Limnology and Oceanography最新文献

The significant contribution of phytoplankton to global primary production is regulated by several abiotic and biotic factors, which are often difficult to account for in natural systems. To address these challenges, we perturbed a summer plankton community from Narragansett Bay, Rhode Island, USA, by manipulating the temperature and nutrient conditions in a controlled short-term incubation experiment and tracked changes in phytoplankton community structure in response to fluctuations in phytoplankton physiology and microzooplankton grazing. Water was incubated at the in situ temperature (22°C) and at deviations from that temperature (± 4°C) with both macronutrient amendments (N, P, and Si addition) and unamended controls. We found nutrient availability and microzooplankton grazing to have pronounced and opposite impacts on phytoplankton size composition, with nutrient amendments shifting the phytoplankton community toward larger cells and grazing rates correlated with smaller phytoplankton communities and low-nutrient conditions. Nutrient amendments also altered cellular elemental ratios by increasing chlorophyll : C. Conversely, temperature was not found to have a direct impact on size or elemental stoichiometry, but did influence community composition. These findings paralleled prior observations from lab and field studies in Narragansett Bay which together suggest that over shorter timescales, nutrient availability may have a greater impact on phytoplankton community composition than temperature or grazing, altering phytoplankton nutritional value for higher trophic levels and as a result, secondary production. Thus, understanding underlying nutrient dynamics will be necessary to decipher how short-term changes in temperature or grazing may impact phytoplankton communities and the ecosystems they support.

Coccolithophores are abundant marine microalgae that produce a cell cover (coccosphere) composed of calcium carbonate platelets (coccoliths), and that play a unique role in the global carbon cycle. Gephyrocapsa huxleyi is the most prevalent coccolithophore in the oceans. Experiments in vitro indicate that the coccosphere size in G. huxleyi increases or decreases during phosphorus (P) or nitrogen (N) deprivation, respectively. To test whether coccosphere size variation occurs in native populations and relates to specific macronutrient availability, we examined communities at the open sea “station A” in the Gulf of Aqaba, northern Red Sea, and at the open sea station THEMO-2 in the Eastern Mediterranean Sea. At station A, the average diameter of G. huxleyi coccospheres was larger during the stratified, oligotrophic season (6.98 ± 1.29 μm), and smaller during the mesotrophic winter (6.16 ± 1.04 μm), and along the deep-chlorophyll maximum during stratified periods (6.36 ± 1.09 μm). A similar seasonal variation in coccosphere diameter was observed in the sister species Gephyrocapsa ericsonii. Complementary bioassays indicate that G. huxleyi was primarily P-limited during the stratified period, while non-limited or weakly P-limited in winter. The seasonal pattern in G. huxleyi diameter was the opposite at the Mediterranean THEMO-2 station. Larger coccospheres were detected in winter and smaller ones during summer. However, this pattern coincided with the prevalent patterns of inorganic macronutrient availability in the Eastern Mediterranean. Therefore, inorganic phosphorus availability is a key driver of coccosphere size variations in marine ecosystems.

The metabolism of heterotrophic bacteria acts as a key control on the turnover of organic matter in the ocean. However, much remains unknown about how nutrient availability, particularly iron concentration, impacts bacterial growth. In the dimly lit waters of the lower photic and upper mesopelagic zones, the attenuation of sinking particulate flux is intense, due in part to remineralization by heterotrophic bacteria. In the North Pacific Subtropical Gyre, dissolved iron concentrations display a subsurface minimum near the base of the photic zone, and this is also a region where bacteria have elevated cellular iron demands. In a series of field experiments, we examined how the availability of iron, nitrogen, and organic carbon impacts bacterial metabolism in the lower photic zone. Results of these experiments suggest that low iron conditions limit the turnover of organic carbon by bacteria, potentially enhancing the efficiency of organic carbon export to the deep sea. Uptake of both dissolved iron and inorganic nitrogen by heterotrophic bacteria was greatest when bacteria metabolized carbon-rich organic substrates, typical of the lower photic zone, compared to particulate material collected in the near-surface ocean. Vertical changes in the composition of organic matter may be an important control on bacterial cellular iron requirements, potentially pushing this community toward iron limitation in the interior waters of the ocean.

