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Some Effects of Spring Snowmelt Runoff on Aquatic Invertebrate Populations in a High Mountain Stream


This study was an investigation of the abiotic and biotic factors acting on a community of benthic invertebrates in a high mountain stream during the spring snowmelt runoff period. All data used were collected in 1970. The study area was a 1 km section of Nash Fork Creek near 3,000 m in the Snowy Range, Wyoming.

The runoff began in mid-May when the discharge was 6 c.f.s. (.17 c.m.s.), rose erratically to a peak of June 25, at 125 c.f.s. (3.54 c.m.s.), then dropped through the rest of the summer. Water temperatures increased through the runoff from 0C. in mid-May to 4.5-11.5C. on June 25 and to 7-16C. by July 21. The general snow cover over the stream present in mid-May was gone by June 4.

For each invertebrate species in the study area which could be separated, population estimates were made, size ranges were recorded, and when possible, hatching and breeding periods were noted. Of 30 species for which the life cycle could be defined, 6 were non-seasonal, 12 were slow seasonal, and 12 were fast seasonal, based on the system of Hynes (1970).

The median values for the total standing crop biomass were 3,600 mg per m2 for June 1-4 and 5,980 mg per m2 for July 27-29. These values were not different at the .05 level using the Mann-Whitney u-statistic.

Drift samples were taken between the benthos sampling periods. From the drift samples of June 12-13, 17-18, and 22-24, drift rates for all invertebrates together increased with the increase in discharge. The same trend was found for the drift of oligochaetes and tipulid larvae considered separately.

In the daytime drift, the percentage of drifting oligochaete biomass which occurring as fragments was correlated with discharge (r = .9717).

The species which could be sorted with a live-sorting apparatus were considered as a group. The drift rates of this group increased with discharge through June 12-13 and 17-18 until the peak on June 22-24. They then decreased to lower levels on July 6-7 and 13-14, and finally increased, only at night, on July 21-22.

A net downstream loss of drifting invertebrates from the study area could not be confirmed.

A "Nocturnal Drift Index," which

Night Drift Rate - Day Drift Rate
Night Drift Rate + Day Drift Rate
was correlated with water temperature (positive correlation) and discharge (negative correlation). For the multiple regression of the index on water temperature and discharge the coefficient of determination was .86. This was interpreted as showing an increase in behavioral drift with increasing water temperature, but an inhibition of behavioral drift with high discharge.

Samples taken to determine the percentage of drifting dead invertebrates contained 11 percent dead invertebrates.

Ratios of drift rate/benthos density were measured for a few species. The species most highly modified to resist current had the lowest drift/benthos ratios.

Predation on drifting invertebrates by fish was assumed to be a source of mortality. Also the rapid growth of the net-spinning Arctopsyche sp. larvae was taken to indicate that it was a very effective predator on drift during the runoff.

Adjustment of the life cycle was found to be a common means of adaptation to the runoff and was manifested in several ways:

  1. The Diptera avoided exposing pupae to the peak of the runoff.
  2. Many insect species were found to breed during the runoff. Of 30 selected species, 18 did breed, 4 (all fast seasonal species) were presumed not to breed, and the breeding period of 8 was not known.
  3. Prolonged hatching of eggs and avoidance of hatching were recognized as runoff-survival strategies. Of 48 selected, species, 5 were hatching only in the rising (June 1-24) phase, 6 only in the falling (July 6-29) phase, 21 species hatched in both phases, and 16 species did not hatch in either phase.

The coexistence of very similar congeneric species in the general Baetis, Alloperia, Nemoura, Paraleuctra, and Rhyacophila, was hypothesized as possible because the congeners may have different runoff-survival strategies which vary in effectiveness due to the large yearly differences in the timing and severity of the runoff.

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