Interannual fluctuations in primary production: Meteorological forcing at two subalpine lakes

Meteorological factors are associated with most of the interannual variability in primary production at both Castle Lake, California (4 l”N, 122%‘) and Lake Tahoe, California-Nevada (39”N, 12O”W). At Castle Lake, extreme values of annual primary production, either much higher or lower than the long-term average, are likely to occur during the phenomenon of El Nifio/Southern Oscillation. Two plausible pathways for the impacts of these large-scale climate events at Castle Lake were identified: winter snowfall, acting through its effect on the snow-ice pack and timing of the spring thaw; and total precipitation, acting through its effect on outwash rates. In contrast, no influence of large-scale climate events is apparent at Lake Tahoe, but a plausible pathway involving the impact of synoptic-scale phenomena on interannual variation was identified: local weather events occurring in the late winter-early spring period near the time of minima1 stratification, acting through their cl&t on the depth of spring mixing. The difference between the two lakes can be attributed to absence of an ice cover and long hydraulic retention time at Lake Tahoe. Timeseries models that incorporate meteorological information adequately forecast primary production at both lakes.