environmental sciences research institute - University of Ulster

4.3 FRESHWATER SCIENCE
Toxic metals in the aquatic food chain
Figure 1. The pristine environment of Loch Coire nan Arr in the
north Scottish Highlands (one of the study lakes) where the
aquatic life are subject to toxic metal burden.
The growth of the global economy since the early 20th
Century has been mirrored by atmospheric trace metal
burden. Consequently, toxic metals such as lead (Pb),
cadmium (Cd) and mercury (Hg) have entered freshwater
lakes. This has posed a threat to ecosystems and humans
as these metals are accumulated by the primary producers
(the
phytoplankton) and subsequently transferred through
the aquatic food chain. Therefore the United Nations
Economic Commission for Europe adopted the Heavy
Metals Protocol to encourage modelling, research and descriptions of metal pathways. However, accurate modelling of
these pathways in freshwater lakes has been inhibited by a lack of knowledge on metal uptake by the phytoplankton. This
is particularly due to the temporal and spatial variations in phytoplankton biomass that stems from their photosynthetic
nature and variations in their growth limiting nutrient, phosphorus.
This project has focused on quantifying the short-term changes in the uptake of Pb, Cd and Hg by the phytoplankton in
lakes of varying nutrient status. Three remote Scottish lakes that have received high, medium or low metal contamination
from the atmosphere were selected for analysis. Phytoplankton specimens were collected and the behaviour of metals in
the sediment was investigated with continuous sediment traps and sediment cores. The technique of diffusive gradients
in thin film was employed to assess the distribution of dissolved metals. Concentrations of Pb, Cd and Hg in the
phytoplankton and sediment were analysed with Inductively Coupled Plasma-Mass Spectrometry.
A redox sensitive release of metals from the sediment was found during the early spring as dissolved oxygen in the lakes
declined due to higher community respiration rates. The concentration of metals then accumulated by the phytoplankton
varied between the taxonomic groups and their metabolic demand or sensitivity of each metal. Furthermore, the
phytoplankton were more concentrated with Pb, Cd and Hg in poor nutrient status waters that receive low metal
contamination from the atmosphere than in nutrient rich (eutrophic) waters receiving high metal contamination.
Considering the possibility of these metals transferring through the aquatic food chain, the ecosystems in nutrient poor
waters may be under a greater threat of toxicity.
DEL Funded PhD Studentship, Supervisor: Dr Richard Douglas, Student: Áine Gormley
References.
Gormley, A. M., The Impact of Algae Growth on the Transport of Toxic Metals in Lakes, in Proceedings of the Thermo
Scientific ICP-MS Users Meeting, The UK National Space Centre, England, 2007
Gormley, A. M., The Impact of Phytoplankton Growth on the Biogeochemical Cycling of Metals in Lakes, in Proceedings
of the 30th Congress of the International Association of Theoretical and Applied Limnology in Montreal, Canada, 2007
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