Under the background of global change, the climate and ecosystems in the ice-free areas around the Antarctica are undergoing rapid changes. A better understanding of the impact of climate change on the ecosystem helps us predict possible changes in eco-ecosystems of the Antarctic in the future. Penguins are the most representative animals in the Antarctic continent. They are also considered as "biological indicators" of climate change. The response of the penguin population to climate change in historical periods has always been the study focus for many scholars. The Antarctic continent has experienced significant climatic and environmental changes since the onset of the Holocene, resulting in significant changes in the penguin population and lake ecosystems in ice-free areas. Due to the complex environment and human influences, characteristics of climate and environmental changes in different ice-free areas around the Antarctic in the past did not follow a uniform pattern. In particular, the climate and environmental changes in the Middle and Late Holocene showed significant spatial differences, and this may be a result of chronology uncertainty and the unfavorable resolution of the natural archives. In addition, the response of penguins to climate change in the Antarctica is complex. It is essential to reconstruct more records of penguin’s populations in different ice-free areas, and to obtain its temporal and spatial characteristics, so as to fully understand the impact of climate change on penguin ecology. However, according to current research, our understanding of late Holocene climatic change and its ecological response process in the Antarctic ice-free region remains limited, especially in the ice-free area in Victoria Land of the Ross Sea. Based on previous field investigations in Victoria Land, this study analyzed several lake sedimentary profiles. Environmental geochemical proxies such as basic physical and chemical parameters, inorganic elements, sedimentary pigments, carbon and nitrogen isotopes were analyzed, and their climatic and eco-environmental significance were discussed. Based on such data, reliable chronological frameworks were established for each sediment profile. Positive Matrix Factorization models (PMF) and Generalized Additive Models (GAMs) were employed to reconstruct the late Holocene climatic changes, ecological records of penguins, as well as aquatic algal community structures on Inexpressible Island, the Ross Sea. Their coupling mechnism were further examined. The main results obtained are as follows:

1. Record of climate and environmental changes over the past 3,000 years in Inexpressible Island.
The basic physicochemical properties and isotopic geochemistry of the four lake sediment profiles collected from Inexpressible Island were analyzed, and the sources of organic matter in the lake sediments were identified. Typical lake sedimentary profiles with aquatic algae (or microbial mats) and penguin guano as the main sources of organic matter were selected for chronological analysis. A two-endmember mixture model for stable carbon isotopes was used to quantify the relative contributions of the microbial mats and the penguin guano to the bulk carbon, and then the AMS¹⁴C dating results were calibrated by appropriate age calibration database. The IIL1 and IIL4 profiles were strongly affected by the penguin guano, and their bottom (oldest) samples were dated to 1659 and 4820 a BP, respectively, using a Mixed Marine SoHem mode. By contrast, the organic matter of IIL3 and IIL9 sediments was predominantly derived from aquatic microbial mats, and the bottom ages of these two cores were 3179 and 2945 a BP, respectively, based on a SHCal13 mode. The dating results revealed that the ice sheet in the ice-free area of the Inexpressible Island melted no later than 4,800 years ago, since then sediments began to accumulate in the lakes. Based on the Bayesian model, chronological frameworks of four sedimentary profiles were established, and mass accumulation rates (MAR) of the sedimentary profiles were further obtained. The MAR of the four sediment profiles showed peaks during ~1400-800 a BP, indicating a relatively warm period occurred in the study area. This time most likely corresponds to a well-recognized climate event, i.e. the Medieval Climate Anomaly (MCA). This provides new evidence for understanding the late Holocene climate and environmental changes in the Ross Sea region and the ice-free areas around the Antarctica.

