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Selenium in plankton from the northwestern Mediterranean Sea
~ Pergamon P l h S0043-1354(96)00155-.8
Beat. Res. Vol. 30, No. 11, pp. 2593-2600, 1996 Copyright © 1996ElsevierScienceLtd Printed in Great Britain,All rights reserved 0043-1354/96$15.00+ 0.00
SELENIUM IN P L A N K T O N FROM THE NORTHWESTERN M E D I T E R R A N E A N SEA F. BOISSON* and M. ROMEO@ Laboratoire de Toxicologie Marine, Facult6 de Medecine, Universit6 de Nice-Sophia Antipolis, 06107 Nice, Cedex 02, France (First received July 1994; accepted in recised form April 1996) Abstract--Selenium is a multiple oxidation state trace element whose redox chemistry and concentration in sea water can be modified by biological activity in the euphotic zone. A field study was undertaken to examine the effects of oceanographic and biological parameters on the selenium concentration of the coastal plankton. For this purpose, plankton was collected in the Ligurian Sea from October 1990 to December 1991. The samples analyzed were composed of mixed phytoplankton and microzooplankton ranging in size from 50 to 200 #m. The medium selenium concentration measured in microplankton was 1.57#g Se.g -~ carbon or 0.62#g Se.g -~ dry weight (n= 15). Seasonal fluctuations in selenium concentration of microplankton were influenced by hydrodynamic conditions. During strong vertical mixing of the water column, deeper water with high selenite and selenate concentrations may reach the surface and selenium concentration in plankton was found to increase by a factor of eight compared to the median annual value. Under usual hydrodynamic conditions, seasonal fluctuations of the microplankton selenium content were typically low. Such low variations appeared to be due to the specific composition of microplankton. In the case of a phytoplankton bloom, selenium concentration in microplankton increased in proportion to the diatom biomass, whereas in other cases a negative linear relationship was found between selenium content and zooplankton biomass. The biogeochemicalcycling of selenium may be influenced by seasonal fluctuations of the plankton selenium content which is controlled by hydrodynamic processes and plankton composition. Copyright © 1996 ElsevierScience Ltd Key words--phytoplankton, zooplankton, selenium, bioaccumulation, water pollution, Mediterranean Sea
INTRODUCTION Selenium is a metalloid belonging to the V I A group of elements. Because of its physical and chemical similarities with sulphur, it can behave as a sulphur analogue in algae (Shrift, 1954; Wrench, 1978). Selenium is an essential element for some algae (Price et aL, 1987), but it is also toxic at elevated concentrations (Anonymous, 1987). Selenium enters the Mediterranean Sea from atmospheric and terrestrial inputs (natural or anthropogenic). The atmospheric input is estimated to be five times higher than that from terrestrial runoff (Xiao Ming, 1989). Xiao Ming (1989) reported that approximately 110 tons of selenium per year are added to the northwestern Mediterranean Sea of which 9.7 tons enter from the Rh6ne river in France. Total selenium concentration in surface water of the northwestern Mediterranean Sea is approximately 523 pM.l -j (Dao Ming and Martin, 1991). These authors reported that selenium concentration was similar in the coastal area and in the open sea. *Present address: I.A.E.A. Marine Environment Laboratory, P.O. Box 800, MC-98012 Monaco [Fax: (377) 92 05 77 44; Email: [email protected]].
Biological processes are known to play a central role in establishing the redox state and spatial distribution of dissolved trace elements in the oceanic environment. In the case of selenium, Wrench and Measures (1982) provided direct evidence that biological activity in the euphotic zone can modify both redox balance and the concentrations of this multiple oxidation state trace element (Se -2, Se°, Se +4, Se+6). At present, there are few reported measurements of selenium in marine plankton (Sandholm et al., 1973; Terraria et al., 1975; Wrench and Measures, 1982; Cutter and Bruland, 1984; Liu et al., 1987) and only one in the plankton from the Mediterranean Sea (Fowler and Benayoun, 1976). No data are apparently available on temporal variations in selenium content of plankton. Thus, in order to better understand the biogeochemical cycling of selenium in marine systems, a spatial and temporal survey of selenium in plankton was undertaken in the northwestern Mediterranean Sea. The aims of the study were to establish (1) what type of quantitative data can be obtained from a simple sampling scheme; (2) how hydrodynamic conditions, plankton biomass and specific composition may produce significant changes in the selenium content of plankton; (3) how
2593
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F. Boisson and M. Romeo
the spatial a n d t e m p o r a l variations in the selenium c o n c e n t r a t i o n can be interpreted in Ligurian sea plankton. SAMPLING AND METHODOLOGY
Study area The Ligurian Sea is located in the northeastern portion of the western Mediterranean Sea bounded by the French and Italian coasts. The water layers present in the Ligurian Sea have been described according to B&houx and Prieur (1983): surface water 0-200m, T = 12.4-22.8°C, S = 3%5-38.1 PSU; intermediate water 200-800 m, T = 13.50°C, S > 38.45 PSU and deep water (Gostan, 1968) with T = 12,90°C and S = 35.5PSU. The surface and intermediate waters flow north along both sides of Corsica, join to the north of Cap Corse and form the Ligurian current which parallels the shores of the Italian and French Riviera. 91% of the flux of this current is channelled in a coastal band 32 to 48 km wide with the major part of the flux occurring in the first 13 km (B6thoux et al., 1988). Thus the Ligurian current plays an important role in the circulation of the western Mediterranean Sea and in the associated transport of pollutants. The circulation pattern in the Ligurian Sea is shown in Fig. 1. Strategy and data acquisition Eight surveys involving physical and biological measurements were made at two stations, B (43°41'10"N; 07°19'00"E) and 4V (43°35'08"N; 07°18'15"E), from October 1990 to December 1991. The location of the stations is shown in Fig. 1. Station B is at the entrance of the Bay of Villefranche-sur-mer with a depth of 80 m; station 4V is in the Ligurian current, 11 km off-shore with a depth of 1151 m. The physical measurements included temperature, salinity, oxygen, chlorophyll a and nutrients. Due to technical reasons, dissolved selenium in sea water was not measured. Only selenium concentrations in
10~
20~ !
Villefranch( •
4V
GEN,~
i
43o
!
7"
f
8"
I 9'
!
10"
Fig. 1. Liguro-Proven~;al basin in the northeastern part of the western Mediterranean Sea. The Ligurian current is formed by the surface and intermediate waters which flow north along both sides of Corsica and join just north of Cap Corse. Sampling stations (B and 4V) are indicated.
plankton were considered taking into account hydrological and biological parameters. For this study, a sample was collected at each station by horizontally towing a plankton net of 53-#m mesh size, at 5 m depth. The mean volume of sea water filtered through the net was 60 m 3. The plankton collected was sieved on board through 500 pm to remove large particles and kept in a cooler. In the laboratory, plankton samples were successively sieved through 200 #m and 50 #m. Only the 50-200 #m size range was analyzed. This size fraction is considered to be microplankton (Sieburth et al., 1978). Samples were then rinsed with buffered ammonium formate (1.4 M) to remove salt and dehydrated at 50°C to a constant dry weight. For selenium measurements, triplicate analyses were performed using the following technique: dried samples (between 1 and 3 g) were digested under pressure with 1.5 ml suprapur concentrated nitric acid (Merck) using a microwave oven. Samples were placed in a teflon PFA-lined digestion vessel and the volume of the final solution was adjusted to 5 ml with deionized water. Samples were stored in "nalgene" tubes at 4°C, in darkness, until analyzed. Selenium in biological material was determined by duplicate analyses at 196nm (Boisson, 1992) using an atomic absorption spectrometer (Pye Unicam SP9) equipped with a coated graphite furnace and an automatic sampler. Deuterium background correction was used in order to eliminate non-specific absorbance. The detection limit of the method was 2.54 pg.l-' with a coefficient of variation of 3.5%. The accurancy of the procedure was previously checked by analysis of a standard reference material (lobster hepatopancreas--CNR Canada). Selenium concentration obtained by triplicate analysis was 6.28 + 0.14mg.kg ' D.W., in agreement with the certified value (6.88 + 0.47 mg.kg-' D.W.). An aliquot of each sample was preserved and resuspended in filtered (0.2 #m) seawater for plankton species determination. The species composition and enumeration of each sample was microscopically determined by the Uterm6hl method (Uterm6hl, 1958) after Lugol fixation.
