Thursday, May 29, 2008 - 4:30 PM
595

The frequency of drought disturbance influences the structure and dynamics of stream benthic algal communities

Mark E. Ledger1, Rebecca M.L. Harris1, Patrick D. Armitage2, and Sandy Milner1. (1) School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, United Kingdom, (2) Centre for Ecology and Hydrology, c/o Freshwater Biological Association, East Stoke, Wareham, Dorset, United Kingdom

Fluctuating low flows caused by supra-seasonal drought and water management periodically dewater habitat patches, potentially creating heterogeneity in the taxonomic composition and successional dynamics of benthic communities. The frequency of disturbance induced by low flows is contingent upon the topography of the river bed and thus varies among patches. We investigated whether the frequency of patch dewatering influenced the structure and temporal dynamics of benthic algal communities attached to the upper surfaces of stones in stream mesocosms (4 m2). In a 693-day disturbance experiment, we applied short dewatering disturbances (6-d) at high (33 day cycles) and low frequency (99 day cycles) and compared algal assemblages with undisturbed controls at 21 endpoints. In the absence of disturbance, epilithic space was dominated by the green encrusting alga Gongrosira incrustans. However, drying disturbances consistently reduced the dominance of the green alga, and crust abundance decreased with increasing disturbance frequency, thereby opening space for a diversity of mat-forming diatoms. The response of mat diatoms to disturbance varied markedly during the experiment, from strong reductions in the abundance of loosely attached mats in the first year of the experiment, to exploitation of open space by closely adhering mats in the second year. Contrary responses were attributed to changes in the species composition of mat diatoms which influenced the physiognomy and hence stress-resistance and resilience of the assemblage. Our results indicate that patchy dewatering of habitat patches during periods of low flow influences the successional dynamics of algae, thereby creating distinctive mosaics on the stream bed.


Web Page: Drought, algae, community structure

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