Wiley

Limnology and Oceanography

Published by Wiley and Association for the Sciences of Limnology and Oceanography

Online ISSN: 1939-5590

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Print ISSN: 0024-3590

Disciplines: Environmental studies

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112 reads in the past 30 days

Average total nitrogen (Tot N, a), total phosphorus (Tot P, b), dissolved inorganic nitrogen (DIN, c), dissolved inorganic phosphorus (DIP, d), DIN : DIP (e), humic substances (HS, f), dissolved organic carbon (DOC, g), and photosynthetically active radiation (PAR, h), in the different treatments. Values are means of three replicates from days 7 to 28 ± standard error.
Average phytoplankton primary production (PP) (a), heterotrophic bacterial production (BP), (b) and trophic balance (PP–BP, c) in the different treatments. Values are means of three replicates from days 7 to 28 ± standard error.
Average fish production (FP, a), food web efficiency (FWE, b), δ¹⁵N Fish‐Seston (c), and δ¹³C Fish‐Seston (d) in the different treatments. Values are means of three replicates ± standard error. (c) and (d) represent endpoint values. Error bars denote standard error.
Principal component analysis (PCA) of factors such as dissolved organic carbon (DOC), dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorous (DIP), and photosynthetically active radiation (PAR) influencing phytoplankton primary production (PP), heterotrophic bacterial production (BP), ciliate biomass (Cil), mesozooplankton biomass (ZP) and fish production (FP) for Day 28 in the different treatments.
Relation between trophic balance (a), food web efficiency (FWE), (b), δ¹³C Fish‐Seston (c), and dissolved organic carbon (DOC). Trophic balance, food web efficiency, and DOC are average values during the experiment (from days 7 to 28). δ¹³C Fish‐Seston values are from the end of the experiment.
Climate change–induced terrestrial matter runoff may decrease food web production in coastal ecosystems

January 2025

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116 Reads

Owen F. Rowe

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110 reads in the past 30 days

Climate oscillations drive nutrient availability and seagrass abundance at a regional scale

January 2025

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110 Reads

Aims and scope


Limnology and Oceanography (L&O) publishes research on all aspects of the sciences of limnology and oceanography. Submissions are judged on their originality and intellectual contribution to the journal’s unifying theme—understanding of aquatic systems.
We welcome articles, reviews, and comments that are physical, chemical, or biological in nature, empirical or theoretical in method, and from elemental to geological, ecological to evolutionary, species to ecosystem, or system to global in scale.

Recent articles


Recovery from drought‐induced dieback may lead to modified salt marsh vegetation composition
  • Article
  • Full-text available

February 2025

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8 Reads

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Andrea D'Alpaos

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Marco Marani

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Sonia Silvestri

Salt marshes are vital but vulnerable ecosystems. However, our understanding of disturbance‐induced dieback and recovery processes in multi‐specific marshes remains limited. This study utilized remote sensing data (2001–2021) to analyze a dieback event and subsequent recovery in the multi‐specific San Felice marsh within the Venice lagoon, Italy. A significant dieback of Spartina maritima (Spartina) was identified in 2003, likely triggered by a drought event and heat stress. This resulted in a conversion of 4.6 ha of marsh predominantly colonized by Spartina (fractional cover of Spartina > 50%) in 2001 to bare soil in 2003. These bare areas were then gradually encroached by vegetation, indicating the occurrence of the recovery. Despite gradually gaining ground, Spartina only dominated 6.4 ha marshes in 2021, significantly lower than its pre‐dieback area (21.3 ha). However, other species also encroached on the dieback area, such that the aboveground biomass returned to pre‐dieback levels, indicating that the shift in marsh species composition that occurred as a consequence of the event compensated for this ecosystem service. Vegetation recovery, spanning from 1 yr to more than 18 yr, was found to be slowest in areas of lowest elevation. This study provides evidence that dieback and recovery can modify the species composition of multi‐specific marshes over decades. These insights contribute to a better understanding of marsh resilience to drought and elevated temperature, both of which are likely to increase in the future.


Study area and sampling site: (a) Location of Lake Constance in Central Europe. (b) Catchment of Upper Lake Constance, Geology for Switzerland and Liechtenstein from swisstopo (2005), Germany and Austria from Donnini, Marchesini, and Zucchini (2020). Sampling sites from Blattmann et al. (2019) are indicated with blue (soil) and red (bedrock) circles. (c) Sampling location for this study. Bathymetry (Wessels et al. 2016) and extent of Rhine interflow in black, following Wessels et al. (1995). The green triangle indicates the location of the sediment trap sampled by Blattmann et al. (2019). White circles show locations of sediment core samples from Blattmann et al. (2019). Black circles show locations of sediment cores with quantified accumulation rate from Dominik, Mangini, and Müller (1981) and Förstner, Müller, and Reineck (1968).
(a) Line scan photograph of core BO21‐14. (b) Five main trophic phases; modified from Ibrahim et al. (2021). (c) Intensity (as a proxy for concentration) of lead (Pb), measured with X‐ray fluorescence, in counts per second. Gray bars correspond to flood layers. Layers used for age‐model are labeled with the corresponding age on the right y‐axis. (d–h) Analyses of quasi‐annual subsamples for (d) total organic carbon (TOC) content in weight %, (e) TOC to total Nitrogen (TN) ratio, (f) stable C isotope signature as δ¹³C in ‰, (g) F¹⁴CTOC as Δ¹⁴C in ‰, (h) ratio of residual oxidizable OC (ROC) over TOC. Error bars in the y‐direction indicate the thickness of the subsample. Gray symbols represent samples taken from flood layers. The colored line shows a 10‐point moving average of non‐flood layers.
(a) Frequency distribution of modeled transit time (τ) of OCAq and OCSoil. Different line styles denote individual sampling chains (n = 6). (b) Modeled ¹⁴C signal for DIC and OCSoil over time (94% confidence interval in shaded area). Blue circles indicate independently measured samples of DI¹⁴C (Kölle 1969; Blattmann et al. 2018). Atmospheric curve from Graven, Keeling, and Rogelj (2020). (c) Modeled OC pool contribution over time. The shaded area indicates the 94% confidence interval. Dashed lines indicate the linear trend per pool. (d) Mass flux of OC per pool in g OC m⁻² yr⁻¹ with shaded band showing the standard deviation. The period of eutrophication is highlighted in green. OC, organic carbon.
Results of the ordinary kriging interpolation of the organic carbon (OC) pool contribution. (a) Total organic carbon (TOC) content of the deep (> 60 cm) sediment in weight %. Black dots show data points used, taken from Blattmann et al. (2019). (b) TOC content of the surface (0–2 cm) sediment in weight %. (c) Mass accumulation rate (MAR) in g cm⁻² yr⁻¹. Black dots show data points used from Blattmann et al. (2019), Dominik, Mangini, and Müller (1981) and Förstner, Müller, and Reineck (1968). (d) MAR of aquatic OC (OCAq) in the surface sediment in gC m⁻² yr⁻¹. (e) MAR of soil OC (OCSoil) in the surface sediment in gC m⁻² yr⁻¹. (f) MAR of petrogenic OC (OCPet) in the surface sediment in gC m⁻² yr⁻¹. (g) MAR of OCAq in the deep sediment in gC m⁻² yr⁻¹. (h) MAR of OCSoil in the deep sediment in gC m⁻² yr⁻¹. (i) MAR of OCPet in the deep sediment in gC m⁻² yr⁻¹.
Pre‐aged organic matter dominates organic carbon burial in a major perialpine lake system

