Fate of Planktothrix-derived toxins in aquatic food webs: A case study in Lake Mindelsee (Germany).

Blooms of the red, filamentous cyanobacterium Planktothrix rubescens occur frequently in pre-alpine lakes in Europe, often with concomitant toxic microcystin (MC) production. Trophic transfer of MCs has been observed in bivalves, fish, and zooplankton species, while uptake of MCs into Diptera species could facilitate distribution of MCs into terrestrial food webs and habitats. In this study, we characterized a Planktothrix bloom in summer 2019 in Lake Mindelsee and tracked possible trophic transfer and/or bioaccumulation of MCs via analysis of phytoplankton, zooplankton (Daphnia) and emergent aquatic insects (Chaoborus, Chironomidae and Trichoptera). Using 16 S rRNA gene amplicon sequencing, we found that five sequence variants of Planktothrix spp. were responsible for bloom formation in September and October of 2019, and these MC-producing variants, provisionally identified as P. isothrix and/or P. serta, occurred exclusively in Lake Mindelsee (Germany), while other variants were also detected in nearby Lake Constance. The remaining cyanobacterial community was dominated by Cyanobiaceae species with high species overlap with Lake Constance, suggesting a well-established exchange of cyanobacteria species between the adjacent lakes. With targeted LC-HRMS/MS we identified two MC-congeners, MC-LR and [Asp3]MC-RR with maximum concentrations of 45 ng [Asp3]MC-RR/L in lake water in September. Both MC congeners displayed different predominance patterns, suggesting that two different MC-producing species occurred in a time-dependent manner, whereby [Asp3]MC-RR was clearly associated with the Planktothrix spp. bloom. We demonstrate an exclusive transfer of MC-LR, but not [Asp3]MC-RR, from phytoplankton into zooplankton reaching a 10-fold bioconcentration, yet complete absence of these MC congeners or their conjugates in aquatic insects. The latter demonstrated a limited trophic transfer of MCs from zooplankton to zooplanktivorous insect larvae (e.g., Chaoborus), or direct transfer into other aquatic insects (e.g. Chironomidae and Trichoptera), whether due to avoidance or limited uptake and/or rapid excretion of MCs by higher trophic emergent aquatic insects.

Export of dietary lipids via emergent insects from eutrophic fishponds.

Fishponds, despite being globally abundant, have mainly been considered as food production sites and have received little scientific attention in terms of their ecological contributions to the surrounding terrestrial environment. Emergent insects from fishponds may be important contributors of lipids and essential fatty acids to terrestrial ecosystems. In this field study, we investigated nine eutrophic fishponds in Austria from June to September 2020 to examine how Chlorophyll-a concentrations affect the biomass of emergent insect taxa (i.e., quantity of dietary subsidies; n = 108) and their total lipid and long-chain polyunsaturated fatty acid content (LC-PUFA, i.e., quality of dietary subsidies; n = 94). Chironomidae and Chaoboridae were the most abundant emergent insect taxa, followed by Trichoptera, Ephemeroptera, and Odonata. A total of 1068 kg of emergent insect dry mass were exported from these ponds (65.3 hectares). Chironomidae alone exported 103 kg of total lipids and 9.4 kg of omega-3 PUFA. Increasing Chl-a concentrations were associated with decreasing biomass export and a decrease in total lipid and LC-PUFA export via emergent Chironomidae. The PUFA composition of emergent insect taxa differed significantly from dietary algae, suggesting selective PUFA retention by insects. The export of insect biomass from these eutrophic carp ponds was higher than that previously reported from oligotrophic lakes. However, lower biomass and diversity are exported from the fishponds compared to managed ponds. Nonetheless, our data suggest that fishponds provide crucial ecosystem services to terrestrial consumers by contributing essential dietary nutrients to consumer diets via emergent insects. Supplementary Information: The online version contains supplementary material available at 10.1007/s10750-022-05040-2.

Climate change shifts the timing of nutritional flux from aquatic insects.

Climate change can decouple resource supply from consumer demand, with the potential to create phenological mismatches driving negative consequences on fitness. However, the underlying ecological mechanisms of phenological mismatches between consumers and their resources have not been fully explored. Here, we use long-term records of aquatic and terrestrial insect biomass and egg-hatching times of several co-occurring insectivorous species to investigate temporal mismatches between the availability of and demand for nutrients that are essential for offspring development. We found that insects with aquatic larvae reach peak biomass earlier in the season than those with terrestrial larvae and that the relative availability of omega-3 long-chain polyunsaturated fatty acids (n-3 LCPUFAs) to consumers is almost entirely dependent on the phenology of aquatic insect emergence. This is due to the 4- to 34-fold greater n-3 LCPUFA concentration difference in insects emerging from aquatic as opposed to terrestrial habitats. From a long-sampled site (25 years) undergoing minimal land use conversion, we found that both aquatic and terrestrial insect phenologies have advanced substantially faster than those of insectivorous birds, shifting the timing of peak availability of n-3 LCPUFAs for birds during reproduction. For species that require n-3 LCPUFAs directly from diet, highly nutritious aquatic insects cannot simply be replaced by terrestrial insects, creating nutritional phenological mismatches. Our research findings reveal and highlight the increasing necessity of specifically investigating how nutritional phenology, rather than only overall resource availability, is changing for consumers in response to climate change.

Spatial and length-dependent variation of the risks and benefits of consuming Walleye (Sander vitreus).

Restricted fish consumption due to elevated contaminant levels may limit the intake of essential omega-3 fatty acids, such as eicosapentaenoic (EPA; 20:5n-3) and docosahexaenoic (DHA; 22:6n-3) acids. We analyzed lake- and length-specific mercury and EPA+DHA contents in Walleye (Sander vitreus; Mitchell 1818) from 20 waterbodies in Ontario, Canada, and used this information to calculate the theoretical intake of EPA+DHA when the consumption advisories are followed. The stringent consumption advisory resulted in decreased EPA+DHA intake regardless of the EPA+DHA content in Walleye. Walleye length had a strong impact on the EPA+DHA intake mainly because it was positively correlated with the mercury content and thereby consumption advisories. The highest EPA+DHA intake was achieved when smaller Walleye (30-40cm) were consumed. The strong relationship between the consumption advisory and EPA+DHA intake enabled us to develop a more generic regression equation to estimate EPA+DHA intake from the consumption advisories, which we then applied to an additional 1322 waterbodies across Ontario, and 28 lakes from northern USA for which Walleye contaminant data are available but fatty acid data are missing. We estimate that adequate EPA+DHA intake (>250mgday-1) is achieved in 23% of the studied Ontario lakes, for the general population, when small (30-40cm) Walleye are eaten. Consumption of medium- (41-55cm), and large-sized (60-70cm) Walleye would provide adequate EPA+DHA intake from only 3% and 1% of the lakes, respectively. Our study highlights that mercury contamination, which triggers consumption advisories, strongly limits the suitability of Walleye as the sole dietary source of EPA+DHA to humans.