Polychlorinated biphenyls (PCBs), DDT, DDT transformation products, and several other organochlorine substances (OCS) have been analysed in numerous species and matrices from different parts of the world including Arctic environments. However, knowledge on levels and distribution of OCS is still lacking for many regions world-wide including the Eastern Europe in general. Hence the aim of this thesis is to evaluate the environmental situation concerning OCS in the aquatic environment of Latvia, as a part of the Eastern Baltic region. Two species - perch (Perca fluviatilis) and grey heron (Ardea cinerea) - were studied in detail as biomarkers of contaminant exposure.

The effects of fish size, age, sex, and trophic positioning on the OCS concentrations in perch were investigated. There were no pronounced differences in OCS concentrations between the sexes. Perch up to a length of 20 cm showed no increase in contaminant concentrations with increased length, despite a clear raise in trophic positioning as shown by the analyses of stable nitrogen isotopes and gut contents. In perch exceeding 20 cm the concentrations increased with length. Thus perch below this length may be used as contaminant exposure biomarkers regardless of the omnivorous nature of the species.

Further, to be able to measure OCS concentrations in a matrix at a higher trophic level than fish, the use of blood of grey heron chicks was evaluated. The chicks were sampled repeatedly, with a week in between, to investigate the contaminant concentration variation. The results showed that heron blood is a useful matrix for OCS exposure assessment. Blood sampling allows a nonbiased specimen for analysis and does not affect the birds' health. An optimal heron chicks' age for sampling of about three weeks is recommended.

For the first time OCS contamination in the inland waters of Latvia is assessed. Lipid normalised total PCB concentrations in perch muscle from rural lakes were 200 to 600 ng/g. Conversely, somewhat higher PCB levels were determined in perch from some Riga lakes and the river mouth of Daugava. DDE concentrations were generally the highest of all individual OCS determined in the perch and heron samples. Bis(4-chlorophenyl) sulfone, DDT, DDD, hexachlorobenzene, hexachlorocyclohexanes, and trans-nonachlor were also analysed. On the whole, the OCS contamination is in the same range as in comparable samples from Swedish lakes and is generally similar to other parts of the Baltic Sea region.