SAMPLING STRATEGIES FOR MEASURING MACROALGAL BIOMASS IN THE SHALLOW WATERS OF THE VENICE LAGOON

A number of procedures were tested to set up a reliable method for determining macroalgal standing crops in the shallow waters of the Venice Lagoon, populated by macroalgae with different densities and variously distributed over the bottom. Accurate and precise measures of biomass were obtained using a box of 1m2 and a grid sampling scheme. Three areas (15 x 15 m2) characterized by high (5-20 kg m-2), low (0.1-1.0 kg m-2) and intermediate (1-10 kg m-2) biomass densities were studied. Twenty sub-samples were taken in each area and the results obtained were considered as the best estimate of the true mean of the population in the area. The difference between the best estimate of the total macroalgal population mean (BEM) and the mean of different sets of sub-samples (SM) divided by the best estimate of the mean: \BEM-SM\/BEM was proved the most efficient parameter to attain a value which differs by a given percentage -at 95% probability-from the true macroalgal biomass, considering all the possible sampling sequences. This difference was below 10% after only 3 sub-samplings. To obtain an accuracy within 5%, 4-14 sub-samplings, according to the different area tested, were needed. In the field, during the sampling campaigns, the per cent ratio (HL-SM)/SM, (HL = 95% higher confidence interval limit) was routinely used as a guide for establishing the number of sub-samples needed to obtain a certain precision. It was demonstrated that, when the per cent ratio (HL-SM)/SM was within the predetermined value of 30% or 15-20%, the accuracy of the obtained sub-sample mean was also differing less than 10% or 5%, respectively, from the best estimated population mean. Finally, a very rapid procedure is suggested for classifying macroalgal biomass into pre-established ranges and/or for mapping wide areas.