Effectiveness of common benthic macrofaunal sampling methodology in boulder and cobble reefs

Many rigorous methods have been developed to manipulate and sample rocky benthic species, but development of field methodology is less advanced for disturbance-prone rocky habitats e.g. cobbles and boulders. These habitats can have ecological and economic importance but research is complicated by their complex physical structure that necessitates causing disturbance when sampling, possibly resulting in mobile species being lost from samples. We used simulated/controlled sampling conditions to determine the proportions of mobile species successfully captured or lost during sampling. Boulders and cobbles were lifted out of shallow water (i.e. “littoral” sampling) and sampled in simulated sublittoral conditions, targeting biota attached to the undersurfaces. Variation in sample losses associated with different methods was tested – sampling in shallow vs deep water and small vs large boulders/cobbles. Different species and locations were also compared. Tests were done to determine contexts producing minimal losses and thus deserving prioritisation. Experiments were replicated in southern Australia and NE Baltic Sea. Large proportions of littoral amphipods and isopods were not successfully captured during sampling in most contexts (overall mean loss = 35%). This means lifting boulders from shallow water cannot accurately sample them. This finding is important especially for highly researched boulder or cobble associated invasive amphipods with large ecological impacts. Seventeen percent of littoral gastropods were lost from samples in both regions; in Australia greater proportions were occasionally lost when sampling larger boulders. Chitons and one gastropod (Theodoxus fluviatilis) could be sampled from shallow water with consistently high reliability. During sublittoral sampling, mean sample loss was 13% with no significant interspecific variability. Boulder/cobble size and water depth rarely caused influence. The results reveal sampling contexts where more sophisticated methods may be used for highly accurate abundance measurements in recent research programmes of ecological restoration and conservation of rare species involving marine but also freshwater habitats.