7th International Conference on Mercury as a Global Pollutant

Bioavailability of mercury in saline waters; Field experiment

N.Odžak1,2, M.Matić1, Z.Kljaković-Gaspić1, T. Zvonarić1 and A.Barić1

1Institute of Oceanography and Fisheries, Split, Croatia

2Swiss Federal Institute for Environmental Science and Technology, Duebendorf, Switzerland

Abstract:Trace metal concentrations in the water column (dissolved and particulate), environmental and biological conditions are the parameters determining metal accumulation in marine organisms. To study bioavailability of mercury, marine mussels Mytilus galloprovincialis were transferred from the clean area (Mali Ston Bay, Croatia) to the experimental station in polluted area (Kastela Bay, Croatia). The experiments were conducted twice (in summer and in winter). During those two experimental periods, each lasting 6 weeks, the mussels, suspended matter (sediment traps), biofouling organisms (plexiglass plates) and water were sampled weekly. At the same station, temperature, pH, O2, salinity and sea level were measured. Also, meteorological situation (wind direction and velocity, rain quantity and air pressure) was monitored. The results of this study showed that food is the main source of Hg for filter-feeding marine mussels Mytilus galloprovincialis, while water is the main source of Hg for the communities of biofouling organisms. Bioavailability of Hg was higher for mussels and lower for biofouling organisms in winter experimental period. Bioavailability is highly dependent on meteorological and oceanographic characteristics of the experimental area, because they influence both metal content and speciation in water, and metabolic rate and physiological behavior of an organism. Further, biological factors (e.g. age, size) affect mercury uptake in marine organisms.

Key words: Bioavailability, Mercury, mussels Mytilus galloprovincialis, biofouling organisms.

Introduction

Primary concern to water quality is how readily toxicants such as trace metals are taken up by biota. Bioavailability, which refers to the fraction of the total quantity of a metal in the environment that is potentially available for biological effects, is affected by numerous physico-chemical (e.g. temperature, salinity, dissolved O2, pH) and biotic factors (e.g. age, size, reproductive cycle). It depends on the molecular structure of the metal species, their thermodynamic stability and the chemical kinetics of their interaction in a biological membrane.

Trace metals may be taken up by aquatic organisms either from solution, or from food. Mussels, as filter feeders, effectively filter particulate matter out of a suspension (Cossa, 1989). Suspended matter, therefore, may be a source of trace metals if ingested by mussels. The extent to which ingested particulate matter can be the source of trace metals depends not only on the trace metal concentration in suspended matter, but also on the quantity of ingested suspended matter and the absorption efficiency of ingested trace metals. Among the chemical characteristics of suspended particles that might influence bioavailability of trace metals, the organic content have been shown to play a pronounced effect (Gagnon and Fisher, 1997)

Obviously, trace metal concentrations in the water column (dissolved and particulate), environmental and biological conditions are the parameters determining metal accumulation in marine organisms.

Materials and methods

Three age groups of marine mussel Mytilus galloprovincialis were transferred from the clean area (Mali Ston Bay, Middle Adriatic coast, Croatia) to the experimental station in relatively polluted area (Kastela Bay, Croatia) (Fig.1). From the laboratory experiments, it was suggested that younger mussels accumulate trace metals from the water faster than the older ones (Odzak et al 1994). That is why, to study bioaccumulation kinetics, we have chosen three different age groups.

According to the most recent results of biomonitoring conducted in the bay during the last three years, the season has very big influence onto metal bioaccumulation into the mussels (Odzak et al 2000; Odzak et al 2001). For that reason, the experiments were conducted in summer (21 June – 27 July 2001) and in winter (23 January – 07 March 2002). During each of those two experimental periods the mussels, suspended matter (sediment traps) and biofouling organisms (plexiglas plates) were sampled six times (Fig. 2). Biofouling organisms (benthic algae) were not transplanted to the experimental station. They were growing for two months at the site on a plexiglas plates before the experiment started.

At the same station, temperature, pH, O2, salinity and sea level were measured. Also, meteorological situation (wind direction and velocity, rain quantity and air pressure) was monitored.

