UC San Diego

Anthropogenic climate change is expected to continue increasing temperatures globally as well as alter the nutrient composition of many ecosystems. Higher air temperatures have been causing lakes to warm, and dissolved organic carbon (DOC) concentrations are rising in many freshwater systems due to increased input of terrestrial organic matter, a phenomenon known as “browning.” The combined effects of warming and browning on fish populations is unclear. I investigated the effects of warming and browning on brook trout (Salvelinus fontinalis), an ecologically and economically important species in California’s Eastern Sierra. A previous study found antagonistic effects of temperature and DOC on fish growth rates, with higher temperatures increasing fish growth rates and higher DOC decreasing fish growth rates. This study aimed to investigate whether these relationships could be replicated and still applied years later as global temperatures have continued to increase. I found that fish growth rates and body condition were higher in lower elevations. Fish body condition also increased with chlorophyll-a concentration and decreased with DOC concentration and the δ13C signature of fish muscle. These patterns suggest that autochthonous production along with increased terrestrial insect consumption may promote increased body condition. Fish grew faster and had better body condition at warmer temperatures, likely contributing to the higher fish growth seen in lower elevations. My results suggest that two major symptoms of climate change, higher temperatures and greater allochthonous carbon concentrations, will have opposing effects on trout growth and productivity in mountain lakes.