CNR-IRSA activities related to INHABIT

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Positioning and season

The groups of SA are attached along a wire (preferably stainless steel made) and ballasted with a weight fixed at about 2.5m from the substrates.
Groups of SA will be, where possible, submerged to a depth of at least 0.5-1.5m. Optimal position is close to the river bottom, at about 2/3 of the depth. For a correct positioning, possible variations in river discharge causing the SAs to sink too close to the bottom must be considered.
A good knowledge of the study site and of the general river dynamics are needed for a correct SA positioning. However, it is difficult to suggest precise rules generally valid for all the sites. Positioning of SA is strictly dependant from site characteristics, in particular in terms of water depth, distance from the bank, floating support stability, discharge variability, type of debris carried by the river, positioning within he channel, etc.
In ideal conditions SA are fixed to floating already existing structures such as, piers or barges anchored to the bank. In this case the technique is called ‘positioning to piers’. This type of positioning is generally easy and requires few minutes. Practically different SA can be placed both on the external side of the pier, in high velocity flow, and internally, where current is slower. When permanent, or reliable, floating structures are not present, SA can be submerged by hanging them to a floating tank. In this case, two steps must be considered for the correct positioning: 1) preparation of the group SA/tank; 2) anchorage of the tank to the bank. The distance from the bank will be in this case kept by a stick (so called ‘spacer stick’). Technique is called in this case ‘positioning through sticks’.

Indications related to season for SA positioning are only indicative and very general. In order to correctly define  taxonomic composition, individuals abundances and diversity at a site, sampling seasons should be clearly planned. Regarding benthic communities in large rivers, late winter season appears to be a critical period when chemical pollutants can be present in high concentrations and represent a significant alteration for biotic communities. Also, benthic organisms show different life cycles, with some of the species exclusively present in the spring/summer period. This would suggest to space out the sampling periods along the year, in order to adequately describe the community in the different fluvial areas, also considering possible input of fauna from different tributaries. Such variations do suggest 4 periods along the year for benthic fauna sampling through SA: late winter (February/March), late spring/early summer (May/June/July) and late summer (August/September) can be considered as guide periods. Where possible, one extra sampling can be performed in autumn (November/December).