Observer Variation in River Macrophyte Surveys

Abstract

In summer 2008 we carried out multiple-observer sampling trials on two stretches of the Western Cleddau, a lowland river in south-west Wales. These trials were arranged to establish the rates of variation between surveyors, specifically in relation to macrophyte species detection and estimates of vegetation cover. The sampling trials were carried out in one 500 m section of river, and one 100 m section of river.

Eleven experienced professional botanists were asked to record a cover estimate for each species of alga, lichen, bryophyte and vascular plant in each of the two sections of river.

The results showed that no surveyor recorded more than 64% of the species present in the 500 m section and that no surveyor recorded more than 51% of the species in the 100 m section. Only 25% of the species had a detection rate of <75%, with these comprising species that were either abundantly distributed or forming considerable areas of cover. Approximately half of the species had a detection rate of >25%. Estimates of vegetation cover differed between observers by a mean of 24%.

Several common and readily identifiable species had low detection rates, including Iris pseudacorus, Juncus effusus and Potamogeton perfoliatus. The scale of observer variation reflects the inability of the surveyors, no matter how experienced, to critically survey the required areas of search: 6,500 m² for the 500 m section of river and 800 m² for the 100 m section of river.