Qiuqi Lin

Carnivory and active hunting by the planktonic testate amoeba Difflugia tuberspinifera

Difflugia tuberspinifera, a testate amoeba found in the open water plankton of Liuxi He and other south Chinese reservoirs during summer, is one of six or more species that occasionally live a pelagic life. Here, we suggest that its incentive to leave the bottom might be the abundance of food in the water column rather than temperature. This Difflugia (and perhaps the other pelagic species as well) is indeed an actively hunting carnivore that catches small rotifers and other prey in the same size range. In Liuxi He, it readily feeds on Collotheca cf. mutabilis, which it catches and consumes with remarkable agility: it first inspects the jelly tube that protects the prey, then moves to the bottom of it, perforates the jelly near the prey’s foot, and finally ingests the rotifer foot-first.

Does stocking of filter-feeding fish for production have a cascading effect on zooplankton and ecological state? A study of fourteen (sub)tropical Chinese reservoirs with contrasting nutrient concentrations

Stocking of filter-feeding fish is a common tool used in Chinese reservoirs to increase fish production because of low natural recruitment. Whether such stocking has important negative effects on zooplankton with cascading effects on phytoplankton is debated. We compared the zooplankton communities in fourteen reservoirs with different nutrient concentrations and fish densities. Both chlorophyll a (Chla) and fish catch were positively related with total phosphorus (TP), whereas zooplankton biomass did not show a similar relationship with TP. Zooplankton seemed to be influenced by fish as high fish catches coincided with a low proportion of calanoids of the total copepod biomass, a high proportion of rotifers of the total zooplankton biomass, a low zooplankton:phytoplankton biomass ratio, and the absence of Daphnia irrespective of TP concentration. Both zooplankton biomass and most of the zooplankton:phytoplankton biomass ratios were among the lowest reported in the literature for the nutrient range studied. Furthermore, the Chla:TP ratio was higher than what is typically observed in temperate lakes. We conclude that top-down control of zooplankton is of key importance in reservoirs in South China where frequent stocking of filter-feeding fish seems to contribute to poor water quality in the form of higher algal biomass and reduced clarity.

Pelagic flatworm predation on daphniids in a subtropical reservoir: different effects on Daphnia galeata and on Ceriodaphnia quadrangula

Following a reduction in fish populations in 2004–2005, a new, single annual pulse of pelagic flatworms was observed in early summer during 2006–2009 in Liuxihe, a freshwater reservoir in South China. As soon as these worms appear, Daphnia galeata retreats into dormancy, one month earlier (July) in the presence of flatworms than previously observed (August) with fish, while the population of the related Ceriodaphnia quadrangula tends to increase. We show, through in situ lake sampling, in large enclosures and by laboratory observations, that Ceriodaphnia, although perhaps competitively inferior in its ability to acquire algal food, has a higher tolerance to flatworm toxins. As a result, Ceriodaphnia manages to coexist with and proliferate in the presence of the flatworm. Observations in the laboratory suggest that flatworm population autoregulates by being sensitive to their own toxins and that Ceriodaphnia, even if prey to the worms, likely incurs more benefit than cost from their presence.

Predation pressure induced by seasonal fishing moratorium changes the dynamics of subtropical Cladocera populations

Seasonal fishing moratorium with selective fish harvesting is a common means of increasing fish production in Asia. Such manipulation might be expected to lead to a seasonal variation in abundance and composition of fish and predation pressure on zooplankton, and consequently a seasonal change in the Cladocera populations. In this study, we examined the seasonal dynamics of cladocerans in a subtropical plateau lake in southwestern China, Lake Erh, in which a fishing moratorium was implemented in spring and early summer. Cladocerans showed a bimodal seasonal distribution linked to variations in predation pressure induced by the seasonal fishing moratorium. When predation was low in winter because of cool water temperatures and decreasing fish density, Daphnia galeata appeared in the water column and increased towards spring. The cladoceran peaked in the middle of the fishing moratorium, coinciding with a clear water phase. The feeding rate increased with temperature in the late moratorium, allowing planktivorous fish to strongly suppress large daphnids. Small cladocerans benefited from the reduced competition with large daphnids by increasing their numbers rapidly, which resulted in a second peak of cladocerans in the fishing season.