Data conversion For the whole sample, data expressed as pg Se.g -t D.W. were converted to #g Se'g -~ carbon as follows: from the plankton enumeration of the aliquot and using the results reported by Lins Da Silva (1991) for phytoplankton (ng C.cells-') and Nassogne (1972) for zooplankton, the number of individuals of each species was transformed into ng of carbon. Carbon biomass of phytoplankton (Wp), zooplankton (Wz) and total plankton (W,) were then calculated for each aliquot. Carbon biomass (W) was transformed to dry weight biomass (D.W.) using the following relations: for phytoplankton D.W. = 1/0.35 Wp (Bougis, 1974); for zooplankton D.W. -- 1/0.43 Wz (Bougis, 1974); for the copepod Oithona sp. which was found to be the dominant organism in July D.W. = 1/0.4 W (Nassogne, 1972). The percentage of dry weight in the aliquot attributed respectively to phytoplankton and to zooplankton was then deduced. These percentages were applied to the dry weight of the whole sample. After obtaining the dry weight, the carbon biomass of phytoplankton (Wp), zooplankton (Wz) and total plankton (WJ was computed. Selenium concentrations in the plankton (expressed as #g Se.g-~ C) were also calculated for the whole sample. RESULTS
Hydrodynamic characteristics of the sampling periods T h e h y d r o d y n a m i c structure of the Ligurian Sea is determined by the presence or absence o f t h e r m a l stratification with a thermocline limited by the 15°C
S e l e n i u m c o n c e n t r a t i o n in c o a s t a l p l a n k t o n
isotherm (Gostan, 1968; Nyffeler et al., 1980). Data for the physical and chemical parameters are presented in Table 1. In February 1991, the hydrodynamic conditions were very particular. Sea water temperature was at a minimum (12.82°C, winter minimum) and corresponded to a period of homogeneity in the water column between 0 and 40m. Salinity values ranged from 38.126PSU at station B to 38.143 PSU at station 4V. Temperature and salinity data show that water mass displayed winter characteristics (B6thoux and Prieur, 1983). In winter, the density of surface water increases leading to a mixing of the water column due to the influence of cold and dry air masses. Therefore, the formation of a water mass with a temperature lower than 13°C and a salinity higher than 38.000 PSU indicates a non-steady state condition and mixing of the water column. The hypothesis of deeper water coming up to the surface was supported by the high values obtained for the nutrients.
2595
The samples of June 1991 corresponded to a spring situation, i.e. stratification of the surface layer. The densities were lower at surface (26.90kg-m -3 at station B and 27.12 kg'm -3 at station 4V) than at 4 0 m depth (28.39kg.m -3 at station B and 28.40 kg.m -3 at station 4V). In July 1991, the thermal stratification was well established and the density difference between water at the surface and at 40 m depth was important. The 15°C isotherm at the base of the thermocline was located at 40 m depth. This sampling was carried out during summer conditions. Samples from September, October and December 1991 were collected under fall conditions. In September the thermocline decreased below 40 m. In October, temperature and salinity had the same value at the surface as at 40 m depth. These conditions caused a vertical instability in the water column as described by Nival and Corre (1976). In December, the seawater became colder due to water mixing and
Table 1. Temperature (T, ~'C), salinity (S, particular salinity unit), density (kg.m-3), chlorophyll a (Chl a, mg.m 3), oxygen (02, ml.1 ~) and nutrients ( p M ' l - ' ) at station B and 4V for each collection date between October 1990 and December 1991. In December, only Station B could be sampled (n.d. = not determined) Sample
Station
15/10/90 5m 40m 15/10/90 5m 40m 18/01/91 5m 40m 18/01/91 5m 40m 26/02/91 5m 40 m 26/02/91 5m 40m 18/06/91 5m 40m 18/06/91 5m 40m 11/07/91 5m 40m 11/07/91 5m 40m 3/09/91 5m 40m 3/09/91 5m 40m 28/10/91 5m 40m 28/10/91 5m 40m 6/12/91 5m 40m
B
T (°C)
Salinity (PSU)
Density (kg.m -J)
20.64 20.39
37.664 37.722
26.62 26.73
20.50 20.06
37.374 37.809
13.98 13.99
Chl a (mg.m -J)
O2 (mid-')
SiO2 (,um.I -~)
PO4 (/.tm.l-')
0.09 0.11
5.10 5.14
1.20 1.20
0.15 0.10
26.44 26.89
0.11 0.06
5.10 5.16
1.00 1.00
38.050 38.160
28.56 28.65
0.08 0.08
5.70 5.84
14.02 13.95
n.d. n.d.
n.d. n.d.
0.13 0.06
13.09 13.09
38.126 38.131
28.80 28.82
12.82 13.00
38.143 38.139
19.72 14.67
NH4 (,um.l-D
NO2 (#m.l ')
NO~ (pm.l ')
0.30 0.30
0.35 0.30
0.30 0.30
0.10 0.10
0.20 0.20
0.15 0.15
0.10 0.10
1.40 1.40
0.10 0.10
<0.10 <0.10
0.10 0.10
0.70 0.70
5.54 5.80
1.40 1.40
0.10 0.10
<0.10 <0.10
0.10 0.15
0.80 0.90
0.30 0.15
n.d. n.d.
1.60 1.60
0.15 0.30
0.10 0.20
0.25 0.45
0.90 2.00
28.87 28.83
n.d. n.d.
n.d. n.d.
1.60 1.60
0.15 0.15
0.10 0.10
0.20 0.20
0.60 1.10
37.709 38.036
26.90 28.39
0.14 0.05
5.41 5.60
1.40 1.40
0.10 0.13
0.10 0.10
0.10 0.15
0.40 1.10
19.25 i4.59
37.826 37.033
27.12 28.40
0.10 0.07
5.20 5.64
1.60 1.60
0.10 0.10
0.10 0.10
0.10 0.15
0.30 0.20
24.09 14.98
37.552 38.026
25.55 28.31
0.03 0.01
5.18 5.69
1.40 1.60
0.15 0.10
0.10 0.10
0.20 0.10
1.30 1.30
23.74 15.01
37.692 38.030
25.76 28.31
0.04 0.01
4.98 6.07
1.40 1.40
0.05 0.05
< 0.10 <0.10
0.15 0.15
0.40 0.50
25.19 16.18
38.278 37.995
25.76 28.01
0.02 0.13
4.87 6.00
1.20 1.20
0.10 0,10
0.40 1.00
0.20 0.40
0.80 1.00
24.20 16.50
38.334 38.319
26.12 28.23
0.04 0.13
5.34 5.39
1.00 1.00
0.10 0.10
0.60 2.30
0.10 0.15
0.60 0.50
19.09 19.08
38.994 38.037
27.29 27.33
0.25 0.32
5.38 5.39
1.20 1.60
0.10 0.10
0.10 0.10
0.15 0.15
0.20 0.20
18.85 18.87
37.960 37.992
27.33 27.33
0.17 0.24
n.d. n.d.
1.20 1.10
0.10 0.10
0.10 0.10
0.15 0.15
0.90 0.20
15.82 15.86
37.994 37.947
28.06 28.05
0.18 0.17
n.d. n.d.
1.40 1.40
<0.05 <0.05
<0.10 <0.10
0.15 0.15
0.30 0.40
4V
B
4V
B
4V
B
4V
B
4V
B
4V
B
4V
B
2596
F. Boisson and M. Romeo
BIO~
(%C)
(A)
I00
~" ~
75-
•%.,~
"~g$
.~,.¢. "~'~ .... ~"~"~ ~liil .~ ~.~ :a:
50-
25
.~.~ .~ •~
I~.~ ~.~ ~,"~2 ~'~
•.~.~
~,."
! JUL
SEP
.o*0'
I JAN
OCT
! FEB
JUN
OCT
I DEC
[]
DIATOMS
•
TINTINNIDS
[]
DINOFLAGELLATES
[]
COPEPODS
(%C}
BIOI~
(B)
100
No data were obtained in December for station 4V. Zooplankton comprised the majority of the plankton collected (between 56 and 94%): Tintinnids represented a low proportion of the biomass and were only present in October 1990 and 1991, and February, September and December 1991. The zooplankton biomass was mainly comprised of copepods and copepodites which were present all through the year. The biomass of the samples collected in July 1991 was only composed of the copepod Oithona sp. For phytoplankton, diatoms (Chaetoceros sp. and Chaetoceros curvisetus) were the dominant group present in January and December 1991. Dinoflagellates were present all year and they never represented more than 43.3% of the carbon biomass. Samples collected in December 1991 were mainly phytoplankton dominated by the diatom Chaetoceros curvisetus (74% of total C) and the dinoflagelate Ceratium sp. (12% of total C).
Seasonal variations of selenium concentration in microplankton '
3
Selenium concentration (#g Se.g -s C) in the microplankton collected just outside the Bay of
,~_~', ,??~.',
,,i!
I
~g S e / g c
¢:
;J..q, 12
.% 8
0 OCT
I JAN
FEB
I JUN
I JUL
I SEP
I
I
OCT
DEC
Fig. 2. Seasonal variations of the percentage of carbon attributed to diatoms, dinoflagellates, tintinnids and copepods in the microplankton biomass at station B (A) and station 4V (B). Only species with at least 1% of the whole sample carbon biomass were considered. OCT
a concomitant decrease in air temperature. Bougis (1968) reported that during this period and under strong SW winds (parallel to the coast), deeper water rich in nutrients could upwell to the surface leading to the formation of a phytoplankton bloom.
JAN
FEB
JUN
JUL
SEP
OCT
DEC
se/g c 14 -t'[
(B)
12-
Seasonal variations in plankton biomass composition
1 0 -~
The seasonal variation of the plankton biomass was in the range 162-487 mg D.W. respectively in July 1991 and January 1991 at station B, and 150-467 mg D.W. respectively in October 1991 and February 1991 at station 4V. Only microplankton species representing at least 1% of the total carbon biomass were taken into consideration. They were primarily the diatoms, dinoflagellates, tintinnids and copepods. Seasonal variations of the biomass composition (expressed as percent of total carbon) are shown in Fig. 2A (station B) and 2B (station 4V).
86 4-
OCT
JAN
FEB
JUN
~
SEP
OCT
Fig. 3. Selenium concentration in the microplankton biomass between October 1990 and December 1991 (A) at station B and (B) at station 4V.
