Jiawei Fan

Spatial heterogeneity of the population age structure of the ostracode Limnocythere inopinata in Hulun Lake, Inner Mongolia and its implications

Most existing studies of lacustrine ostracodes have been focused on species composition, and little attention has been paid to population structure. In this study, 39 surface-sediment samples from Hulun Lake, Inner Mongolia, were analyzed for ostracode species composition and 17 of them also for population structure of Limnocythere inopinata, which is the dominant ostracode in the lake. A total of nine ostracode species were recovered, most of which show high abundances at sites with rich plant detritus, implying the possible control of food supply on ostracode species occurrence in the lake. On the other hand, both the subfossil and the living populations of L. inopinata have greater numbers of early instars in deeper waters, while middle and late instars occur mostly in shallower waters, which is related to the grainsize composition of the substrate, denoting a hydraulic control on population structure. Pre- and post-mortem transport of ostracodes may be responsible for the observed within-lake changes in the population structure, although other factors such as migration, intrinsic population dynamics, and life cycle could also be involved. Our data provide insights into the interpretation of lake conditions when using ostracodes in paleolimnological research of lakes with similar natural settings.

Differential transport and preservation of the instars of Limnocythere inopinata (Crustacea, Ostracoda) in three large brackish lakes in northern China

Population age structure, preservation, and carapace abundance of the valve remains of Limnocythere inopinata in 88 surface-sediment samples from Lakes Daihai, Dali, and Hulun were examined to better understand taphonomy of limnic ostracods. The spatial changes in population structure of L. inopinata match the within-lake hydraulic gradients, with the older instars preferentially deposited in the littoral zone while the younger instars being more abundant in deeper waters. We propose that molt and mortem remains of ostracods, especially those of the small juveniles, experience downslope transport. The magnitude of transport in a lake is controlled by wind-driven hydrodynamics. There is no chemical dissolution of ostracod valves in these alkaline lakes, and degradation is mainly represented by physical breakage. The preservation of L. inopinata is related to sedimentation rate and hydraulic stability, and the spatial pattern of preservation varies between lakes. Spatial and seasonal changes in sedimentation rate and hydraulic condition, and the ostracod life cycle may have contributed to this complexity. Adult and A-7 carapaces are the most frequent among the instars. We suggest that the preferential preservation of juvenile carapaces is either related to high mortality rate or to the good preservation in deep waters.

The manifestation of the Younger Dryas event in the East Asian summer monsoon margin: New evidence from carbonate geochemistry of the Dali Lake sediments in northern China:

The processes and mechanisms of the Younger Dryas (YD) event in the modern northern margin of the East Asian summer monsoon (EASM) are still heatedly debated. This study presents new high-resolution (~25 years) records of elements and stable isotopes of <38-µm calcites from a sediment core from Dali Lake in order to investigate the climatic change in the EASM margin at the last glacial–interglacial transition. The <38-µm calcites in the Dali Lake sediments are cubical or rhombohedral, implying that they are predominated by endogenic calcites precipitated within the water body of the lake. High values of Ca and Mg concentrations of the endogenic calcites are interpreted as strong evaporation and low dissolved CO2 concentration of the lake water related to high regional temperature. Concurrent increases in δ13C and δ18O values of the endogenic calcites are interpreted as the result of intensified evaporation associated with high temperature or low precipitation in the region. These data indicate that the climate in the Dali Lake region was relatively warm and wet from 15,500 to 12,800 and from 11,550 to 10,000 cal. yr BP, and cold and dry from 12,800 to 11,550 cal. yr BP, which was generally supported by the evidence from the data of sedimentary organic matter from the same sediment core. In addition, the abruptness of the temperature change in the Dali Lake region from 12,800 to 11,550 cal. yr BP could be corresponded, within age uncertainties, to the YD cold reversal occurring over northern high latitudes. The atmospheric coupling between the North Atlantic region and the EASM margin was proposed as the dominant pattern influencing the climatic change in the Dali Lake region during the YD event.

Phenotypic plasticity of the gastropod Melanoides tuberculata in the Nile Delta: A pollution-induced stabilizing selection

To understand the effect of metal pollution on the speciation process, we conducted comparative analyses of six populations of the gastropod Melanoides tuberculata, which dominated the Manzala lagoon (Nile Delta, Egypt). Geometric morphometric analysis was implemented to quantify the phenotypic plasticity of the species. The results from both Canonical Variate Analysis and Relative Warp indicated an overall decrease in the morphological breadth of M. tuberculata in the polluted sites. The favored phenotypes in the polluted sites have moderate whorl section, moderate ovate aperture, less-prominent radial ornament, and overall moderate-spired shells. Lack of morphological variations and dominance of intermediate phenotypes in the polluted sites indicate that stabilizing selection is driving the morphological pattern of this species. Moreover, analysis by using the partial least square model confirmed that metal pollution is the major predictor of the observed shape variations, whereas other biotic/abiotic traits are a minor predictor.