This doctoral dissertation improvement project will study the movement and transformation of carbon in streams. Streams and rivers are not passive pipes that merely transport carbon. Active uptake of carbon through photosynthesis and subsequent release of carbon through respiration takes place in streams and can lead to complex patterns of carbon exchange with the atmosphere as water moves from land to the oceans. This study will measure the amount of carbon in mountain streams that is respired, stored in living organisms, and transported downstream.
Current freshwater carbon budgets include the role of terrestrial carbon processing, but have not addressed the role of in-stream carbon fixation and processing and, consequently, may underestimate rates of carbon dioxide transfer from streams to the atmosphere, carbon burial, and carbon export downstream. This study will help complete our understanding of the role streams and rivers play in carbon movement in the landscape.
Stream bacteria, invertebrates, and fish depend on plants and algae (primary producers) within the stream for a large portion of their energy and nutrient requirements, even in ecosystems that receive substantial energy and nutrient subsidies from the surrounding landscape. The fate of internally fixed carbon (C) by stream primary producers is difficult to measure directly because of how quickly it is consumed and cycled within food webs. Possible fates include: moving up the food web as organic C, respired as CO2 after being fixed or consumed, and exported downstream without being eaten. We measured the fixation and fate of primary producer C by adding 13C to a mountain stream in Medicine Bow National Forest (Wyoming, USA). 13C is a stable isotope of C that exists naturally at low concentrations (relative to 12C) and is fixed into primary producers during photosynthesis. We measured changes in the ratio of 12:13C in primary producers (algae, moss, and rock biofilm), dissolved organic C (DOC), dissolved inorganic C (DIC), suspended particulate organic C (POC), and POC on the stream bed for 44 days after adding the C tracer. Stream primary producer C cycled very rapidly: it was respired as DIC by primary producers and associated bacteria, released by primary producers as DOC, stored as POC, and exported downstream as POC. Very little DOC was exported downstream, suggesting it was quickly consumed by stream bacteria. Using the decline in tracer C over our sampling period, we estimated that the residence time of primary producer C in our study reach was 49 days for algae, 58 days for rock biofilm, and 76 days for aquatic mosses. More than half of the fixed C did not remain in primary producers long after fixation, and was quickly released as DOC and respired as CO2.
