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- Rolff, Carl, 1958-
(författare)
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Stable isotope studies of contaminant and material transport in Baltic pelagic food-webs
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Both carbon and nitrogen appear in nature as different stable isotopes. Many natural processes affect the relative proportions of heavy and light isotopes in predictable ways. The isotopic composition of organisms and other organic materials can therefore serve as records of the processes involved in their formation. Materials with origin in different production systems like land, lakes, rivers and oceans differ in this respect. Inferences about the origin of different materials can therefore be made from their isotopic composition. The application of stable isotope methods in ecology turns nature into a vast tracer experiment in which we are free to participate, observe, deduce and learn.Heavy isotopes accumulates in tissue of organisms. The isotopic composition thereby reflects the trophic positions of organisms in a food web. Many environmental contaminants are also accumulated by organisms from their food and thereby come to appear in higher concentrations in consumers. Environmental contaminants, found in low levels in primary producers may, by consecutive food chain transfers, be boosted to concentrations that are detrimental, or even fatal, to the top consumers. A method is presented by which the food web position of organisms (estimated by heavy nitrogen isotope content) can be used to quantitatively estimate the food web accumulation of persistent organic contaminants. The method can be used as an "early warning system" to identify compounds that accumulate in food chains. It was used to show that the highly toxic 2378-substituted dibenzo-p-dioxins and dibenzofurans increased in concentration with increasing trophic position in a Baltic food web from phytoplankton to cod.Filamentous blue-green algae are often toxic and are generally considered to be unpalatable to animals. The characteristic summer blooms of filamentous nitrogen-fixing blue-green algae in the Baltic proper appear, from results presented here, to play a more important role in the food web than previously assumed. Because they assimilate gaseous nitrogen, these blue-green algae have considerably lighter nitrogen isotopic composition than other phytoplankton. The light nitrogen isotope signal of blue-green algae could be detected in all size classes of plankton. The role of blue-green algae as a food resource and as providers of fixed nitrogen in the Baltic ecosystem may therefore have to be re-evaluated.The Baltic is influenced by dissolved organic matter transported by rivers. The role of this material in plankton production is largely unknown. If extensively utilised it should cause the food web structure to differ between the basins of the Baltic Sea. Generally, organic material produced in the marine environment is heavier in isotopic composition than terrestrial materials. The isotopic composition can therefore be used to estimate the relative utilisation of land derived material in marine food webs. In 10 size classes of plankton the relative content of heavy carbon isotope was found to increase linearly with salinity from the Bothnian Bay to the northern Baltic proper. A two component model suggested that 16-35% of zooplankton production in the Bothnian Bay is derived from material with terrestrial origin.In situations when a sampling site can only be accessed on few occasions, the stable isotope composition of organisms may integrate process information over time. The statistical design of sampling programs must provide data, that are representative of the spatio-temporal scales of interest. The appropriate design for basin wide estimates of isotopic composition in surface sediment, benthic organisms, phyto- and zooplankton was studied by hierarchic analysis of variance components. An aggregated sampling design with replication on 50 and 5 nautical miles difference was found to be adequate for both carbon and nitrogen isotopes in all sample types in the Bothnian Sea. The seasonal changes of carbon and nitrogen isotopic composition in size-fractionated plankton was described for a complete annual production cycle, and the importance of time-lags for the interpretation of results was discussed.
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