Development of a lake pollution monitoring system using periphyton in the littoral zone of Lough Gill
Feeney, Declan M. J.
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Periphyton is a complex mat, predominantly made up of algae, found attached to submerged surfaces in the photic zone of a waterbody. In the early 1990's this brown substance was observed to cover plants, stones and most submerged surfaces around the shoreline of Lough Gill in NW Ireland. Analysis of the water indicated a mesotrophic system, however such a substantial growth of periphyton warranted further investigation. A monitoring programme was established with six sample sites spread around the littoral zone. Three artificial substrates were used to measure periphyton; glass slides, trays of washed stone and a plastic substrate (to simulate the macrophyte Littorella uniflora). Substrates were submerged for periods of 1 month from February 1997 to May 1998. Phytoplankton samples were also collected. Diatoms always dominated the biomass on slides, with peaks during spring and autumn. Green and blue/green algae became prominent in the summer and autumn. Diatom genera included Cymbella, Gomphonema, Nitzschia and Synedra. Chlorophyta included Chaetophora, Stigeodonium and Utothrix, and the main Cyanophyta were Anabaena and Aphanocapsa. During 1997 periphyton biomass from glass slides ranged from 25 g/m2 (May) to <1 g/m2 (November), in the same period AFDW ranged between 14 g/m2 and <1 g/m2 with algal numbers ranging between 16,200 cells/mm2 and 124 cells/mm2. During April 1998 periphyton biomass exceeded anything seen during 1997 with dry weight from 24 g/m2 to 36 g/m2, AFDW from 11 g/m2 to 19 g/m2 and cell numbers were greatly increased (18,850 cells/mm2 to 41700 cells/mm2). A substantial proportion of cells suspended in the waters of the littoral zone were periphytic in origin. These diatoms dominated littoral phytoplankton during spring and considerably influenced phytoplankton populations throughout the rest of the year. In periods of peak periphyton growth, clots of algae became suspended through wind and wave action during stormy weather which temporarily reduced water clarity. Considerable spatial variation was observed between the sites. This would seriously effect site selection in a monitoring program. Wind patterns and associated water movement may influence growth variability on substrates; those sites with greater exposure having greater levels of growth. Glass slides suspended in the water column were more indicative of periphyton on natural substrates, whilst trays of washed stones and artificial Littorella were found to trap excessive amounts of inorganic sediment. The quantity of periphyton, irrespective of spatial and temporal trends, appears to be remarkably greater than other lakes in the west of Ireland.
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