Faunal relationships to abiotic factors along the River Dan in northern Israel

Abstract

A 4.5 km section of the River Dan in northern Israel was sampled at intervals of 6 weeks, between April 1983-March 1984, using standardized mesh bags. The river is characterized by extremely stable temperature and other physical and chemical conditions. The effects of a water diversion project 2400 m from the spring were investigated.The relationships between fauna and depth, current velocity and distance from the spring sources were evaluated. Of 48 common taxa analyzed, 27 showed relativity to the distance from the sources, 18 to current velocity and 12 to depth. The lowest density of invertebrates, but the highest number of taxa and highest variety of species were found at the spring sources. The sampling site immediately downstream from the water diversion project was characterized by intermediate densities, lowest varieties of species and highest evenness of distribution.Results are compared with the predictions of the Intermediate Disturbance Hypothesis; but the influence of invertebrate drift caused this hypothesis to be inapplicable to lotic waters. Invertebrate drift is in addition suggested as an important factor governing the structure of the fauna at the spring sources.

Relationship between current velocity, depth and the invertebrate community in a stable river system

“…The velocity of the water, combined with other parameters, such as depth, availability of food, light intensity, plant distribution, chemical composition of water, dissolved oxygen and dissolved materials, influences species diversity and size. These parameters create various and varied niches (Hynes, 1970;Statzner & Higler, 1986;Degani et al, 1989Degani et al, , 1992. In many situations it is not only the absolute value of an abiotic factor that influences distribution, but also its periodicity and variability, i.e.…”

“…Monthly discharge varies less than 20%) and owing to its extremely small catchment area (8 km2), flooding contributes less than 1 y0 to annual discharge. Preliminary work (Por et al, 1986;Allan et al, 1988;Degani et al, 1987Degani et al, , 1989Degani et al, , 1992 has shown that annual as well as diurnal water temperatures remain virtually constant (15.5 "C kO.5"). In addition, dissolved oxygen concentration remains near saturation.…”

“…A previous study in our laboratories (Degani et al, 1992) examined, in particular, the relationships of fauna to abiotic factors along the Dan River, but other parameters were also examined. One section of the river was selected, in order to gain a better understanding of niche selection by the fauna.…”

“…Annual temperature variation was 0.5 "C, dissolved oxygen was uniformly high, and conductivity varied from 0.30-0.36 mS cm-i at 25 "C. During the period of the study (Dec. 1982-March 1984 river discharge varied by approximately 20%. No flood events were observed: changes in the river are mainly annual, between summer and winter (Degani et al, 1992).…”

“…The weight of the gravel held them down. The size and shape of the cages, the type of filler (gravel size, natural vs introduced substrates) and the period of case incubation were determined from preliminary experiments, conducted in the river during the autumn of 1982 (Degani et al, 1992). The cage design allowed for maximum colonization rate and species richness, and retained the smallest number of invertebrates upon removal from the river, with subsequent shaking (2-7 y0 retention, mostly Chironomidae and Simulium spp.).…”