Biomass and Drift of the River Macroinvertebrates

Drift flux per unit bottom area (DF) of different river reaches can be expediently estimated as biomass of transported organisms at a particular collection time through the cross sectional area of water flow for width of 1 m and height equals to the depth of the river part (H, m) at the time of sampling (DF, g/m²/day). This method allows one to calculate specific drift rate (SDR), which takes into account benthic density on the river bottom and is estimated with units of time^-1, i.e., the fraction of benthic biomass that enters the drift in 24 hours. So SDR = DF/B, where B is biomass of benthos, g/m². The drift flux (DF) and the specific drift rate (SDR) calculated for various groups of invertebrates showed a positive correlation between DF and B, but a negative correlation between SDR and B. Thus, our results contradict the generally accepted hypothesis describing drift as competition among aquatic organisms for food and substratum.

 Considering the drift of benthos in the investigated river reach being a result of distribution of the whole upstream benthic invertebrates productivity (BP, g/m²/day), than the share of benthic invertebrates productivity (SBP, %) removed by drift benthic organisms from the river community (DB, g/m²/day) during a certain time period would be calculated as: SBP = (DB/BP) 100%. Parameter SBP for amphipods and mayflies larvae – over a 24 h period between floods – amounts to an insignificant part of the daily productivity of population (1-8%). And decreases further as advancing away from the river source, while increasing parameter DB considerably. During the floods, the daily value of SBP intermittently increases, and the multiple exceeds the value of SP, which results in the obvious decrease of the hydrobionts biomass on the river bottom.