Limnology and Oceanography Letters最新文献

This study examines suspended sediment transport affected by flocculation settling in a highly stratified tidal estuary. In situ observation recorded two estuarine front passages during strong-ebb and flood-slack tides, respectively. The strong-ebb front enhanced turbulence, increasing sediment concentrations (~ 5 ppm), macrofloc size (~ 300 μm) and settling efficiency. The surface plume combined with middle layer together thickened from 1.5 to 2.5 m, facilitating particles mixing. The plankton and organic debris were transported into bottom water, forming fluffy, porous macroflocs. In contrast, the flood-slack front developed stable middle barrier layer (~ 2 m) that restricted mixing and organic input, leading to denser, compact flocs (~ 150 μm) in bottom waters. Floc size distributions shifted from microfloc-dominated to macrofloc-dominated during strong-ebb front passing, then returned to smaller, denser flocs. Dissolved-oxygen changes due to sediment oxygen depletion varying. These findings help to improve predictive models for sediment transport and hypoxia dynamics in estuarine systems.

Lake ice cover is declining globally with important implications for lake ecosystems. Ice loss studies often rely on small numbers of lakes with long-term data. We analyzed variation and trends in ice cover phenology from 1213 lakes over 74 yr (1949–2022) in Minnesota (USA), during which ice cover duration declined at a rate of 2 d per decade (14 d total) and became more variable. Despite variation in phenology, just 10–20% of lakes differed from statewide phenological trends. Accounting for synchronous annual variation and estimating trends over long time periods (e.g., > 40 yr) were critical for obtaining robust estimates of ice loss. The constant rates estimated here were consistent with recent global estimates (1.7–1.9 d per decade) and suggest that, even if present, accelerating rates of ice loss could be difficult to detect in the midst of shorter-term periods of warming and increasing variability.

The Kuroshio Current, a western boundary current in the North Pacific Ocean, is regarded as a hotspot of nitrogen fixation that drives marine primary productivity and biogeochemical cycles. However, this assumption is based on limited, spatiotemporally biased data. We curated nitrogen fixation data and applied a generalized additive model to revisit the distribution and total amount of nitrogen fixation in the Kuroshio region. We found a substantial spatial variation, with lower nitrogen fixation in southern Japan and higher rates in the more upstream areas. These variations were explained by nitrate concentration, sea surface temperature, and the distance along the Kuroshio from its origin. We calculated the total nitrogen fixation for the Kuroshio region to be approximately half of the previous estimate. This study provides a more precise estimate of the regional nitrogen fixation and highlights its biogeochemical importance in the Kuroshio region and the western North Pacific Ocean.

Macroalgae are important primary producers in the coastal ocean, and they release a large fraction of their net primary production as dissolved organic carbon (DOC). It is assumed that much of this DOC is recalcitrant and results in the sequestration of large amounts of carbon. We lack sufficient knowledge about the bioavailability of this material and the role of other sinks such as photooxidation. We conducted dark remineralization and photooxidation experiments on DOC derived from an abundant brown macroalga and quantified the fraction of amended DOC that was remineralized by both processes. The bioavailability of the amended DOC ranged from 14% to 99% and was significantly negatively correlated with its phenolic content. Upon exposure to light, the biologically recalcitrant compounds were quickly oxidized to CO₂, indicating that photooxidation is an important sink for recalcitrant brown macroalgal DOC. These results are especially important as macroalgae cultivation is being considered to sequester atmospheric CO₂.

We resolve mangrove nitrous oxide (N₂O), methane (CH₄), and carbon dioxide (CO₂) vertical exchange with the atmosphere and lateral exchange with the ocean. Our new observations in Brazil were combined with literature data to reassess the overall mangrove carbon sequestration potential. The pristine mangrove creek was a source of CO₂ and CH₄, and a sink for N₂O. The CO₂-equivalent N₂O uptake offset up to 71% of local CH₄ emissions. Global mangrove N₂O sinks offset 34% of water–air CH₄ emissions, potentially absorbing 0.6 Tg CO₂ equivalents per year. Overlooking lateral exchange led to a large underestimation of mangrove N₂O and CH₄ fluxes. Previous observations in mangroves receiving nitrogen-rich freshwater may have misattributed N₂O sources. Pristine seawater-dominated mangroves typically act as N₂O sinks while those influenced by anthropogenic inputs are sources. Overall, the observed N₂O sink further enhances the net climate benefit of mangroves.