Selenium concentration in coastal plankton Villefranche (Fig. 3A) or at the edge of the Ligurian current (Fig. 3B) showed clear temporal variations (no data for station 4V in December). The minimum Se concentrations, 0.37/~g Se.g -~ C at station B (Fig. 3A) and 0.82/tg Se.g -j C at station 4V (Fig. 3B), were observed in January; the maximum Se concentrations were measured in February, i.e. 12.76 #g Se.g -~ C at station B (Fig. 3A) and 10.91/~g Se.g-~ C at station 4V (Fig. 3B). The variation range between the minimum and the maximum Se concentration observed covers two orders of magnitude. In order not to give a major importance to these range limits exceptionally observed, median values of the selenium concentrations measured in the microplankton between October 1990 and December 1991 were calculated. Values obtained were 1.76 ktg Se'g -~ C ( n = 8 ) and 1.57#g Se.g -t C ( n = 7 ) at stations B and 4V respectively. Stations B and 4V are 10 km apart, the former being near shore and the latter located in the Ligurian current. The sampling location and the hydrology of the area may influence the selenium concentration in microplankton even if there were no obvious differences in the species composition between the two stations. A paired t-test was performed with the results (#g Se.g -~ D.W.) from the two stations for all the sampling dates excepted the last one (only station B results available). No significant difference was found at the 0.05 level (d.f. = 6) between selenium concentration in microplankton from both stations. Based on 15 results, the median value of selenium concentration in microplankton from the Ligurian Sea was 1.57 pg Se.g -~ C or 0.62 #g Se.g -~ D.W. July samples from station 4V, mainly comprised of the copepod Oithona sp., contained a selenium concentration of 1.57#g Se.g -~ C. Thus, this genus of copepod had the same selenium content as the median value found for total microplankton. Likewise, the biomass of January sample from station 4V collected under typical hydrodynamic conditions contained only phytoplankton (11% dinoflagellates and 89% diatoms). Therefore, a selenium concentration of 0.82#g Se.g -~ C may be considered as typical for microphytoplankton from the Ligurian Sea. In general terms, we hypothesize that the selenium concentration in microplankton should be the mean value of the selenium concentration in phytoplankton and in zooplankton, weighed by their respective fraction of biomass,
[Se], = (w./w,) [Se]. + (WdW,) [Seh where W, is the whole sample carbon biomass (g), Wp is the carbon biomass ofphytoplankton in the sample (g), Wz is the carbon biomass of zooplankton in the sample (g), [Se]p is the mean selenium concentration in phytoplankton (#g.g-' C), [Se]z is the mean selenium concentration in zooplankton (#g.g-' C) and [Se], is the selenium concentration in the whole sample (/~g-g-~ C).
2597
Then, W,[Se], = Wp[Se]0 + Wz[Se]z therefore, Q, = Wp [Se]p + wz[Se],
(1)
where Q, (#g) is the quantity of selenium present in the whole sample. Equation (1) is a multiple linear regression in which the descriptors (Wp and Wz) are the phytoplankton and zooplankton biomasses in the sample and the coefficients represent a mean concentration of selenium for phytoplankton (fl~) and zooplankton (f12) respectively (stations B and 4V were considered together). This relation can be written as: Q, = fl, Wp + f12 Wz + fl0
(2)
Calculations of equation (2), with d.f. = 14, gave Q, = 4.353 10 -6 Wp + 9.882 l0 -6 Wz-0.001 (r = 0.648; F = 4.337; P < 0.05). Probabilities associated with the coefficents were p(fl0=0.419 and p(fl0=0.014. The coefficient associated with zooplankton biomass (rio was significant at the 0.05 level whereas the coefficient associated with phytoplankton biomass (fi~) was not significant. Selenium concentration in phytoplankton did not significantly influence selenium concentration in the total microplankton. Therefore, selenium concentrations in microplankton (on C basis) were plotted against zooplankton biomass (Fig. 4). Samples containing only phytoplankton (January 1991 at station 4V and December 1991 at station B) were not considered in this evaluation. The regression equation, obtained for d.f. = 10, was [Se], (pg-g-' C) = -2.911 10 -2 Wz + 4.074 (r = -0.881; F = 30.913; P < 0.001)
lSei (~IgC) 4
2-
i 0
: ,.% i 8O
75
100
125
150
w zoo (m~ c}
Fig. 4. Relationship between selenium concentration in the microplankton and zooplankton biomass. The equation is
[Self (.ug'g-* C)=-2.911 10-2 Wz +4.074 F = 30.913; P < 0.001).
(r = --0.881;
2598
F. Boisson and M. Romeo
There was a significant negative linear relationship between selenium concentration in microplankton and zooplankton biomass in these samples.
%C
4.z o ~ g s e / ~
2.ng ugse/gc
[]
I0
Diatoms
Ill Dinoflagellates
Influence of selenium species on selenium concentration Selenite and selenate concentration in sea water may influence the selenium concentration in microplankton. Dao Ming and Martin (1991) established a polynomial relationship between SeO~ (selenite) and SiO2 on the one hand and SeO] (selenate) and PO 3- on the other. Given the SiO£ and PO] concentrations we measured in sea water at each sampling date (Table 1) and considering that selenium concentration in microplankton depends on zooplankton biomass (Fig. 4), selenium concentration in microplankton can be computed from the equation [Se] (#g.g ' C) = fl, Wz (g C) +/32 [SiO2 l
[]
50-100
Tintinn/ds
100-200
SIZE FRACTION (gm)
Fig. 5. Influence of diatom, dinoflagellate and tintinnid proportions of the carbon biomass of the December 1991 sample on microplankton selenium concentration.
( p M . l - ' ) +/33 [PO 3 l (pM.I ') +/30 Substituting the appropriate values, this multiple regression gives [Se] = 2.877 10 8 Wz + 7.863 10 _6
[sio:] + 4.548
10 -5 [PO4s-] - 1.495 10 ' (r = 0.810; F = 6.997; P < 0.01; d.f. 14). The probabilities associated with the coefficients were p(/3,) = 0.514, p(/3:) = 0.046 and P(/33) = 0.070. Thus, only the coefficient associated with SiO2 concentration was significant at the 0.05 level. Thus, according to the polynomial relationship established by Dao Ming and Martin (1991), selenium concentration in the Ligurian microplankton appears to be more closely correlated with selenite than selenate concentration in sea water. Influence o f species composition on selenium concentration For a given selenium concentration in sea water, the selenium content of microplankton may also be influenced by the species composition. In the case of the sample collected in December at station B, the percentages of biomass of each species for the two fractions 50-100 pm and 100-200/~m are shown in Fig. 5. This figure indicates that selenium concentration in microplankton (mainly composed of
phytoplankton) was higher in the smaller size fraction which contained the highest proportion of diatoms. Selenium concentration was lower in the 100-200 #m size fraction which contained the highest proportion of dinoflagellates. Therefore, diatoms would appear to have a higher selenium concentration than dinoflagellates. DISCUSSION AND C O N C L U S I O N S
Selenium concentration in the northwestern Mediterranean microplankton; comparison with literature The selenium concentrations measured in Ligurian Sea microplankton (#g Se-g -~ D.W.) reported here may be compared with previously published data on selenium content in the same size-class of marine plankton, usually called "phytoplankton" by the authors even if samples were more or less contaminated by zooplankton (Table 2). When comparing the data obtained with a similar sampling method (net tow, 50-70-#m mesh), it appears that selenium content varies between sampling locations, ranging from 1.20/~g Se.g ~ D.W. in Xaimen Bay, China (Liu et al., 1987) to 1.95 #g Se.g ' D.W. in the Baltic Sea (Sandholm et al., 1973) and 2.72#g Se.g -t D.W. in the Mediterranean Sea (Fowler and Benayoun, 1976).
Table 2. Selenium concentration in marine phytoplankton Sample location
Collection method
[Se] (pg.g- ' D.W.)