Organotrophic denitrification is an important nitrogen (N) removal process in lakes, but alternative N reduction processes such as lithotrophic sulfur (S)-oxidizing denitrification may be greatly underappreciated. We studied the redox transition zone (RTZ) in the meromictic water column of the North Basin of Lake Lugano (Switzerland) to characterize N transformation pathways coupled to the S and carbon (C) cycles. Incubations with ¹⁵N-labeled and unlabeled nitrate (NO₃⁻) revealed low denitrification rates and a general limitation of organic electron donors. The most accessible fractions of exported primary production biomass may have been largely consumed in the oxic water column during sedimentation and did not reach the RTZ at ~ 100 m depth. Conversely, sulfide (H₂S) and methane (CH₄), major end products of anaerobic degradation of the more recalcitrant organic carbon fractions in the sediment, represent a continuous source of energy to the RTZ, fostering the establishment of a community of S- and CH₄-dependent NO₃⁻ reducers, dominated by Sulfuritalea and Candidatus Methylomirabilis over several years of observation. Anoxic incubation experiments with H₂S amendments revealed a strong stimulation of dissimilatory NO₃⁻ reduction to ammonium (NH₄⁺) (DNRA), but not denitrification. High relative abundances of the archaeal NH₄⁺ oxidizer Candidatus Nitrosopumilus and bacterial nitrifiers indicate intense NO₃⁻ regeneration by nitrification in the upper RTZ. The potential interaction between nitrification and S-driven DNRA is unclear. However, their co-occurrence suggests that, at least under conditions of carbon limitation, N recycling between the NO₃⁻ and ammonium pools predominates over N removal via complete denitrification.

Coastal wetlands are critical in global carbon sequestration, but their biogeochemical cycling is highly sensitive to sediment mixing. Here, we quantified the impacts of storm-induced mixing on organic carbon (OC) storage across China's coastal wetlands using multiple radionuclides and machine learning (SHAP model). Field observations and SHAP results identified the storm surge as a key driver of sediment mixing, whose impact weakens with increasing offshore distance and vegetation cover. The vegetated, supratidal site in the Yellow River estuary wetland was less affected by the storm surge with a sediment mixing depth of only 18 cm. Sediment mixing was observed at 80% of stations from the literature in China's coastal wetlands, mainly driven by river discharge and waves. Furthermore, a negative correlation between sediment mixing depth and soil organic carbon (SOC) density (r = −0.42) in the top meter was found. For regions with SOC density > 5 kg m⁻², the depths of sediment mixing are lower than 40 cm. Under the medium-forcing scenario SSP2-4.5 and the high-forcing scenario SSP5-8.5, the average sediment mixing depth in China's coastal wetlands is expected to increase by 19% and 28% by 2100, leading to corresponding declines in SOC density of 2.5% and 3.5%. The findings show that intensified sediment mixing caused by climate warming will weaken the carbon storage capacity of coastal wetlands, providing a crucial scientific basis for the sustainable management of coastal wetlands and the formulation of carbon sink enhancement strategies.

Nitrification is a key process in the aquatic nitrogen (N) cycle, but its products, nitrate (NO₃⁻) and nitrous oxide (N₂O), contribute to eutrophication and greenhouse gas emissions, particularly in eutrophic lakes. Variations in in-lake N cycling and N₂O production pathways, as a function of seasonality and artificial oxygenation, remain poorly understood. We investigated nitrification in the artificially oxygenated eutrophic Lake Baldegg, by analyzing NO₃⁻ and N₂O concentrations and isotope ratios, and measuring ammonium oxidation rates via ¹⁵N tracer incubations over one year. An N isotope mass-balance model revealed that nitrification sustained only 5.3 ± 0.7% of total NO₃⁻ consumption in the epilimnion, where external N loadings were influential, and considerably more in the hypolimnion (81.6 ± 18.5%) during stratification. Dual NO₃⁻ isotope signatures (Δδ¹⁸O : Δδ¹⁵N ~ 1.5–1.73) and associated negative NO₃⁻ isotope anomalies confirmed epilimnetic nitrification, though external inputs partly obscured this signal. During stratification, relatively high hypolimnetic nitrification rates correlated with organic matter export, and seemed linked to sediment resuspension and artificial oxygenation. While sedimentary denitrification/DNRA dominated hypolimnetic NO₃⁻ reduction (with negligible effects on δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻), transient suboxic conditions enabled water column denitrification during stratification (24.1–30.2% of the total hypolimnetic denitrification). High N₂O isotope site-preference values (30–35‰) confirmed hypolimnetic ammonium oxidation as the main N₂O production pathway. During winter overturn, N₂O transport from the hypolimnion caused epilimnetic N₂O oversaturation and atmospheric emissions up to 3.52 μmol m⁻² d⁻¹. Comparison with other lakes suggests that artificial oxygenation enhances N turnover, manifesting in greater ambient N₂O backgrounds and fluxes to the atmosphere.

