Stable isotopes reveal the importance of terrestrially derived resources for the diet of the freshwater pearl mussel (Margaritifera margaritifera)

Abstract

The freshwater pearl mussel (FPM) is among the most endangered freshwater species worldwide. The few remaining populations suffer from low recruitment rates and are subject to habitat fragmentation, pollution, siltation, decline or loss of host fish populations, and climate change. Successful conservation strategies for FPM require a holistic understanding of its ecological requirements, life history, population dynamics, and habitat prerequisites. Although habitat requirements are well described, food requirements at different life stages have received less attention. Stable isotope analyses of FPM and potential food resources in three German streams were combined with mixing model analysis to quantify organic matter resources assimilated by juvenile (first year after encystment from host fish) and semi‐adult (10 years old, immature) individuals. There were only slight differences in dietary contributions between the two life stages, and terrestrial particulate organic matter and benthic organic matter contributed substantially to the diet. Tissue type was more important in explaining variation in dietary contributions than individual variation for semi‐adult FPM. The strong reliance on terrestrial resources sheds new light on the functional role of unionid mussels and the connection of streams to their riparian area. The dependence of FPM on terrestrial resources also emphasizes the need for a stronger focus on the restoration and protection of intact riparian areas, including wetlands with their specific vegetation, when planning conservation and management strategies for threatened FPM populations.

Riparian detritus vs. stream detritus: food quality determines fitness of juveniles of the highly endangered freshwater pearl mussels (Margaritifera margaritifera)

“…Obviously, riparian detritus entering the stream will contribute to the stream detritus and therefore will be part of the available food resources for FPM. A stable isotope analysis conducted by Brauns et al (2021) showed that terrestrial particulate organic matter as well as benthic organic matter contributed substantially to the diet of FPM supporting this assumption. The partial dependency on allochthonous organic matter in the headwater streams in which FPM occur would fit to the River Continuum concept (Vannote et al, 1980).…”

“…One explanation for this could be a rather low flow through of these sources and therefore a higher water temperature as it is assumed in previous studies that algae from temperate headwater streams probably contain more long-chained PUFAs than algae from warmer waters (Guo et al, 2016a(Guo et al, , 2017. That terrestrial organic material contributes significantly to the nutrition of FPM and that therefore intact riparian areas are of great importance for FPM has already been shown by Brauns et al (2021). Once riparian detritus enters the stream, it is most likely processed further by macroinvertebrates and the microbial community and becomes mixed with other organic compounds from the stream, e.g.…”

Thermal Tolerance and Vulnerability to Climate Change of a Threatened Freshwater Mussel

“…The studied streams are not oligotrophic, but most stretches correspond to good ecological status concerning the Water Framework Directive (2000/60/EC). The headwater streams meander through meadows with riparian trees, agricultures, forests (mainly spruce plantations), and few small villages [44]. The five investigated streams (referred to as S1, S2, S3, S4, S5) are situated in close proximity to each other, and show similar abiotic and hydrological conditions such as width (1-3 m) or flow velocity (0.05-1.3, mean 0.44 ± 0.26 m/s).…”

“…Buddensiek [39] stated that temperature effects are difficult to determine in field studies because the accelerated growth might be caused by temperature effects on metabolism or changes in food quality or uptake as observed later by Brauns et al [44] or Grunicke et al [45]. Therefore, our study combines field mesocosm studies in five streams and across four years with laboratory experiments at summer and winter conditions in semi-natural but controlled and replicated conditions.…”

Assessing the Reliability of Quantitative Fatty Acid Signature Analysis and Compound-Specific Isotope Analysis-Based Mixing Models for Trophic Studies

“…Stable isotope analysis (SIA) involves the measurement of the isotopic compositions of consumer tissues and probable food sources followed by calculation of consumer diets using isotope mixing models [ 16 ]. This method was used to establish the important role of terrestrial food sources in the diet of the freshwater pearl mussel ( Margaritifera margaritifera ) [ 17 ]. Additionally, isotope mixing models have enabled estimation of the total contribution of invasive quagga mussel ( Dreissena rostriformis bugensis ) to the diet of the invasive fish round goby ( Neogobius melanostomus ) and the native fish ruffe ( Gymnocephalus cernuus ) [ 18 ].…”