Colonisation and community structure of benthic diatoms on artificial substrates following a major flood event: A case of the Kowie River (Eastern Cape, South Africa)

T Dalu,Nb Richoux,Pw Froneman,Ld Chari

doi:10.4314/wsa.v40i3.10

Abstract

A major flooding event that occurred during October–November 2012 caused major changes in the Kowie River hydromorphology and aquatic communities. The aim of our study was to identify the environmental variables that structure riverine benthic diatom communities at upstream and downstream locations 25 km apart on the Kowie River, South Africa. This was undertaken using tiles as artificial substrates so that we could study how the communities developed after the flood disturbance. The diatom community structure was assessed over a 28-day period following a flood event in October 2012. The Mann Whitney test indicated that there was a statistically significant difference (p < 0.05) in total dissolved solids, salinity, pH and oxygen reduction potential between the two sites. In total, 58 diatom species belonging to 30 genera were identified over the 28-day study. Achnanthidium minutissimum, Fragilaria biceps, F. ulna var. acus, Pinnularia borealis and P. acrosphaeria were the most numerically dominant on Day 7 and were considered as early colonisers, while on Day 28, Achnathidium minutissimum, F. capucina, Craticula buderi, C. vixnegligenda, Diploneis subovalis and Gomphonema venusta, the late colonisers, were dominant. The species richness increased from 13 (upstream location) on Day 7 to 22 (both locations) by Day 21. A redundancy analysis showed that total suspended solids, salinity, resistivity, pH and oxygen reduction potential were the most significant physico-chemical variables explaining diatom composition. The results from this relatively small-scale tile experiment indicate the complexity of freshwater benthic diatom community structure and development.Keywords: water flow, total dissolved solids, Fragilaria sp., tile substrate, resistivity

Highlights

Numerous studies have been conducted on the seasonal dynamics of diatom communities in lotic and lentic ecosystems (Ruth, 1977; Gamier et al, 1995; Ha et al, 1998; Lane 2003; Claquin et al, 2006; Wojtal and Sobczyk, 2012), with diatom community structure periodicity being largely described more in lentic than lotic systems (Gamier et al, 1995; Claquin et al, 2006)
Benthic diatoms are dominant contributors to primary production in rivers and streams (Claquin et al, 2006) and are an important food source for many invertebrates (Lane, 2003), we know surprisingly little about their community structure and dynamics
Along the Kowie River, the upstream location had significantly decreased conductivity, TDS, pH, water flow, salinity, temperature, phosphates and DO due to the pristine nature of the site and minimal human influences compared to the downstream location, which was influenced by agricultural activities and wastewater discharge

Summary

Introduction

Numerous studies have been conducted on the seasonal dynamics of diatom communities in lotic and lentic ecosystems (Ruth, 1977; Gamier et al, 1995; Ha et al, 1998; Lane 2003; Claquin et al, 2006; Wojtal and Sobczyk, 2012), with diatom community structure periodicity being largely described more in lentic than lotic systems (Gamier et al, 1995; Claquin et al, 2006). Benthic diatoms are dominant contributors to primary production in rivers and streams (Claquin et al, 2006) and are an important food source for many invertebrates (Lane, 2003), we know surprisingly little about their community structure and dynamics. Variation in river discharge as a result of freshwater input has a significant influence on primary production by benthic diatoms as shifts in flow affect diatom community composition (Stevenson, 1990). An increase in water flow (current) can positively affect the benthic algal community by increasing turbulent flux and the transport of nutrients to individual algal cells, thereby stimulating their metabolism (Miller et al, 1987; Ács and Kiss, 1993; Wehr and Sheath, 2003; Song 2007). Flood effects on benthic diatoms increase with flood intensity, with effects ranging from scouring most algae from substrates to having little effect on standing crops (Stevenson, 1990; Steinman and Mclntire, 1990)

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