 6 2001

a

s

450

550

650 , nm

Fig. 1. Spectrum of the light absorption by phytoplankton suspension taken from the central part of the Baltic Sea. The absorption values are averaged over the spectral bands 400-550 nm (afl) and 400700 nm (as).

A STUDY OF THE VARIABILITY OF THE PARAMETERS FOR THE MODEL

827

Calculation of the km and I1/2m values at the stations in the Baltic Sea. Numbers of cruises and stations, dates, areas, and time ranges (hours of local time) are presented km в 105, relative units Cruise Stations Date Area 36 1 1 2 3 4 5 6 7 2 8 9 10 11 3 12 13 14 4 15 16 17 5 18 19 20 21 22 23 June 23, 1993 June 26, 1993 June 28, 1993 June 30, 1993 July 1, 1993 July 9, 1993 July 10, 1993 May 13, 1993 May 14, 1993 May 15, 1993 May 10, 1993 September 28, 1993 September 29, 1993 September 30, 1993 May 9, 1994 May 11, 1994 May 13, 1994 September 8, 1995 September 9, 1995 September 10, 1995 September 12, 1995 September 13, 1995 September 14, 1995 6 6 4 3 3 2 2 1 1 4 6 6 6 6 1 6 5 3 3 1 1 1 1 5.60 5.28 5.60 5.00 4.72 5.00 Time range, h 69 4.64 4.92 5.48 5.32 5.52 4.60 4.80 5.40 5.60 912 5.48 4.8 6.20 4.32 5.88 4.44 4.92 5.68 5.80 8.44 5.48 7.24 7.96 5.60 8.60 5.28 6.08 5.32 5.48* 5.88 5.76* 5.12 1215 5.60 5.20 4.44 5.60 5.00 5.48 6.20* 7.44* 36 134 170 176 113 188 170 I1/2m , µE m2 s1 Time range, h 69 155 164 134 116 100 95 134 148 180 912 167 135 144 107 134 140 124 118 178 145 164 104 110 125 115 140 98 145 190* 133 165* 131 1215 161 128 124 139 155 127 121* 138

Note: Areas: 1--central waters; 2--Gulf of Riga; 3--Coast of Lithuania; 4--Gulf of Gdansk; 5--Coast between Gulf of Gdansk and Pomorskaya Inlet; 6--Pomorskaya Inlet. * Results averaged over several measurements.

we can admit that, within the upper water layers (not deeper than 1 m), where the spectrum of the underwater irradiance is close to that of solar light, the E value for natural phytoplankton varies from 0.6 to 0.75. The changes in the spectrum of the underwater irradiance with depth are accompanied by changes in the E value; namely, it occurs according to the equation E(z) = E(0)(z), where E(0) corresponds to the E value at the water surface and (z) describes the E value variations with depth. In clear waters, the contribution of red light to the spectrum of the underwater irradiance decreases with depth and has to cause a rise in the E value from 0.600.75 at the surface to about 1.00 at a depth of 20 m and deeper, where the spectrum of the fluorescence excitation and that of the underwater irraOCEANOLOGY Vol. 41 No. 6 2001

diance almost coincide (the data are not presented). Therefore, we can expect that the E values averaged over the water column, on which the km values depend, vary in the range from 0.7 to 1.0. Within the upper intensively mixed layer of the Baltic Sea, the taxonomic composition of phytoplankton within the water column is homogeneous, and the pattern of the vertical distribution of the chlorophyll a concentration represents variations in the phytoplankton concentration and light absorption over the profiles. At low irradiance at the surface (I(0) < I1/2), one can admit that Fo(z) is constant, therefore, E(z) = const Chl(z)/F0(z). We estimated the E value at a depth of 1 m using the results of 11 measurements of Fo and

828 Amount (a) 15 SD = ± 18% 10 (b) SD = ± 21%

ANTAL et al.

5

0 4.0 4.8 5.6 6.4 7.2 8.0 8.8 100 120 140 160 180 km в 105, relative units I1/2m, µE m2 s1
Fig. 2. Histograms of the distributions of (a) km and (b) I1/2m values calculated by formula (8) from experimental data on fluorescence, underwater irradiance, and phytoplankton productivity (by the radiocarbon method) at 23 stations in the Baltic Sea.

