Response of grazing impacts of an algivorous fish

Response of grazing impacts of an algivorous...

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Response of grazing impacts of an algivorous Fsh ( Pseudogastromyzon myersi : Balitoridae) to seasonal disturbance in Hong Kong streams GRACE Y. YANG AND DAVID DUDGEON Division of Ecology & Biodiversity, School of Biological Sciences, The University of Hong Kong, Hong Kong, China SUMMARY 1. Manipulative experiments were carried out in four Hong Kong streams (two shaded, two unshaded) to investigate the impact of grazing by an algivorous fish, Pseudogastr- omyzon myersi , on benthic algal biomass and assemblage composition. Experiments were conducted and repeated during both the dry and wet seasons to determine whether spate-induced disturbance modiFed any grazing effect. Treatments comprised Fsh exclusion and inclusion via closed and open cages, with a no-cage treatment used as a control for the cage effect. Treatments were maintained for 4 weeks in each experimental run. 2. Grazing by P. myersi reduced benthic algal biomass and the organic matter content of periphyton in open cages and the no-cage treatment relative to closed cages. The similarity between open-cage and no-cage treatments was evidence that the overall difference among treatments was caused by limiting Fsh access to closed cages and not merely an artifact of caging. Grazing effects were broadly similar in all streams, but there was a signiFcant statistical interaction between treatments and seasons. 3. Analysis of dry-season data matched the overall trend in inter-treatment differences, confirming the effects of grazing by P. myersi on algal biomass and periphyton organic matter. SigniFcant differences in algal assemblage composition between closed-cage and no-cage treatments during the dry season re±ected reductions in the abundance of erect, stalked diatoms ( Gomphonema ) and Flamentous cyanobacteria ( Homeothrix ). Removal of these vulnerable overstorey algae by P. myersi resulted in greater abundance of understorey diatoms ( Achnanthes and Cocconeis ) in the no-cage treatment in all streams during the dry season. The composition of algal assemblages in open cages was intermediate between the other two treatments. 4. Although fish densities were greater in all streams during the wet season, spate- induced disturbance obscured grazing effects and there were no significant differences among treatments attributable to fish grazing. Seasonal variation in impacts of P. myersi grazing provides support for the harsh-benign hypothesis, and conFrms that biotic factors are less important controls of stream algal biomass and assemblage structure during periods (i.e. the wet season in Hong Kong) when abiotic disturbances are frequent or intense. Keywords : cyanobacteria, diatoms, loach, periphyton, spates, tropical Correspondence: David Dudgeon, Division of Ecology & Biodiversity, School of Biological Sciences, The University of Hong Kong, Hong Kong, China. E-mail: [email protected] Freshwater Biology (2010) 55, 411–423 doi:10.1111/j.1365-2427.2009.02290.x ȑ 2009 Blackwell Publishing Ltd 411
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Introduction Grazers have multiple effects on benthic algae (Steinman, 1996). The presence of grazers often causes
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This note was uploaded on 04/30/2011 for the course EEMB 142C taught by Professor Alldredge/osborn during the Spring '08 term at UCSB.

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Response of grazing impacts of an algivorous...

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