Disturbance frequency influences patch dynamics in stream benthic algal communities

Abstract

Disturbance is integral to the organisation of riverine ecosystems. 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 days) at high (33-day cycles) and low frequencies (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 mid-late 2000 to the exploitation of open space by closely adhering mats in 2001. 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.

Climate change and freshwater ecosystems: impacts across multiple levels of organization

“…In the absence of disturbance, epilithic space was dominated by the green encrusting alga, Gongrosira incrustans. However, droughts consistently reduced the dominance of the green alga, and crust abundance decreased, opening up space for a diversity of mat-forming diatoms (Ledger et al 2008). The invertebrate community structure also shifted in the most stressed treatments relative to the controls, with a decrease in large, rare species higher in the food web.…”

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

“…The strength of grazing pressure upon epibenthic algae can vary according to flow conditions (Poff & Ward, 1995;Chester & Norris, 2006), potentially influencing the intensity of topdown control of assemblage composition and biomass (Opsahl et al, 2003). Because of the complex interactions among abiotic factors, such as discharge and nutrients, and biotic factors (Peterson & Stevenson, 1992;Ledger et al, 2008), the effects of flow on epibenthic algal assemblages vary substantially according to stream conditions (Jowett & Duncan, 1990;Downes et al, 2003;Wu et al, 2010).…”

Quantifying biostabilisation effects of biofilm-secreted and synthetic extracellular polymeric substances (EPS) on sandy substrate

“…The diverse flora was 15 dominated by five main species: a) Nitzschia pellucida, b) Nitzschia sigma, c) Mastogloia sp, d) Navicula perminuta, and e) Amphora pediculus. The Nitzschia species are considered early colonisers (Ledger et al 2008;Ros, Marín-Murcia, and Aboal 2009), and were indeed found primarily in the samples of the early stages of the experiment. Furthermore, all taxa were benthic rather than planktonic, as expected in lotic conditions (Passy 2001;Schmidt et al 2016).…”