February 2025

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12 Reads

Benedict V. A. Mittelbach

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Alexander S. Brunmayr

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Margot E. White

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Timothy I. Eglinton

Organic carbon (OC) burial in lake sediments is comparable to that in marine sediments globally. However, climatic and carbon cycle implications depend on the origin of buried OC. This study utilizes high‐resolution radiocarbon (¹⁴C) measurements in combination with stable carbon isotopes (¹³C) and total organic carbon/total nitrogen ratios to constrain sources and ages of OC deposited since the early 20th century in Lake Constance, the second‐largest lake in central Europe. We differentiate between aquatic, pre‐aged soil, and fossil rock‐derived (petrogenic) OC. The shape and magnitude of the ¹⁴C bomb spike recorded in the sediment profile indicate the sequestration of recently synthesized biospheric OC with a complex overlay from different OC sources. We find that soil‐derived OC is the dominant component of sedimentary OC, with a mean transit time in the catchment of around 110 yr. Additionally, we quantified the ¹⁴C dynamics of dissolved inorganic carbon in the lake, which can be modeled with a mean transit time of around 10 yr. An ordinary kriging spatial analysis revealed that the Alpine Rhine delta and the profundal areas are the primary loci for allochthonous OC deposition. Lake‐wide surface sediment OC fluxes were spatially heterogeneous but averaged 52.0 gC m⁻² yr⁻¹, where 26.7 gC m⁻² yr⁻¹ of mostly stable, allochthonous OC are buried long term. This study highlights the necessity of accounting for both pre‐aged and fossil OC sources, as well as spatial heterogeneity, when assessing the response of lakes and, more broadly, source‐to‐sink systems to ongoing climate and ecosystem change.


The appendicularian, Oikopleura dioica, visualized with dark field microscopy shows artificial microspheres collected on food concentrating filter (tiny white particles) (a), scale bar 500 μm. Appendicularians use their tail to drive fluid flow (blue arrows, b) over mucous mesh filters to concentrate microscopic prey particles (b). Particles with different properties (shown as black, gray, or white particles) may be represented in different proportions in each fate pathway (e.g., gut, discarded houses, fecal pellets, or rejected particles). While rejected particles may remain and recirculate in surface waters, those bound by mucous houses or in fecal pellets are transported to depth (black arrows).
Fluorescent particles in the Oikopleura dioica gut (a) and house (b). The appendicularian is situated with anterior up (a); particles in gut are visible in several groups due to mechanical compression of animal between microscope slide and cover slip. Two lobes of the food concentrating filter can be seen in upper region of image (b) and orange amine‐modified (arrowhead, A) and green carboxylate‐modified (C) particles are also visible. Image of an un‐expelled fecal pellet shows numerous green (carboxylate‐modified) particles (inset, b). Scale bars 100 μm (a, b) and 10 μm (inset, b).
Different fates of amine‐modified (gray bars) and carboxylate‐modified (black bars) particles. Particles observed in the gut (a and b) and in the mucous house (c and d) of Oikopleura dioica following 10‐min feeding incubations. Results are shown for experiments containing two particle sizes with similar concentrations (a and c) and three particle sizes with varying concentrations (b and d). Relative concentrations of each particle type are represented as gray and black circles. Significance values shown for comparisons between particle surfaces (*p < 0.05; **p < 0.001; ***p < 0.0001).
Chesson's alpha electivity indices of amine‐modified (gray bars) and carboxylate‐modified (black bars) particles observed in the gut (a and b) and in the mucous house (c and d) of Oikopleura dioica following 10‐min feeding incubations. Results are shown for experiments containing two particle sizes with similar concentrations (a and c) and three particle sizes with varying concentrations (b and d). Relative concentrations of each particle type are represented as gray and black circles. Horizontal lines indicates no selection, above which is positive selection and below, negative selection. Significant values for selection shown (**p < 0.001; ***p < 0.0001).
Theoretical interactions of two differently charged particles and mucous fibers under three scenarios. In direct interception (a), particles following fluid flow within one radius of the mucous fiber are captured. In electrostatic attraction (b), particle capture may be stronger or capture distance greater for particles that have greater attractive forces (positive particles, b), allowing particles that are greater than one radius away to be captured. Appendicularian feeding depends on initial particle attachment (a and b) and the subsequent detachment (c) that is necessary for particles to reach the mouth. The detachment of particles is caused by the elasticity of the mucous fibers and drag force on filter fibers (red arrowheads) (see Conley et al. 2018). The detachment force required to overcome adhesion forces and detach a particle may be greater for some particles (e.g., positive particles, c). (After Rubenstein and Koehl 1977).
Prey particle surface property mediates differential selection by the ubiquitous appendicularian Oikopleura dioica

February 2025

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13 Reads

Cell surface properties can strongly mediate microbial interactions with predators in soil and host‐pathogen systems. Yet, the role of microbial surface properties in avoiding or enhancing predation in the ocean is less well known. Appendicularians are globally abundant marine suspension feeders that capture marine microorganisms in a complex mucous filtration system. We used artificial microspheres to test whether the surface properties of prey particles influenced selection by the appendicularian, Oikopleura dioica. We used a range of microsphere sizes (0.5, 1, 2, and 3 μm), concentrations (~ 10³–10⁶ particles mL⁻¹), and two charges (amine‐modified, more positive vs. carboxylate‐modified, more negative) to represent open‐ocean microbial communities. We found that appendicularians selected between the particles of different charge. More negatively charged particles were enriched in the gut by up to 3.8‐fold, while more positive particles were enriched in the mucous filters by up to 4.7‐fold, leading to different particle fates. These results expand understanding of the mechanisms by which filter‐feeders select between prey and reveal a mechanism by which marine bacteria could rapidly alter their susceptibility to predation, either through adaption or acclimation.


Baroclinic instability‐induced intensification of phytoplankton blooms at submesoscales in eutrophic frontal regions

February 2025

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34 Reads

Off‐coast phytoplankton blooms occur frequently in the frontal region of the eutrophic Taiwan Strait during the northeasterly monsoon relaxation period, as consistently revealed by extensive cruise and satellite observations. Realistic model simulations have shown that restratification by frontal baroclinic instability (BCI) plays a crucial role in triggering blooms under nutrient‐rich conditions. This study deciphered the distinct contributions of submesoscale and mesoscale BCIs to bloom development using sensitivity tests of an idealized model of the Taiwan Strait featuring an intense alongshore front with ample nutrients. In three‐dimensional fine simulations with both submesoscale and mesoscale BCIs present, blooms were triggered by the cessation of a down‐front wind. Chlorophyll a was higher in submesoscale front regions than in mesoscale regions, primarily because of the higher upper‐ocean stability resulting from more effective restratification by submesoscale BCI. In three‐dimensional coarse simulations, mesoscale BCI led to relatively lower upper‐ocean stability and weaker blooms following wind relaxation, consistent with those in mesoscale regions in corresponding three‐dimensional fine simulations. In two‐dimensional simulations without submesoscale and mesoscale BCIs, blooms could not be triggered despite the cessation of a down‐front wind, primarily because of the absence of significant near‐surface restratification by BCIs. Furthermore, although symmetric instability was present in two‐dimensional fine simulations, its contribution to blooms was limited because of its minimal restratification effect. These results show that BCIs play the predominant role in triggering off‐coast blooms in eutrophic coastal front regions such as the Taiwan Strait.