Cold seeps are biodiversity hotspots that significantly affect sediment geochemistry in marine environments. Although seepage-driven substrate modifications are ecologically significant, their interactions with benthic community structure remain poorly understood. This knowledge gap largely reflects the challenge of obtaining high-resolution seafloor data to capture the fine-scale organisms-substrate relationships. Here, we used high-resolution seafloor imagery to investigate a seepage area offshore northern Svalbard (~ 150 m of water depth). Two orthomosaics (spanning ~ 2261 m² and generated through photogrammetry applied to underwater videos collected using a remotely operated vehicle) were analyzed to classify visible epibenthic fauna and describe seafloor substrate changes. Epibenthic fauna was annotated to the lowest possible taxonomic level, while object-based image analysis facilitated a quantitative and repeatable classification of substrates into four distinct classes. Integrating faunal and substrate data allowed us to quantify community patterns relative to seafloor morphometric parameters. The network plot revealed substrate class similarities and faunal colonization preferences, particularly where methane-derived authigenic carbonates are present. Our results demonstrate that seep-associated substrates play a crucial role in shaping benthic community structure. However, methane-derived authigenic carbonate formation further amplifies these effects, although its relationship with specific sediment types (e.g., coarse- or fine-grained) remains unclear. This study demonstrates a robust framework for future ecological assessments of seep environments, emphasizing the interplay between gas seepage, sedimentological attributes, and their combined impact on benthic community structure.

In aquatic ecosystems, greater food web complexity is theorized to increase persistence and resilience of primary production to pulse disturbances, yet experimental evidence is limited. We simulated two storm-induced pulse disturbances by adding nutrients (~ 3%–5% increase in ambient concentrations) to three ponds with low, intermediate, and high food web complexity and compared to reference ponds. We evaluated the ecological stability of primary production by quantifying persistence as the number of days it took chlorophyll-a or ecosystem metabolism to deviate significantly from reference conditions and resilience as the time to recover to reference conditions following each disturbance. We also evaluated if a critical transition occurred following the disturbance. The high complexity pond did not significantly deviate from reference conditions following either nutrient pulse, suggesting high ecological stability. The intermediate complexity pond had lower stability, with persistence relatively consistent at 18 and 24 d after each nutrient pulse, and resilience trending toward a substantial increase from 23 d to less than a week before the experiment concluded. Stability was lowest in the low complexity pond where persistence decreased from 24 d to just 8 d and resilience decreased from 5 to 22 d. There was also evidence of a critical transition after the first pulse in the low complexity pond, but not for higher complexity ponds. This experiment provides strong support that food web connectivity and food chain length can aid in buffering aquatic ecosystems against increasing and intensifying by influencing persistence and resilience to repeated nutrient pulses.

Sandy beaches are an important passage for the transport of various forms of nitrogen from land to sea. However, export fluxes of these forms of nitrogen and the mechanisms controlling their transformation remain elusive. Using the ²²⁴Ra/²²⁸Th disequilibrium approach, we estimated export fluxes of dissolved inorganic carbon and dissolved inorganic nitrogen at three intertidal sandy beaches with distinct slopes along the southeast China's coast. We identified that sandy beaches are a hotspot of nitrogen loss in the coastal ecosystem, with nitrogen removal rates reaching up to 87.1 mmolN m⁻² d⁻¹ and removal efficiencies varying between 7% and 82%. Notably, nitrogen removal rates peaked at intermediate seawater percolation fluxes, reflecting the optimal balance of oxygen consumption, marine organic matter remineralization, and nitrate production for fueling denitrification in the beach's interior. In addition, total nitrogen removal increased with beach slopes. This is likely due to the fact that steeper beaches facilitate seawater to percolate more efficiently into the beach's interior and travel along a longer flow path before it drains out, thus allowing denitrification to prevail. Overall, our field observations reveal that instead of the surface “skin circulation,” the “body circulation” system within an intertidal beach governs fluid transport and solute exchange between land and sea. We conclude that intertidal sandy beaches function as an efficient biogeochemical reactor, which attenuates anthropogenic nitrogen inputs to the coastal ocean.