Long-term decrease in Baltic Sea blue mussel shell length

Multi-decadal monitoring data and archived biological samples make the rapidly changing Baltic Sea an ideal system for quantifying alterations in ecosystem function. The Baltic Sea blue mussel (Mytilus edulis trossulus) acts as a keystone species by providing food and habitat for other species, and through its filtering activity clearing the water and promoting organic matter and nutrient cycling between benthic and pelagic ecosystems. A decrease in biomass of blue mussels along with altered environmental conditions has been observed in the northern Baltic Sea over a 24-year period (1993-2016), but the size distribution of the mussels was unknown. The present study focuses on retrospective measurements of the individual shell lengths of these archived blue mussels. The mean shell length was found to be significantly lower at the end of the time series than in the beginning (11.1-11.5 mm for the years 1993-1996 compared to 10.3-10.9 mm during 2012-2015), with the proportion of larger mussels decreasing in the population. This 6% decrease in shell length translated into a significantly lower (16%) mean filtration capacity at the end of the time series compared to the beginning, influencing the ecosystem services blue mussels provide.

Automated summary

The Baltic Sea is an ideal system for quantifying changes in ecosystem function, and the blue mussel plays a key role in this system. A decrease in blue mussel biomass and altered environmental conditions have been observed over a 24-year period. The study focuses on retrospective measurements of shell lengths of archived blue mussels and finds a significant decrease in the mean shell length and a decrease in the proportion of larger mussels in the population. This decrease in shell length has led to a lower mean filtration capacity and has implications for the ecosystem services provided by the blue mussels.