Lake chlorophyll responses to drought are related to lake type, connectivity, and ecological context across the conterminous United States

February 2025

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12 Reads

Local and regional‐scaled studies point to the important role of lake type (natural lakes vs. reservoirs), surface water connectivity, and ecological context (multi‐scaled natural settings and human factors) in mediating lake responses to disturbances like drought. However, we lack an understanding at the macroscale that incorporates multiple scales (lake, watershed, region) and a variety of ecological contexts. Therefore, we used data from the LAGOS‐US research platform and applied a local water year timeframe to 62,927 US natural lakes and reservoirs across 17 ecoregions to examine how chlorophyll a responds to drought across various ecological contexts. We evaluated chlorophyll a changes relative to each lake's baseline and drought year. Drought led to lower and higher chlorophyll a in 18% and 20%, respectively, of lakes (both natural lakes and reservoirs included). Natural lakes had higher magnitudes of change and probabilities of increasing chlorophyll a during droughts than reservoirs, and these differences were particularly pronounced in isolated and highly‐connected lakes. Drought responses were also related to long‐term average lake chlorophyll a in complex ways, with a positive correlation in less productive lakes and a negative correlation in more productive lakes, and more pronounced drought responses in higher‐productivity lakes than lower‐productivity lakes. Thus, lake chlorophyll responses to drought are related to interactions between lake type and surface connectivity, long‐term average chlorophyll a, and many other multi‐scaled ecological factors (e.g., soil erodibility, minimum air temperature). These results reinforce the importance of integrating multi‐scaled ecological context to determine and predict the impacts of global changes on lakes.


(a) May 1 to August 1, 2015 discharge rate (m³ s⁻¹) of the five Texas rivers with highest discharge among all Texas rivers in 2015. Vertical blue lines indicate timing of the 2015 Texas–Oklahoma flood (May 22–25) and Tropical Storm Bill landfall at Matagorda Island, Texas (June 16); vertical green line indicates date of glider deployment (July 1) and red line indicates glider recovery date (July 20); (b) map of glider trajectories, green circle indicating location of glider deployment and red the point of glider recovery; orange circle represents the location of National Data Buoy Center Buoy 42035; location of United States Geological Survey river discharge gauges are labeled and colored according to their corresponding line in (a), the rivers themselves are also colored likewise. Note that one line is shown on the map at this scale to represent the trajectory of both Missions 13 and 14 because of how closely their respective trajectories overlapped. Approximate Gulf of Mexico bathymetric depths are indicated by black contour lines on the map, with depths from 10 to 50 m being labeled accordingly.
(a) Result of the k‐means clustering process upon observations of chromophoric dissolved organic matter (CDOM) (mg L⁻¹) as a function of salinity for the entirety of the Mission 13 (M13) glider record (n = 407,294), individual points are colored according to cluster classification (nearshore surface cluster = green; nearshore bottom cluster = blue; pycnocline cluster = purple; offshore surface cluster = gold; and offshore bottom cluster = red), white circles indicate cluster centroid; (b) M13 glider measurement depth as a function of time; points are colored according to their cluster classification as depicted in (a). Note that, as a function of how Slocum gliders operate, the glider does not go near the surface until its scheduled data transmission time, resulting in the data gaps observed at depths < 5 m. Additionally, the glider typically descended to a depth ~ 2–4 m above the seafloor; therefore, the seafloor is just beneath the colored portions of each of the profiles. For this figure with density contours, see Supporting Information Fig. S1.
Biplot of component 2 loadings (20.1% variance explained) as a function of component 1 loadings (69.9% variance explained) derived from the principal component analysis of the composite Mission 13 glider record for the six glider parameters: salinity (Sal), chlorophyll a (Chl a), turbidity (Turb), dissolved oxygen (Oxy), chromophoric dissolved organic matter (CDOM), and water temperature (Temp).
Biplot of component 2 loadings as a function of component 1 loadings derived from the principal component analysis of the Mission 13 (a) nearshore surface cluster (NSC), (b) pycnocline cluster (PYC), (c) offshore surface cluster (OSC), (d), nearshore bottom cluster (NBC), and (f) offshore bottom cluster (OBC), for the six glider parameters: salinity (Sal), chlorophyll a (Chl a), turbidity (Turb), dissolved oxygen (Oxy), chromophoric dissolved organic matter (CDOM), and water temperature (Temp); biplot lines in (a–e) are colored according to cluster classification in Fig. 2.
(a) Result of the k‐means clustering process upon observations of chromophoric dissolved organic matter (CDOM) (mg L⁻¹) as a function of salinity for the entirety of the Mission 14 (M14) glider record (n = 445,500), individual points are colored according to cluster classification (nearshore surface cluster = green; nearshore bottom cluster = blue; pycnocline cluster = purple; offshore surface cluster = gold; and offshore bottom cluster = red), white circles indicate the Mission 13 cluster centroids used for cluster generation; (b) M14 glider measurement depth as a function of time; points are colored according to their cluster classification as depicted in (a). Note that, as a function of how Slocum gliders operate, the glider does not go near the surface until its scheduled data transmission time, resulting in the data gaps observed at depths < 5 m. Additionally, the glider typically descended to a depth ~ 2–4 m above the seafloor; therefore, the seafloor is just beneath the colored portions of each of the profiles. For this figure with density contours, see Supporting Information Fig. S2.
Diagnosing coastal processes using machine learning and ocean buoyancy gliders

February 2025

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16 Reads

Ocean buoyancy gliders provide a comprehensive view of the water column, offering more than simply a snapshot of a single moment in time or space. In this study, we applied the established machine learning method, k‐means clustering, to a glider dataset collected in the summer of 2015 in the northern Gulf of Mexico. Clustering analysis of chromophoric dissolved organic matter and salinity revealed the physical structure of water masses, both vertically within the water column and horizontally along the shelf. Supplementary statistical analyses, including principal component analysis and ANOVA, of individual clusters confirmed the clusters were statistically distinct from one another and provided insights into the factors contributing to their differentiation. The clusters identified in the glider dataset represent water masses variously distinguished by river plumes, wind‐induced upwelling effects, shifts in currents, density‐induced stratification, and biological processes. This study demonstrates that applying machine learning clustering methods to subsurface glider data is a novel technique that enhances the analytical capabilities of both glider and other oceanographic datasets.


The schematic diagram for the model framework. (a) The model shows multiple incorporations of microplastics into marine snow is the main mechanism for transporting buoyant microplastics to the deep ocean. The processes include the rising and settling of microplastics due to the density difference between microplastics with ambient seawater (from routes 0 to 9). (b) The numerical concentration of marine snow aggregates (MSAs) is based on the dataset from North Pacific Ocean (Subhas et al. 2020) and the mass of marine snows (Alldredge and Gotschalk 1988). (c) Shear rates with different depths (Alemán, Pelegrí, and Sangrà 2006). (d) The marine snow attenuation (d⁻¹) with different depths (Fischer et al. 2022). Euphotic zone (0–100 m), Twilight zone (100–1000 m), and Deep ocean (> 1000 m).
Modeling output for the trajectory of the microplastics (75 μm) in the ocean from surface to sediment. (a) The trajectory of microplastics in the ocean interior with time. (b) Enlarged area of the microplastic trajectory in the depth range of 0–1000 m.
The microplastic (75 μm) dynamic in the ocean water column. (a) The rising distance of each settling cycle for microplastics. (b) The settling distance of each settling cycle for microplastics in the marine snows. (c) The net settling distance for microplastics in each settling cycle.
The settling behaviors of microplastics and nanoplastics with different sizes. The green line is for 150 μm, the red line is for 75 μm, the purple line is for 25 μm, and the blue line is for nanoplastics of 100 nm.
Marine snow as vectors for microplastic transport: Multiple aggregation cycles account for the settling of buoyant microplastics to deep‐sea sediments

