The usefulness of large body-size macroinvertebrates in the rapid ecological assessment of Mediterranean lagoons

The success of a monitoring programme depends on the precision and accuracy of assessment tools, on the costs of sampling and laboratory procedures, on the time lag between sampling and the availability of the results. Aquatic ecosystem monitoring programmes using macroinvertebrates, developed in compliance with the EC Water Framework Directive, are constrained by assessment tools which require economically expensive activities and long time lags. This research investigates the adequacy of simplified sampling procedures, based on the selection of large body-size macroinvertebrates, retained by a 2 mm mesh size sieve, to detect the ecological status of a Mediterranean lagoon. To this aim, various tools for assessment of lagoon ecological status were compared, using three macroinvertebrate datasets obtained by field sieving with 2 mm, 1 mm and 0.5 mm sizes of mesh sieves. Simple community descriptors (numerical density, taxonomic richness, biomass density and individual body-size), taxonomic diversity indices (Shannon–Weaver, Margalef, Pielou, Average Taxonomic Distinctness) and ecological indicators sensu WFD (AMBI, BENTIX, BITS, M-AMBI) were used as assessment tools. In September 2009, three soft bottom samples of 0.1 m2 were collected, sieved through the column of sieves, at seven study sites located along a Total Pressure gradient of Lesina lagoon (Italy). Three datasets were obtained with the macroinvertebrates retained by the 2 mm mesh sieve (2mm), the 1 mm and 2 mm sieves (1mm) and the 0.5 mm, 1 mm and 2 mm sieves (0.5mm). The manual sorting time was also recorded for each sample and compared across the three datasets. Taxonomic richness, numerical and biomass densities were significantly higher in the 0.5mm than the 1mm and 2mm datasets, as expected. Nevertheless, the ecological indicators classified the study sites in the same EQS class with all datasets in 64% of cases; comparing the 2mm and 0.5mm datasets, the 2mm and 1mm datasets and the 0.5mm and 1mm datasets, the percentages were 64%, 80% and 86%, respectively. Individual body-size, AMBI, BENTIX were significantly correlated with Total Pressure at each dataset, M-AMBI at 1mm and 2mm and biomass density only at 2mm. The manual sorting time was 56% shorter for the large body-size macroinvertebrate fraction than for the 1mm, and 72% shorter than for the 0.5mm. Finally, the larger body-size macroinvertebrates seem to be advantageous and useful for accurate, rapid and cost effective biomonitoring. Further investigations to define reference conditions for each mesh are still needed, mainly for body-size based ecological indicators.