Coastal waters play a pivotal role in the global carbon cycle, showing increased short-term variability of dissolved oxygen saturation (DOsat) and partial pressure of greenhouse gases like carbon dioxide (pCO₂), especially in underrepresented tropical eutrophic environments. Here, we conducted high-frequency (1-min interval) diel measurements of surface DOsat and pCO₂ in Guanabara Bay, Brazil, a highly nutrient-enriched coastal ecosystem. The predominant metabolic controls on pCO₂ were revealed by its strong negative correlation with DOsat. Air–sea CO₂ fluxes derived from high-frequency diel sampling showed emissions of 533 mmol C m⁻² annually. Conventional estimates based on daylight-only measurements were ~73% and 319% higher in the morning (10:00–12:00 h and sunrise–8:00 h, respectively) or ~172% and 244% lower in the afternoon (12:00–14:00 h and 14:00–16:00 h, respectively). Our findings indicate that rapid diel shifts between CO₂ sinks and sources in eutrophic coastal waters can introduce significant uncertainty in estimating air–water CO₂ fluxes from regional to global carbon budgets.

The formation of initial bivalve shell is sensitive to ocean acidification, encoding the basis of shell formation and environmental information. Here, we demonstrated how the initial shell building processes were affected under various acidified conditions. With decreasing pH, larvae showed smaller shells and higher incidences of deformity. Shell elemental and isotopic profiles suggested that larvae almost exclusively used seawater dissolved inorganic carbon to calcify and exhibited diminished ability to maintain the calcifying fluid homeostasis. Compared to those reared at pH_NBS 8.1, larvae exposed at pH_NBS 7.7 downregulated the expression of genes related to transport of calcification substrates and regulation of carbonate chemistry, all of which were subsequently upregulated at pH_NBS 7.4. This integrated finding advances the application of sclerochronology by providing insights into the initial shell formation, a crucial phase that is overlooked in sclerochronological studies, particularly in how environmental stressors affect the interpretation of geochemical proxies in adult shells.

Adjustments in the metabolism of marine fish are associated with the complexity of resource availability, prey–predator relationships, and biotic and abiotic interactions in the natural environment. To investigate the relationship between metabolism and body mass, this study used a conventional method to estimate the oxygen consumption rate (reflecting the resting metabolic rate) in black porgy, Acanthopagrus schlegelii, over a year of rearing. In addition, we developed a novel metabolic proxy using the δ¹³C values of vertebral structural carbonates to monitor lifelong metabolic changes. The oxygen consumption measurements followed a decreasing mass-dependent trend and yielded a mass-specific allometric exponent scaling (−0.24). By integrating the oxygen consumption with the advanced δ¹³C metabolic proxy, we established a decay model in an increasing form to describe the relationship of the two measurements, and it could be further used in wild fishes and broaden the metabolic studies in marine vertebrates.

The Japanese turban snail Lunella coreensis is sensitive to ocean currents due to its short pelagic larval stage and moderate dispersal ability, making it an ideal model for studying genetic diversity shaped by paleoclimatic shifts. In this study, we analyzed the mitochondrial genes COI and 12S of museum samples collected from various coasts across Japan and identified 10 haplogroups divided into Pacific Ocean and Japan Sea clades, influenced by Kuroshio and Tsushima currents. Divergence time estimates indicate radiation between 3000 and 77,000 yr ago, coinciding with the last ice age, supported by fossil evidence in Japan. Glaciation cycles likely caused genetic isolation and exchange. Rapid radiation between 18,000 and 1000 yr ago aligns with climatic changes during the last glacial maximum. Effective population size estimates indicate past bottlenecks. These findings reveal how historical environmental events shaped L. coreensis genetic diversity, laying the groundwork for future sclerochronological research on marine biodiversity.

Gas transfer velocity ($��k_{600}�$) controls gas fluxes between aquatic ecosystems and the atmosphere. In streams, $��k_{600}�$ is controlled by turbulence and, thus, local hydrology and geomorphology. Resultantly, variability in $��k_{600}�$ can be large and modeling $��k_{600}�$ from physical parameters can have large uncertainty. Here, we leverage a large dataset of $��k_{600}�$ estimates derived from tracer-gas experiments in 22 US streams across a range of discharges. Our analysis shows that $��k_{600}�$ was highly variable both spatially across and temporally within streams, with estimates of $��k_{600}�$ spanning three orders of magnitude. Overall, $��k_{600}�$ scaled with discharge in steep streams due to relatively high stream power, but not in low-slope streams, where stream power was relatively low even at high flows. Understanding how $��k_{600}�$ responds to stream discharge in a wide variety of streams is key to creating temporally and spatially resolved estimates of biogeochemical processes in streams.