February 2025

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61 Reads

Many studies have reported the paradoxical observation of high concentrations of low‐density microplastics (plastic particles < 5 mm) in deep‐sea sediments despite their buoyancy. The incorporation of buoyant microplastics into marine snow has been observed to enhance microplastic settling. Previous studies on the vertical movement of buoyant microplastics have been unable to theoretically account for these ocean observations and no study has comprehensively elucidated microplastic transport pathways in the ocean from the surface to seafloor. Here, we establish a one‐dimensional theoretical model, that embraces key elements of the flocculation process, to explain how marine snow acts as a vector to transport buoyant microplastics to deep water and the ocean bottom. Microplastics reach the ocean floor through multiple cycles of aggregation, settling, and disaggregation between marine snow and microplastics. Each settling cycle results in a net settling of 200–400 m. We demonstrate that microplastics with different sizes show distinct vertical settling behaviors and only microplastics less than 100 μm in diameter can reach the ocean bottom. This theoretical model refines our ability to predict and understand the global and long‐term fate, transport, and inventory of microplastics in the ocean interior, the influence of microplastics on the biological carbon pump and the efficacy of plastic management policies.


Midwater anoxia disrupts the trophic structure of zooplankton and fish in an oxygen deficient zone

February 2025

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22 Reads

Anoxic waters in the ocean's oxygen deficient zones (ODZs) limit the vertical migrations of zooplankton and mesopelagic fish impacting their ecology and influence on biogeochemical processes. Using an oxypleth‐tracking, nighttime‐only sampling protocol, this research reconstructed the trophic interactions of fish larvae and adults, and zooplankton, across the Eastern Tropical North Pacific ODZ. Bulk zooplankton δ¹⁵N increased latitudinally by ~ 3.3‰ from Costa Rica to Baja California due to anoxia‐derived denitrification and consequent enrichment of nitrogen sources for producers. Zooplankton δ¹⁵N also increased with depth, with an abrupt 3.4‰ increase below the anoxic core (~ 900 m depth), indicating a distinct trophic structure in the resident zooplankton community. Above the anoxic core, δ¹⁵N was similar for fish larvae (10.1‰) and zooplankton (10.5‰), reflecting a shared food source. An exception was the hypoxia‐tolerant myctophid Diogenichthys laternatus (δ¹⁵N = 7.5‰) that possibly feeds on chemoautotrophy‐derived material at the oxic‐anoxic interface. The δ¹⁵N of fish adults residing below the anoxic core, like the meso‐bathypelagic Notolychnus valdiviae (17.11‰) and Cyclothone spp. (15.89‰), was, on average, 4.8‰ higher than larval stages sampled at shallower depths, and 1.2‰ higher than zooplankton below the anoxic core. This stark increase in fish and zooplankton δ¹⁵N directly below the anoxic core suggests that anoxic waters act as a barrier for the downward trophic transfer by vertical migrants into the deep sea. Considering the current trends of ocean deoxygenation, this anoxia‐derived disruption of the migrant pump could limit the carbon sequestration potential of ODZs.


Molecular composition of dissolved organic matter from young organic‐rich hydrothermal deep‐sea sediments

February 2025

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57 Reads

Hydrothermal transformations of dissolved organic matter (DOM) are governed by temperature and sedimentary organic carbon content, resulting in the release of hydrothermal DOM containing bioavailable compounds fueling benthic microbes. However, the temperature‐dependent molecular changes in porewater DOM from organic‐rich hydrothermal sediments, and the extent to which these changes contribute to the marine recalcitrant DOM, remain largely unexplored. Here we investigated the DOM composition of hydrothermal porewater and bottom water samples from the Guaymas Basin, Gulf of California, where basaltic sill intrusions generate hydrothermal petroleum in organic‐rich sediments. Samples containing hydrothermal petroleum with in situ temperatures from 4°C to > 106°C were analyzed using Fourier‐transform ion cyclotron resonance mass spectrometry and parallel factor analysis of excitation‐emission matrices from fluorescent DOM (FDOM). We found that the porewater DOM composition was strongly influenced by temperature and petroleum dissolution, evidenced by the enrichment of hydrothermal DOM with highly unsaturated, oxygen‐depleted aromatic, sulfur‐containing molecular formulae and petroleum‐associated FDOM compared to a cold reference site. In bottom waters, hydrothermal DOM accounted for ~ 26% of the DOM molecular formulae, with 82% exhibiting hydrogen‐to‐carbon ratios < 1.5, indicating their recalcitrance. The remaining ~ 18% of the hydrothermal molecular formulae were aliphatic and saturated, representing the release of bioavailable DOM to the ocean. Our results show that hydrothermal sediments are a source of both bioavailable and recalcitrant DOM, releasing water‐soluble petroleum‐derived compounds to the deep ocean. Our study highlights the need for more quantitative research on the contribution of hydrothermal sediments to deep‐sea DOM cycling.


Processes and microorganisms driving nitrous oxide production in the Benguela Upwelling System

February 2025

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52 Reads

Upwelling systems and their associated oxygen deficient zones (ODZs) are hotspots of nitrous oxide (N2O) production in the ocean. The Benguela Upwelling System (BUS) is a highly productive region and an important, yet variable source of N2O to the atmosphere. This study examined underlying processes and microbial key players governing N2O production in the BUS during the austral winter. ¹⁵N‐tracer incubation experiments were conducted to track N2O production from NH4⁺ oxidation and denitrification. N2O production and consumption mechanisms over a longer temporal scale were determined through natural‐abundance isotope analyses. Metagenomics and 16S rRNA gene amplicon sequencing were used to identify potential key prokaryotes driving N2O production. Our results showed that, compared with permanent ODZs, the BUS is characterized by a higher oxidative and a lower reductive N2O production, both of which exhibit substantial spatial variability. N2O production peaked in low‐oxygen (O2) waters, with nearly equal contributions of oxidative and reductive processes, suggesting their co‐occurrence across an O2 concentration range broader than previously thought. However, the observed N2O isotope signatures implied a legacy of recent and extensive N2O reduction to N2. Metagenomic and 16S rRNA gene data identified denitrifiers belonging to Thioglobaceae and the archaeal ammonia‐oxidizers Nitrosopumilaceae among the potential key drivers of N2O production. Our study provides a comprehensive picture of N2O production in the BUS, revealing significant variability in the N‐cycling regime and underlying N2O production mechanisms, and demonstrating the value of combining direct rate measurements with more integrative approaches, such as molecular omics and natural‐abundance stable isotope tracers.


