Freshwater Algae: Identification and Use as Bioindicators

3.5 ESTUARIES 135potential role of epipelic diatoms as monitors of localporewater salinity/nutrient concentrations has beennoted above, and diatom bioindicators of coastal wetlandsalination were discussed in Section 3.3.Detection of large-scale eutrophication is a morepressing and general problem, since it is a feature ofalmost all estuaries. Nutrient increases are acceleratingwith rising levels of human population, and haveimportant economic impacts in relation to fisheries.The use of algae as bioindicators of general estuarineeutrophication is considered in relation to two mainaspects: monitoring algal populations using pigmentconcentrations and molecular detection of freshwaterspecies in river plumes.Eutrophication: analysis of pigmentconcentrationsAnalysis of algal populations as bioindicators in estuarineand related coastal systems typically involvesmonitoring a large area of water surface. The speedwith which environmental changes can occur alsomeans that relatively frequent sampling may be necessary.Because of these requirements, many estuarinemonitoring programmes have combined rapidsample analysis (using pigments as diagnostic markers)with automated collection (from fixed sites or byremote sensing).The combined use of HPLC (pigment separation),spectrophotometry (pigment quantitation) and a matrixfactorization program such as ChemTax R○ (calculationof biomass proportions) is described in Section2.3.3 and provides a speedy and accurate assessmentof the algal community. Although pigment analysisonly resolves phytoplankton communities to the majorgroup (phylum) level, the results obtained giveclear information on the algal response to environmentalchange and enable these organisms to be usedas bioindicators.The use of HPLC/ChemTax R○ analysis is reportedby Paerl et al. (2005) for various southern UnitedStates estuarine systems (Table 3.15), including theNeuse River Estuary (North Carolina), Galveston Bay(Texas) and St. Johns River (Florida). In all of thesecases, eutrophication occurs due to flushing of highvolumes of nutrient-rich freshwater into the lowerreaches of the estuary and into the surrounding ocean.The source of nutrients primarily results from agricultural,urban and industrial activities in the watershed,and flushing occurs due to high rainfall – which maybe seasonal (summer wet season) or episodic (hurricaneand storm effects).Neuse River Estuary This aquatic system is subjectto alternations of high river discharge (seasonaland hurricane rainfall) with periods of low discharge(no rainfall). Algal bioindicators indicate phases ofeutrophication resulting from high freshwater flowin relation to biomass increase (up to 19 µg chl-al −1 ) and a switch in phytoplankton populations –promoting growth of chlorophytes and diatoms, butreducing levels of blue-green algae and dinoflagellates.This apparent reversal of the normal effects ofeutrophication is due to the conditions of high waterflow (reduced residence time), where algae with rapidgrowth and short generation time (r-selected organisms)are able to dominate more slow growing algae(K-selected organisms).Galveston Bay Eutrophication resulting fromtropical storms in 2000 led to increased algal growthin the bay, with HPLC analysis indicating growthsparticularly of cryptophytes and peridinin-containingdinoflagellates. The location of the algal bloomsoverlapped with commercial oyster reefs, leading toingestion of phycoerythrin-containing cryptophytesand a commercially disastrous red/pink coloration ofthe oysters.St Johns River System This is a 300 mile-longestuarine system composed of lakes, tributaries, riverinesegments and springsheds. Eutrophication wasindicated by extensive blooms of blue-green algae,which dominated the relatively static parts of the systemsuch as freshwater lakes and various oligohaline(low salinity) sites. These blooms have been associatedwith fish kills, loss of macrophytes and wildlifemortalities.Eutrophication: molecular detection ofindicator algaeFreshwater plume Outflow of freshwater fromrivers creates a fluctuating zone of fresh (brackish)