Changes in carbon sources fueling benthic secondary production over depth and time: coupling Chironomidae stable carbon isotopes to larval abundance

Stable isotope evidence from archived fish scales indicates carbon cycle changes over the four-decade history of the Římov Reservoir (Czechia)

Using archived fish scale samples together with long-term monitoring data, this study investigates the potential of fish scales to record historical changes in the aquatic environment. We analysed stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes in the scales of two planktivorous cyprinid species collected from the meso-eutrophic Římov Reservoir, Czechia, over its entire four-decade history (1979-2016). The δ¹³C of the fish scales varied greatly throughout the reservoir history. The lowest δ¹³C values were observed immediately after the reservoir was filled in 1979, indicating that fish production at that time was likely partially supported by ¹³C-depleted CO₂ released from the inundated soil. During the 1980s, due to the high levels of phytoplankton production stimulated by high phosphorus inputs from the catchment, the δ¹³C values substantially increased. However, since 1990, the δ¹³C values have generally decreased, reflecting a gradual reduction in reservoir primary production caused by the decreasing input of phosphorus and increasing input of dissolved organic carbon from the catchment. The δ¹³C of fish scales was also used to reconstruct the CO₂ concentration of the surface water. The reconstructed CO₂ varied significantly during the four-decade history, but it was always below the air-equilibrium concentration, suggesting that the surface water of the reservoir has consistently absorbed atmospheric carbon. The fish-scale δ¹⁵N values remained relatively stable, while slightly increasing within three years after impoundment, likely because the nitrogen supply was high throughout the studied period. Our study contributes to the growing body of literature demonstrating that stable isotope analysis of archived biological samples is a promising approach for understanding historical trends in the biogeochemistry of aquatic environments. In particular, our results highlight the potential of δ¹³C in archived fish scales in reconstructing carbon cycle changes and evaluating human impacts on aquatic ecosystems.

Effects of climate warming and nitrogen deposition on subtropical montane ponds (central China) over the last two centuries: Evidence from subfossil chironomids

Many remote montane ecosystems are experiencing biogeochemical changes driven by warming climate and atmospheric pollution. Compared with circumpolar and temperate lakes, the responses of subtropical montane lakes to these external stressors have been less investigated. Here we present sedimentary multi-proxies records (i.e. chironomids, elements and stable isotope of carbon and nitrogen) in ²¹⁰Pb-dated cores from two montane ponds (central China). Before the 1900s, low biomass and the dominance of opportunistic species (e.g. Chironomus anthracinus-type) in both ponds might be in response to cold and harsh condition. Thereafter, chironomid communities in both ponds experienced pronounced shifts. Nutrient-tolerant/warm-adapted species (e.g. Chironomus sp., Polypedilum nubeculosum-type and Endochironomus impar-type) proliferated and biomass increased synchronously after the 1900s, suggestive of favorable condition for chironomid growth. Redundancy analyses revealed that changes in chironomid communities in both ponds were significantly correlated with rising temperature and δ¹⁵N depletion. Prolonged growing season and nitrogen subsidy would increase primary productivity, and hence enhancing food availability for chironomids. Catchment-mediated indirect effects of warming and nitrogen deposition, such as hydrological changes and terrestrial organic matter inputs, would impose further influences on chironomid communities. Taken together, the combined effects of climate warming and nitrogen deposition have caused significant shifts in primary consumers of these montane ponds, and imposed cascading effects on structure and function of subtropical montane aquatic ecosystems.