Results and discussion

Organic matter concentration was in a very good positive correlation with the Hg concentration in suspended matter throughout summer and winter experimental period. Obviously, Hg was mainly bound to the organic part of suspended matter. Relatively good correlation existed among sedimentation rate, Hg concentration and Hg quantity in suspended matter. In winter experimental period the average sedimentation rate, average Hg concentration and total Hg content in suspended matter were higher than in summer (Table 1). At the same time, average dissolved Hg concentration was lower in winter.

Compared to the summer experiment, average Hg concentrations were higher in the older mussels (B and C age group) and slightly lower in the youngest age group. The average Hg concentrations in biofouling organisms were lower during winter experiment (Table 2). In summer, the concentrations in mussels were increasing from the beginning of the experiment and it seems age groups A and B achieved dynamic equilibrium after three weeks (Table 3). The oldest mussels achieved the equilibrium one week later. Contrary to the summer experiment, in winter the oldest mussels achieved dynamic equilibrium in a second week.

Table 1. Average Hg concentrations in the water and suspended matter

SEASON / WATER / SUSPENDED MATTER
Hg
(ng/L) / Sed.rate
(g/m2/d) / Hg
(ng/g) / Hg
(g) / Org. matter
(%)
Summer
Winter / 2.4
1.9 / 4.5
5.5 / 474
595 / 2.1
2.8 / 12.9
14.1

Table 2. Average Hg concentrations in the mussels and biofouling organisms

SEASON / MUSSELS / BIOFOULING
Age group:A
(ng/g) / Age group:B
(ng/g) / Age group:C
(ng/g) / Hg
(ng/g)
Summer
Winter / 138
133 / 111
159 / 111
162 / 240
158

Table 3. Mercury concentrations accumulated in mussels from the beginning of the experiment until establishment of dynamic equilibrium

SEASON / MUSSELS
Age group:A
(ng/g) / Age group:B
(ng/g) / Age group:C
(ng/g)
Summer
Winter / 34 (3th week)
0 / 23 (3th week)
27 (3th week) / 33 (4th week)
63 (2nd week)

Conclusions

  • Hg in suspended matter mostly bound to organic matter,
  • Food is the main source of Hg for mussels,
  • Water is the main source of Hg for biofouling organisms,
  • In winter bioavailability of Hg is higher for mussels, but not for biofouling organisms.
  • Older (bigger) mussels accumulate higher mercury concentrations, but slower than younger mussels,
  • Hydro- and meteo- conditions (temperature, air pressure-sea level, wind, rain) significantly influence mercury speciation in the water column and consequently availability to the marine organisms,
  • Transplantation has the influence mostly on the youngest mussels.

The results of this study showed clearly the importance of all tree aquatic compartments (organisms, particulate matter and water) on bioavailability of trace metals. Further, biological factors (e.g. age, size) affect trace metal uptake in marine organisms. Also, bioavailability is highly dependent onto meteorological and oceanographic data because they influence metal content and speciation in a water and suspended matter, but also metabolic rate and physiologic behavior of an organism.

References

Cossa, D. (1989) A review of the use of Mytilus spp. as quantitative indicators of cadmium and mercury contamination in coastal waters. Oceanologica Acta, Vol. 12, pp. 417-432.

Gagnon, C. and Fisher, N.S. (1997) The bioavailability of sediment-bound Cd, Co and Ag to the mussel Mytilus galloprovincialis.Can. J. Fish. Aquat. Sci., Vol. 54, pp. 147-156.

Odzak, N., Martincic, D., Zvonaric, T. and Branica, M. (1994). Bioaccumulation rate of Cd and Pb in Mytilus galloprovincialis foot and gills. Marine Chemistry, Vol. 46, pp 119-131.

Odzak, N., Zvonaric, T., Kljakovic-Gaspic, Z., Horvat, M. and Baric, A. (2000). Biomonitoring of mercury in the Kastela Bay using transplanted mussels. Sci.Tot. Environ., Vol. 261, pp 61-68.

Odzak, N. Zvonaric, T., Kljakovic-Gaspic, Z., Horvat, M. and Baric, A. (2001). Biomonitoring of copper, cadmium, lead, zinc and chromium in the Kastela bay using transplanted mussels. Fres. Environ. Bull., Vol. 10, pp 37-41.