Reference
Baltic Sea
Net tow (25 pm) Net tow (50 #m) ? Net tow (65 ~tm) Niskin bottle (glass fibre filter) Sediment trap
1.41 ± 0.45 (n = 2) 1.95 + 0.68 (n = 2) 0.50 (n = ?) 2.72 (n = I) 1.90 ± 0.42 (n > 10)
Sandholm et al. (1973) Terrada et al. (1975) Fowler and Benayoun (1976) Wrench and Measures (1982)
1.69 (n = 1)
Cutter and Bruland (1984)
1.20 (n = 1) 0.62 (n = 15) [0.11 to 5.30]
Liu et al. (1987) This paper
Sea of Japan Mediterranean Sea Bedford Bassin N. Pacific Ocean
(50 m) S. China Sea Mediterranean Sea
Net tow (70 pm) Net tow (53 #m)
Selenium concentration in coastal plankton Nevertheless, our results indicate that plankton selenium content may show important temporal variations (0.11-5.30 /1g Se.g-' D.W.). No obvious difference between the selenium content in plankton samples from different oceans can be predicted until their seasonal variations are understood. The value, measured by Fowler and Benayoun (1976) off the coast of Monaco not far from our sampling station B, was in the range reported in this study (0.11-5.30 /~g Se.g i D.W.). Samples collected with a phytoplankton net (mesh size 53/~m) generally contain a varying proportion of zooplankton, e.g. small copepods or nauplii. However the sample collected in July at station 4V was composed almost entirely (94%) of copepods of the genus Oithona. Under these conditions we can consider that selenium concentration in these copepods from the Ligurian Sea is 1.57/~g Se.g -~ C. This value may be compared to those obtained by Liu et al. (1987) in Xaimen Bay for three samples: (1) Labidocera euchaeta and Centropages tenuiremis (dominant) + Acartia pacifica (minor) 1.29/~g Se.g-' C; (2) Tortonus derjuginii (dominant) + Acartia pacifica (intermediate) + Brachyuran zoea 1.60/~g Se'g -~ C; (3) Copepod nauplii + small copepods (Corycaeus, Harpacticus) 3.13/lg Se.g-~ C. Samples (1) and (2) are composed of large copepods. Oithona is a small copepod approximately 0.5mm length (Nassogne, 1972) but the selenium concentration approaches the value of samples (1) and (2) and does not, as it might be supposed, approximate the concentration in the third sample. Liu et al. (1987) and Fowler and Benayoun (1976) found that nauplii and smaller copepods contain higher concentrations of selenium than adults and larger individuals. They hypothesized that this could be attributed to the higher metabolic rate of these smaller organisms. The fact that our Oithona sample contained a minor proportion of copepod nauplii may possibly explain the lower selenium concentration observed. Ligurian Sea: an area contaminated by selenium? The terrestrial contribution of selenium input to the oceans may have, in part, an anthropogenic origin, e.g. in San Francisco Bay where selenium concentrations can reach 3/~g 1-~ (Cutter, 1989). In our sampling area, the effluents originate from France or from the industrial zone of Genoa (Italy), and are subsequently transported by the Ligurian current. Atmospheric input which is estimated to be five times higher than terrestrial sources for the Mediterranean Sea (Xiao Ming, 1989), is more important near the coast than off-shore (Cutter and Bruland, 1984). The similarity between the selenium concentration in microplankton at the two stations examined may suggest that there are no anthropogenic effects affecting the selenium concentration in the microplankton, or that the anthropogenic effluents exert an infuence so far off-shore that they
2599
affect microplankton selenium concentrations similarly at both stations. Further work studying stations extending over the entire Ligurian current should provide an answer to this question. Contribution to assess the biogeochemical cycling of selenium The selenium concentration in the microplankton samples from February 1991 (stations 4V and B) was 40 times higher than in the January sample (station B), even if the biomass was similar at both stations. In February 1991, the hydrological parameters revealed strong vertical mixing of the water column which enriched the intermediate water with nutrients. In the Mediterranean Sea, Dao Ming and Martin (1991) found that vertical profiles of selenite and selenate increase between 0 and 400 m with a maximum value in the intermediate water (at 400 m for selenite and at 320 m for selenate). Thus, in the case of strong vertical mixing of the water masses, surface water would become enriched in nutrients along with selenium. Thus the increasing selenium concentration in sea water may also enhance the selenium content of the microplankton. When vertical mixing of the water column is less important (0-100m), e.g. in December 1991 following strong SW winds parallel to the coast (Bougis, 1968), selenium concentration in the deeper water layer may be similar to that in the surface and not sufficiently different to lead to a significant increase in selenium concentration in microplankton. In addition, selenium content in microplankton can be dependent upon plankton biomass and species composition. Selenium concentration was found to be negatively correlated with zooplankton biomass and not correlated with phytoplankton biomass. The reason may be that the biomass collected for this survey was mainly attributed to zooplankton and that copepods have a high assimilation capacity for selenium (Fisher and Reinfelder, 1991). The linear regression obtained between zooplankton biomass and selenium concentration in the microplankton was negative, i.e. selenium concentration decreased with increasing zooplankton biomass. This process, to our knowledge never reported before for selenium, is likely due to biological dilution. Therefore, in the case of enhanced copepod production, selenium concentration is lower in each organism and thus, selenium concentration in marine organisms feeding on copepods will be lower unless they ingest more biomass. Concerning the effect of species composition, our data suggest that selenium concentration is higher in diatoms than in dinoflagellates. These results are in agreement with those obtained in a laboratory study by Fisher and Frood (1980) who noted that diatoms were more sensitive to selenium than dinoflagellates. Moreover, diatoms were found to have more reactive
2600
F. Boisson and M. Romeo
surfaces than green algae and cyanobacteria (Fisher et al., 1983). Nevertheless, our results were obtained using only the December sample (station B) and further study is needed to confirm this trend. In conclusion, this work reports the first spatial and temporal survey of selenium concentration in plankton from the coastal Mediterranean waters. Our results show that in the northwestern Mediterranean Sea hydrodynamic processes exert the main control on selenium content of plankton, but in some cases, selenium concentration depends on plankton biomass and most likely on species composition. In the case of a large zooplankton biomass composed mainly of copepods, selenium concentrations were found to be low. Consequently, this phenomenon may influence selenium concentrations in organisms feeding on them. Such "biological dilution" of selenium may decrease selenium uptake throughout the marine food chain. N o anthropogenic effect on plankton selenium content was evident in the area studied, even though the Ligurian current is a well-known source of contaminants. Acknowledgements--We thank T. Juhel for technical assistance and S. W. Fowler for helpful comments on the manuscript. REFERENCES
Anonymous (1987) In Ambient water quality criteria for selenium. US Environmental protection agency, Washington DC, 108pp. Brthoux J. P. and Prieur L. (1983) Hydrologie et circulation en M~diterranre Nord-Occidentale. Pbtrole et Techniques 299, 25-34. Brthoux J. P,, Prieur L. and Bong J. H. (1988) Le courant Ligure au large de Nice. Oceanologica Acta/no. S.P. Ocranographie prlagique mrditerranrenne, pp. 59-67. Boisson F. (1992) Le selenium dans le microplancton de la mer Ligure (Mrditerranre Nord-Occidentale). Toxicit6 et bioaccumulation chez une microalgue Cricosphaera elongata et une macroalgue Bryopsis maxima. PhD. University P. & M. Curie (Paris). 185pp. Bougis P. (1968) Le probl~me des remontres d'eaux profondes ~. ViUefranche-sur-Mer. Cah. Ocean. 10 (7), 597-603. Bougis P. (1974) H, Le Zooplancton (Edited by Masson. Paris). Collection d'rcologie No. 3. 200pp. Cutter G. A. (1989) The estuarine behaviour of selenium in San Francisco Bay. Estuarine, Coast. and ShelfSci. 28 (1), 13-34. Cutter G. A. and Bruland K. W. (1984) The marine biogeochemistry of selenium: A re-evaluation. Limnol. Oceanogr. 29, 1179-1192. Dao Ming G. and Martin J. M. (1991) Selenium distribution in the Rhone delta and the Gulf of Lion. Mar. Chem. 36, 303-316. Fisher N. S. and Frood D. (1980) Heavy metals and marine diatoms: influence of dissolved organic compounds on
toxicity and selection for metal tolerance among four species. Mar. Biol. 59, 85-93. Fisher N. S. and Reinfelder J. R. (1991) Assimilation of selenium in the marine copepod Acartia tonsa studied with a radiotracer ratio method. Mar. Ecol. Prog. Ser. 70, 157-164. Fisher N. S., Bjerregaard P. and Fowler S. W. (1983) Interactions of marine plankton with transuranic elements. 1. Biokinetics of neptunium, plutonium, americium and californium in phytoplankton. Limnol. Oceanogr. 28, 432-447. Fowler S. W. and Benayoun G. (1976) Influence of environmental factors on selenium flux in two marine invertebrates. Mar. Biol. 37, 56-68. Gostan J. (1968) Contribution fi l'rtude hydrologique du bassin Liguro-Provenqal entre la Riviera et la Corse. Th~se Doct. Etat. Univ. P. & M. Curie. 206pp. Lins Da Silva N. M. (1991) Etude de la rrpartition spatio-temporelle des peuplements microbiens planctoniques en Mer Ligure (Mrditerranre Nord-Occidentale). PhD. Univ. P. & M. Curie. l l9pp. Liu D. L., Yang Y. P., Hu M. H., Harrison P. J. and Price N. M. (1987) Selenium content of marine food chain organisms from the coast of China. Mar. Environ. Res. 22 (2), 151-165. Nassogne A. (1972) l~tude prrliminaire sur le rrle du zooplancton dans la constitution et le transfert de la matirre organique au sein de la cha]ne alimentaire marine en mer Ligure. Thrse Doct. Etat. Univ. Paris 6. 237pp. Nival P. and Corre M. C. (1976) Variation annuelle des caractrristiques hydrologiques de surface dans la rade de Villefranche-sur-Mer. Ann. Inst. Oceanogr. 52, 57-78. Nyffeler F,, Raillard J. and Prieur L. (1980) Le bassin Liguro-Provenqal, 6tude statistique des donnres hydrologiques 1950-1973. Rapp. Sci. Technol. CNEXO 42, 163pp. Price N. M., Thompson P. A. and Harrison P. J. (1987) Selenium: an essential element for growth of the coastal marine diatom Thalasiosira pseudonana. J. Phycol. 23, 1-9. Sandholm M., Oksanen H. E. and Pesonen L. (1973) Uptake of selenium by aquatic organisms. Limnol. Oceanogr. 18, 496-499. Shrift A. (1954) Sulfur-selenium antagonism I. Antimetabolite action of selenate on the growth of Chlorella vulgaris. Am. J. of Botany 41, 223 230. Sieburth J. H. N., Smetacek V. and Lenz J. (1978) Pelagic ecosystem structure: heterotrophic compartments of the plankton and their relationship to plankton size fractions. Limnol. Oceanogr. 23, 1256-1263. Terrada K., Ooba T. and Kiba T. (1975) Separation and determination in rocks, marine sediments and plankton by direct evolution with bromid-condensed phosphoric acid reagent. Talanta 22, 41-49. Utermrhl H. (1958) Zur Vervollkommung der Quantitativen Phytoplankton Methodik. Comm. Ass. int. Limnol. theor, appl. 9, 1-38. Wrench J. J. (1978) Selenium metabolism in the marine phytoplankters Tetraselmis tetrahele and Dunaliella minuta. Mar. Biol. 49, 231-236. Wrench J. J. and Measures C. I. (1982) Temporal variation in dissolved selenium in a coastal ecosystem. Nature 299, 431-433. Xiao Ming Z. (1989) Comportements biogrochimiques du srlrnium et de l'arsenic dans les milieux fluviaux, estuariens et marins. Th~se Univ. Paris 6. 220pp.
