Increasing air temperatures may result in stronger lake stratification, potentially altering nutrient and biogenic gas cycling. We assessed the impact of climate forcing by comparing the influence of stratification on oxygen, nutrients, and global-warming potential (GWP) of greenhouse gases (the sum of CH4, CO2, and N2O in CO2 equivalents) emitted from a shallow productive lake during an average versus a heat-wave year. Strong stratification during the heat wave was accompanied by an algal bloom and chemically enhanced carbon uptake. Solar energy trapped at the surface created a colder, isolated hypolimnion, resulting in lower ebullition and overall lower GWP during the hotter-than-average year. Furthermore, the dominant CH4 emission pathway shifted from ebullition to diffusion, with CH4 being produced at surprisingly high rates from sediments (1.2-4.1 mmol m(-2) d(-1)). Accumulated gases trapped in the hypolimnion during the heat wave resulted in a peak efflux to the atmosphere during fall overturn when 70% of total emissions were released, with littoral zones acting as a hot spot. The impact of climate warming on the GWP of shallow lakes is a more complex interplay of phytoplankton dynamics, emission pathways, thermal structure, and chemical conditions, as well as seasonal and spatial variability, than previously reported.
Sediment porewater was analyzed at several sampling dates in two adjacent basins of an oligotrophic boreal lake, one basin perennially oxygenated (Basin A) and the other occasionally anoxic (Basin B). Depth concentration profiles of methane (CH4), dissolved inorganic carbon (DIC), and electron acceptors were modeled with a one‐dimensional transport‐reaction equation to constrain the depth intervals (zones) where solutes are produced/consumed in the top 10 cm of the sediment column, and to obtain the net reaction rates in each zone. This multicomponent geochemical modeling reveals that CH4 was produced below 4–7 cm depth at lower rates in Basin A (250–800 fmol cm−2 s−1) than in Basin B (1900–6500 fmol cm−2 s−1) and that methanogenesis accounted for 30–64% and 84–100% of the sediment organic matter (OM) mineralization in Basins A and B, respectively. We show that methanogenesis did not always yield equimolar amount of CH4 and DIC, as would be expected from the fermentation of the model molecule CH2O. While ∼50% of the CH4 produced in Basin A is oxidized in the sediment column, this proportion decreases to ∼20% in Basin B. Dioxygen is by far the main electron acceptor for CH4 and OM oxidations in both basins. Methanotrophy in the sediment, however, is not limited to the ∼4‐mm thick surface layer in which O2 diffuses from bottom water but occurs down to 4–7 cm depth where O2 is transported through bioirrigation. Thermodynamic calculations suggest that, in addition to O2, Fe oxyhydroxides, and sulfate may serve as oxidants for methanotrophy in that zone. We predict that Basin B sediments release more CH4 than DIC whereas Basin A sediments mainly export DIC. This study highlights that small changes in hypolimnetic O2 levels may significantly alter the magnitude of OM mineralization pathways and the fate of CH4 in boreal lake sediments.
Microplastic research,
initially focusing on marine environments,
left freshwater ecosystems largely unexplored. Freshwaters are also
vulnerable to microplastics and are likely the largest microplastic
supplier to the ocean. However, microplastic sources, transport pathways,
and fluxes at the catchment level remain to be quantified, compromising
efficient actions toward mitigation and remediation. Here we show
that 70–90% of microplastics reaching Norway’s largest
lake, originating primarily from urban waste mismanagement and sludge
application on crops, continue their journey toward the ocean without
being buried. Indeed, our microplastic budget for the catchment shows
that out of the 35.9 tons (7.4–119.4 t) of microplastics annually
released into the lake, only 3.5 tons (1.3–8.8 t) are settling
to the lake bottom. The spatial and vertical microplastic distribution
and diversity in lake sediments, the socio-economic modeling of plastic
fluxes and spatial information on land use and potential plastic sources
all point toward urban and agricultural areas as emission hotspots
of increasing importance. We conclude that the degree to which lake
sediments represent a net microplastic sink is likely influenced by
the nature of microplastics the lake receives, and ultimately on their
origin.
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