Ecosystem effects of a tropical cyclone on a network of lakes in northeastern North America.

Here we document the regional effects of Tropical Cyclone Irene on thermal structure and ecosystem metabolism in nine lakes and reservoirs in northeastern North America using a network of high-frequency, in situ, automated sensors. Thermal stability declined within hours in all systems following passage of Irene, and the magnitude of change was related to the volume of water falling on the lake and catchment relative to lake volume. Across systems, temperature change predicted the change in primary production, but changes in mixed-layer thickness did not affect metabolism. Instead, respiration became a driver of ecosystem metabolism that was decoupled from in-lake primary production, likely due to addition of terrestrially derived carbon. Regionally, energetic disturbance of thermal structure was shorter-lived than disturbance from inflows of terrestrial materials. Given predicted regional increases in intense rain events with climate change, the magnitude and longevity of ecological impacts of these storms will be greater in systems with large catchments relative to lake volume, particularly when significant material is available for transport from the catchment. This case illustrates the power of automated sensor networks and associated human networks in assessing both system response and the characteristics that mediate physical and ecological responses to extreme events.

Estimating attenuation of ultraviolet radiation in streams: field and laboratory methods.

We adapted and tested a laboratory quantitative filter pad method and field-based microcosm method for estimating diffuse attenuation coefficients (K(d)) of ultraviolet radiation (UVR) for a wide range of stream optical environments (K(d320) = 3-44 m(-1)). Logistical difficulties of direct measurements of UVR attenuation have inhibited widespread monitoring of this important parameter in streams. Suspended sediment concentrations were manipulated in a microcosm, which was used to obtain direct measurements of diffuse attenuation. Dissolved and particulate absorption measurements of samples from the microcosm experiments were used to calibrate the laboratory method. Conditions sampled cover a range of suspended sediment (0-50 mg L(-1)) and dissolved organic carbon concentrations (1-4 mg L(-1)). We evaluated four models for precision and reproducibility in calculating particulate absorption and the optimal model was used in an empirical approach to estimate diffuse attenuation coefficients from total absorption coefficients. We field-tested the laboratory method by comparing laboratory-estimated and field-measured diffuse attenuation coefficients for seven sites on the main stem and 10 tributaries of the Lehigh River, eastern Pennsylvania, USA. The laboratory-based method described here affords widespread application, which will further our understanding of how stream optical environments vary spatially and temporally and consequently influence ecological processes in streams.