The mitochondrial genome of Atrocalopteryx melli Ris, 1912 (Zygoptera: Calopterygidae) via Ion Torrent PGM NGS sequencing

Abstract The mitochondrial genome of Atrocalopteryx melli was sequenced and assembled via Next-Generation Sequencing (NGS) and iteratively assembly process with a reference seed. This genome is 15,562 bp long and A + T biased (71%), with 37 genes arranged in common order of Odonata. All protein-coding genes are initiated by typical “ATN” codon, and 9 genes are terminated with a complete stop codon, except nad4, nad5, cox2, and cox3, which are terminated with an incomplete codon “T(aa)”. The S5 intergenic spacer is absent in this genome, supporting that lacking of S5 as a specific character for damselflies. The A + T rich region of A. melli is 267 bp longer than that of A. atrata. This mitogenome provides new molecular information for understanding of A. melli and Atrocalopteryx.

The mitochondrial genome of Diaphanosoma dubium with comparison with Daphnia magna

Abstract Diaphanosoma has been called “tropical Daphnia” for its strong ecological role in tropical freshwater as Daphnia in temperate waters. The present study sequenced and annotated the mitochondrial genome (MG) of Diaphanosoma dubium. The MG of Diaphanosoma dubium is 16,362 bp in length, with typical metazoan gene composition. Phylogenetic analysis confirms an earlier finding that Neodiaphanosoma can be separated from Diaphanosoma as a subgenera. One unknown extra CDS region and different arrangement of tRNA were identified when this MG was compared to that of Daphnia magna. This is the first non-daphnia MG of Cladocera, and information on MG sequence and tRNA order provide valuable molecular data in understanding phylogeny of Diaphanosoma and Cladocera.

Seasonal Dynamics of Daphnia galeata in a Reservoir at the Edge of the Tropics Before and After Yearly Stocking with Bighead Carp

We studied the seasonal dynamics of Daphnia galeata from April 2001 to August 2009 in oligo-mesotrophic Liuxihe Reservoir (South China). Until 2004, fish (mainly bighead carp, Hypophthalmichthys nobilis) was released yearly and accounted for 85% of total fish catch. The commercial bighead carp catch, 21 kg ha−1 year−1, suggests a substantial predatory pressure on zooplankton. In 2004, most of the fish were removed and commercial introductions ceased. The abundance of D. galeata was low (from 0 to 1,500 individuals m−3) and with a clear-cut seasonal pattern: a decline in summer, absence in autumn, appearance in early winter, a peak in late winter (February), and a decline towards early summer. Edible nanophytoplankton biomass (<20 μm) was low and was mostly composed of Cyclotella spp. and Peridinium pusillum. D. galeata peaked at low but non-zero abundances of edible nanophytoplankton, indicating that food limitation was not the direct cause of its absence in summer and autumn, when a relatively high edible nanophytoplankton biomass was present. In a first phase, from 2001 to 2004, D. galeata negatively correlated with water temperature. The high surface temperature (32°C) during summer suggests that its development time and life span both shortened so strongly that its lifetime fecundity at that time sank below the threshold, at prevailing needed for maintaining an active population against the fish predation pressure. In 2005, a pelagic flatworm (Rhynchomesostoma sp.), supposedly suppressed by fish predation earlier, appeared. It fed on all zooplankton that it could catch and paralyze using a toxin that it also continuously lost to the environment and that remained active there for several months. Daphnia initially became more common, but its phenology became even more restrictive than before, and it now disappeared 1 month earlier, in July instead of August. Its negative correlation with temperature broke down, because by the time the higher temperatures in the reservoir were reached, it had already disappeared. Its place in the pelagic was taken by Ceriodaphnia quadrangula. Culture and enclosure work showed that the latter was less sensitive to flatworm toxin than Daphnia. Interestingly, the flatworms were susceptible to their own toxin, a possible auto-regulation mechanism.