Beat. Res. Vol. 30, No. 11, pp. 2593-2600, 1996 Copyright © 1996ElsevierScienceLtd Printed in Great Britain,All rights reserved 0043-1354/96$15.00+ 0.00
SELENIUM IN P L A N K T O N FROM THE NORTHWESTERN M E D I T E R R A N E A N SEA F. BOISSON* and M. ROMEO@ Laboratoire de Toxicologie Marine, Facult6 de Medecine, Universit6 de Nice-Sophia Antipolis, 06107 Nice, Cedex 02, France (First received July 1994; accepted in recised form April 1996) Abstract--Selenium is a multiple oxidation state trace element whose redox chemistry and concentration in sea water can be modified by biological activity in the euphotic zone. A field study was undertaken to examine the effects of oceanographic and biological parameters on the selenium concentration of the coastal plankton. For this purpose, plankton was collected in the Ligurian Sea from October 1990 to December 1991. The samples analyzed were composed of mixed phytoplankton and microzooplankton ranging in size from 50 to 200 #m. The medium selenium concentration measured in microplankton was 1.57#g Se.g -~ carbon or 0.62#g Se.g -~ dry weight (n= 15). Seasonal fluctuations in selenium concentration of microplankton were influenced by hydrodynamic conditions. During strong vertical mixing of the water column, deeper water with high selenite and selenate concentrations may reach the surface and selenium concentration in plankton was found to increase by a factor of eight compared to the median annual value. Under usual hydrodynamic conditions, seasonal fluctuations of the microplankton selenium content were typically low. Such low variations appeared to be due to the specific composition of microplankton. In the case of a phytoplankton bloom, selenium concentration in microplankton increased in proportion to the diatom biomass, whereas in other cases a negative linear relationship was found between selenium content and zooplankton biomass. The biogeochemicalcycling of selenium may be influenced by seasonal fluctuations of the plankton selenium content which is controlled by hydrodynamic processes and plankton composition. Copyright © 1996 ElsevierScience Ltd Key words--phytoplankton, zooplankton, selenium, bioaccumulation, water pollution, Mediterranean Sea
INTRODUCTION Selenium is a metalloid belonging to the V I A group of elements. Because of its physical and chemical similarities with sulphur, it can behave as a sulphur analogue in algae (Shrift, 1954; Wrench, 1978). Selenium is an essential element for some algae (Price et aL, 1987), but it is also toxic at elevated concentrations (Anonymous, 1987). Selenium enters the Mediterranean Sea from atmospheric and terrestrial inputs (natural or anthropogenic). The atmospheric input is estimated to be five times higher than that from terrestrial runoff (Xiao Ming, 1989). Xiao Ming (1989) reported that approximately 110 tons of selenium per year are added to the northwestern Mediterranean Sea of which 9.7 tons enter from the Rh6ne river in France. Total selenium concentration in surface water of the northwestern Mediterranean Sea is approximately 523 pM.l -j (Dao Ming and Martin, 1991). These authors reported that selenium concentration was similar in the coastal area and in the open sea. *Present address: I.A.E.A. Marine Environment Laboratory, P.O. Box 800, MC-98012 Monaco [Fax: (377) 92 05 77 44; Email: [email protected]].
Biological processes are known to play a central role in establishing the redox state and spatial distribution of dissolved trace elements in the oceanic environment. In the case of selenium, Wrench and Measures (1982) provided direct evidence that biological activity in the euphotic zone can modify both redox balance and the concentrations of this multiple oxidation state trace element (Se -2, Se°, Se +4, Se+6). At present, there are few reported measurements of selenium in marine plankton (Sandholm et al., 1973; Terraria et al., 1975; Wrench and Measures, 1982; Cutter and Bruland, 1984; Liu et al., 1987) and only one in the plankton from the Mediterranean Sea (Fowler and Benayoun, 1976). No data are apparently available on temporal variations in selenium content of plankton. Thus, in order to better understand the biogeochemical cycling of selenium in marine systems, a spatial and temporal survey of selenium in plankton was undertaken in the northwestern Mediterranean Sea. The aims of the study were to establish (1) what type of quantitative data can be obtained from a simple sampling scheme; (2) how hydrodynamic conditions, plankton biomass and specific composition may produce significant changes in the selenium content of plankton; (3) how
2593
2594
F. Boisson and M. Romeo
the spatial a n d t e m p o r a l variations in the selenium c o n c e n t r a t i o n can be interpreted in Ligurian sea plankton. SAMPLING AND METHODOLOGY
Study area The Ligurian Sea is located in the northeastern portion of the western Mediterranean Sea bounded by the French and Italian coasts. The water layers present in the Ligurian Sea have been described according to B&houx and Prieur (1983): surface water 0-200m, T = 12.4-22.8°C, S = 3%5-38.1 PSU; intermediate water 200-800 m, T = 13.50°C, S > 38.45 PSU and deep water (Gostan, 1968) with T = 12,90°C and S = 35.5PSU. The surface and intermediate waters flow north along both sides of Corsica, join to the north of Cap Corse and form the Ligurian current which parallels the shores of the Italian and French Riviera. 91% of the flux of this current is channelled in a coastal band 32 to 48 km wide with the major part of the flux occurring in the first 13 km (B6thoux et al., 1988). Thus the Ligurian current plays an important role in the circulation of the western Mediterranean Sea and in the associated transport of pollutants. The circulation pattern in the Ligurian Sea is shown in Fig. 1. Strategy and data acquisition Eight surveys involving physical and biological measurements were made at two stations, B (43°41'10"N; 07°19'00"E) and 4V (43°35'08"N; 07°18'15"E), from October 1990 to December 1991. The location of the stations is shown in Fig. 1. Station B is at the entrance of the Bay of Villefranche-sur-mer with a depth of 80 m; station 4V is in the Ligurian current, 11 km off-shore with a depth of 1151 m. The physical measurements included temperature, salinity, oxygen, chlorophyll a and nutrients. Due to technical reasons, dissolved selenium in sea water was not measured. Only selenium concentrations in
10~
20~ !
Villefranch( •
4V
GEN,~
i
43o
!
7"
f
8"
I 9'
!
10"
Fig. 1. Liguro-Proven~;al basin in the northeastern part of the western Mediterranean Sea. The Ligurian current is formed by the surface and intermediate waters which flow north along both sides of Corsica and join just north of Cap Corse. Sampling stations (B and 4V) are indicated.
plankton were considered taking into account hydrological and biological parameters. For this study, a sample was collected at each station by horizontally towing a plankton net of 53-#m mesh size, at 5 m depth. The mean volume of sea water filtered through the net was 60 m 3. The plankton collected was sieved on board through 500 pm to remove large particles and kept in a cooler. In the laboratory, plankton samples were successively sieved through 200 #m and 50 #m. Only the 50-200 #m size range was analyzed. This size fraction is considered to be microplankton (Sieburth et al., 1978). Samples were then rinsed with buffered ammonium formate (1.4 M) to remove salt and dehydrated at 50°C to a constant dry weight. For selenium measurements, triplicate analyses were performed using the following technique: dried samples (between 1 and 3 g) were digested under pressure with 1.5 ml suprapur concentrated nitric acid (Merck) using a microwave oven. Samples were placed in a teflon PFA-lined digestion vessel and the volume of the final solution was adjusted to 5 ml with deionized water. Samples were stored in "nalgene" tubes at 4°C, in darkness, until analyzed. Selenium in biological material was determined by duplicate analyses at 196nm (Boisson, 1992) using an atomic absorption spectrometer (Pye Unicam SP9) equipped with a coated graphite furnace and an automatic sampler. Deuterium background correction was used in order to eliminate non-specific absorbance. The detection limit of the method was 2.54 pg.l-' with a coefficient of variation of 3.5%. The accurancy of the procedure was previously checked by analysis of a standard reference material (lobster hepatopancreas--CNR Canada). Selenium concentration obtained by triplicate analysis was 6.28 + 0.14mg.kg ' D.W., in agreement with the certified value (6.88 + 0.47 mg.kg-' D.W.). An aliquot of each sample was preserved and resuspended in filtered (0.2 #m) seawater for plankton species determination. The species composition and enumeration of each sample was microscopically determined by the Uterm6hl method (Uterm6hl, 1958) after Lugol fixation.
