Population Persistence in Rivers and Estuaries

Abstract

A wide variety of organisms inhabit streams, rivers, and estuaries where they are continually subjected to downstream drift. It is well known that when this is the only transport process, extinction is inevitable (the ''drift paradox''). Using a series of analytical and numerical models, representing a range of hydrodynamic scenarios, we demonstrate that the action of diffusive dispersal can permit persistence in an advective environment. The mechanism underlying this phenomenon is that diffusive dispersal can allow a proportion of the population to reproduce close to their natal location. For well-and poorly mixed non-tidal systems we establish approximate analytic conditions for diffusionmediated persistence both throughout the water column and in a benthic boundary layer. Although tidal forcing results in residual landward flow near the base of the water column, we find that this has little effect on persistence, which is respectably approximated by our analytic results. We apply these analytic results to four hydrodynamically disparate systems: a stream (Broadstone Stream [UK]), a river (Christiana Creek [USA]), a shallow estuary (Ythan [UK]) and a deep fast-flowing estuary (Saco River [USA]). Using parameters derived from published studies we examine the persistence of a number of real and hypothetical organisms in these systems and identify those for which diffusively mediated persistence is a realistic possibility. We note that such persistence is only likely when advection is low or horizontal dispersal is high.

Development of an in-stream migration model for Gammarus pulex L. (Crustacea, Amphipoda) as a tool in river restoration management

“…But how does one explain the persistence of the many species that are commonly found to drift but do not have an aerial adult stage (Humphries and Ruxton 2002)? Speirs and Gurney (2001) and Humphries and Ruxton (2002) demonstrated that long-distance flight by aerial adults is not required to prevent extinction of upstream reaches. It can be achieved by very small upstream movements of individuals along the substrate.…”

Microfaunal communities in three lowland rivers under differing flow regimes

“…The downstream movement of microfauna in Australia has not been investigated but has been recorded from many other systems. Drift is a characteristic of many invertebrate taxa and allows animals to recolonise newly inundated slackwaters following periods of high flows (Speirs & Gurney, 2001). Studies of microfaunal drift have suggested that animals found regularly in the drift are not likely to be benthic in origin (Campbell, 2002) but may be derived from the plankton of still water zones along the river edge (Braioni, 1981).…”

Effects of Heterogeneity on Spread and Persistence in Rivers

“…Recent modeling efforts for the drift paradox identified a "critical flow speed" below which a population can persist on a long enough stretch of a river and also spread upstream in form of a moving front (Speirs and Gurney, 2001;Lutscher et al, 2005;Pachepsky et al, 2005). These models neglect spatial heterogeneities and assume a homogeneous habitat.…”