Freshwater Algae: Identification and Use as Bioindicators

136 3 ALGAE AS BIOINDICATORSTable 3.15 Algal Bioindicators of Estuarine Pollution: Episodic and Seasonal Events in Southern USA Estuaries(Paerl et al., 2005)Estuarine System Source of Eutrophication Algal Analysis: Results*Neuse RiverestuaryNorth CarolinaGalveston Bay.TexasSt Johns Riverestuarinesystem, FloridaHigh rainfall – hurricanes in 1999Pollution source – agriculturalurban and industrial activitiesHigh rainfall – tropical storm in 2000High summer rainfall Acceleratingpoint and diffuse nutrient loadingfrom watershedRiver discharge into estuary – elevated biomass (chl-a),increased chlorophytes/diatom populations, seasonalchanges-reduced dinoflagellate bloomFreshwater flushed into bay, resulting in reduced salinity,increased DIN plus blooms of cryptophytes anddinoflagellates. Adverse effects on oyster bedsSeasonal nutrient enrichment from watershed leading toblue-green algal blooms, fish kills,submerged-vegetation loss, wildlife mortalities*HPLC diagnostic photopigment determinations coupled to ChemTax R○ analysis.water within the surrounding marine environment.This plume of water extends into the surroundingocean, and is characterized by reduced salinity, highlevels of inorganic nutrients, high levels of suspendedmaterial and a distinctive phytoplankton population.The freshwater tends to lie on the surface of the seawater(forming a ‘freshwater lens’), so that the locationof the freshwater plume can be considered bothin terms of horizontal and vertical distribution.A good example of this is the Amazon River plume,which extends into the Western Tropical North Atlantic(WTNA) ocean (Foster et al., 2007). The AmazonRiver represents the world’s largest fluvial outflow,releasing an average 1.93 × 10 5 m 3 s −1 of waterto the WTNA. The influence of the Amazon riverplume is geographically far-reaching, with freshwaterlenses containing enhanced nutrients and distinctphytoplankton populations reported as far away as>1600 km from the river mouth (Borstad, 1982). TheAmazon plume brings silica, phosphorus, nitrogenand iron into the WTNA, promoting high levels ofphytoplankton productivity within the plume area.This productivity tends to be nitrogen limited, promotingthe growth of nitrogen-fixing (diazotrophic)blue-green algae.Molecular detection The detection of diazotrophicblue-greens can be used to map regionsof eutrophication resulting from freshwater outflow.Studies by Foster et al. (2007) on the occurrence ofnitrogen-fixing algae in the WTNA targeted the verticaland horizontal distribution of seven diazotrophicpopulations, including Trichodesmium (typical ofopen ocean environments), three symbioticallyassociatedalgae and three unicellular freeliving organisms(phylotypes). Algal distributions were examinedin terms of the presence of the nif H gene,which encodes for the iron protein of the nitrogenaseenzyme (responsible for nitrogen fixation),and is highly conserved. The presence of speciesspecificnif H gene sequences was determined usingDNA quantitative polymerase chain reaction (QPCR)technology.The studies showed that one particular algal symbiont(Richelia), associated with the diatoms Hemiaulushauckii and Rhizosolenia clevei, was distributedwithin the freshwater lens of the Amazonplume – with abundances of H. hauckii–Richelia nif Hgenes being particularly prominent. The QPCR studyshowed the dominance of H. hauckii–Richelia symbiosesin the Amazon plume waters, implying thatthese associations had an ecological advantage overother diazotrophs and establishing them as indicatororganisms of the plume environment. Free livingunicellular blue-green diazotrophs were particularlyabundant in the saline waters outside the plume,where inorganic nutrients were at minimal levels ofdetection.