2. Impact of climate and environmental changes on the paleoecology of penguins in Inexpressible Island.
According to the source identification of sedimentary organic matter in the lacustrine sediments, profiles IIL1 and IIL4 were significantly influenced by the penguin guano, and thus they were selected to reconstruct the historical record of penguin’s paleoecological evolution. According to distribution characteristics of inorganic elements in the sediment samples, the PMF model and some statistical methods were used to analyze the changes in the proportions of source materials including the penguin guano and other substances in the sediments. Combined with the AMS¹⁴C-based dating results of penguin subfossils, the paleoecological process of penguins was reconstructed by the proxy of the penguin guano input proportion in the ornithogenic sediments. Penguin populations recorded from the IIL1 and IIL4 profiles peaked around 1300-650 a BP, and ages of penguin bone and feather obtained from the ornithogenic sediments were also concentrated at 1350-1200 a BP. This corresponded to the warmer climatic conditions that occurred during the MCA on the Inexpressible Island. During the MCA, the positive state of the Southern Annular Mode (SAM) and the frequent occurrence of the El-Niño enhanced the upwelling of the Modified Circumpolar Deep Water (MCDW) and reduced the coastal sea ice extent, providing favorable conditions for penguins. At the same time, the expanded ice-free area during the MCA provided more habitats for penguins’ breeding. Our results revealed that the paleoecological evolution process of the penguins on the Inexpressible Island was closely linked with historical atmospheric and oceanic circulation, temperature and sea ice area, which provided a basis for examining and predicting the regional response of the Antarctic penguin population to future global climate change.

3. Aquatic algal community changes in the Ross Sea ice-free area and its controlling factors.
The sedimentary profiles receiving different impacts of penguin guano and modern microbial mats were selected from the Ross Sea region of East Antarctica, and the contents of various sedimentary pigments were quantitatively analyzed by HPLC-MS. The results showed that the pigments in the lake sediments with different organic matter sources have generally been well preserved, although some sedimentary pigments were degraded a little bit. Green algae, cyanobacteria, diatoms, pyrrophyta and cryptophytes were determined in the lake sediments, in consistent with the species of modern microbial mats. However, the changes in sedimentary pigments on the profile were complicated and this is mainly related to the nutrient input via bio-transportation and climate changes. GAMs models were used to evaluate the impact of climate change and nutrients input associated with penguin’s bio-transportation on lake algal community structure and primary productivity. The results indicated that nutrient input promoted the growth of green algae, cyanobacteria, diatoms and cryptophytes to varying degrees, thereby enhancing the primary productivity of the lake, among which the contribution to the biomass of green algae was the most obvious. The warming climatic conditions had promoted the growth of algae over the past 200 years. The results of GAMs quantitative analysis also showed that the composition of aquatic algae community in the Antarctic lakes affected by penguin feces was largely controlled by the guano nutrient input. However, for the lakes without the penguin guano, in which the ecological evolution of aquatic algae was controlled by complex factors, climate change might not be the only controlling factor.

4. Ecological evolution process and impact mechanism of modern penguins at Adélie Cove.
The chronology was established from ¹³⁷Cs and AMS¹⁴C analysis on the AC2 sedimentary profile collected from Adélie Cove, and the results showed that the core has a history of approximately 270 years old. Variations of 15 inorganic elements including Cu, Zn, Co, Ni, Cr, Al, Fe, Ca, Mg, K, Na, Mn, Ba, Ti, and P in the AC2 profile were analyzed, and the relative proportion change of sedimentary material sources such as the penguin guano in the AC2 profile was reconstructed by PMF model. Furthermore, factor score changes of the guano source in the sedimentary profile were obtained through the Principal Component Analysis (PCA); the change in guano input ratio was calculated based on a two-endmember mixing model; the lake primary productivity changes was reconstructed by sedimentary pigments. GAMs were used to comprehensively analyze the above four sedimentary records related to the guano input, and then the record of penguin population over the past ~270 years at Adélie Cove was reconstructed. The results showed that the population of Adélie penguins in the study area had changed significantly in the past. The rapid increase in the penguin population during 1775-1875 AD might be related to the expansion of coastal polynyas and the enhancement of upwelling; the slow increase of the penguin population from 1875 to 1950 might be attributed to the increase in poleward heat flux and favorable temperature. After 1950, the penguin population decreased, which can be explained by large-scale climate and environmental changes and enhanced human activities.