Identification of Stream Drift Mechanisms: An Experimental and Observational Approach

Abstract

Laboratory experiments were conducted with the periphyton—grazing mayfly Baetis tricaudatus to test the hypothesis that stream drift results from the passive removal of individuals from substrates. This hypothesis accounts for entry into the water column and diel drift periodicities, based on changes in the positioning of individuals on substrates and changes in their activity rates. A factorial design that incorporated food abundance (low or high), food distribution (low, intermediate, or high degree of patchiness), and forager hunger level (starved or well fed) was used to produce contrasting diel behavioral patterns necessary to test the hypothesis. Nymph behavior was monitored during the day and night using time—lapse cinematography and direct visual observations. Nymphs fed ad libitum had greater proportions of individuals on top surfaces at night that during the day. Starved nymphs showed little day—night variation in activity or positioning behavior, and their behavior during the day was similar to that of fed nymphs at night. Therefore, if drift—entry was passive, diurnal drift of starved nymphs should have been greater than or equal to nocturnal drift of fed nymphs in comparisons made within food levels. In all cases this prediction was not supported. These results refute the passive drift—entry hypothesis and support the alternative hypothesis of active entry into the water column. Drift patterns and foraging behaviors in these experiments and in an additional set of experiments were analyzed to identify proximate factors responsible for active drift—entry by Baetis. Drift was greatest at night and at the low food level. Periphyton patchiness and nymph hunger level did not significantly affect drift in the factorial experiments. In additional experiments, over a wide range of food abundance (2.5 x 105 to 107 diatom cells/cm2), the proportion drifting at night was significantly greater than zero when food was patchily distributed on substrates, but not when food was uniformly distributed. Based on these results and behavioral observations suggesting that food patches were less readily encountered with increasin degree of patchiness, I suggest that active drift is a component of Baetis food searching behavior and that individuals abandon habitats by enterin the water column when their estimate of habitat quality falls to some threshold level.

Causes and consequences of invertebrate drift in running waters: from individuals to populations and trophic fluxes

“…Active drift, in contrast, results from deliberate behaviours, including benthic predator avoidance (Peckarsky 1980;Malmqvist and Sjostrom 1987;Kratz 1996;Huhta et al 2000), active patch selection while foraging (Hildebrand 1974;Kohler 1985), or escape from unfavourable abiotic conditions (Lauridsen and Friberg 2005;Gibbins et al 2007b;James et al 2009;Larsen and Ormerod 2010). Density dependence may also increase drift entry owing to increased competition for space (Corkum 1978;Hildrew and Townsend 1980;Kohler 1992) or resource limitation (Dimond 1967;Richardson 1991;Fonseca and Hart 1996;Rowe and Richardson 2001;Siler et al 2001).…”

The influence of recruitment on within-generation population dynamics of a mayfly

“…The specific mechanisms explaining the pattern consistent with post-recruitment densitydependent losses of these mayflies in both types of streams are unresolved, but potentially include dispersal or mortality caused by predation, crowding, food limitation or competition for space, as have been demonstrated in other systems (Kohler 1985, Forrester 1990, Morse 1993, Fonseca and Hart 1996, Wilson and Osenberg 2002. While mermithid parasites have been reported to cause mortality in early instar B. bicaudatus, rates of infection did not increase with population density (Vance and Peckarsky 1996).…”

“…Initial population sizes are established when eggs are oviposited in streams by winged adult females (Elliott and Humpesch 1980). Subsequently, larvae drift downstream in search of food (Kohler 1985), via fluid mediated dispersal (Fonseca 1999) often to avoid predation (Peckarsky 1996).…”

An Irrigation Canal as a Lotic Mesocosm: Examining the Relationship between Macroinvertebrate Benthos and Drift

“…He found that only after the benthos reached a "recovered" density (compared to pre-insecticide levels) did drift become significant. Others, too, have suggested that drift may be a densitydependent mechanism of the benthos (Kohler 1985, Anholt 1995, Turner and Williams 2000.…”