Abstract
Thermal regime warming and increased variability can result in human developed watersheds due to runoff over impervious surfaces and influence of stormwater pipes. This study quantified relationships between tree canopy, impervious surface, and water temperature in stream sites with 4 to 62% impervious land cover in their “loggersheds” to predict water temperature metrics relevant to aquatic species thermal stress thresholds. This study identified significant (≥0.7, p < 0.05) negative correlations between water temperature and percent tree canopy in the 5 m riparian area and positive correlations between water temperature and total length of stormwater pipe in the loggershed. Mixed-effects models predicted that tree canopy cover in the 5 m riparian area would reduce water temperatures 0.01 to 6 °C and total length of stormwater pipes in the loggershed would increase water temperatures 0.01 to 2.6 °C. To our knowledge, this is the first time that the relationship between stormwater pipes and water temperature metrics has been explored to better understand thermal dynamics in urban watersheds. The results highlight important aspects of thermal habitat quality and water temperature variability for aquatic species living in urban streams based on thermal thresholds relevant to species metabolism, growth, and life history.
Highlights
Water temperature has long been recognized as an important aquatic environmental variable [1,2,3] that directly and indirectly affects numerous ecological processes [4,5,6] and as such is regulated in the United States under the Clean Water Act, Section 303 (d) [7,8]
Thermal regime is a term that refers to the stream temperature characteristics and dynamics that we describe based on stream temperature data collected over time [24]
We focused on April to October water temperatures, with April to June generally representing the time for fish spawning and growth of early life stages; July and August as the most likely time of year for aquatic species thermal stress and reduced baseflow; and September to October as a transition period when water temperatures cool and various aquatic species migrate to spawning grounds or move to other habitats to feed [59,60,61]
Summary
Water temperature has long been recognized as an important aquatic environmental variable [1,2,3] that directly and indirectly affects numerous ecological processes [4,5,6] and as such is regulated in the United States under the Clean Water Act, Section 303 (d) [7,8].Increasing water temperature values and variability are known to induce thermal stress in aquatic species that can affect growth, reproductive success, and mortality [9,10,11,12]. Numerous studies have focused on quantifying a stream’s thermal regime and drivers of water temperature variability [20,21,22,23,24]. Important factors that affect stream temperature include riparian vegetation [25], hydrology (e.g., discharge, groundwater source volume, and hyporheic exchange) [1,26,27], and locations where tributaries enter the main channel [1,28]. Local scale variability related to groundwater and tributary connection are relative to baseflow hydrology according to stream size and volume [29,30]. Factors that may affect water temperature variability at the catchment and watershed scales include climate, elevation, and land cover, and geology [1,31,32]
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