Spatial structure and β-diversity of phytoplankton in Tibetan Plateau lakes: nestedness or replacement?

Spatial patterns and β-diversity of phytoplankton assemblages depend on the relative importance of species dispersal capacity and species-sorting. Variability in species composition, composed by differences in species richness (nestedness) and/or species replacement, may be caused by niche availability and environmental selection. A field survey was carried out in Tibetan plateau on 38 lakes. Tibetan plateau lakes, located at high elevation, are harsh ecosystems characterized by low temperatures, low available nutrients, high UV amount, and strong salinity gradients. Only well-adapted species can survive in these environments. We therefore hypothesized that environmental filtering was the main factor determining phytoplankton assemblage structure in these lakes, and that species nestedness largely contributed to β-diversity. Spatial analysis showed that both environmental variables and broad-scale spatial variables accounted for the variation of phytoplankton assemblages. In particular, phytoplankton was mainly shaped by spatially structured environmental heterogeneity, which resulted in low α-diversity and high β-diversity. In contrast to our assumption, species turnover contributed to the majority of β-diversity of the studied assemblages. Local environments selected species with the corresponding stress-tolerant traits. Within the narrow range of available niches, environmental filtering of phytoplankton traits induced species’ replacement along environmental gradients.

Spatial heterogeneity of spring phytoplankton in a large tropical reservoir: could mass effect homogenize the heterogeneity by species sorting?

Reservoirs are river–lake hybrid ecosystems characterized by a marked longitudinal zonation and variable flushing rates depending on the use of stored waters. The structure of their phytoplankton is therefore subjected to the interplay between the environmental conditions of the different zones (species sorting) and the strength of the unidirectional flow (mass effect). The spatial distribution of spring phytoplankton was investigated in a tropical reservoir across its different zones. Phytoplankton displayed heterogeneous spatial patterns from the turbulent, nutrient-rich riverine zones to the relatively stable lacustrine zone. The analysis of this spatial heterogeneity revealed the relative importance of species sorting and mass effect in this morphologically complex reservoir. Different taxonomic groups showed different spatial patterns due to their specific physiological and morphological features, and as a result of the local environmental filtering. In the studied reservoir, the strength of the homogenizing effect of water flow did not smooth the spatial heterogeneity and played a weak role in constraining spring phytoplankton. Actually, water flow contributed to the spatial heterogeneity of phytoplankton by enhancing the local environmental differences caused by an operational drop in water level, and species sorting resulted as the primary forcing in determining spring phytoplankton spatial distribution.

Daphnia diversity on the Tibetan Plateau measured by DNA taxonomy

Abstract Daphnia on the Tibetan Plateau has been little studied, and information on species diversity and biogeography is lacking. Here, we conducted a 4‐year survey using the barcoding fragment of the mitochondrial COI gene to determine the distribution and diversity of Daphnia species found across the Plateau. Our results show that species richness is higher than previously thought, with total described and provisional species number doubling from 5 to 10. Six of the taxonomic units recovered by DNA taxonomy agreed well with morphology, but DNA barcoding distinguished three clades each for the D. longispina (D. galeata, D. dentifera, and D. longispina) and D. pulex (D. pulex, D. cf. tenebrosa, and D. pulicaria) complexes. The sequence divergence between congeneric species varied within a large range, from 9.25% to 30.71%. The endemic D. tibetana was the most common and widespread species, occurring in 12 hyposaline to mesosaline lakes. The lineage of D. longispina is the first confirmed occurrence in west Tibet.