Changes in dissolved inorganic nitrogen (DIN) and phosphorus (PO4³⁻) concentrations during the biological and photochemical degradation experiments. (a, c) Biodegradation experiments contained filtered (0.22 μm) seawater plus 5% unfiltered seawater with some treatments receiving an additional 30 μmol C L⁻¹ (+ C) while others did not receive any additional carbon (− C). (b, d) Photodegradation experiment only contained 0.22 μm filtered seawater. Here Apr‐c, Jul‐c, and Sep‐c represent the dark control group in the photodegradation experiments.
Changes in dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) concentrations during the biological and photochemical degradation experiments (Apr, Jul, and Sep). (a, c, e) Biodegradation experiments which contained filtered (0.22 μm) seawater plus 5% unfiltered seawater, with some receiving additional 30 μmol C L⁻¹ (+ C) while others did not (− C). (b, d, f) Photodegradation experiment only contained 0.22 μm filtered seawater. Here Apr‐c, Jul‐c, and Sep‐c represent the dark control group in the photodegradation experiments.
Changes in colored dissolved organic matter (CDOM) absorption at 350 nm (a350), CDOM spectral slope ratio (SR), and the carbon specific absorbance at 254 nm (SUVA254) during the biological and photochemical degradation experiments. (a, c, e, g) Biodegradation, contained filtered (0.22 μm) seawater plus 5% unfiltered seawater, some treatments received an additional 30 μmol C L⁻¹ (+ C) while others did not (− C). (b, d, f, h) Photodegradation experiment only contained 0.22 μm filtered red seawater. Here Apr‐c, Jul‐c, and Sep‐c represent the dark control group in the photodegradation experiments.
Changes in fluorescent dissolved organic matter (FDOM) fluorescence indexes (BIX, biological index; FI, fluorescence index; HIX, humification index) in the biological and photochemical degradation experiments. (a, c, e) Biodegradation, contained filtered seawater (0.22 μm) plus 5% unfiltered seawater, with some treatments receiving an additional 30 μmol C L⁻¹ (+ C) and others did not (− C). (b, d, f) Photodegradation experiment only contained 0.22 μm filtered seawater. Here Apr‐c, Jul‐c, and Sep‐c represent the dark control group in the photodegradation experiments.
Changes in the identified four PARAFAC components for the colored dissolved organic matter fluorescence (FDOM) pool in the biological and photochemical degradation experiments. (a, c, e, g) Biodegradation, contained filtered (0.22 μm) seawater plus 5% unfiltered seawater, with some treatments receiving an additional 30 μmol C L⁻¹ (+ C) and others did not (− C). (b, d, f, h) Photodegradation experiment only contained 0.22 μm filtered seawater. Here Apr‐c, Jul‐c, and Sep‐c represent the dark control group in the photodegradation experiments.
Limited degradability of dissolved organic carbon, nitrogen, and phosphorus during contrasting seasons in a tropical coastal environment

February 2025

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71 Reads

The biogeochemistry of dissolved organic matter (DOM) is poorly understood in tropical coastal waters. Here, we quantified the biological and photochemical lability of dissolved organic carbon, nitrogen, and phosphorus, in the tropical coastal waters of Singapore. We conducted experiments during the inter‐monsoon, the mid‐southwest monsoon, and the late southwest monsoon seasons, which span the greatest range of biogeochemical conditions found in the area. The DOM lability was quantified as concentration changes during 90‐d biodegradation and 7‐d photoreactor incubations. Overall, DOM showed low lability, even though dissolved organic nitrogen and dissolved organic phosphorus accounted for most of the dissolved nitrogen and phosphorus. In the biodegradation experiments, only 5–15% of dissolved organic carbon, 0–7% of dissolved organic nitrogen, and 8–21% of dissolved organic phosphorus were degraded. The addition of labile dissolved organic carbon, intended to test priming effects and to ensure the microbes were not carbon‐limited, had no measurable impact on the results. During our photochemical experiments only 2–10% of the dissolved organic carbon were degraded, while neither dissolved organic nitrogen nor dissolved organic phosphorus showed consistent photochemical losses. The DOM optical properties (absorbance and fluorescence spectra) showed limited or no changes during the biodegradation experiments but larger declines in the photochemical experiments. Overall, the biodegradation of DOM was highest during the inter‐monsoon, when autochthonous DOM was most dominant, while photolability was greater during the terrestrial DOM‐rich southwest monsoon. Our results illustrate that in some tropical coastal environments, DOM can be fairly resistant to biological and photochemical degradation, and thus does not represent a large stock of potentially available nutrients.


A new dynamic distribution model for Antarctic krill reveals interactions with their environment, predators, and the commercial fishery in the south Scotia Sea region

February 2025

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83 Reads

The management strategy for the Antarctic krill (Euphausia superba) fishery is being revised. A key aim is to spatially and temporally allocate catches in a manner that minimizes impacts to both the krill stock and dependent predators. This process requires spatial information on the distribution and abundance of krill, yet gaps exist for an important fishing area surrounding the South Orkney Islands in the south Scotia Sea. To fill this need, we create a dynamic distribution model for krill in this region. We used data from a spatially and temporally consistent acoustic survey (2011–2020) and year‐specific environmental covariates within a two‐part hurdle model. The model successfully captured observed spatial and temporal patterns in krill density. The covariates found to be most important included distance from shelf break, distance from summer sea ice extent, and salinity. The northern and eastern shelf edges of the South Orkney Islands were areas of consistently high krill density and displayed strong spatial overlap between intense fishing activity and foraging chinstrap penguins. High mean krill density was also linked to oceanographic features located within the Weddell Sea. Our data suggest that years in which these features were closer to the South Orkney shelf were also years of positive Southern Annular Mode and higher observed krill densities. Our findings highlight existing fishery–predator–prey overlap in the region and support the hypothesis that Weddell Sea oceanography may play a role in transporting krill into this region. These results will feed into the next phase of krill fisheries management assessment.


Eddy dipole differentially influences particle‐associated and water column protistan community composition

January 2025

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30 Reads

Ocean eddies are mesoscale features that can extend > 100 km and maintain cohesiveness for months, impacting planktonic community structure and water column biogeochemical cycles. Standing stocks of protists in the water column and on sinking particles were investigated using microscopy, in situ imagery, and metabarcoding across an anticyclonic to cyclonic eddy dipole in the North Pacific Subtropical Gyre during July 2017. The water column was sampled from the surface to 500 m and particle interceptor traps were deployed at 150 m. Protistan assemblage composition varied substantially between sample type and analytical approach across the eddy dipole. Alveolates represented 63% of sequences from water samples. In contrast to water samples, rhizarian protists represented 79% of trap sequences obtained by metabarcoding of sediment trap material. Microscopy of trap material supported the important contribution of Rhizaria to sinking particles and revealed increased relative abundances of ciliates in the anticyclonic eddy and diatoms in the cyclonic eddy. In situ imagery confirmed the presence of relatively large Rhizaria that were not adequately assessed from water samples but contributed significantly to particle flux. Together, these data demonstrate differing perspectives of planktonic protistan community composition and contributions to sinking particles gained from the application of different sampling and analytic approaches. Our observations and analyses indicate a specific subset of the protistan community contributed disproportionately to organic matter downward export.


Wave‐driven plant reconfiguration modifies light availability in seagrass meadows

January 2025

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42 Reads

Seagrasses are considered foundation species in marine and estuarine ecosystems by contributing biomass, providing habitat, and damping waves and currents. Globally, seagrass health and primary productivity are threatened by factors that affect light availability, such as shading by algae and epiphytes, self‐shading, and increased water column turbidity. This study focuses on how plant motion and reconfiguration lead to shading of an individual plant by itself and its neighbors, and how wave conditions, plant material properties, and shoot density affect light availability along a seagrass blade. We use a simple ray‐optics shading model with the plant motion model of Zhu et al. (2020; Journal of Geophysical Research: Oceans 125:e2019JC015517) for a flexible blade under wavy flow to understand how phase‐resolved plant behavior affects light availability as a function of vertical location in the water column. Results show that that shading of a plant by its neighbors occurs more under wave crests and troughs, and that factors that increase blade tip excursion (large wave height or wave period, or high plant flexibility) reduce light exposure. We develop a simplified theory and parameterization for average light exposure as a function of flow and plant conditions (as captured by the Cauchy number, buoyancy parameter, and ratio of stem spacing to blade length). These results help delineate optimal conditions for maximizing light exposure to seagrass' photosynthetic tissue in restoration projects, and facilitate the inclusion of flow‐vegetation interactions in biological models of seagrass production.