km в 105, relative units 9.6 7.2 4.8 2.4 Mean

1 2 1000 1500 2000 I(0), µE m2 s1

As we can see from the histogram of the distribution of km values given in Fig. 2a, the rather great standard deviation of this parameter was, to a great extent, determined by the km > 7 в 105 values, which significantly exceeded the average for the entire station (5.6 в 105). Figure 3 illustrates the relationship between the km values and those of the irradiance at the surface I(0) constructed by the results of the measurements that were conducted at the stations where a light-dependent depression of the primary production and phytoplankton fluorescence was registered near the sea surface. As is shown in the figure, a weak positive correlation between the km values and those of the surface irradiance was observed only in the case when the km values exceeded 7 в 105. These values were obtained at stations 11, 13, 14, 16, and 22 (below, they will be denoted as stations*). Figure 4a shows the relationship between the ratio PP/(FсFv/Fm) and the underwater irradiance typical of stations* and the other 17 measurements performed at the stations with PP and Fo depressions within the upper layers. At high values of the underwater irradiance (I(z) > I1/2), this ratio is proportional to the product (kI1/2), because the equation PP/(FoFv/Fm) ~ const(kI1/2) (see formula (7)) is admitted. As we can see from the figure, at the values I > 400 µE m2 s1, the product (kI1/2) increased at stations*, which caused an overestimation of the calculated km values at these stations. At other stations, the changes in the (kI1/2) product were independent of I(0). The reason for the lightdependent changes in the (kI1/2) product could be the decrease in the Fo values and/or the increase in the I1/2 values under the adaptation of phytoplankton to the hyperoptimal irradiation. In [4] it was shown that the I1/2 parameter depended on the optical depth and had a rise near the water surface. However, this relationship is characteristic of oligotrophic areas and, to a lesser extent, of mesotrophic areas; in eutrophic areas such as the test area in the Baltic Sea it is absent. Therefore, the increase in the product (kI1/2) in the subsurface waters at stations* was most likely related to the vertical variations in the quantum fluorescence yield. This was confirmed when comparing the profiles of Fo and the chlorophyll a concentration. As mentioned above, in the intensively mixed waters, the vertical distribution of the chlorophyll a concentration is proportional to that of the light absorption and, correspondingly, to the phytoplankton fluorescence, namely, Chl(z) = const k(z)Fo(z). The character of the vertical distribution of the chlorophyll a concentration was homogeneous at stations*, where km > 7 в 105, while the subsurface Fo values were twofour times smaller than at a depth of 15 m and deeper (Fig. 4b). This pointed to an increase in the k values in the subsurface waters. The rise in the values of the k parameter was related to the decrease in the fluorescence quantum yield at a high intensity of the underwater irradiation. It is interesting that four of five stations*, at which the
OCEANOLOGY Vol. 41 No. 6 2001

0

Fig. 3. Dependence of the km parameter on the irradiance at the sea surface I(0) and its value averaged over 23 stations in the Baltic Sea. 1--km > 7 в 105; 2-- km < 7 в 105.

the chlorophyll a concentration under low irradiance Chl ( 1 ) Fo ( 20 ) according the formula E(1) = --------------------------------- E(20), Chl ( 20 ) Fo ( 1 ) where E(20) = 1. The values obtained varied from 0.83 to 1.1, which exceeded the E(0) values calculated from the spectra of light absorption by algal cultures. This result can be caused by the marked decrease in the contribution of the red light to the spectrum of the underwater irradiance already at a depth of 1 m. Therefore, the average E value over the water column is close to unity, shows weak variations at different stations, and has no pronounced effect on the dispersion of the km values.

A STUDY OF THE VARIABILITY OF THE PARAMETERS FOR THE MODEL Fo, relative units 60 1800 3000 (a) 0.015 0.010 4 0.005 1 2 400 800 I, µE m2 s1 1200 1 3 5 Chl, mg m3 1 2 3 6 (b) 0 2 Depth, m

829

0.020 Fo*(Fv/Fm), relative units

PP

0

Fig. 4. (a) Relationships between the ratio PP/(FoFv/Fm) and the underwater irradiance I observed at the stations in the Baltic Sea during the midday depression of Fo and PP within the upper water layers typical of the stations with 1-- km > 7 в 105 and 2--km < 7 в 105. (b) Example of the vertical distribution of the Fo values and chlorophyll a concentrations at the stations with km > 7 в 105: 1-- Fo; 2--Chl; 3--result obtained after correcting Fo with regard to Chl.

growth of the k values was observed near the surface, were located in Pomorskaya Inlet (the Odra River estuary). Therefore, the characteristic light-dependent decrease in the Fo values in the subsurface waters was likely to be related to the features of the phytoplankton physiological condition in this area, which are explained by its exposure to anthropogenic factors. After conversion of the km values with regard to the vertical Fo profiles corrected by the chlorophyll a concentration at five stations*, as is shown in Fig. 4b, the standard deviation of this parameter for all the measurements decreased from 17 to 9% as compared to the value calculated before. This points to a reasonably great contribution of the light-dependent variations of the Fo to the dispersion of the km parameter. Therefore, when calculating this parameter, it is desirable to take into account the necessity of a correction of the Fo and chlorophyll a concentration profiles and to carry it out. At other stations at which a Fo depression was registered in the subsurface waters, the chlorophyll a concentration decreased proportionally to the fluorescence value within the upper layers. In this case, the ratio Chl(z)/Fo(z) did not depend on the depth or show insignificant variations. Thus, the vertical variations of the Fo values at most of the stations reflected the changes in the concentration of microalgae and their light absorption capacity rather than the quantum yield of fluorescence, which, in general, agrees with the data published in [32, 33]. They show that the Fo value for the natural phytoplankton is close to constant, in particular, in the Baltic Sea. Thus, only at 5 stations of 23 studied in the Baltic Sea was the dispersion of the km parameter mostly related to the light-dependent variations of Fo. After correction of the fluorescence profiles with respect to those of the chlorophyll a concentration, the dispersion of the km values decreased almost twofold. Similarly,
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the average of the km values for all the measurements, which comprised 5.4 в 105, insignificantly decreased. Later, the above value was used as a constant when calculating the phytoplankton primary production values in the area studied. Variations of the I1/2 value. In column 6 of the table, the calculated values (according to formula (8)) for the I1/2 (I1/2m) averaged over the water column are given. The maximum and minimum I1/2m values were equal to 98 and 190, respectively; the average calculated with regard to all the measurements was 137 µE m2 s1, and the standard deviation was 22%, which points to a greater extent of variation of this parameter as compared to km (see Fig. 2b). The I1/2m value did not show any correlation with the daily dynamics of the irradiance (see also [1]), but it had a tendency to decrease with the growth of the chlorophyll concentration (Fig. 5a). The result of a polynomial fit of the relationship between the I1/2m values and the average chlorophyll a concentration over the water column (Chlm) at the stations of the Baltic Sea is: I
1/2 m

= 171 14.7 Chl m + 0.8 ( Chl m ) .
2

(9)

When comparing Figs. 2b and 5b, we can see that the standard deviation of the I1/2m value de