We monitored the change in the isotope composition of sulfur, carbon, and nitrogen in broad whitefish (Coregonus nasus) tissues in response to a change in the isotope composition of their food. One of two batches of 2.5-yr-old fish raised in the laboratory were given a new food source with different δ34S, δ13C, and δ15N, which were monitored in muscle and liver tissue for 1 yr. A model including change due to tissue accumulation (growth) and metabolic replacement was developed. For all three isotopes, most of the change could be attributed to growth. Metabolic replacement expressed as a turnover rate was only 0.1 −0.2%∙d−1 and was similar for the three isotopes. Although liver tissue was −4.4 and −4.1‰, respectively, for δ34S and δ13C relative to muscle tissue, the response over time to the new food was the same as for muscle. We expect that the complete change in the isotope composition of fish tissue in response to a change in food could take years in slow-growing wild populations. The stable isotope composition would represent a long-term average of the food. In fast-growing fish the rate of change would directly reflect the growth rate.

SUMMARY1. Lake Victoria endured multiple stresses over the past century including population growth, increased cultivation of land, meteorological variability, resource extraction, intensive fishing, introduction of exotic species and more recently climate warming. These stressors became manifest through a fundamental and rapid change in the fish community and fishery in the early 1980s and visible eutrophication. However, the relation of these two phenomena and the possible interaction of the multiple stressors have been difficult to establish because of the temporally fragmented nature of the environmental data. 2. Comprehensive limnological observations from the 1960s were repeated in the 1990s and established the eutrophication of the lake, but these do not provide insight to the time course of when changes in trophic state occurred. Comprehensive fishery catch data from 1965 to the present provide a time course of the change in community composition and yield but cannot be correlated in time with discontinuous and sparse limnological data to determine possible cause-effect relationships. 3. Palaeolimnologic studies were conducted on three cores, two offshore and one nearshore, to establish a time course for the eutrophication of the lake that can be related to time-based data on the fishery. In the 1920s, the cores recorded an increase in nitrogen content of the sediments, but there was no significant response in the paleo-productivity indicators of biogenic Si deposition and change d 13 C of deposited organic matter.Phosphorus deposition began to increase in the 1940s in all three cores after which biogenic Si deposition increased steadily over time. Responses in d 13 C of organic matter begin in the 1960s at the coring sites. In the 1970s, the d 13 C of organic matter at the nearshore site increased nearly 3& in a 10-year period likely as a response to a dramatic increase in internal P loading caused by spreading anoxia. 4. Nile perch, the large predatory fish introduced in 1954, had become established through much of the lake at low abundances by the 1970s. In 1980, the catch of this fish began to increase, and by the end of the decade, the Lake Victoria fishery was the largest lake fishery in the world; and Nile perch dominated the catch. While catches of some other fishes also increased, the endemic haplochromines suffered a catastrophic decline in abundance and loss of biodiversity. 5. The detailed chronostratigraphies for these sediment cores established that the major changes in the trophic condition of the lake were accomplished prior to the change in the fish community and that the increased primary productivity of the lake likely contributed to the increased fish catches after 1980. The increased algal abundance also would have greatly reduced visibility and facilitated the emergence of Nile perch as the dominant top predator. 6. Thematic implications: multiple stresses were present in Lake Victoria over several decades, but transition to a new ecosystem state with a transformed food web and high...

Climate warming is projected to affect forest water yields but the effects are expected to vary. We investigated how forest type and age affect water yield resilience to climate warming. To answer this question, we examined the variability in historical water yields at long-term experimental catchments across Canada and the United States over 5-year cool and warm periods. Using the theoretical framework of the Budyko curve, we calculated the effects of climate warming on the annual partitioning of precipitation (P) into evapotranspiration (ET) and water yield. Deviation (d) was defined as a catchment's change in actual ET divided by P [AET/P; evaporative index (EI)] coincident with a shift from a cool to a warm period – a positive d indicates an upward shift in EI and smaller than expected water yields, and a negative d indicates a downward shift in EI and larger than expected water yields. Elasticity was defined as the ratio of interannual variation in potential ET divided by P (PET/P; dryness index) to interannual variation in the EI – high elasticity indicates low d despite large range in drying index (i.e., resilient water yields), low elasticity indicates high d despite small range in drying index (i.e., nonresilient water yields). Although the data needed to fully evaluate ecosystems based on these metrics are limited, we were able to identify some characteristics of response among forest types. Alpine sites showed the greatest sensitivity to climate warming with any warming leading to increased water yields. Conifer forests included catchments with lowest elasticity and stable to larger water yields. Deciduous forests included catchments with intermediate elasticity and stable to smaller water yields. Mixed coniferous/deciduous forests included catchments with highest elasticity and stable water yields. Forest type appeared to influence the resilience of catchment water yields to climate warming, with conifer and deciduous catchments more susceptible to climate warming than the more diverse mixed forest catchments.

The variability of surface water carbon dioxide concentration, or partial pressure (pCO 2 ), was studied in 11 lakes of greatly varying size (2.4 ha up to 8 million ha) in Northwest Ontario, Canada. Six of these lakes were chosen to be as similar as possible in all respects except surface area (the Northwest Ontario Lake Size Series [NOLSS], which range from 88 to 35,000 ha). Spatial and temporal variability of pCO 2 within a single lake was no greater in the larger lakes than in the smaller lakes. Interannual variability was significant and synchronous, which indicates that weather patterns were important and affected the different lakes within the region in a similar manner. However, annual pCO 2 averages were not related to annual differences in planktonic photosynthetic activity, measured by 14 CO 2 fixation. In the six NOLSS lakes, there was not a significant relationship of average pCO 2 with lake size. For all 11 lakes, however, there was a significant negative correlation of pCO 2 with lake size, which was likely due to several characteristics of the very small and very large lakes that covaried with size. The larger lakes were deeper and had longer water residence times and lower DOC, which suggests lower CO 2 production from allochthonous organic carbon inputs. Also, the ratio of epilimnetic sediment area/epilimnetic volume (A e /V e ) was smaller in the larger lakes, which likely resulted in lower rates of recycling of fixed carbon to CO 2 during summer stratification.

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