Abstract
The bioturbation activity of macroinvertebrates can affect the level of water exchange across the sediment–water interface. The impact of tubificid worm with different densities on the vertical water exchange at the sediment–water interface was investigated based on laboratory flume experiments. Vertical water fluxes, as well as physiochemical parameters, were measured at seven-day intervals, and the maximum penetration depths were obtained by dye injection before and after the tubificid bioturbation experiment, respectively. The bioturbation effects can be summarized in two aspects: (1) when the density was less than (or equal to) 20 individual/10 cm2, the volume of vertical water exchange positively correlated with the tubificid bioturbation. Once the density exceeded (or equaled) 25 individual/10 cm2, the vertical water flux decreased with increasing tubificid bioturbation. After 14 to 21 days, a negative correlation was identified between the bioturbation and the vertical water flux under all biological densities. (2) The maximum depth that the surface water can penetrate the sediment increased with increasing tubificid density. These results revealed that the vertical water was closely related to the biological density. The study has certain reference significance to understanding the spatiotemporal heterogeneity of hyporheic water exchange on a local scale.
Highlights
Among the factors that affect the size and activity of hyporheic zone, hyporheic water exchange is of particular importance, controlling the transfer of heat, the fate of contaminants, the nutrient cycling and the distribution of organic matter between stream and groundwater [1,2]
On day 0, tubificid worms were added into flume 1 to flume 6, spontaneously dispersed on sediment surface and dug into the sediment, which may be a migration behavior to ensure sufficient living space and food
At the end of the experiment, plenty of lateral galleries appeared in the surficial sediments of flume 1 to flume 4 with low-density (Figure 3a,b), while fewer lateral channels were formed in flume 5 and flume 6 with high-density, and part of the worms became vertically dwelling as a cluster in the sediment (Figure 3c,d)
Summary
Among the factors that affect the size and activity of hyporheic zone, hyporheic water exchange is of particular importance, controlling the transfer of heat, the fate of contaminants, the nutrient cycling and the distribution of organic matter between stream and groundwater [1,2]. As an active moving component in the hyporheic sediment, pore water dynamically exchanges with the surface water [3]. Frequent water exchange between the stream and hyporheic sediment can further influence the metabolism and biogeochemical processes near the sediment–water interface (SWI) [4]. The hydrological exchange allows the upwelling pore water to bring some primary producers (i.e., benthic plants and autotrophic bacteria) into the river, which produce organic matter, while the downwelling of surface water enters the sediment, transferring nutrients and. The intensity and size of water exchange occurring at the SWI are closely related to the ecological health of the river
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