UC Santa Barbara

Land use change is a key driver of change in stream ecosystems worldwide and poses a significant risk to streams in areas with high biodiversity and large human populations, such as regions with Mediterranean climates. Two important characteristics of stream ecosystems, whole ecosystem metabolism and water temperature, are sensitive to the changes caused by urban and agricultural development.

In the first section of this dissertation, the effects of urbanization on stream metabolism (gross primary productivity (GPP), community respiration (CR), net ecosystem productivity (NEP), and the ratio of GPP to CR (P/R)) in Santa Barbara, California were assessed by comparing stream metabolism, measured using a single-station diel oxygen change method, across 6 streams with varying catchment urban development between March and July 2014. Environmental variables at each site were measured, including water temperature; light; algal biomass; nutrients (N, P); dissolved and particulate organic carbon; specific conductance; total suspended solids; canopy openness; water width, depth, and velocity; discharge; slope; and elevation. Nitrate concentration was identified as an important driver of GPP, NEP, and P/R, and both GPP and nitrate levels were found to be higher at developed than undeveloped sites. However, elevation was found to be a potential confounding factor in the study, especially for CR, which was significantly higher at higher elevation sites. GPP, NEP, and P/R were related to multiple urban metrics and leaking septic fields were identified as a potential nitrate source driving GPP and P/R.

In the second section of this dissertation, the thermal environment of Santa Barbara streams was characterized using water temperature data collected between 2001 and 2015 in 21 stream locations. Temperatures exceeding the upper tolerance limits (25°C) for the endangered, native southern California steelhead trout (Oncorhynchus mykiss) were identified and the effects of agricultural, urban, and undeveloped land use on water temperatures were assessed. Sub-lethal temperatures for southern California steelhead trout were found at most sites, but a subset of sites in key watersheds for steelhead conservation efforts sometimes exceeded lethal limits. The warmest sites lacked riparian vegetation, the coolest sites were at high elevations, and the sites with small annual temperature ranges were located where significant groundwater influence was expected. Mean and maximum daily temperatures, in the spring and summer, were positively related to percent impervious surface cover, with evidence that the relationship was driven by warm temperatures at channelized sites. Excluding the channelized sites, minimum daily temperatures in the spring and summer were positively related to percent impervious cover, with or without the inclusion of high elevation sites, possibly indicating warmer groundwater inputs to streams in the urban areas of Santa Barbara.