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The spatial distribution of benthic (up to 0.05 m depth) and hyporheic (0.25 and 0.5 m depth) macroinvertebrates from downwelling zones at the heads of riffles and upwelling zones at the tails of riffles was examined in two studies on a 4th order chalk stream in Dorset, England. In the first study, differences in benthic and hyporheic macroinvertebrate community composition between the head and tail of a single riffle were investigated. In the second study, a replicated design involving eight riffles was used to compare benthic and hyporheic macroinvertebrate community composition both between heads and tails of the same riffles and between riffles. In the first (single riffle) study there were significantly higher mean numbers of benthic invertebrates and families at the riffle head (715 individuals and 13.8 families per 0.0225 m2) compared to the tail (192 individuals and 8.7 families). ANOSIM analysis also showed that the community structure of head and tail benthic samples was significantly different. In the second (replicated riffle) study, there were also significantly more benthic invertebrates at riffle heads (\(\bar{x}\) = 594 per 0.0225 m2) compared to tails (\(\bar{x}\) = 417 per 0.0225 m2), although this was not the case for families, and community structure also differed significantly between riffle heads and tails. In contrast, in the hyporheic zone, there were no significant differences between the total numbers of invertebrates in the riffle heads and tails, or between riffles, although a significant difference in family richness between riffle head and tail samples was identified in the first study. Community analysis revealed progressively poorer separation of riffle head and tail samples at 0.25 m and 0.5 m hyporheic depths. Whilst being able to identify clear differences in benthic communities from riffles heads and tails, the physically heterogeneous nature of the riffle habitats studied made it difficult to account for the consistent differences in macroinvertebrate communities observed with the physical variables measured.

The effectiveness of four aquatic plants including water hyacinth (Eichornia crassipes), water lettuce (Pistia stratiotes), zebra rush (Scirpus tabernaemontani) and taro (Colocasia esculenta) were evaluated for their capabilities in removing mercury from water. The plants were exposed to concentrations of 0 mg/L, 0.5 mg/L or 2 mg/L of mercury for 30 days. Assays were conducted using both Microtox (water) and cold vapor Atomic Absorption Spectroscopy (AAS) (roots and water). The Microtox results indicated that the mercury induced acute toxicity had been removed from the water. AAS confirmed an increase of mercury within the plant root tissue and a corresponding decrease of mercury in the water. All species of plants appeared to reduce mercury concentrations in the water via root uptake and accumulation. Water lettuce and water hyacinth appeared to be the most effective, followed by taro and zebra rush, respectively.