Freshwater Algae: Identification and Use as Bioindicators

3.4 RIVERS 1213.4 RiversUntil recently, monitoring of river water qualityin many countries (Kwandrans et al., 1998) wasbased mainly on Escherichia coli titre (sewage contamination)and chlorophyll-a concentration (trophicstatus). Chlorophyll-a classification of the FrenchNational Basin Network (RNB), for example, distinguishedfive water quality levels (Prygiel andCoste, 1996): normal (chlorophyll-a concentration≤10 µgml −1 ), moderate pollution (10–≤60), distinctpollution (60–≤120), severe pollution (120–≤300)and catastrophic pollution (>300).The use of microalgae as bioindicators was pioneeredby Patrick (Patrick et al., 1954) and has concentratedparticularly on benthic organisms, since therapid transit of phytoplankton with water flow meansthat these algae have little time to adapt to environmentalchanges at any point in the river system.In contrast, benthic algae (periphyton and biofilms)are permanently located at particular sites, integratingphysical and chemical characteristics over time,and are ideal for monitoring environmental quality.Examples of benthic algae present on rocks andstones within a fast-flowing stream are shown inFig. 2.23. The use of the periphyton communityfor biomonitoring normally involves either the entirecommunity, or one particular taxonomic group – thediatoms.3.4.1 The periphyton communityAnalysis of the entire periphyton community clearlygives a broader taxonomic assessment of the benthicalgal population compared to diatoms only,but the predominance of filamentous algae makesquantitative analysis difficult. Various authors haveused periphyton analysis to characterize water quality,including a study of fluvial lakes by Cattaneoet al. (1995 – Section 3.2.1). This showedthat epiphytic communities could be monitored bothin terms of size structure and taxonomic composition,leading to statistical resolution of physical(substrate) and water quality (mercury toxicity)parameters.3.4.2 River diatomsContemporary biomonitoring of river water qualityhas tended to concentrate on just one periphyton constituent– the diatoms. These have various advantagesas bioindicators – including predictable tolerancesto environmental variables, widespread occurrencewithin lotic systems, ease of counting and a speciesdiversity that permits a detailed evaluation of environmentalparameters. The major drawbacks to diatomsin this respect is the requirement for complexspecimen preparation and the need for expert identification.Attached diatoms can be found on a variety of substratesincluding sand, gravel, stones, rock, wood andaquatic macrophytes (Table 2.6). The composition ofthe communities that develop is in response to waterflow, natural water chemistry, eutrophication, toxicpollution and grazing.Various authors have proposed precise protocolsfor the collection, specimen preparation and numericalanalysis of benthic diatoms to ensure uniformityof water quality assessment (see below). More generalaspects of periphyton sampling are discussed inSection 2.10.Sample collectionThe sampling procedure proposed by Round (1993)involves collection of diatom samples from a reachof a river where there is a continuous flow of waterover stones. About five small stones (up to 10 cmin diameter) are taken from the river bed, avoidingthose covered with green algal or moss growths. Thediatom flora can be removed from the stones either inthe field or back in the laboratory. As an alternativeto natural communities, artificial substrates can beused to collect diatoms at selected sample sites. Theseovercome the heterogeneity of natural substrata andconsequently standardize comparisons between collectionsites, but presuppose that the full spectrum ofalgal species will grow on artificial media. Dela-Cruzet al. (2006) used this approach to sample diatomsin south-eastern Australian rivers, suspending glassslides in a sampling frame 0.5 m below the water surface.Slides were exposed over a 4-week period to