Controls on fine particle retention in experimental streams

The transport and retention of fine particulate organic matter (FPOM) in headwater streams plays a significant role in organic matter processing and represents a critical link between upstream and downstream systems. Benthic biofilms potentially alter substrate characteristics and dynamics. However, the influence of biofilm colonization on FPOM retention has not been well documented, likely due to inherent heterogeneity of natural systems. We expected that biofilm growth would either enhance or diminish FPOM retention in the streambed, whereas biofilm growth would vary depending on the underlying substrate template. We used a short-term FPOM addition technique to estimate transport distances of a fine particle analog (corn pollen: 70 mu m) in four 50-m experimental streams (Q = 1.5 L/s) at the Notre Dame Linked Experimental Ecosystem Facility. Streams varied in substrate size (1-cm pea gravel vs 10-cm small cobble) and complexity (homogenous 50/50 mix vs alternating sections). We sampled streams on multiple dates over 5 mo (June-October) of biofilm colonization and senescence. While FPOM transport distances generally increased as biofilm colonization occurred, retention was variable across streams and over time. We found no consistent or simple pattern of FPOM retention in our streams. Rather, one of the strongest effects was an indirect relationship between FPOM removal and transient storage, a factor that is strongly driven by biofilm colonization. These results emphasize the dynamic feedback of the physical and biological environments influencing FPOM retention in streams.