Trace Elements and POPs in Pelagic Plankton

A better understanding of the trophic transfer of persistent contaminants requires knowledge of their interactions with phytoplankton cells. In general, the uptake of metals depends on geochemical and biological processes such as metal speciation, metal-metal antagonistic interactions and environmental factors (e.g. temperature or CO2 concentrations; Sunda 1994; Cullen et al. 1999). In two species of diatoms, for instance, the accumulation of Cd was found to depend on ambient nitrogen conditions (i.e. on the physiological status of phytoplankton cells; Wang et al. 2001). As large areas of the Southern

Ocean are characterised by high nutrient bioavailability, this could be a factor enhancing the uptake of Cd in phytoplankton and its transfer to marine food chains. Algae produce extracellular chelating compounds or porphyrin complexes, which play an important role in the sequestration of essential and nonessential elements and other persistent contaminants (Hutchins et al. 1999). Moreover, there is evidence that marine diatoms, like higher plants, respond very rapidly to Cd (and probably to other metal stress) by producing cysteine-rich polypeptides (phytochelatins). These compounds play a major role in the storage of metals and their detoxification. Diatom exposure to high inorganic Cd concentrations determines an efflux of phytochelatins and Cd from their cells. As the Cd-phytochelatin complex does not appear to be very stable in seawater once outside the cell, the exported metal remains available to diatoms and induces phytochelatin synthesis (Lee et al. 1996).

Relatively little effort has been put into determining concentrations of persistent contaminants in organisms from open-ocean waters, particularly in those from the Southern Ocean. In general, open-sea studies are more expensive and are often ancillary to coastal-oriented contamination surveys. Moreover, plankton samples usually include communities of mixed pelagic organisms representing many phyla; owing to different physiologies, life spans and bioaccumulative abilities, it is difficult to compare concentrations of persistent contaminants in samples of mixed plankton. In general, concentration factors of 102-105 relative to seawater concentrations have been reported for most environmental contaminants in phytoplankton. In the 1990s the Southern Ocean water was largely free of radionuclide contamination, but Wood et al. (1990) demonstrated that Antarctic microalgae were able to concentrate otherwise undetectable, artificial and naturally occurring radionuclides. Measured concentrations of 239Pu, 95Nb and 144Ce, for instance, ranged from 0.1-106 fCi (femtocurie) g-1 wet wt.

Joiris and Overloop (1991) analysed organochlorine residues in samples of particulate matter (mainly phytoplankton) from the Indian sector of the Southern Ocean. They found that PCB concentrations (0.7 |gg-1 dry wt.) were similar to those in temperate zones but, when values were expressed per unit water volume rather than dry weight, Antarctic concentrations (1.2 |g m-3) were seven times lower than North Sea ones (8.8 |g m-3). The low phytoplankton density in the sampled marine area had determined high PCB levels per unit of biomass. Thus, the authors suggested the need to adopt a different system of units in order to correctly express the contamination level of a marine area and to identify mechanisms responsible for persistent contaminant accumulation. In netplankton (mainly zooplankton) samples from the same zone, PCB concentrations were comparable to those in phytoplankton on a dry weight basis, lower on a lipid weight basis, and much higher when expressed per unit seawater volume (1.2 |g m-3). Very low concentrations or traces of lindane, heptachlor epoxide, dieldrin, DDE and DDT were detected in various netplankton samples, while these compounds were undetectable in bulk phytoplankton. The high DDT/DDE ratio indicated that Antarctic organochlorines had originated recently in the Southern Hemisphere, and suggested a possible increase in Antarctic contamination by POPs.

Corsolini and Focardi (2000) measured mean PCB concentrations (the sum of 50 congeners) in Ross Sea phytoplankton (mainly diatoms) and mixed zooplankton (copepods, amphipods and krill) of 1 and 4.2 ng g-1 wet wt. respectively. Although the percentage of each congener was a small percentage of the total PCB content (<10 %) in seawater, phytoplankton showed a remarkable increase in PCBs 153, 138, 180 and 195.

