Transport, sources, and quality of seston in a Piedmont headwater stream

Abstract

Streams transport and process particulate organic carbon (POC) within the suspended load (seston) after terrestrially-fixed (allochthonous) carbon enters lotic ecosystems or as instream (autochthonous) production is suspended from the streambed. POC provides a basal resource for upstream food webs, but can also support heterotrophic metabolism in downstream rivers. Yet, the controls on transport, sources, and biological availability (quality) of POC from headwaters are poorly understood. I examined seston and POC dynamics in a 3^rd-order headwater stream in SE Pennsylvania. I studied the temporal controls on seston transport, composition, and sources, and the biological quality of POC.

I present evidence that seasonal patterns in seston and POC transport and composition reflect stream organism activity and cycles of autumnal leaf litter inputs and vernal algal production. This work also provides the first evidence of recurrent nighttime peaks of seston transport; I attribute this pattern to bioturbation of streambed sediments that suspends particles during stream-organism nest digging, foraging, and movement. I present the first effort to demonstrate that stream salamanders contribute to ecosystem level processes such as modification of seston and organic carbon flow.

Mixing model analyses of seston composition indicate that seston in small streams is predominantly mineral-core particles; however, POC fluxes are primarily organic-core particles. Furthermore, the traditional view of headwater POC as leaf detritus should be expanded to include algal-derived particles, even in forested headwater streams. Finally, I report a new method for measuring the quality of suspended POC from aquatic environments using heterotrophic respiration as a metric for lability. Heterotrophic respiration rates for suspended POC measured using this method were an order of magnitude higher than respiration rates previously reported for benthic POC.

The results from this research highlight the importance of seston for both headwater streams and as a longitudinal linkage of allochthonous and autochthonous organic matter to downstream ecosystems. The magnitude of this connection is controlled by the quality, quantity, and timing of POC delivery.