Responses of epibenthic algal assemblages to water abstraction in Hong Kong streams

We studied the effects of flow reduction on epibenthic algal assemblages by comparing up- and down-stream reaches of ten Hong Kong streams subject to different degrees of water abstraction during 2007-2008. Downstream discharge declined by 71 and 54% during the wet and dry seasons, respectively. Algal responses varied seasonally and according to morphological guild and reflecting reductions in discharge and current velocity, or changes in total nitrogen and phosphate, or their combined effects. Significant inter-reach assemblage differences were observed during the both seasons, but wet-season assemblage structure was not directly influenced by any flow-related variable. During the dry season, scouring-tolerant prostrate/adnate diatoms (especially Achnanthes, Cocconeis) were relatively abundant upstream, whereas intolerant stalked (Eunotia, Gomphonema) and mobile diatoms were more numerous downstream. Both low- and high-profile (erect diatoms, Chamaesiphon, Calothrix) guilds were sensitive to changes in nutrients. Low-flow conditions downstream, surprisingly, enhanced algal diversity, and richness. Based on the linear response of Cocconeis (mainly C. placentula) to discharge reduction, we recommend that > 65% of dry-season discharge should be maintained in downstream reaches in order to sustain near-natural algal assemblages; 32% of discharge is required to avoid substantial alterations in assemblage structure.