The spiral does not end with uptake: The turnover length may be more important than we thought

Many measurements of nutrient spiraling in streams have estimated the uptake length, S_W, or average downstream transit distance prior to uptake. In contrast, the turnover length, S_B, which is the downstream distance traveled between uptake and mineralization, i.e., in organic form, whether living or nonliving, has received scant attention. Early work indicated that S_B<<S_W, and hence contributed little to total spiraling length, S= S_W + S_B.  This, together with measurement challenges, may explain its neglect.  It may time, however, for a second look.  Nutrient that travels downstream as part of the turnover length—in particles, drifting organisms, or dissolved organic matter—is unavailable for uptake.  Thus, for a limiting nutrient, longer turnover length may translate into reduced productivity, and the transport of unavailable nutrient may play a regulatory role.  The early evidence for a short turnover length was based largely on a short-term tracer experiment at steady state.  Here we review evidence—from modeling, from seasonal and episodic variations in nutrient transport, and from more recent tracer experiments—that long-run average turnover length, involving time-varying rates of nutrient accumulation and transport, may, in fact, be substantial.  We further suggest that the stoichiometry of particulate transport may strongly affect ratios of S_W / S_B and hence the relative availability of competing nutrients.