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

Impacts of climate warming and atmospheric deposition on recent shifts in chironomid communities in two alpine lakes, eastern China

Climate warming and atmospheric deposition are altering alpine lake ecosystems at unprecedented rates, whereas their direct and indirect effects on primary consumer communities are unclear. This study presents sedimentary multi-proxy records including chironomids, diatoms, elements and stable isotopes of carbon and nitrogen in 210Pb-dated cores from two alpine lakes located above the timberline in the Taibai Mountain, eastern China. Before ∼2000 CE, chironomid communities were co-dominated by Heterotrissocladius marcidus-type and Micropsectra atrofasciata-type in the two lakes. Thereafter, Tanytarsus glabrescens-type increased rapidly to be a dominant species. Redundancy analyses (RDAs) revealed that chironomid fauna shifts were significantly correlated with rising diatom concentrations in both lakes, declining Ti content in the upstream lake and δ13C depletion in the downstream lake. Although temperature, precipitation and δ15N were not significant explanatory variables in RDAs, climate warming and atmospheric deposition likely promoted terrestrial and aquatic primary production, indicated by synchronous increases in organic matter contents and diatom concentrations in the two sediment cores. Since diatoms contain essential polyunsaturated fatty acids that are essential for chironomids, rising diatom concentrations can promote food quantity and quality. In addition, increased primary production would create organic substrates for chironomid larvae. Recent shifts in chironomid fauna driven by indirect effects of global warming and atmospheric deposition might be a widespread phenomenon in alpine lakes, probably triggering regime shifts in headwater lake ecosystems.

Asynchronous multitrophic level regime shifts show resilience to lake browning

Lake browning is widespread due to increased supply of dissolved organic carbon under climate warming and nitrogen deposition. However, multitrophic level responses to lake browning are poorly understood. Our study aims to explore such responses across multitrophic levels based on sedimentary records of diatoms, chrysophyte stomatocysts and chironomids in a remote headwater lake in the Three Gorges Reservoir region, central China. Although all biotic proxies were analysed in the same core, the timing of shifts in chironomids (1886 ± 18 CE) preceded that in chrysophyte stomatocysts (∼1914 ± 10 CE) and diatoms (∼1941 ± 6 CE). Shifts in biotic communities were closely linked to rising temperature, δ15N depletion (a proxy for nitrogen deposition), δ13C enrichment (a proxy for littoral moss expansion), as well as biotic interactions, whereas the relative importance of the driving forces varied among the three biotic groups. Our results suggest that the zoobenthos grazing effect might be more important than bottom-up pathways in humic environments. Additionally, the coexistence of benthic, littoral and pelagic algae after the 1950s suggested that mixotrophic chrysophytes could reduce lake browning through heterotrophic processes and sustain the ecological equilibrium between littoral, pelagic and benthic productivity. Therefore, lake browning ecosystem regime shifts require analyses of multiple trophic levels. Our results suggest that heterotrophy may become more important in lake ecosystem carbon cycling with water brownification in Mulong Lake, as well as similar montane lakes.

Recent trends in stream macroinvertebrates: warm-adapted and pesticide-tolerant taxa increase in richness

Recently, a plethora of studies reporting insect declines has been published. Even though the common theme is decreasing insect richness, positive trends have also been documented. Here, we analysed nationwide, systematic monitoring data on aquatic insect richness collected at 438 sites in Switzerland from 2010 to 2019. In addition to taxonomic richness, we grouped taxa in accordance with their ecological preferences and functional traits to gain a better understanding of trends and possible underlying mechanisms. We found that in general, richness of aquatic insects remained stable or increased with time. Warm-adapted taxa, common feeding guilds and pesticide-tolerant taxa showed increasing patterns while cold-adapted, rarer feeding guilds and pesticide-sensitive taxa displayed stable trends. Both climate and land-use-related factors were the most important explanatory variables for the patterns of aquatic insect richness. Although our data cover the last decade only, our results suggest that recent developments in insect richness are context-dependent and affect functional groups differently. However, longer investigations and a good understanding of the baseline are important to reveal if the increase in temperature- and pesticide-tolerant species will lead to a decrease in specialized species and a homogenization of biotic communities in the long term.

Long-term responses of epiphytic midges (Diptera, Chironomidae) to emergent macrophytes removal and P concentrations in a shallow hypertrophic lake ecosystem

Chironomid larvae are used as indicators of environmental changes in neolimnological and paleolimnological research. In the present study, we evaluated the responses of epiphytic chironomids to changes in environmental conditions over a long time scale. We intended to decipher whether changes in the trophic status of a lake (hypertrophic-eutrophic) would affect the taxonomic structure of epiphytic chironomids by influencing their food availability (structure of periphytic algae) and whether the responses of chironomids are taxa specific. In a shallow hypertrophic lake ecosystem, epiphytic chironomids associated with the emergent macrophyte Phragmites australis were studied from 2001 to 2018. In the autumn of 2006, emergent macrophyte removal led to an improved water transparency and reduced phytoplankton biomass. Epiphytic chironomids responded clearly to the shift from hypertrophic to eutrophic conditions. Under hypertrophic conditions larvae of detritivorous Cricotopus sp. (gr. sylvestris) and filter-feeder larvae of Glyptotendipes sp. prevailed. After macrophyte removal, we observed high relative abundances of Endochironomus albipennis and Paratanytarsus austriacus, which are classified as grazers, utilise periphytic algae (mainly diatoms) as a food source. The results indicated that the density of P. australis, relative abundances of cyanobacteria and diatoms in periphyton communities, Secchi disc depth, and periphytic and planktonic Chl-a are significant determinants of the diversity of epiphytic chironomids. The results can provide a reliable reference to the neolimnological and paleominological studies on chironomid responses to multiple environmental stressors in shallow lakes.