Concrete lined urban streams and macroinvertebrates: a Hong Kong case study

In this study an urban stream network with earthen and concreted sections were studied for different seasons (wet and dry) to investigate the macroinvertebrate composition and the governing factors. The factors considered were water quality (nitrogen species, soluble reactive phosphorus etc.) and stream eco-hydraulics (velocity, wetted depth and width, vegetative indicators, number of mesoscale physical habitats, etc.). In contrast to common perception, results showed that concrete lined sites are not ecologically dead. Even though low, concrete line section had viable populations of macroinvertebrates and importantly a few native species. Interestingly, some macroinvertebrate indices of the concrete lined sites did not show a significant difference with the earthen sites. About 60% of the macroinvertebrates were grazers and filterers, and these two group populations always showed an inverse relationship. Earthen and concrete lined sites had similar diversities, but for different reasons. In general earthen sites diversity and pollution tolerance index of macroinvertebrates (PTI) was positively correlated, but no uniform correlation was observed in concrete lined sites. Some concrete lined sites with high diversity showed low PTIs (i.e. high diversity as a result of many types of pollution tolerant species) whereas in some the high diversity associated with high PTIs. Macroinvertebrate composition and the influencing factors were to a greater degree section dependent and to a lesser extent were dependent on the season. Physical heterogeneity played an important role in the macroinvertebrate responses in earthen sites. Water quality and flow rate explained comprehensively, the variations in the concrete lined sites. Results proved that nutrient levels need to be limited and instream heterogeneity needs to be improved to enhance diversity and populations of pollution intolerant species. Also, controlled vegetation harvesting is recommended in contrast to the current practice of complete removal for flow conveyance.