Stuart E.G. Findlay
Institute of Ecosystem Studies

Microbes play central roles in carbon and nutrient cycling in many ecosystems. Microbial ecologists have made considerable progress in predicting variability among ecosystems in "bulk" microbial characteristics or processes such as biomass and respiration. However, the fundamental mechanism underlying these differences among systems remains poorly described. Changes in bulk processes may be due to physiological shifts in the resident populations such that rates may change with little change in community composition. Alternatively, species substitutions may drive variability in rates, with changes in community composition being responsible for variability in bulk characteristics. Because the bacterial community has a wide diversity of metabolic abilities, and rapid turnover rates, we believe that changes in bulk properties are associated with changes in community composition.

To examine these alternatives Dr. Findlay proposes to take advantage of gradients in subsurface dissolved organic carbon (DOC) that consistently yield differences in microbial functional responses including productivity and extracellular enzyme activity. Knowing that we can sample communities exhibiting differing growth rates and enzymatic activities is an essential prerequisite for asking whether such variability is, or is not, underlaid by shifts in community composition. Dr. Findlay's team has sampled a suite of streams representing a wide range in total DOC and has consistently observed differences among systems in DOC consumption as streamwater perfuses through shallow hyporheic sediments associated with these streams. Patterns range from negligible declines in DOC concentration to reductions of 50%. These differences in consumption of DOC are accompanied by several-fold fluctuations in rates of bacterial productivity and dramatic changes in activities of extracellular enzymes. Additionally, the scientists have constructed and tested a set of nine experimental mesocosms which allow controlled, replicated manipulation of DOC supply to subsurface sediments. When supplied with water from different streams, these mesocosms yield patterns in productivity and enzyme activities completely predictable from the site of source water collection. These field sites and mesocosms provide the perfect opportunity to ask whether these large functional shifts are in fact accompanied by changes in community composition.

Dr. Findlay predicts that communities living near the beginning of hyporheic flowpaths will experience the highest availability of DOC. These communities are hypothesized to be highly diverse and quite distinct from communities existing under conditions of low DOC availability at the tail of the same flowpaths. As DOC availability decreases, he expects community composition among streams to converge parallel to convergence in DOC composition. Comparisons among streams provide a range in expected initial DOC availability, with streams showing large declines in DOC along flowpaths apparently supplying higher quality, more available DOC than streams exhibiting only minimal declines. Arguing as above, he expects subsurface communities from the beginning of hyporheic flowpaths in streams with high DOC bioavailability to be distinct from communities at either the head or tail of flowpaths in streams with low DOC bioavailability.

DOC processing along flowpaths in stream sediments provides a logical ecological template for investigating links between microbial community function and structure. Dr. Findlay proposes mesocosm experiments in which scientists manipulate DOC source water to generate patterns in bacterial functional responses among and along hyporheic flowpaths. These functional responses will be used to predict associated structural responses, specifically changes in community similarity. DNA hybridization techniques will be used to determine similarity among communities at the heads vs. tails of flowpaths and among source waters. This approach will link population-level shifts with variability in physiological and bulk processes. Dr. Findlay believes this linkage extends beyond stream and microbial ecology to help fill the recognized gap between community dynamics and ecosystem-level processes.