Patterns in periphyton biomass and community structure in foothill rivers: A comparison between winter and summer rainfall regions

[摘要] ENGLISH ABSTRACT: South Africa has a semi-arid climate with seasonal rainfall whose runoff is jeopardised by high rates ofevapotranspiration. These conditions decrease the ability of rivers to dilute point and non-point sourcesof pollution, which leads to enrichment and the subsequent eutrophication of water bodies.Eutrophication occurs when periphyton communities proliferate through a shift in community structure,accompanied by greater biomass accrual that deteriorates water quality and impairs aquatic ecosystemfunctioning. The National Eutrophication Monitoring Programme (NEMP) has set water qualityguidelines for concentrations of phosphorous and Benthic chlorophyll a, but periphyton areunderstudied in South African waters, which explains their exclusion from the River Eco-StatusMonitoring programme (REMP). To underpin the causes of eutrophication, this study aimed to broadenthe knowledge on periphyton-environmental relationships in terms of patterns in peripyton biomass andcommunity structure on a regional scale across seasons in a winter (Western Cape) and summer(KwaZulu-Natal) rainfall region. Periphyton biomass and community structure were observed acrossflow and enrichment categories and a suite of environmental metrics comprising flow, nutrients, watertemperature, sunlight and macroinvertebrates. An in situ tool known as the Benthotorch® wasvalidated to potentially be used in future rapid assessments of trophic status. Periphyton samples fromsites representing a range in environmental conditions were collected in autumn and spring which markthe beginning and end of the periphyton growth seasons. Periphyton biomass in the Western Cape wasfound to be influenced predominantly by the availability of TIN in autumn and WTMAX in spring. InKwaZulu-Natal, periphyton biomass was influenced mostly by flow metrics and WTCV. Periphytoncommunity structure in the Western Cape was influenced mostly by TIN and the length of the growingseason in autumn and by EC and the duration of class 1 floods in spring. In KwaZulu-Natal, periphytoncommunity structure was influenced mostly by water temperature and flow metrics in autumn and bythe length of the growing season (Since≥2), the duration of class 2 floods and PO4-P in spring. The flowregime is regarded as the primary regulator of flood prone rivers, which was not the case in this study,and calls for future research. Nutrients accounted minimally towards spatial variation in periphytoncommunities in KwaZulu-Natal possibly due to sites with similar nutrient ranges, or because nutrientsare not a key driver of periphyton communities here. The importance of water temperature metrics inboth regions stresses the need for water temperature monitoring programmes, that are currently lackingin South Africa. The Benthotorch® estimated periphyton biomass and community structure moreaccurately at sites that were dominated by diatoms, overestimated cyanobacteria and did notconsistently recognize green algae.

[发布机构] Stellenbosch University