Ontogenetic shifts by juvenile fishes highlight the need for habitat heterogeneity and connectivity in river restoration

January 2025

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76 Reads

Large‐scale anthropogenic river modifications have caused the loss of critical floodplain nursery habitats for riverine fish, leading to population declines. Restoration efforts have been implemented to recover these habitats, but with varying success. Understanding how larval and juvenile fish use habitats in dynamic river environments is essential for improving restoration strategies. We assessed ontogenetic shifts in habitat use by young‐of‐the‐year fishes in the lower Rhine, analyzing 2167 samples across 18 restored floodplains over three growing seasons (2018–2020). Five distinct nursery habitats were identified: (1) exposed, fast‐flowing habitats with coarse substrate; (2) turbid, nonflowing areas with high turbidity and chlorophyll; (3) shallow, vegetated habitats with macrophytes and shoreline vegetation; (4) deeper, sheltered habitats with structural complexity; and (5) shallow, slow‐flowing areas. Habitat use shifted significantly with ontogeny across species. Larvae generally preferred shallow habitats (< 50‐cm depth), either in slow‐flowing areas (e.g., asp, ide, monkey goby, nase, and whitefin gudgeon) or vegetated zones with macrophytes (e.g., bleak, bitterling, bream, round goby, and zander). Juveniles increasingly used deeper habitats (> 50‐cm depth), favoring fast‐flowing areas (e.g., asp, barbel, ide), or deeper, nonflowing habitats (e.g., bream, zander). Our findings thus highlight the critical importance of habitat heterogeneity and connectivity for riverine fish biodiversity. Restoration strategies should prioritize the creation of a mosaic of shallow, low‐velocity habitats for larvae, alongside deeper, fast‐flowing, or sheltered areas for juveniles. Additionally, the movement of rheophilic species from floodplain habitats to the main river channel emphasizes the need for maintaining continuous connectivity between floodplains and the river.


(a) Observed epiphyton biomass phenology (normalized to the respective maximum biomass) with onset of epiphyton net loss (dashed lines) based on published field data of temperate lakes and experimental ponds: Müggelsee: Roberts et al. (2003); Experimental Lake: Moss (1976); Lake Gulbinas: Karosienė and Kasperovičienė (2008); Lake Balaton: Tóth (2013); Lake Kalgaard: Sand‐Jensen and Søndergaard (1981); and Lake Veluwe: van Dijk (1993). (b) A general model of epiphyton spring–summer phenology on submerged macrophytes in temperate systems derived from field data in panel (a). An initial period of fast growth in spring is followed by a net loss period due to grazing. Toward summer, the epiphyton regains net biomass gain with the highest values observed in mid/late summer. Note that spatial and temporal variability occurs in the cardinal events such as in (1) the start, (2) slope of the growth curve, as well as the (3) onset, (4) duration, and (5) strength of the net loss period.
(a) A load‐response curve of a state variable (here vegetation dry weight [g dry weight (DW) m⁻²]) at equilibrium under a gradually increasing or decreasing driver (here epiphyton shading) showcasing a hysteresis. Internal stabilizing feedback in the clear water state allows submerged vegetation to tolerate high levels of epiphyton‐mediated shading before the vegetation collapses and the lake shifts to a turbid state (forward shift, dashed line). A return of macrophytes to the system (return shift, solid line), however, requires a reduction far below the shading level on which the forward shift occurred as the system is stabilized by a different feedback loop due to high pelagic phytoplankton shading. The hysteresis space between these two tipping points covers the range of shading conditions at which either vegetation state can occur and at which a stronger perturbation can shift the system from one state to another. Assessment of the influence of the onset date of epiphyton shading on the tolerance of submerged vegetation to epiphyton shading showed (b) later onset of epiphyton growth resulted in a much higher tolerance to epiphyton shading by the macrophytes. However, (c) a return of macrophytes from a turbid system required a very strong reduction of epiphyton shading.
Scatter plots along a gradient of epiphyton shading curve midpoints (day [D] 100–150; blue shades) in presence of a grazing event in the control temperature scenario. (a) The 25‐yr average summer biomass of macrophytes (g dry weight [DW] m⁻²) against onset of the grazing period relative to the midpoint of the shading curve. The spread of points along the y‐axis for each shading curve set reflects the influence of the duration of the grazing period (5, 10, 15, and 20 d). (b) The same 25‐yr average summer biomass of macrophyte data against the duration of the grazing event. The spread of points along the y‐axis for each shading curve set reflects the relative onset of the grazing period (− 10, 0, 10, and 20 d). (c) Years until macrophytes disappear during the growth period from the model lake in response to the relative onset of the grazing period. (d) Years until macrophytes disappear during the growth period from the model lake in response to the duration of the grazing period.
Response of submerged macrophyte biomass and resilience in the warmer winter temperature scenario. The 25‐yr average summer biomass of macrophytes (g dry weight [DW] m⁻²) relative to (a) timing of the grazing period, and (b) duration of the grazing event. Years until macrophytes disappear during the growth period from the model lake along a gradient of epiphyton shading curve midpoints (day [D] 80–110; blue shades) in response to (c) the relative onset of the grazing period and (d) the duration of the grazing period.
The 25‐yr average summer pelagic phytoplankton biomass (mg chlorophyll a [Chl a] m⁻³) along a gradient of epiphyton shading curve midpoints (day [D] 100–150; blue shades) in presence of a grazing event and under the control temperature scenario relative to the (a) onset of the grazing period and (b) duration of the grazing event.
Epiphyton phenology determines the persistence of submerged macrophytes: Exemplified in temperate shallow lakes

January 2025

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71 Reads

Submerged macrophytes are key components in many freshwater and marine ecosystems, contributing to ecosystem functions and services. In temperate shallow lakes, spring epiphyton shading can be decisive for submerged macrophyte development, potentially leading to macrophyte collapse and a shift to undesired, turbid conditions. Global change can alter epiphyton phenology; however, the consequences for submerged macrophytes and their stabilizing effects on clear‐water conditions remain to be elucidated. Based on field data, we propose a general epiphyton shading phenology for submerged macrophytes in temperate shallow lake ecosystems. We express the temporal dynamics of epiphyton shading in terms of onset and relative increase (slope) of epiphyton development as well as epiphyton grazing impacts (onset, duration) using a Boltzmann function. This function is added to the ecosystem model PCLake+ as a customizable, macrophyte‐specific shading factor. We then assess how changes in the epiphyton phenology and the presence of grazing on epiphyton affects submerged macrophyte biomass in a generic temperate shallow model lake under control and warm winter scenarios. The results from the model provide a proof‐of‐concept that epiphyton shading can provoke macrophyte loss and shifts between alternative equilibria. Threshold values for critical shifts depend on epiphyton shading phenology. Earlier onset and longer duration of grazing can maintain macrophytes in nutrient or climate conditions under which they would otherwise collapse. Our results show the pivotal importance of epiphyton phenology in determining lake ecosystem‐wide responses stressing the need for better incorporation of epiphyton into both models and monitoring.