Data conversion For the whole sample, data expressed as pg Se.g -t D.W. were converted to #g Se'g -~ carbon as follows: from the plankton enumeration of the aliquot and using the results reported by Lins Da Silva (1991) for phytoplankton (ng C.cells-') and Nassogne (1972) for zooplankton, the number of individuals of each species was transformed into ng of carbon. Carbon biomass of phytoplankton (Wp), zooplankton (Wz) and total plankton (W,) were then calculated for each aliquot. Carbon biomass (W) was transformed to dry weight biomass (D.W.) using the following relations: for phytoplankton D.W. = 1/0.35 Wp (Bougis, 1974); for zooplankton D.W. -- 1/0.43 Wz (Bougis, 1974); for the copepod Oithona sp. which was found to be the dominant organism in July D.W. = 1/0.4 W (Nassogne, 1972). The percentage of dry weight in the aliquot attributed respectively to phytoplankton and to zooplankton was then deduced. These percentages were applied to the dry weight of the whole sample. After obtaining the dry weight, the carbon biomass of phytoplankton (Wp), zooplankton (Wz) and total plankton (WJ was computed. Selenium concentrations in the plankton (expressed as #g Se.g-~ C) were also calculated for the whole sample. RESULTS
Hydrodynamic characteristics of the sampling periods T h e h y d r o d y n a m i c structure of the Ligurian Sea is determined by the presence or absence o f t h e r m a l stratification with a thermocline limited by the 15°C
S e l e n i u m c o n c e n t r a t i o n in c o a s t a l p l a n k t o n
isotherm (Gostan, 1968; Nyffeler et al., 1980). Data for the physical and chemical parameters are presented in Table 1. In February 1991, the hydrodynamic conditions were very particular. Sea water temperature was at a minimum (12.82°C, winter minimum) and corresponded to a period of homogeneity in the water column between 0 and 40m. Salinity values ranged from 38.126PSU at station B to 38.143 PSU at station 4V. Temperature and salinity data show that water mass displayed winter characteristics (B6thoux and Prieur, 1983). In winter, the density of surface water increases leading to a mixing of the water column due to the influence of cold and dry air masses. Therefore, the formation of a water mass with a temperature lower than 13°C and a salinity higher than 38.000 PSU indicates a non-steady state condition and mixing of the water column. The hypothesis of deeper water coming up to the surface was supported by the high values obtained for the nutrients.
2595
The samples of June 1991 corresponded to a spring situation, i.e. stratification of the surface layer. The densities were lower at surface (26.90kg-m -3 at station B and 27.12 kg'm -3 at station 4V) than at 4 0 m depth (28.39kg.m -3 at station B and 28.40 kg.m -3 at station 4V). In July 1991, the thermal stratification was well established and the density difference between water at the surface and at 40 m depth was important. The 15°C isotherm at the base of the thermocline was located at 40 m depth. This sampling was carried out during summer conditions. Samples from September, October and December 1991 were collected under fall conditions. In September the thermocline decreased below 40 m. In October, temperature and salinity had the same value at the surface as at 40 m depth. These conditions caused a vertical instability in the water column as described by Nival and Corre (1976). In December, the seawater became colder due to water mixing and
Table 1. Temperature (T, ~'C), salinity (S, particular salinity unit), density (kg.m-3), chlorophyll a (Chl a, mg.m 3), oxygen (02, ml.1 ~) and nutrients ( p M ' l - ' ) at station B and 4V for each collection date between October 1990 and December 1991. In December, only Station B could be sampled (n.d. = not determined) Sample
Station
15/10/90 5m 40m 15/10/90 5m 40m 18/01/91 5m 40m 18/01/91 5m 40m 26/02/91 5m 40 m 26/02/91 5m 40m 18/06/91 5m 40m 18/06/91 5m 40m 11/07/91 5m 40m 11/07/91 5m 40m 3/09/91 5m 40m 3/09/91 5m 40m 28/10/91 5m 40m 28/10/91 5m 40m 6/12/91 5m 40m
B
T (°C)
Salinity (PSU)
Density (kg.m -J)
20.64 20.39
37.664 37.722
26.62 26.73
20.50 20.06
37.374 37.809
13.98 13.99
Chl a (mg.m -J)
O2 (mid-')
SiO2 (,um.I -~)
PO4 (/.tm.l-')
0.09 0.11
5.10 5.14
1.20 1.20
0.15 0.10
26.44 26.89
0.11 0.06
5.10 5.16
1.00 1.00
38.050 38.160
28.56 28.65
0.08 0.08
5.70 5.84
14.02 13.95
n.d. n.d.
n.d. n.d.
0.13 0.06
13.09 13.09
38.126 38.131
28.80 28.82
12.82 13.00
38.143 38.139
19.72 14.67
NH4 (,um.l-D
NO2 (#m.l ')
NO~ (pm.l ')
0.30 0.30
0.35 0.30
0.30 0.30
0.10 0.10
0.20 0.20
0.15 0.15
0.10 0.10
1.40 1.40
0.10 0.10
<0.10 <0.10
0.10 0.10
0.70 0.70
5.54 5.80
1.40 1.40
0.10 0.10
<0.10 <0.10
0.10 0.15
0.80 0.90
0.30 0.15
n.d. n.d.
1.60 1.60
0.15 0.30
0.10 0.20
0.25 0.45
0.90 2.00
28.87 28.83
n.d. n.d.
n.d. n.d.
1.60 1.60
0.15 0.15
0.10 0.10
0.20 0.20
0.60 1.10
37.709 38.036
26.90 28.39
0.14 0.05
5.41 5.60
1.40 1.40
0.10 0.13
0.10 0.10
0.10 0.15
0.40 1.10
19.25 i4.59
37.826 37.033
27.12 28.40
0.10 0.07
5.20 5.64
1.60 1.60
0.10 0.10
0.10 0.10
0.10 0.15
0.30 0.20
24.09 14.98
37.552 38.026
25.55 28.31
0.03 0.01
5.18 5.69
1.40 1.60
0.15 0.10
0.10 0.10
0.20 0.10
1.30 1.30
23.74 15.01
37.692 38.030
25.76 28.31
0.04 0.01
4.98 6.07
1.40 1.40
0.05 0.05
< 0.10 <0.10
0.15 0.15
0.40 0.50
25.19 16.18
38.278 37.995
25.76 28.01
0.02 0.13
4.87 6.00
1.20 1.20
0.10 0,10
0.40 1.00
0.20 0.40
0.80 1.00
24.20 16.50
38.334 38.319
26.12 28.23
0.04 0.13
5.34 5.39
1.00 1.00
0.10 0.10
0.60 2.30
0.10 0.15
0.60 0.50
19.09 19.08
38.994 38.037
27.29 27.33
0.25 0.32
5.38 5.39
1.20 1.60
0.10 0.10
0.10 0.10
0.15 0.15
0.20 0.20
18.85 18.87
37.960 37.992
27.33 27.33
0.17 0.24
n.d. n.d.
1.20 1.10
0.10 0.10
0.10 0.10
0.15 0.15
0.90 0.20
15.82 15.86
37.994 37.947
28.06 28.05
0.18 0.17
n.d. n.d.
1.40 1.40
<0.05 <0.05
<0.10 <0.10
0.15 0.15
0.30 0.40
4V
B
4V
B
4V
B
4V
B
4V
B
4V
B
4V
B
2596
F. Boisson and M. Romeo
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No data were obtained in December for station 4V. Zooplankton comprised the majority of the plankton collected (between 56 and 94%): Tintinnids represented a low proportion of the biomass and were only present in October 1990 and 1991, and February, September and December 1991. The zooplankton biomass was mainly comprised of copepods and copepodites which were present all through the year. The biomass of the samples collected in July 1991 was only composed of the copepod Oithona sp. For phytoplankton, diatoms (Chaetoceros sp. and Chaetoceros curvisetus) were the dominant group present in January and December 1991. Dinoflagellates were present all year and they never represented more than 43.3% of the carbon biomass. Samples collected in December 1991 were mainly phytoplankton dominated by the diatom Chaetoceros curvisetus (74% of total C) and the dinoflagelate Ceratium sp. (12% of total C).
Seasonal variations of selenium concentration in microplankton '
3
Selenium concentration (#g Se.g -s C) in the microplankton collected just outside the Bay of
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OCT
DEC
Fig. 2. Seasonal variations of the percentage of carbon attributed to diatoms, dinoflagellates, tintinnids and copepods in the microplankton biomass at station B (A) and station 4V (B). Only species with at least 1% of the whole sample carbon biomass were considered. OCT
a concomitant decrease in air temperature. Bougis (1968) reported that during this period and under strong SW winds (parallel to the coast), deeper water rich in nutrients could upwell to the surface leading to the formation of a phytoplankton bloom.
JAN
FEB
JUN
JUL
SEP
OCT
DEC
se/g c 14 -t'[
(B)
12-
Seasonal variations in plankton biomass composition
1 0 -~
The seasonal variation of the plankton biomass was in the range 162-487 mg D.W. respectively in July 1991 and January 1991 at station B, and 150-467 mg D.W. respectively in October 1991 and February 1991 at station 4V. Only microplankton species representing at least 1% of the total carbon biomass were taken into consideration. They were primarily the diatoms, dinoflagellates, tintinnids and copepods. Seasonal variations of the biomass composition (expressed as percent of total carbon) are shown in Fig. 2A (station B) and 2B (station 4V).
86 4-
OCT
JAN
FEB
JUN
~
SEP
OCT
Fig. 3. Selenium concentration in the microplankton biomass between October 1990 and December 1991 (A) at station B and (B) at station 4V.
