SAMPLE-SIZE IN THE MONITORING OF BENTHIC MACROFAUNA IN THE PROFUNDAL OF LAKES - EVALUATION OF THE PRECISION OF ESTIMATES

We discuss here the influence of sample size (number of replicates) on the accuracy and precision of the results when sampling profundal ben thos with an Ekman grab according to the Finnish standard, SFS 5076, w hich is equivalent to the Swedish and Norwegian standards. The aim was to find criteria for choosing a sample size which would avoid any pow erful influence of chance on the results without entailing an unreason able amount of work for monitoring purposes. Lake Haukivesi (area 620 km(2), total phosphorus 13 mu g l(-1) and colour 35 Pt mg l(-1)), Lake Paasivesi (116 km(2), 5 mu g l(-1) and 35 Pt mg l(-1)) and Lake Puruv esi (322 km(2), 4 mu g l(-1) and 5 Pt mg l(-1)) were sampled randomly in June and October 1991. 25 Replicate samples were taken on each occa sion from the deep profundal area of each lake, defined here as 60-100 % of the maximum depth. The sedimentation areas studied were fairly ho mogeneous, since the animal-communities were not markedly affected by the variations in depth. Distribution estimates for the statistics stu died, such as number of individuals, expected number of species, diver sity and benthic quality indices, were calculated for a large set of r andom samples taken from the empirical data by computer (bootstrap sam pling). The sample variance, s(2), correlated with the mean animal den sity, m (ind, m(-2)), according to the equation s(2) = 31.77 m(1.247). The simple size required to achieve the desired precision in mean ani mal density (D, expressed as the ratio standard error/mean) can thus b e estimated as n = 31.77 m(-0.753) D-2. Th, number of replicate sample s needed to achieve a standard error of 20% of the mean density was 10 in Lake Haukivesi, seven in Lake Paasivesi and 11 in Lake Puruvesi. T he accuracy and precision of the estimated number of species, Shannon' s diversity and Benthic Quality Index improved markedly as the sample size was increased to 10 replicates. As a compromise between work load and statistical reliability, a figure of 10 replicate Ekman samples i s proposed here for the monitoring of profundal benthos. The proposed sample size usually produces individual numbers which are high enough for practical purposes, probably at least 100 individuals, which is re commended as a minimum in the standard. The lower number of replicate samples recommended in recent Finnish handbook, 3-5, usually produces inadequate data, and this may detract from the comparability of the re sults and leave the changes in profundal communities undetected.