EFFICIENCY OF A BRUSH SAMPLER TO MEASURE PERIPHYTON IN STREAMS AND LAKES

Natural periphytic communities are usually sampled by scraping all the algae from a small area. Such a protocol seems more likely to sample loosely attached forms than those which grow tightly attached to the substratum, but this bias has never been quantified. To this end, chlorophyll estimates were obtained by sampling periphyton on cobbles with a brushing syringe sampler in a series of streams and lakes in southern Quebec and compared with estimates obtained by immersing the cobble in extractant solvent. In streams, the syringe sampler underestimated chlorophyll concentration, and the severity of this underestimation increased with the current speed. in lakes, the situation was reversed: the brushing syringe gave higher values than those obtained by immersion. These results likely reflect the relative proportions of loosely and tightly attached subcommunities within the periphyton. The latter fraction, which cannot be quantitatively removed by brushing, appears especially important in fast-flowing streams. Ineffective sampling of this fraction may invalidate comparisons obtained with different methods.