Fronts, convergences and divergences are permanent features of the Southern Ocean, separating water masses with different physico-chemical and biological characteristics. As individual plankters remain within a particular body of water, Hennig et al. (1985) analysed samples of zooplankton collected between New Zealand and McMurdo Sound to determine whether their elemental composition could be used to identify their provenance. Although the results of this study may have been affected by the use of formalin-fixed materials, the most widespread species (i.e. the amphipod Themisto gaudichaudii, the chaetognath Eukronia hamata and the euphausiid Euphausia triacantha) had very different metal concentrations, and amphipods generally showed the highest values. There were also striking differences among samples of the same species collected in different water masses. Large variations in metal concentrations were also found among samples collected in the same area but on different legs of the cruise (i.e. collected 2 months apart). In addition to the species-specific and spatio-temporal variability of metal concentrations in zooplankton, the study by Henning et al. (1985) showed that metal concentrations in Antarctic organisms are not necessarily low. Fe and Zn concentrations in E. triacantha, for instance, were higher in samples from the Ross Sea than in those from the deep-ocean basin and New Zealand continental shelf.

Table 13 reports average trace metal concentrations in some species of pelagic crustaceans from the Weddell Sea. Rainbow (1989) found geographical variations in metal concentrations, and a significant negative regression between the size of the amphipod T. gaudichaudii and its Fe and Zn contents. This relationship suggests that surface-adsorbed metals may constitute a significant proportion of the whole-body metal content in small individuals with a higher surface area/body weight ratio. Furthermore, smaller amphipods are faster growing and probably have greater physiological requirements for essential trace elements. In general, when compared with related species from other seas, crustaceans from the Southern Ocean have lower Pb concentrations, comparable Zn and Ni contents, and much higher Cu and especially Cd contents (particularly in caridean decapods and some species of hyperiid amphipods). The bioaccumulation of Cd has an ecological significance because amphipods of the genus Themisto, for instance, are an important component in the diet of petrels (Croxall et al. 1988) and squid which, in turn, are eaten by other seabirds and marine mammals.

Table 13. Mean trace metal concentrations (|ig g-1 dry wt.) in some species of pelagic crustaceans from the Southern Ocean

Species Region Cd Co Cu Ni Pb Zn

Table 13. Mean trace metal concentrations (|ig g-1 dry wt.) in some species of pelagic crustaceans from the Southern Ocean

Species Region Cd Co Cu Ni Pb Zn

Amphipoda3

Themisto gaudichaudii

60° 30'S/56° 04'W

53

-

31

-

-

45-83

53° 55'S/37° 57'W

19

-

28

-

-

32-77

Eusirus properdentatus

60° 49'-65° 45'S

95

-

107

-

0.44

49

55° 24'-68° 15'W

Decapodab

Notocrangon antarcticus

73° 21'-77° 31'S

13

-

67

-

0.78

46

Chorismus antarcticus

21°24'-42° 12'W

13

-

93

-

1.60

44

Metridia gerlachei

66° 0'-74 0 30'S

10

0.06

26

11.3

0.72

518

Copepodac

Calanoides acutus

53° 55'E-38° 32'W

4.6

0.02

10

4.3

0.31

183

Calanus propinquus

5.6

0.02

26

4.8

0.51

191

a Rainbow (1989) b Petri and Zauke (1993) c Kahle and Zauke (2003a)

As discussed in the preceding chapter, Cd concentrations in Southern Ocean surface waters are higher than in other ocean waters. The bioaccumulation of Cd in several species of Antarctic crustaceans could be due (e.g. Petri and Zauke 1993; Kahle and Zauke 2003a) to their slower growth rates, later sexual maturity and especially to their longer moult cycles (an efficient way of eliminating metals). Moreover, as the Southern Ocean is an environment with unique biological and geochemical features, it cannot be excluded that the poor bioavailability of some essential trace elements may promote the accumulation of Cd and other potentially toxic trace elements.

Although seawater samples from polar seas have similar Cu concentrations, Kahle and Zauke (2003a) found that Cu contents in Antarctic copepods are higher than in related species from Arctic seas. Since Cu is an essential element involved in several enzymatic activities, it was supposed that differences in Cu contents were mainly due to different life stages (i.e. samples from the Greenland Sea were caught at depths below 500 m at the end of the diapause, while those from the Weddell Sea were caught in the upper water layer during the austral summer). Toxicokinetic studies on the uptake of water-borne metals revealed that the Antarctic copepod Metridia gerlachei and the copepod Orchomene plebs accumulate Co, Cu, Ni, Pb and Zn during exposure and depurate them in uncontaminated seawater (Kahle and Zauke 2002, 2003b). The two species turned out to be very sensitive biomonitors for these metals, even at minimal increments in ambient exposure concentrations (from 0.2-0.8 |g l-1). However, Cd bioaccumulation was not observed, suggesting that the uptake of this metal does not occur in the soluble phase. These results seem to confirm the prominent role of phytoplankton in the uptake of Cd and its transfer to Antarctic marine food chains.

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