Normally, 14–17 days is enough to achieve good growth on the substrates before significant
sloughing takes place. Of course this varies from river to river, and with season. After your
first experiment you may need to adjust the length of any subsequent incubations. Keep the
periods as consistent as is practical. It is best to avoid leaving the samplers to develop thick
mats as sloughing may occur, and this would distort the results. Also large volumes of ethanol
are required for chlorophyll extraction from thick mats.