Selenium concentration in coastal plankton Villefranche (Fig. 3A) or at the edge of the Ligurian current (Fig. 3B) showed clear temporal variations (no data for station 4V in December). The minimum Se concentrations, 0.37/~g Se.g -~ C at station B (Fig. 3A) and 0.82/tg Se.g -j C at station 4V (Fig. 3B), were observed in January; the maximum Se concentrations were measured in February, i.e. 12.76 #g Se.g -~ C at station B (Fig. 3A) and 10.91/~g Se.g-~ C at station 4V (Fig. 3B). The variation range between the minimum and the maximum Se concentration observed covers two orders of magnitude. In order not to give a major importance to these range limits exceptionally observed, median values of the selenium concentrations measured in the microplankton between October 1990 and December 1991 were calculated. Values obtained were 1.76 ktg Se'g -~ C ( n = 8 ) and 1.57#g Se.g -t C ( n = 7 ) at stations B and 4V respectively. Stations B and 4V are 10 km apart, the former being near shore and the latter located in the Ligurian current. The sampling location and the hydrology of the area may influence the selenium concentration in microplankton even if there were no obvious differences in the species composition between the two stations. A paired t-test was performed with the results (#g Se.g -~ D.W.) from the two stations for all the sampling dates excepted the last one (only station B results available). No significant difference was found at the 0.05 level (d.f. = 6) between selenium concentration in microplankton from both stations. Based on 15 results, the median value of selenium concentration in microplankton from the Ligurian Sea was 1.57 pg Se.g -~ C or 0.62 #g Se.g -~ D.W. July samples from station 4V, mainly comprised of the copepod Oithona sp., contained a selenium concentration of 1.57#g Se.g -~ C. Thus, this genus of copepod had the same selenium content as the median value found for total microplankton. Likewise, the biomass of January sample from station 4V collected under typical hydrodynamic conditions contained only phytoplankton (11% dinoflagellates and 89% diatoms). Therefore, a selenium concentration of 0.82#g Se.g -~ C may be considered as typical for microphytoplankton from the Ligurian Sea. In general terms, we hypothesize that the selenium concentration in microplankton should be the mean value of the selenium concentration in phytoplankton and in zooplankton, weighed by their respective fraction of biomass,
[Se], = (w./w,) [Se]. + (WdW,) [Seh where W, is the whole sample carbon biomass (g), Wp is the carbon biomass ofphytoplankton in the sample (g), Wz is the carbon biomass of zooplankton in the sample (g), [Se]p is the mean selenium concentration in phytoplankton (#g.g-' C), [Se]z is the mean selenium concentration in zooplankton (#g.g-' C) and [Se], is the selenium concentration in the whole sample (/~g-g-~ C).
2597
Then, W,[Se], = Wp[Se]0 + Wz[Se]z therefore, Q, = Wp [Se]p + wz[Se],
(1)
where Q, (#g) is the quantity of selenium present in the whole sample. Equation (1) is a multiple linear regression in which the descriptors (Wp and Wz) are the phytoplankton and zooplankton biomasses in the sample and the coefficients represent a mean concentration of selenium for phytoplankton (fl~) and zooplankton (f12) respectively (stations B and 4V were considered together). This relation can be written as: Q, = fl, Wp + f12 Wz + fl0
(2)
Calculations of equation (2), with d.f. = 14, gave Q, = 4.353 10 -6 Wp + 9.882 l0 -6 Wz-0.001 (r = 0.648; F = 4.337; P < 0.05). Probabilities associated with the coefficents were p(fl0=0.419 and p(fl0=0.014. The coefficient associated with zooplankton biomass (rio was significant at the 0.05 level whereas the coefficient associated with phytoplankton biomass (fi~) was not significant. Selenium concentration in phytoplankton did not significantly influence selenium concentration in the total microplankton. Therefore, selenium concentrations in microplankton (on C basis) were plotted against zooplankton biomass (Fig. 4). Samples containing only phytoplankton (January 1991 at station 4V and December 1991 at station B) were not considered in this evaluation. The regression equation, obtained for d.f. = 10, was [Se], (pg-g-' C) = -2.911 10 -2 Wz + 4.074 (r = -0.881; F = 30.913; P < 0.001)
lSei (~IgC) 4
2-
i 0
: ,.% i 8O
75
100
125
150
w zoo (m~ c}
Fig. 4. Relationship between selenium concentration in the microplankton and zooplankton biomass. The equation is
[Self (.ug'g-* C)=-2.911 10-2 Wz +4.074 F = 30.913; P < 0.001).
(r = --0.881;
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F. Boisson and M. Romeo
There was a significant negative linear relationship between selenium concentration in microplankton and zooplankton biomass in these samples.
%C
4.z o ~ g s e / ~
2.ng ugse/gc
[]
I0
Diatoms
Ill Dinoflagellates
Influence of selenium species on selenium concentration Selenite and selenate concentration in sea water may influence the selenium concentration in microplankton. Dao Ming and Martin (1991) established a polynomial relationship between SeO~ (selenite) and SiO2 on the one hand and SeO] (selenate) and PO 3- on the other. Given the SiO£ and PO] concentrations we measured in sea water at each sampling date (Table 1) and considering that selenium concentration in microplankton depends on zooplankton biomass (Fig. 4), selenium concentration in microplankton can be computed from the equation [Se] (#g.g ' C) = fl, Wz (g C) +/32 [SiO2 l
[]
50-100
Tintinn/ds
100-200
SIZE FRACTION (gm)
Fig. 5. Influence of diatom, dinoflagellate and tintinnid proportions of the carbon biomass of the December 1991 sample on microplankton selenium concentration.
( p M . l - ' ) +/33 [PO 3 l (pM.I ') +/30 Substituting the appropriate values, this multiple regression gives [Se] = 2.877 10 8 Wz + 7.863 10 _6
[sio:] + 4.548
10 -5 [PO4s-] - 1.495 10 ' (r = 0.810; F = 6.997; P < 0.01; d.f. 14). The probabilities associated with the coefficients were p(/3,) = 0.514, p(/3:) = 0.046 and P(/33) = 0.070. Thus, only the coefficient associated with SiO2 concentration was significant at the 0.05 level. Thus, according to the polynomial relationship established by Dao Ming and Martin (1991), selenium concentration in the Ligurian microplankton appears to be more closely correlated with selenite than selenate concentration in sea water. Influence o f species composition on selenium concentration For a given selenium concentration in sea water, the selenium content of microplankton may also be influenced by the species composition. In the case of the sample collected in December at station B, the percentages of biomass of each species for the two fractions 50-100 pm and 100-200/~m are shown in Fig. 5. This figure indicates that selenium concentration in microplankton (mainly composed of
phytoplankton) was higher in the smaller size fraction which contained the highest proportion of diatoms. Selenium concentration was lower in the 100-200 #m size fraction which contained the highest proportion of dinoflagellates. Therefore, diatoms would appear to have a higher selenium concentration than dinoflagellates. DISCUSSION AND C O N C L U S I O N S
Selenium concentration in the northwestern Mediterranean microplankton; comparison with literature The selenium concentrations measured in Ligurian Sea microplankton (#g Se-g -~ D.W.) reported here may be compared with previously published data on selenium content in the same size-class of marine plankton, usually called "phytoplankton" by the authors even if samples were more or less contaminated by zooplankton (Table 2). When comparing the data obtained with a similar sampling method (net tow, 50-70-#m mesh), it appears that selenium content varies between sampling locations, ranging from 1.20/~g Se.g ~ D.W. in Xaimen Bay, China (Liu et al., 1987) to 1.95 #g Se.g ' D.W. in the Baltic Sea (Sandholm et al., 1973) and 2.72#g Se.g -t D.W. in the Mediterranean Sea (Fowler and Benayoun, 1976).
Table 2. Selenium concentration in marine phytoplankton Sample location
Collection method
[Se] (pg.g- ' D.W.)
Reference
Baltic Sea
Net tow (25 pm) Net tow (50 #m) ? Net tow (65 ~tm) Niskin bottle (glass fibre filter) Sediment trap
1.41 ± 0.45 (n = 2) 1.95 + 0.68 (n = 2) 0.50 (n = ?) 2.72 (n = I) 1.90 ± 0.42 (n > 10)
Sandholm et al. (1973) Terrada et al. (1975) Fowler and Benayoun (1976) Wrench and Measures (1982)
1.69 (n = 1)
Cutter and Bruland (1984)
1.20 (n = 1) 0.62 (n = 15) [0.11 to 5.30]
Liu et al. (1987) This paper
Sea of Japan Mediterranean Sea Bedford Bassin N. Pacific Ocean
(50 m) S. China Sea Mediterranean Sea
Net tow (70 pm) Net tow (53 #m)
Selenium concentration in coastal plankton Nevertheless, our results indicate that plankton selenium content may show important temporal variations (0.11-5.30 /1g Se.g-' D.W.). No obvious difference between the selenium content in plankton samples from different oceans can be predicted until their seasonal variations are understood. The value, measured by Fowler and Benayoun (1976) off the coast of Monaco not far from our sampling station B, was in the range reported in this study (0.11-5.30 /~g Se.g i D.W.). Samples collected with a phytoplankton net (mesh size 53/~m) generally contain a varying proportion of zooplankton, e.g. small copepods or nauplii. However the sample collected in July at station 4V was composed almost entirely (94%) of copepods of the genus Oithona. Under these conditions we can consider that selenium concentration in these copepods from the Ligurian Sea is 1.57/~g Se.g -~ C. This value may be compared to those obtained by Liu et al. (1987) in Xaimen Bay for three samples: (1) Labidocera euchaeta and Centropages tenuiremis (dominant) + Acartia pacifica (minor) 1.29/~g Se.g-' C; (2) Tortonus derjuginii (dominant) + Acartia pacifica (intermediate) + Brachyuran zoea 1.60/~g Se'g -~ C; (3) Copepod nauplii + small copepods (Corycaeus, Harpacticus) 3.13/lg Se.g-~ C. Samples (1) and (2) are composed of large copepods. Oithona is a small copepod approximately 0.5mm length (Nassogne, 1972) but the selenium concentration approaches the value of samples (1) and (2) and does not, as it might be supposed, approximate the concentration in the third sample. Liu et al. (1987) and Fowler and Benayoun (1976) found that nauplii and smaller copepods contain higher concentrations of selenium than adults and larger individuals. They hypothesized that this could be attributed to the higher metabolic rate of these smaller organisms. The fact that our Oithona sample contained a minor proportion of copepod nauplii may possibly explain the lower selenium concentration observed. Ligurian Sea: an area contaminated by selenium? The terrestrial contribution of selenium input to the oceans may have, in part, an anthropogenic origin, e.g. in San Francisco Bay where selenium concentrations can reach 3/~g 1-~ (Cutter, 1989). In our sampling area, the effluents originate from France or from the industrial zone of Genoa (Italy), and are subsequently transported by the Ligurian current. Atmospheric input which is estimated to be five times higher than terrestrial sources for the Mediterranean Sea (Xiao Ming, 1989), is more important near the coast than off-shore (Cutter and Bruland, 1984). The similarity between the selenium concentration in microplankton at the two stations examined may suggest that there are no anthropogenic effects affecting the selenium concentration in the microplankton, or that the anthropogenic effluents exert an infuence so far off-shore that they
2599
affect microplankton selenium concentrations similarly at both stations. Further work studying stations extending over the entire Ligurian current should provide an answer to this question. Contribution to assess the biogeochemical cycling of selenium The selenium concentration in the microplankton samples from February 1991 (stations 4V and B) was 40 times higher than in the January sample (station B), even if the biomass was similar at both stations. In February 1991, the hydrological parameters revealed strong vertical mixing of the water column which enriched the intermediate water with nutrients. In the Mediterranean Sea, Dao Ming and Martin (1991) found that vertical profiles of selenite and selenate increase between 0 and 400 m with a maximum value in the intermediate water (at 400 m for selenite and at 320 m for selenate). Thus, in the case of strong vertical mixing of the water masses, surface water would become enriched in nutrients along with selenium. Thus the increasing selenium concentration in sea water may also enhance the selenium content of the microplankton. When vertical mixing of the water column is less important (0-100m), e.g. in December 1991 following strong SW winds parallel to the coast (Bougis, 1968), selenium concentration in the deeper water layer may be similar to that in the surface and not sufficiently different to lead to a significant increase in selenium concentration in microplankton. In addition, selenium content in microplankton can be dependent upon plankton biomass and species composition. Selenium concentration was found to be negatively correlated with zooplankton biomass and not correlated with phytoplankton biomass. The reason may be that the biomass collected for this survey was mainly attributed to zooplankton and that copepods have a high assimilation capacity for selenium (Fisher and Reinfelder, 1991). The linear regression obtained between zooplankton biomass and selenium concentration in the microplankton was negative, i.e. selenium concentration decreased with increasing zooplankton biomass. This process, to our knowledge never reported before for selenium, is likely due to biological dilution. Therefore, in the case of enhanced copepod production, selenium concentration is lower in each organism and thus, selenium concentration in marine organisms feeding on copepods will be lower unless they ingest more biomass. Concerning the effect of species composition, our data suggest that selenium concentration is higher in diatoms than in dinoflagellates. These results are in agreement with those obtained in a laboratory study by Fisher and Frood (1980) who noted that diatoms were more sensitive to selenium than dinoflagellates. Moreover, diatoms were found to have more reactive
2600
F. Boisson and M. Romeo
surfaces than green algae and cyanobacteria (Fisher et al., 1983). Nevertheless, our results were obtained using only the December sample (station B) and further study is needed to confirm this trend. In conclusion, this work reports the first spatial and temporal survey of selenium concentration in plankton from the coastal Mediterranean waters. Our results show that in the northwestern Mediterranean Sea hydrodynamic processes exert the main control on selenium content of plankton, but in some cases, selenium concentration depends on plankton biomass and most likely on species composition. In the case of a large zooplankton biomass composed mainly of copepods, selenium concentrations were found to be low. Consequently, this phenomenon may influence selenium concentrations in organisms feeding on them. Such "biological dilution" of selenium may decrease selenium uptake throughout the marine food chain. N o anthropogenic effect on plankton selenium content was evident in the area studied, even though the Ligurian current is a well-known source of contaminants. Acknowledgements--We thank T. Juhel for technical assistance and S. W. Fowler for helpful comments on the manuscript. REFERENCES
Anonymous (1987) In Ambient water quality criteria for selenium. US Environmental protection agency, Washington DC, 108pp. Brthoux J. P. and Prieur L. (1983) Hydrologie et circulation en M~diterranre Nord-Occidentale. Pbtrole et Techniques 299, 25-34. Brthoux J. P,, Prieur L. and Bong J. H. (1988) Le courant Ligure au large de Nice. Oceanologica Acta/no. S.P. Ocranographie prlagique mrditerranrenne, pp. 59-67. Boisson F. (1992) Le selenium dans le microplancton de la mer Ligure (Mrditerranre Nord-Occidentale). Toxicit6 et bioaccumulation chez une microalgue Cricosphaera elongata et une macroalgue Bryopsis maxima. PhD. University P. & M. Curie (Paris). 185pp. Bougis P. (1968) Le probl~me des remontres d'eaux profondes ~. ViUefranche-sur-Mer. Cah. Ocean. 10 (7), 597-603. Bougis P. (1974) H, Le Zooplancton (Edited by Masson. Paris). Collection d'rcologie No. 3. 200pp. Cutter G. A. (1989) The estuarine behaviour of selenium in San Francisco Bay. Estuarine, Coast. and ShelfSci. 28 (1), 13-34. Cutter G. A. and Bruland K. W. (1984) The marine biogeochemistry of selenium: A re-evaluation. Limnol. Oceanogr. 29, 1179-1192. Dao Ming G. and Martin J. M. (1991) Selenium distribution in the Rhone delta and the Gulf of Lion. Mar. Chem. 36, 303-316. Fisher N. S. and Frood D. (1980) Heavy metals and marine diatoms: influence of dissolved organic compounds on
toxicity and selection for metal tolerance among four species. Mar. Biol. 59, 85-93. Fisher N. S. and Reinfelder J. R. (1991) Assimilation of selenium in the marine copepod Acartia tonsa studied with a radiotracer ratio method. Mar. Ecol. Prog. Ser. 70, 157-164. Fisher N. S., Bjerregaard P. and Fowler S. W. (1983) Interactions of marine plankton with transuranic elements. 1. Biokinetics of neptunium, plutonium, americium and californium in phytoplankton. Limnol. Oceanogr. 28, 432-447. Fowler S. W. and Benayoun G. (1976) Influence of environmental factors on selenium flux in two marine invertebrates. Mar. Biol. 37, 56-68. Gostan J. (1968) Contribution fi l'rtude hydrologique du bassin Liguro-Provenqal entre la Riviera et la Corse. Th~se Doct. Etat. Univ. P. & M. Curie. 206pp. Lins Da Silva N. M. (1991) Etude de la rrpartition spatio-temporelle des peuplements microbiens planctoniques en Mer Ligure (Mrditerranre Nord-Occidentale). PhD. Univ. P. & M. Curie. l l9pp. Liu D. L., Yang Y. P., Hu M. H., Harrison P. J. and Price N. M. (1987) Selenium content of marine food chain organisms from the coast of China. Mar. Environ. Res. 22 (2), 151-165. Nassogne A. (1972) l~tude prrliminaire sur le rrle du zooplancton dans la constitution et le transfert de la matirre organique au sein de la cha]ne alimentaire marine en mer Ligure. Thrse Doct. Etat. Univ. Paris 6. 237pp. Nival P. and Corre M. C. (1976) Variation annuelle des caractrristiques hydrologiques de surface dans la rade de Villefranche-sur-Mer. Ann. Inst. Oceanogr. 52, 57-78. Nyffeler F,, Raillard J. and Prieur L. (1980) Le bassin Liguro-Provenqal, 6tude statistique des donnres hydrologiques 1950-1973. Rapp. Sci. Technol. CNEXO 42, 163pp. Price N. M., Thompson P. A. and Harrison P. J. (1987) Selenium: an essential element for growth of the coastal marine diatom Thalasiosira pseudonana. J. Phycol. 23, 1-9. Sandholm M., Oksanen H. E. and Pesonen L. (1973) Uptake of selenium by aquatic organisms. Limnol. Oceanogr. 18, 496-499. Shrift A. (1954) Sulfur-selenium antagonism I. Antimetabolite action of selenate on the growth of Chlorella vulgaris. Am. J. of Botany 41, 223 230. Sieburth J. H. N., Smetacek V. and Lenz J. (1978) Pelagic ecosystem structure: heterotrophic compartments of the plankton and their relationship to plankton size fractions. Limnol. Oceanogr. 23, 1256-1263. Terrada K., Ooba T. and Kiba T. (1975) Separation and determination in rocks, marine sediments and plankton by direct evolution with bromid-condensed phosphoric acid reagent. Talanta 22, 41-49. Utermrhl H. (1958) Zur Vervollkommung der Quantitativen Phytoplankton Methodik. Comm. Ass. int. Limnol. theor, appl. 9, 1-38. Wrench J. J. (1978) Selenium metabolism in the marine phytoplankters Tetraselmis tetrahele and Dunaliella minuta. Mar. Biol. 49, 231-236. Wrench J. J. and Measures C. I. (1982) Temporal variation in dissolved selenium in a coastal ecosystem. Nature 299, 431-433. Xiao Ming Z. (1989) Comportements biogrochimiques du srlrnium et de l'arsenic dans les milieux fluviaux, estuariens et marins. Th~se Univ. Paris 6. 220pp.