Elliott, Scott. 2000. The impact of riparian disturbance on physicochemical characteristics of seston. M.S.
Suspended particulate matter (seston) plays an important role in the trophic ecology of forested streams. Forest management in and along the riparian corridor alters existing levels of allochthonous and autochthonous contributions to the lotic system, ultimately changing various physicochemical characteristics of seston. This thesis is divided into two chapters, the first providing a subject background on seston dynamics and the second detailing the results of my study of physicochemical parameters of seston at summer baseflow. In this study, my objectives were to describe the effects of riparian condition on physicochemical parameters of seston at summer baseflow, to assess the utility of seston analysis as an indicator of riparian recovery, and to develop a conceptual model of baseflow seston dynamics following logging disturbance in the riparian corridor. To achieve this, I sampled seston at summer baseflow from 19 streams on the Olympic Peninsula of Washington state. The riparian condition of each stream was characterized by harvest history and other parameters to determine the successional condition of the canopy. I analyzed total seston concentration, percent organic matter, chlorophyll-a concentration, and the carbon to nitrogen (C:N) ratio at baseflow discharge, to determine trends related to riparian condition. Seston was visually characterized using the scanning electron microscope (SEM). Consistent with other studies, summer baseflow seston concentrations and percent of organic matter were not correlated with any single or combination of riparian factors, consistent with other studies. Chlorophyll-a concentrations were generally low (<1.0 µg Chl-a·L¹), and were found to be correlated with canopy closure (p<0.01, r² = 0.45), particularly when streams of 1st and 2nd order were considered alone (p<0.01, r² = 0.64). C:N ratios ranged from 7.6 to 25.6, with average values at coniferous sites significantly higher than those at alder-dominated deciduous sites (p<0.01, ANOVA) or at sites with no appreciable riparian cover (p<0.02, ANOVA). Visually, diatoms were a significant proportion of the total seston at each riparian cover type (range = 47-66% of the number of particles), although significantly higher concentrations of diatoms were found in sites lacking cover. Wood particles were not well-represented (range 2-4% of seston particles) and concentrations did not vary among site cover types nor by any other potentially related parameter. Visual characterization of seston using the Shannon-Wiener diversity index (H') for each site, revealed a greater diversity of particles (by type and size) in the mature coniferous sites (p<0.01, ANOVA). Physicochemical parameters of seston at summer baseflow, specifically those which are directly influenced by riparian characteristics, could prove valuable as a tool for assessing riparian integrity.