Brief communication: ICESat-2 reveals seasonal thickness change patterns of Greenland Ice Sheet outlet glaciers for the first time

Dynamic changes of marine-terminating outlet glaciers are projected to be responsible for about half of future ice loss from the Greenland Ice Sheet. However, we lack a unified, process-based understanding that can explain the observed dynamic changes of all outlet glaciers. Many glaciers undergo seasonal dynamic thickness changes and classifying the patterns of seasonal thickness change can improve our understanding of the processes that drive glacier behavior. The Ice, Cloud and land Elevation Satellite (ICESat-2) provides the first space-based, seasonally repeating altimetry measurements of the ice sheets, allowing us to quantify near-termini seasonal dynamic thickness patterns of 34 outlet glaciers around the Greenland Ice Sheet. We classify the glaciers into seven common patterns of seasonal thickness change over a two-year period from 2019 to 2020. We find small groupings of neighboring glaciers with similar seasonal thickness change patterns but, within larger sectors of the ice sheet, seasonal thickness change patterns are heterogeneous. Comparing the seasonal thickness changes to average glacier ice flow speeds, we find that faster glaciers typically undergo patterns of spring and summer dynamic thickening, while slower glaciers exhibit a variety of thickness change patterns. Future studies can build upon our results by comparing seasonal dynamic thickness changes with external forcings, such as ocean temperature and meltwater runoff, and with other dynamic variables such as seasonal glacier velocity and terminus position changes.

Seasonal variation of Microcystis aeruginosa and factors related to blooms in a deep warm monomictic lake in Mexico

The occurrence of cyanobacterial blooms has increased globally over the last decades, with the combined effect of climate change and eutrophication as its main drivers. The seasonal dynamic of cyanobacterial blooms is a well-known phenomenon in lakes and reservoirs in temperate zones. Nevertheless, in the tropics, most studies have been performed in shallow and artificial lakes; therefore, the seasonal dynamic of cyanobacterial blooms in deep and eutrophic tropical lakes is still under research. We studied the seasonal variation of the phytoplankton community and the factors associated with Microcystis aeruginosa blooms along the water column of Lake Alberca de Tacámbaro, a warm monomictic crater lake located in Mexico, during 2018 and 2019. According to previous studies performed in 2006 and 2010, this lake was mesotrophic-eutrophic, with Chlorophyta and Bacillariophyta as the dominant groups of the phytoplankton community. During 2018 and 2019, the lake was eutrophic and occasionally, hypertrophic, a phenomenon likely associated with the increase of farmland area around the lake. The dominant species was M. aeruginosa, forming blooms from the surface to 10 m depth in winter, in the hypolimnion in spring and summer, and along the full water column in autumn. These findings suggest that M. aeruginosa in Lake Alberca de Tacámbaro displays seasonal and spatial population dynamics. Total phosphorus, dissolved inorganic nitrogen, water temperature and photosynthetically active radiation were the environmental factors related to M. aeruginosa blooms. Our results suggest that the changes in the structure of the phytoplankton community through time, and M. aeruginosa blooms in Lake Alberca de Tacámbaro, are mainly related to changes in land use from forest to farmland in areas adjacent to the lake, which promoted its eutrophication in the last years through runoffs. Comparative studies with other deep and eutrophic lakes will allow us to gain a deeper understanding of the dynamic of cyanobacterial blooms in natural and artificial water reservoirs strongly stressed by human activities.

A framework for seasonal variations of hydrological model parameters: impact on model results and response to dynamic catchment characteristics

Previous studies have shown that the seasonal dynamics of model parameters can compensate for structural defects of hydrological models and improve the accuracy and robustness of the streamflow forecast to some extent. However, some fundamental issues for improving model performance with seasonal dynamic parameters still need to be addressed. In this regard, this study is dedicated to (1) proposing a novel framework for seasonal variations of hydrological model parameters to improve model performance and (2) expanding the discussion on model results and the response of seasonal dynamic parameters to dynamic characteristics of catchments. The procedure of the framework is developed with (1) extraction of the dynamic catchment characteristics using current data-mining techniques, (2) subperiod calibration operations for seasonal dynamic parameters, considering the effects of the significant correlation between the parameters, the number of multiplying parameters, and the temporal memory in the model states in two adjacent subperiods on calibration operations, and (3) multi-metric assessment of model performance designed for various flow phases. The main finding is that (1) the proposed framework significantly improved the accuracy and robustness of the model; (2) however, there was a generally poor response of the seasonal dynamic parameter set to catchment dynamics. Namely, the dynamic changes in parameters did not follow the dynamics of catchment characteristics. Hence, we deepen the discussion on the poor response in terms of (1) the evolutionary processes of seasonal dynamic parameters optimized by global optimization, considering that the possible failure in finding the global optimum might lead to unreasonable seasonal dynamic parameter values. Moreover, a practical tool for visualizing the evolutionary processes of seasonal dynamic parameters was designed using geometry visualization techniques. (2) We also discuss the strong correlation between parameters considering that dynamic changes in one parameter might be interfered with by other parameters due to their interdependence. Consequently, the poor response of the seasonal dynamic parameter set to dynamic catchment characteristics may be attributed in part to the possible failure in finding the global optimum and strong correlation between parameters. Further analysis also revealed that even though individual parameters cannot respond well to dynamic catchment characteristics, a dynamic parameter set could carry the information extracted from dynamic catchment characteristics and improve the model performance.

Seasonal Dynamic of CDOM in a Shelf Site of the South-Eastern Ligurian Sea (Western Mediterranean)

Chromophoric dissolved organic matter (CDOM) is the fraction of the Dissolved Organic Carbon (DOC) mainly absorbing UV and blue radiation, influencing water optical properties, light availability for primary production, and water-leaving radiance. In open seas, phytoplankton is the main source of organic carbon and CDOM. Despite this, the direct or indirect phytoplankton role in CDOM production is not yet fully clarified. From studies about the relationship between CDOM and phytoplankton biomass as Chlorophyll a (Chl) in the epipelagic layer, positive correlations have been highlighted with regional differences and high levels of variability. Seven years of seasonal dynamic and vertical distribution of CDOM in the Ligurian Sea continental shelf waters have been analyzed in order to evidence the main environmental and/or biological factors determining CDOM dynamic, focusing on the CDOM/Chl relationship. CDOM optical properties (absorption at 440 nm, aCDOM (440), and spectral slope, S) allowed to distinguish different pools and to debate their origin. Four different pools were characterized and two of them were directly or indirectly related to phytoplankton biomass and taxonomic composition. Nevertheless, CDOM/Chl confirm a high level of variability These findings suggest some inputs to improve Mediterranean satellite estimates of Chl and CDOM, such as the seasonal differentiation of optical properties, especially S and CDOM/Chl relationships.