|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.