โ Scribed by Walton C. Poole; Kenneth W. Stewart
No coin nor oath required. For personal study only.
Vertical stratification samplers were developed for sampling the grave-sand substrate of a Brazos River, Texas riffle. Fifteen of 25 species recovered, occurred below Io cm. Mean percentages of total organisms recovered were 66.4%, 20%, 6.1% and 7.5% per lo cm level, respectively, from the surface down. Dominant insects were Neochoroterpes mexicanas naiads and chironomid, Simulium, Cheumatopsyche and Stenelmis larvae. Seasonal population peaks of these five groups in the top o cm correspond with observed emergence peaks. The smaller size classes were generally predominant in the o-Io cm level. Larvae of Stenelmis were the most evenly distributed among the various to cm levels in all size classes. A movement of Cheumatopsvche and Neochoroterpes to lower levels was observed following a large flood, suggesting an escape response to increased silt load and scouring.
Dissolved oxygen ranged from saturation at the surface down to 0.4-0.7 ppm at 30-40 cm, indicating that it was possibly limiting at lower levels. Maximum temperature difference between io cm levels was only 3 C. Flow was negligible below Io cm.
The vertical stratification sampler recovered significantly greater populations in the surface 20 cm, but not in the total 40 cm. than a modified Hess sq. ft. sampler.
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