Ecosystem State as Assessed by Water Temperature: A Guide to Restoring River Patchiness

Abstract:

River restoration projects commonly idealize the free-flowing state, despite the fundamental understanding that rivers are patchy systems. This talk presents temperature data from rivers in the northeastern U.S. to further demonstrate the nature of this patchiness and to suggest that a better restoration target would be ecosystem heterogeneity. These temperature data were collected at 15-minute intervals using 27 onset water temperature pro v2 loggers in Hubbard Brook, in central New Hampshire, and Pisgah State Park and Hosley Brook, in southwestern New Hampshire, all of which are in watersheds without direct modern human influence. These loggers captured the temperature of the river water entering, leaving, and within nine beaver ponds. Analysis of these data show many different aspects of the patchiness of these ecosystems. The range of temperatures recorded in the free-flowing reaches—across a day, season, or year—is significantly more constrained than in the beaver ponds, some of which have higher highs and others of which have lower lows. The onset of warming in the springtime shows a similar significantly tighter grouping for the free-flowing reaches, with both earlier and later warming dates within the beaver ponds. Spectral analysis shows a cluster of similar signatures for the free-flowing reaches, with a peak at the 24-hour period. In contrast, some beaver ponds show additional peaks at 6-hour and 4-hour periods, demonstrating variability of flow paths. Despite these significant differences, the temperature signatures of the beaver ponds are quickly lost downstream, where the temperature regimes upstream and downstream of the ponds are nearly indistinguishable. These temperature data demonstrate that these ponds create water conditions considered unsuitable for target species of river restoration; this talk will also present data from a companion study showing dissolved oxygen and nitrogen concentrations also in excess of river management regulations. Because these conditions quickly resolve downstream, and because these areas are known as hotspots for desired aquatic species, these data demonstrate the importance of patchiness rather than a single desired restoration target.