Study species and experimental timeline. (a) Representative images of larvae (top row) and juveniles (bottom row) from the three study species: Nematostella vectensis (left), Galaxea fascicularis (middle), and Porites astreoides (right). Red borders around images indicate life stages that possess dinoflagellate endosymbionts (family Symbiodiniaceae). (b) Timeline of the experiment depicting the dissolved oxygen level of seawater in which organisms were held during and after the hypoxia treatment (control animals experienced normoxia throughout the experiment), organism life stages (larva = green, juvenile = purple), and sampling time points (arrowheads) colored by life stage at the time of sampling. Hours are relative to the end of the oxygen treatments (i.e., h post‐treatment).
Effects of hypoxia on larval swimming and settlement. (a–c) Percent of (a) Nematostella vectensis, (b) Galaxea fascicularis, and (c) Porites astreoides larvae (n = 600–1200 larvae treatment⁻¹ cohort⁻¹ species⁻¹) swimming at the conclusion of the 6‐h treatment period (dotted lines) or settled (final percentage; dashed lines) for the hypoxia (yellow) and normoxia (blue) treatments. Points with error bars depict means ± standard error of the mean, and asterisks indicate statistical significance (p < 0.05) of pairwise comparisons (hypoxia vs. normoxia).
Effects of hypoxia on animal growth and metabolism. (a–c) Size (larval length or juvenile polyp diameter in mm), (d–f) ash‐free dry weight (AFDW; normalized to size; μg mm⁻¹), and (g–i) respiration rates (pmol O2 consumed min⁻¹ μg AFDW⁻¹) of Nematostella vectensis (left), Galaxea fascicularis (middle), and Porites astreoides (right) larvae and juveniles (n = 60–90 treatment⁻¹ time point⁻¹ cohort⁻¹ species⁻¹; dashed line separates life stages) over time following hypoxia (yellow) and normoxia (blue) treatments. Purple arrowheads indicate the approximate timing of settlement, points with error bars depict means ± standard error of the mean, and asterisks indicate statistical significance (p < 0.05) of pairwise comparisons (hypoxia vs. normoxia).
Effects of hypoxia on endosymbiont density and photophysiology. (a) Photosynthesis rates (pmol O2 produced min⁻¹ μg AFDW⁻¹) of Porites astreoides larval and juvenile (n = 60–90 treatment⁻¹ time point⁻¹ cohort⁻¹ species⁻¹; dashed line separates life stages) holobionts (i.e., animal and dinoflagellate endosymbionts together). (b, c) Symbiont density (cells μg AFDW⁻¹) of (b) P. astreoides and (c) Galaxea fascicularis animals (n = 60–90 treatment⁻¹ time point⁻¹ cohort⁻¹ species⁻¹). (d) Photochemical yield (Fv/Fm) and (e) chlorophyll content (pg cell⁻¹) of P. astreoides endosymbionts (n = 60–90 treatment⁻¹ time point⁻¹ cohort⁻¹ species⁻¹). All panels depict data over time following the hypoxia (yellow) and normoxia (blue) treatments. Purple arrowheads indicate the approximate timing of settlement, points with error bars depict means ± standard error of the mean, and asterisks indicate statistical significance (p < 0.05) of pairwise comparisons (hypoxia vs. normoxia).
Effects of hypoxia on larval thermal tolerance. (a–i) Survival (%) of Nematostella vectensis (top), Galaxea fascicularis (middle), and Porites astreoides (bottom) larvae over time at 36°C for heat tolerance assays initiated at 0 (left), 12 (middle), and 36 (right) hours post‐treatment (h; n = 30 larvae treatment⁻¹ time point⁻¹ cohort⁻¹ species⁻¹). (j–l) Photochemical yield (yield; Fv/Fm) of P. astreoides dinoflagellate endosymbionts over time at 36°C for heat tolerance assays initiated at 0 (left), 12 (middle), and 36 (right) h (n = 30 larvae treatment⁻¹ time point⁻¹ cohort⁻¹ species⁻¹). Data from larvae previously exposed to hypoxia are in yellow, while those from normoxia controls are in blue. Points represent raw data, while vertical dotted lines with shaded ribbons represent mean lethal doses 50 (LD50s) ± standard error of the mean. Asterisks indicate significance (p < 0.05) for pairwise comparisons of LD50s (hypoxia vs. normoxia).
Hypoxia threatens coral and sea anemone early life stages

January 2025

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17 Reads

Seawater hypoxia is increasing globally and can drive declines in organismal performance across a wide range of marine taxa. However, the effects of hypoxia on early life stages (e.g., larvae and juveniles) are largely unknown, and it is unclear how evolutionary and life histories may influence these outcomes. Here, we addressed this question by comparing hypoxia responses across early life stages of three cnidarian species representing a range of life histories: the reef‐building coral Galaxea fascicularis, a broadcast spawner with horizontal transmission of endosymbiotic algae (family Symbiodiniaceae); the reef‐building coral Porites astreoides, a brooder with vertical endosymbiont transmission; and the estuarine sea anemone Nematostella vectensis, a non‐symbiotic broadcast spawner. Transient exposure of larvae to hypoxia (dissolved oxygen < 2 mg L⁻¹ for 6 h) led to decreased larval swimming and growth for all three species, which resulted in impaired settlement for the corals. Coral‐specific responses also included larval swelling, depressed respiration rates, and decreases in symbiont densities and function. These results indicate both immediate and latent negative effects of hypoxia on cnidarian physiology and coral–algal mutualisms specifically. In addition, G. fascicularis and P. astreoides were sensitized to heat stress following hypoxia exposure, suggesting that the combinatorial nature of climate stressors will lead to declining performance for corals. However, sensitization to heat stress was not observed in N. vectensis exposed to hypoxia, suggesting that this species may be more resilient to combined stressors. Overall, these results emphasize the importance of reducing anthropogenic carbon emissions to limit further ocean deoxygenation and warming.


How ecological regimes and emergent macrophytes determine sediment microbial communities: A new insight into typical eutrophic shallow lakes

Understanding the response of microbial communities to different ecological regimes in eutrophic lakes and the underlying assembly mechanisms is of great significance for revealing the biodiversity maintenance mechanisms of lake ecosystems under alternative stable states. However, our current understanding of the response of sediment microbial communities under emergent macrophytes to regime shifts remains limited. Here, we demonstrated, for the first time, the asynchronous variations of littoral sediment bacterial and fungal communities, regarding the microbial diversities, assembly mechanisms, and inter‐kingdom interactions across three lake regional regimes: macrophyte‐dominated, transitional, and phytoplankton‐dominated. We found the alpha diversities of the bacterial and fungal communities showed opposite trends, as the transitional regime had the highest bacterial but lowest fungal diversities. Stochastic processes, dominated by dispersal limitation, determined fungal community assembly, whereas deterministic processes, especially variable selection, shaped the bacterial community. The highest number of species–environment interactions and proportion of intra‐kingdom interactions were observed in the co‐occurrence network of the transitional regime; however, this network had the lowest proportion of inter‐kingdom (bacteria–fungi) interactions among the three lake regional regimes. Furthermore, the macrophyte‐dominated regime was observed to have the most complex network structure and maintain the highest microbial community stability. The rhizosphere of Phragmites australis enhanced the inter‐kingdom interactions of bacterial and fungal communities. These findings provide a preliminary ecological perspective for understanding the hysteresis of regimes in response to environmental stress at the microbial community level and emphasize the importance of distinguishing ecologically distinct microbial taxa in future studies focused on alternative stable states.


