ABSTRACT Land‐use activities within catchments alter surface runoff dynamics, potentially influencing food quality and quantity available to consumers. Essential fatty acids are crucial biomolecules for consumers, which they usually need to uptake from their diet. Globally, freshwater mussels are facing severe declines, yet their nutritional ecology and its link to catchment land‐use pressures remain poorly understood. Freshwater pearl mussel ( Margaritifera margaritifera , FPM), a long‐lived critically endangered keystone species, inhabits oligotrophic rivers. We investigated how dietary resource availability and fatty acid retention vary along environmental gradients of water quality and catchment characteristics, by analysing fatty acid compositions in FPMs' stomach content and their diet (seston) collected from 23 rivers across Finland. Seston fatty acid compositions and content were compared to the ingested fatty acids in mussel stomach content. Our results highlighted a clear shift in dietary availability associated with anthropogenic pressures, characterized by a decreased contribution of terrestrial fatty acids to the FPM diet and an increased contribution of phytoplankton. Furthermore, the comparison between fatty acid profiles of seston and stomach contents indicated selective feeding by FPM, with a preference for phytoplankton, particularly diatoms. However, rivers with lower anthropogenic pressures exhibited reduced phytoplankton fatty acids and higher reliance on bacterial sources. These findings suggest that FPM actively retain essential fatty acids, indicating a high dietary reliance. Variation in fatty acid retention correlated with environmental gradients, emphasizing the significant role of surrounding environments in nutrient acquisition and retention, potentially impacting mussel health.
Hajisafarali et al. (Wed,) studied this question.