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The field data from four humic lakes suggested that water colour may have both direct and indirect effects on inter- and intra-specific interactions of perch Perca fluviatilis and roach Rutilus rutilus. The results agree with suggestions that, compared with R. rutilus, P. fluviatilis may be an inferior forager on zooplankton in highly coloured water. As an indirect effect, water colour decreases the coverage of macrophytes and limits suitable littoral habitats, benefiting R. rutilus over P. fluviatilis. Perca fluviatilis benefiting from complex habitats does not have the advantage in macrophyte-poor highly coloured water.

We evaluated the potential utility of stable isotope analysis of tissues commonly archived by aquatic biologists. Previous studies with chemically preserved samples have shown contradictory results, which present an obstacle for the use of archived sample materials. We tested the effects of ethanol and formalin preservation on zooplankton and of ethanol on benthic macroinvertebrate d 13 C and d 15 N values. We found that neither formalin nor ethanol had a significant effect on d 13 C and d 15 N values of preserved zooplankton. Nor did ethanol significantly affect d 13 C or d 15 N values of macroinvertebrates. However, ethanol preservation slightly, but significantly decreased C:N ratios of both zooplankton and macroinvertebrates, probably reflecting some extraction of lipids. Overall, the effects of preservatives on d 13 C and d 15 N values that we observed were minor. We also compared d 13 C and d 15 N values analysed from roach scales and perch operculum bones with those analysed from muscle tissue. Decalcification of scales and operculum bones only slightly improved our comparison to muscle tissue d 13 C and d 15 N values. Decalcified scales had slightly higher d 13 C and lower d 15 N values. Similarly, decalcified operculum bones showed slightly increased d 13 C and decreased d 15 N values to those for fish muscle. Our results confirm that scales and operculum bones can provide a suitable proxy for fish muscle in isotope studies with minor correction. We conclude that various archived sample materials can indeed be used with confidence for historical reconstructions of freshwater food webs by stable isotope analysis.

In the Enonselkä and Laitialanselkä basins of Lake Vesijärvi, perch Perca fluviatilis and roach Rutilus rutilus were abundant in the littoral and in the pelagic zones throughout the summer. In the littoral zone, roach was always more numerous than perch, while perch dominated in the open water. Intraspecific diet overlap values were higher than interspecific values. In the pelagic zone, perch <155 mm fed mainly on the cladoceran Leptodora kindtii, while small bosminids were most important food items for roach. Large perch were piscivorous, feeding mainly on smelt Osmerus eperlanus. In the littoral zone small perch foraged on zooplankton and chironomid larvae and large perch on chironomids and fish (small perch). Small roach fed mainly on bosminids and detritus, while for roach >185 mm macrophytes (Elodea canadensis, Lemna trisulca) were also of importance. Detritus was more common in the food of roach in Laitialanselkä than in Enonselkä. The slower growth rate of roach in Laitialanselkä compared with Enonselkä was probably connected with this. However, considering the latitude of the lake, the growth rate of both roach and perch was relatively fast in both basins. The results indicated that in a large lake both perch and roach are able to utilize effectively the different habitats and diverse food resources. By segregation in food resource utilization they are able to co-exist in large quantities, at the same time maintaining a relatively fast growth rate. 2000 The Fisheries Society of the British Isles M % J J A M J J A          61 M % J J A A Bosmina Daphnia Leptodora Other cladoc. Copepoda M J J A

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Changes in water pH and colour since the late 1980s were studied in 35 small boreal lakes of varying hydrological and landscape settings but similar climate and acid deposition. The data was collected during the autumnal overturn on the annual basis except in lake with weekly sampling during the ice-free period. In addition to the deposition data information about catchment soil types as well as local meteorological and hydrological conditions were used for the long-term data interpretation. The lakes are situated in a small area in southern Finland, 130 km north from Helsinki, where sulphate deposition declined by [60% in one decade since the mid1980s. The results showed that water colour increased in most lakes while pH did not. In lakes dominated by surface runoff there was a distinct upward shift in colour, with an initial increase after the mid-1990s and a second increase in 2004. The first shift appeared when the sulphate deposition reached a level ca. 25% of that in 1988. However, the upward shift in colour also coincided with a change in hydrological conditions after several dry summers. In contrast, the second shift in colour clearly coincided with a switch in hydrology due to the abnormally wet summer of 2004 after severe drought in 2002-2003. Although the hydrological conditions indisputably had a key role in determining the annual variability in colour, a distinct negative relationship between acid deposition and water colour in 90% of the lakes strongly suggested that reduction in sulphate deposition fostered the leaching of coloured organic substances from the catchment soils. Increase in colour, in turn, strongly influenced lake water pH, and the present day higher organic matter concentrations seemingly depress pH values more than in the 1980s, before the reduction in acid deposition.

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