Contribution of riparian soils to hyporheic dissolved organic matter in a large, floodplain river
Sandra Clinton, PhD Project

Microbial processes

This study is designed to describe the structure and function of the riparian floodplain microbial community, and determine what factors control microbial abundance and activity. During summer 1999, an additional 20 wells were added to the terrace site where the main well transects are located. This site is the same as that used for the soil dynamics study so that our understanding of soil leaching processes can be used to better understand the effect of soil inputs on hyporheic microbial communities. New wells were added to 1) improve our understanding of subsurface flow through the terrace and 2) provide additional sampling points associated with key vegetation patches. Wells were surveyed and tied into the original grid. Wells were sampled for hydraulic conductivity. We are continuing to sample seasonally for microbial parameters and DOC chemistry.

Results to date:

Water is flowing from the top of the terrace and exiting at the bottom of the terrace. Regardless of season, dissolved oxygen concentration decreases by 70-80% as water moves through the terrace, indicating a high degree of microbial activity. Dissolved organic carbon (DOC) concentration is low in subsurface waters (0.5-2 mg/L). Higher concentrations of DOC occur in wells overlain by older vegetation patch types than younger vegetation patch types. However, this pattern is not significant. During summer and fall, DOC concentration increases as water moves from the head to bottom of the terrace. This increase indicates other sources of DOC other than exist must exist. We propose that inputs of DOC originated from overlying riparian soils. During winter, DOC decreases as water flows through the terrace, indicating the diminished influence of riparian soils to subsurface chemistry. For the same head, there is a high degree of variability in DOC concentration. This variability increases as water flows down the head gradient. This variability reflects the difference in the path of water as water moves through the terrace.

We are currently measuring seasonal changes in microbial production using the H3-leucine technique. Microbial production will be correlated with hydrology and overlying vegetation. We expect production to be higher in wells overlain with older vegetation patches. We are also currently analyzing samples for exoenzyme activity. A suite of 8 microbial enzymes (alpha-glucosidase, beta-glucosidase, N-acetyl-amineglucosidase, xylosidase, phosphatase, leucine aminopeptidase, esterase, and guanindizoate) has been measured. The combination of how these enzymes are expressed will give us an indication of the types of carbon microorganisms are using for growth. This research forms the basis of a PhD thesis for Sandra Clinton.