Freshwater pearl mussel as indicators of Swedish streams - a comparison between freshwater pearl mussel and biological quality elements

Sammanfattning: The freshwater pearl mussel (Margaritifera margaritifera) is a large freshwater mussel. Because of its sensitivity to human impact, such as eutrophication, acidification and alterations of hydromorphology, the species is considered as a robust indicator of stream water quality and biodiversity. Moreover, due to the species complex life cycle, including a larval stage on the gills of salmon and trout, and sensitivity of early life stages, regeneration is often used as an indicator. The freshwater pearl mussel is also used in the expert judgement of status classification of benthic invertebrates according to the European Water Framework Directive. The purpose of this study was to gain a deeper understanding of the freshwater pearl mussel as an indicator for Swedish streams. It was made with two types of association analyses. Regeneration of freshwater pearl mussel populations was compared with several indices and parameters of the biological quality elements benthic fauna, diatoms and fish. Then, values and classes for indices and parameters were compared across water bodies where freshwater pearl mussels were recorded as being present or not present. The association analyses were made with appropriate statistical methods and the results were considered significant for a p-value <0,05. The study found no significant results to support the expert judgement of benthic fauna using freshwater pearl mussel. However, significant relationships between regeneration of freshwater pearl mussel populations and the status classifications using diatoms were noted, suggesting the use of freshwater pearl mussels to complement or replace classifications using diatom metrics. Surprisingly, no clear relationships were found between regeneration of freshwater pearl mussel populations and parameters for salmon and trout. This finding was unexpected as the freshwater pearl mussel life cycle is dependent on host salmon and trout for growth and survival. The study supports that the freshwater pearl mussel can indicate biodiversity since the diversity of benthic invertebrates and diatoms were higher where the freshwater pearl mussels were recorded as being present.