The abundance of the microalgal film was determined using chlorophyll a as an
index of standing crop at one exposed shore ŽPort St. Mary. and one sheltered shore
Žinner Langness.. Only the patches with manipulated densities of limpets were sampled
at inner Langness. Two sampling dates were chosen at random from within each of two
seasons, summer ŽJune–August 1998. and winter ŽDecember 1998–March 1999.. Three
chips of rock with a minimum surface area of 4 cm2 were taken randomly using a
hammer and fine chisel Žblade size, 1–2 cm. from within the area of each plot and
patch. Barnacle and algal Žincluding lithothamnion. encrusted rock was avoided. This
amount of rock represented only 0.1% of exposed shore plots and between 0.4% and
2.4% of patches on sheltered shores over the whole year of sampling. The level of
chlorophyll a in each sample was estimated using the method for rock substrata
described by Thompson et al. Ž1999.. Samples were initially washed and hydrated in
filtered sea water. Extraction was made using cold methanol over a period of between 5
and 24 h and absorbance of the resulting solution determined at 665 and 750 nm. The
surface area of each rock sample was determined by image analysis and the level of
chlorophyll a per unit area of rock surface calculated using the following equation: