Unraveling long-term changes in lake color based on optical properties of lake sediment

Brownification Increases the Abundance of Microorganisms Related to Carbon and Nitrogen Cycling in Shallow Lakes

Brownification in aquatic ecosystems under global change has attracted attention. The composition and quantity of dissolved organic matter transported from various land use types to lakes differ significantly, causing varying ecological effects of lake brownification by region. Bacterial communities make a significant contribution to the material cycle of ecosystems and are sensitive to environmental changes. In this study, a series of mesocosm systems were used to simulate forest lakes and urban lakes with different degrees of brownification, and a high-throughput amplicon sequencing technique was used to explore the changes in the composition, structure, and function of bacterial communities in shallow lakes undergoing brownification. Principal coordinate analysis (PCoA) and Jensen‒Shannon distance typing analysis both indicated significant differences in bacterial communities between forest lakes and urban lakes. The α diversity of bacterial communities in urban lakes increased with the degree of brownification. However, whether forest lakes or urban lakes, brownification increased the abundance of carbon cycling-related bacterial phyla (Proteobacteria, Poribacteria, and Chloroflexi) and nitrogen cycling-related bacterial genera (Microbacteriaceae, Limnohabitans, Comamonadaceae, Bacillus, and Rhizobiales_Incertae_Sedis). Additionally, the carbon and nitrogen cycling functions of bacterial communities in forest lakes are dominant, while those in urban lakes are dominated by functions related to light. Our study has preliminarily revealed that lake brownification promotes the growth of carbon and nitrogen cycling microorganisms, providing a new paradigm for understanding the response of lake ecosystems in different catchment areas to environmental changes and the carbon and nitrogen cycling processes in shallow lake ecosystems.

Properties and metal binding behaviors of sediment dissolved organic matter (SDOM) in lakes with different trophic states along the Yangtze River Basin: A comparison and summary

The nature of sediment dissolved organic matter (SDOM) can reflect the environmental background, nutritional status and human activities and is an important part of lakes. The differences in the binding capacity of heavy metals and organic matter in lake sediments with different trophic states at the catchment scale and the mechanism of the differences in binding are still unclear. To solve this problem, we collected bulk SDOMs (< 0.7 μm) from 6 respective lakes (from upstream to downstream) in the Yangtze River Basin (YRB) to qualitatively and quantitatively characterize their properties and metal binding behaviors using excitation-emission matrix spectroscopy combined with parallel factor analysis (EEM-FARAFAC) and two-dimensional correlation spectroscopy of synchronous fluorescence spectroscopy and Fourier transform infrared spectroscopy (2D-SF-COS and 2D-FTIR-COS). The results showed that sediment dissolved organic carbon (SDOC) was mainly enriched in low molecular weight (LMW: < 1 kDa) fractions. The total fluorescence intensity (F_max) of SDOM from upstream was larger than that from downstream (p = 0.033), and humic-like fluorophores were dominant in these lakes. The F_max of sediment humic-like components (C1+C2) was closely related to the trophic levels of the lakes. Protein-like substances and oxygen-containing functional groups (C-OH, C=O, and C-O) were preferred in the reaction between SDOM and copper (Cu²⁺) or cadmium (Cd²⁺), while a unique binding path was exhibited in the moderately eutrophic DCL. In terms of fluorophore types, higher Cu²⁺-binding abilities (LogK_Cu) were observed in the humic-like matter for the lakes in the upper reaches and tryptophan-like matter for the lakes from the midstream and downstream areas of the YRB. Although Cd²⁺ complexed only with humic-like matter, LogK_Cd was higher than LogK_Cu. In terms of molecular weight (MW), the LogK_Cu/Cd of components were enhanced after MW fractionation. The HMW (0.7 μm - 1 kDa) components possessed higher LogK_Cu in most lakes (except for CHL and C4). The different fluorophores and molecular weight fractions in SDOM make an important contribution to reducing the ecological risks of heavy metals in lakes.

A review of long-term change in surface water natural organic matter concentration in the northern hemisphere and the implications for drinking water treatment

Reduced atmospheric acid deposition has given rise to recovery from acidification - defined as increasing pH, acid neutralization capacity (ANC), or alkalinity in surface waters. Strong evidence of recovery has been reported across North America and Europe, driving chemical responses. The primary chemical responses identified in this review were increasing concentration and changing character of natural organic matter (NOM) towards predominantly hydrophobic nature. The concentration of NOM also influenced trace metal cycling as many browning surface waters also reported increases in Fe and Al. Further, climate change and other factors (e.g., changing land use) act in concert with reductions in atmospheric deposition to contribute to widespread browning and will have a more pronounced effect as deposition stabilizes. The observed water quality trends have presented challenges for drinking water treatment (e.g., increased chemical dosing, poor filter operations, formation of disinfection by-products) and many facilities may be under designed as a result. This comprehensive review has identified key research areas to be addressed, including 1) a need for comprehensive monitoring programs (e.g., larger timescales; consistency in measurements) to assess climate change impacts on recovery responses and NOM dynamics, and 2) a better understanding of drinking water treatment vulnerabilities and the transition towards robust treatment technologies and solutions that can adapt to climate change and other drivers of changing water quality.

Assessing the current water clarity status of ~100,000 lakes across southern Canada: A remote sensing approach

Canada has more lakes than any other country, making comprehensive monitoring a huge challenge. As more and more satellite data become readily available, and as faster data processing systems make massive satellite data operations possible, new opportunities exist to use remote sensing to develop comprehensive assessments of water quality at very large spatial scales. In this study, we use a published empirical algorithm to estimate Secchi depth from Landsat 8 reflectance data in order to estimate water clarity in lakes across southern Canada. Combined with ancillary information on lake morphological, hydrological, and watershed geological and landuse characteristics, we were able to assess broad spatial patterns in water clarity for the first time. Ecological zones, underlying geological substrate, and lake depth had particularly strong influences on clarity across the whole country. Lakes in western mountain ecozones had significantly clearer waters than those in the prairies and plains, while lakes in sedimentary rock formations tended to have lower clarity than lakes in intrusive rock. Deep lakes were significantly clearer than shallow lakes over most of the country. Water clarity was also significantly influenced by human impact (urbanization, agriculture, and industry) in the watershed, with most lakes in high impact areas having low clarity or very low clarity. Finally, we used in situ measured data to help interpret the underlying optical water column constituents influencing clarity across Canada, and found that chlorophyll-a, total suspended solids, and color dissolved organic matter all had strong but varying underlying effects on water clarity across different ecozones. This research provides an important step towards further research on the relationship between water column optical properties and the health and vulnerability status of lakes across the country.

Ecology and extent of freshwater browning - What we know and what should be studied next in the context of global change

Water browning or brownification refers to increasing water color, often related to increasing dissolved organic matter (DOM) and carbon (DOC) content in freshwaters. Browning has been recognized as a significant physicochemical phenomenon altering boreal lakes, but our understanding of its ecological consequences in different freshwater habitats and regions is limited. Here, we review the consequences of browning on different freshwater habitats, food webs and aquatic-terrestrial habitat coupling. We examine global trends of browning and DOM/DOC, and the use of remote sensing as a tool to investigate browning from local to global scales. Studies have focused on lakes and rivers while seldom addressing effects at the catchment scale. Other freshwater habitats such as small and temporary waterbodies have been overlooked, making the study of the entire network of the catchment incomplete. While past research investigated the response of primary producers, aquatic invertebrates and fishes, the effects of browning on macrophytes, invasive species, and food webs have been understudied. Research has focused on freshwater habitats without considering the fluxes between aquatic and terrestrial habitats. We highlight the importance of understanding how the changes in one habitat may cascade to another. Browning is a broader phenomenon than the heretofore concentration on the boreal region. Overall, we propose that future studies improve the ecological understanding of browning through the following research actions: 1) increasing our knowledge of ecological processes of browning in other wetland types than lakes and rivers, 2) assessing the impact of browning on aquatic food webs at multiple scales, 3) examining the effects of browning on aquatic-terrestrial habitat coupling, 4) expanding our knowledge of browning from the local to global scale, and 5) using remote sensing to examine browning and its ecological consequences.

Hypolimnetic assimilation of ammonium by the nuisance alga Gonyostomum semen

Gonyostomum semen is a bloom-forming freshwater raphidophyte that is currently on the increase, which concerns water managers and ecologists alike. Much indicates that the recent success of G. semen is linked to its diel vertical migration (DVM), which helps to overcome the spatial separation of optimal light conditions for photosynthesis at the surface of a lake and the high concentration of phosphate in the hypolimnion. I here present data from a field study conducted in Lake Lundebyvannet (Norway) in 2017-2019 that are consistent with the idea that the DVM of G. semen also allows for a hypolimnetic uptake of ammonium. As expected, microbial mineralization of organic matter in a low-oxygen environment led to an accumulation of ammonium in the hypolimnion as long as G. semen was absent. In contrast, a decreasing or constantly lower concentration of hypolimnetic ammonium was found in presence of a migrating G. semen population. In summer of 2019, a short break in the DVM of G. semen coincided with a rapid accumulation of hypolimnetic ammonium, which was equally rapidly decimated when G. semen resumed its DVM. Taken together, these data support the idea that G. semen can exploit the hypolimnetic pool of ammonium, which may be one reason for the recent success of the species and its significant impact on the structure of the aquatic food web.