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The stable isotope composition of lacustrine sediments is routinely used to infer Late Holocene changes in precipitation over Scandinavia and, ultimately, atmospheric circulation dynamics in the North Atlantic realm. However, such archives only provide a low temporal resolution (ca. 15 years), precluding the ability to identify changes on inter-annual and quasi-decadal timescales. Here, we present a new, high-resolution reconstruction using shells of freshwater pearl mussels, Margaritifera margaritifera, from three streams in northern Sweden. We present seasonally to annually resolved, calendar-aligned stable oxygen and carbon isotope data from 10 specimens, covering the time interval from 1819 to 1998. The bivalves studied formed their shells near equilibrium with the oxygen isotope signature of ambient water and, thus, reflect hydrological processes in the catchment as well as changes, albeit damped, in the isotope signature of local atmospheric precipitation. The shell oxygen isotopes were significantly correlated with the North Atlantic Oscillation index (up to 56 % explained variability), suggesting that the moisture that winter precipitation formed from originated predominantly in the North Atlantic during NAO+ years but in the Arctic during NAO− years. The isotope signature of winter precipitation was attenuated in the stream water, and this damping effect was eventually recorded by the shells. Shell stable carbon isotope values did not show consistent ontogenetic trends, but rather oscillated around an average that ranged from ca. −12.00 to −13.00 ‰ among the streams studied. Results of this study contribute to an improved understanding of climate dynamics in Scandinavia and the North Atlantic sector and can help to constrain eco-hydrological changes in riverine ecosystems. Moreover, long isotope records of precipitation and streamflow are pivotal to improve our understanding and modeling of hydrological, ecological, biogeochemical and atmospheric processes. Our new approach offers a much higher temporal resolution and superior dating control than data from existing archives.

Physiologically-driven rhythms in bivalve molluscs are predicted to vary as a function of metabolic rate and temperature, in contrast to genetically predisposed biological clocks. These rhythms can be evaluated using long-term video monitoring techniques under controlled conditions in laboratory aquaria. The bivalves Anodonta cygnea and Margaritifera falcata were used to evaluate the effect of temperature on rhythms in gape and the formation of siphons at the mantle edge. Frequency and duration of shell closure vary with temperature in both species, but with different responses. Mean duration of intervals of valve closure decreases as temperature rises in both species, and is consistent with physiological limitation by increased biological oxygen demand. For A. cygnea, cumulative gape duration peaks at 25°C, with less time spent closed than at any other temperature, but increasing temperatures correspond to an increase in gape frequency with a strong increase observed at 31°C. In contrast, frequency of adduction and valve closure peak at 25°C in M. falcata, and continuous gaping is observed above 29.5°C. This physiological stress is consistent with evidence from sclerochronologically-calibrated stable isotope studies of shells, where growth breaks in many marine taxa coincide with maximum temperatures above 31°C as derived for δ 18 O carbonate . The results of this study suggest that these growth breaks may be due to physiological limitations in oxygen uptake and metabolic activity, rather than being a direct consequence of elevated temperature alone.

We present the results of sclerochronologi- cally calibrated growth and stable isotope analyses of the freshwater bivalve Margaritifera falcata collected from an agricultural, suburban setting near Vancouver, BC. The oxygen isotope range of shell aragonite can be explained by the temperature range during the growing season, assuming the water d 18 O composition remained constant. However, shell growth is strongly influenced by local summer precipitation and potentially runoff of nutrient-rich stormwater. About 44% of the variability of annual shell growth can be explained by amounts of local summer (June-September) rainfall. Local winter precipitation and El Nino-Southern Oscillation (ENSO) strength during the preceding year exert a weak, but significant control on shell growth. In com- bination, summer and winter precipitation can explain up to 50% of the variability in annual shell growth. Spectral analyses substantiate the effect of precipita- tion on shell growth and demonstrate that shell growth and ENSO are coupled by precipitation. Common spectral density was found at periods of 6.5-9 years, particularly between 1985 and 2004. Higher frequency oscillation corresponding to periods of 3-5 years occurred during the early 1970s, early to mid 1980s, and later 1990s. These results suggest that skeletal records of bivalve mollusks provide suitable archives of ENSO-coupled precipitation in areas where other climate proxies such as tree-rings and speleothems may not be available.

In a recent analysis of d 13 C in the aragonitic shells of the bivalve Margaritifera margaritifera, Geist et al. (2005) argue that variation in the carbon isotope composition of contemporaneous shells from one locality over periods of decades cannot be correlated with one another, and thus do not record environmental signals. Instead, they ascribe the observed fluctuations to respiratory loss of light carbon and consequent enrichment of 13 C in the youngest part of the shell. However, we note two significant weaknesses in the study that make it difficult to accept their conclusions. First of all, we note the absence of reliable and long-established sclerochronological techniques in the analysis of their shells needed to determine the timing of shell growth and construct reliable chronologies. Second, the authors do not report relevant environmental and physiological data needed to establish an independent metabolic control on shell d 13 C.