Benthic bacterial communities are shaped by browning in boreal headwater streams

Owing to the rapid progress of high‐throughput sequencing technologies, microbial assemblages have gained growing interest in environmental impact assessment. However, research on microbial community responses, particularly those of benthic biofilm, to browning (increased concentrations of dissolved organic carbon [DOC]), is scarce. We used data from 55 boreal streams to examine if biofilm bacterial communities exhibit changes in diversity and community composition along a gradient of browning (3.6–27 mg DOC L⁻¹). Species richness increased slightly with increasing DOC, whereas community composition changed markedly across the gradient, especially in the active community. Pseudomonadota and Bacteroidota were overall dominant bacterial phyla. In the active community, Bacteroidota became relatively less abundant and Pseudomonadota more abundant with increasing DOC. Nitrate‐N (NO3‐N) and DOC were the most important predictors of bacterial community turnover. The greatest change in community composition occurred between 75 and 100 μg NO3‐N L⁻¹. For DOC, the first change point was at the low‐end of the gradient, followed by a major change in strongly brownified waters (> 20 mg L⁻¹). Bacterial communities became phylogenetically more similar than expected by chance as DOC increased. Concordance between bacterial and benthic invertebrate communities was very high, indicating that browning exerts a strong control over both taxonomic groups. Our results suggest that microbial communities, particularly the active portion of the community, may provide a sensitive and reliable tool for stream bioassessment. We defined a threshold‐type response in bacterial assemblages to water browning but more research is needed on microbial responses to multiple simultaneous stressors related to global warming and land‐use intensification.


Dynamics of marine inorganic carbon and silica: A field study of the mechanisms controlling seawater major element concentrations

January 2025

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44 Reads

A highly resolved time series of dissolved major element (calcium, strontium, magnesium, and lithium) concentrations in the north Gulf of Aqaba, Red Sea, reveals variability in major cation concentrations beyond analytic uncertainties. This variability is composed of an interannual component that is most important for calcium, and a short‐term daily‐timescale component that is most important for lithium. As evident from covariation in calcium, potential alkalinity, and Sr/Ca, the calcium carbonate cycle of the Gulf of Aqaba is dominated by coral calcification, and there was an increase in calcification rates between 2017 and 2018. Variability in lithium concentrations, and larger changes in magnesium concentrations than expected from magnesium distribution coefficients in carbonate minerals, suggest an active cycle of aluminosilicate mineral dissolution, and precipitation of secondary silicate minerals.


Production and transfer of essential fatty acids in a man‐made tropical lake ecosystem

Essential biomolecules, such as physiologically essential fatty acids, can critically influence consumers' performance and the ecosystem's functioning. Eicosapentaenoic (EPA; 20:5ω3) and docosahexaenoic (DHA; 22:6ω3) fatty acids are physiologically crucial for consumers, and they must be either obtained from the diet or bioconverted from precursors. We monitored the synthesis of EPA and DHA by primary producers in the largest man‐made ecosystem (Lake Kariba) and in situ fatty acid production, trophic transfer, and endogenous production of EPA and DHA in the tropical lake food web using ¹³C‐labeling, compound‐specific isotopes, and gene expression of fads2 and elovl5 genes in most abundant fish species. Seston pigment analysis and 23S rRNA sequencing revealed that cyanobacteria dominated primary producers throughout three seasons, and the biosynthesis rate of EPA and DHA was under the detection limit. Moreover, due to the low zooplankton densities and EPA and DHA content in zooplankton, the transfer of EPA and DHA from phytoplankton–zooplankton to upper trophic levels is low. The low production of EPA and DHA by primary producers is mitigated by bioconversion of α‐linolenic acid to EPA and DHA in two tilapia species, especially by Nile tilapia (Oreochromis niloticus) known to feed on cyanobacteria. Compound‐specific isotope analysis revealed that tigerfish (Hydrocynus vittatus), the main predatory fish on the lake, was more closely related to Nile tilapia than to lake planktivorous fish (Limnothrissa miodon). Therefore, trophic interaction between cyanobacteria and algivorous fish has replaced traditional phytoplankton and zooplankton trophic interaction in the synthesis and transfer of EPA and DHA to upper trophic levels.


Climate oscillations drive nutrient availability and seagrass abundance at a regional scale

January 2025

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110 Reads

Seagrasses are increasingly recognized for their ecosystem functions and services. However, both natural and anthropogenic stressors impact seagrass functional traits, for example by altering nutrient regimes. Here, we synthesize 27 yr of data from regional, long‐term seagrass and water quality monitoring programs of south Florida to investigate the impacts of relative nutrient availability on seagrass abundance (as expressed by percent cover) across an oligotrophic seascape. We employ linear mixed‐effect models and generalized additive models to show that seagrass abundance is driven by interannual variations in nutrient concentrations, which are ultimately controlled by climate oscillations (El Niño Southern Oscillation Atlantic Multidecadal Oscillation) via regional rainfall‐runoff relationships. Our study suggests that climate oscillations drive interannual variations in seagrass cover on a regional scale, with high‐rainfall years leading to increased nitrogen availability and higher seagrass abundance in typically nitrogen‐limited backreef meadows. Conversely, these periods are associated with reduced seagrass cover at the more P‐limited inshore sites and in Florida Bay, with yet unknown consequences for the provision of seagrass ecosystem services. We show that nutrient delivery from runoff can have diverging impacts on benthic communities, depending on spatial patterns of relative nutrient limitation, with some N‐limited seagrass meadows showing resilience to periodic nutrient enrichment.


Upwelling of cold water in the South Yellow Sea alleviates phosphorus and silicon limitations

Upwelling in the South Yellow Sea is a phenomenon that plays an important role in nutrient transport and biological productivity. Based on remote sensing data from 2000 to 2022 and in situ observations from 2012 to 2022, we investigated the interannual variability of cold‐water mass frontal upwelling and its contribution to the transport of nutrients in the South Yellow Sea. The results showed that the upwelling positions during summer were consistent with the fronts of the cold‐water masses. The influence of upwelling on nutrient distribution and transport varied interannually, and the El Niño‐Southern Oscillation during winter might influence the intensity of summer frontal upwelling by modulating summer winds. Nutrient fluxes via upwelling from 2012 to 2022 were estimated: 0.08 × 10⁸–25.9 × 10⁸ mol month⁻¹ of dissolved inorganic nitrogen, 0.003 × 10⁸–0.67 × 10⁸ mol month⁻¹ of dissolved inorganic phosphate, and 0.12 × 10⁸–40.8 × 10⁸ mol month⁻¹ of dissolved silicate. Nutrient fluxes during summer were comparable to the summer inputs from the Changjiang River. The dissolved inorganic nitrogen/dissolved inorganic phosphate ratio in frontal upwelling decreased from 38.6 in 2012 to 20.0 in 2022, and the dissolved silicate/dissolved inorganic nitrogen ratio increased from 0.93 in 2012 to 2.48 in 2022. Nutrient composition and fluxes carried by upwelling can alleviate the limitations of both phosphorus and silicon in the South Yellow Sea. Upwelling nutrients in the frontal zone of the Yellow Sea Cold Water Mass could promote local phytoplankton growth and contribute to the development of Ulva prolifera.


Specific growth rate (a), cell volume (b), N2 fixation rate (c), and respiration rate (d) of Crocosphaera watsonii under three nutrient conditions: nutrient (Fe and P)‐replete (761 pM Fe′ and 10 μM PO43−$$ {\mathrm{PO}}_4^{3-} $$), Fe‐limited (21 pM Fe′ and 10 μM PO43−$$ {\mathrm{PO}}_4^{3-} $$), and P‐limited (761 pM Fe′ and ~ 0.02 μM PO43−$$ {\mathrm{PO}}_4^{3-} $$; Supporting Information Table S2), under both ambient (400 μatm) and acidified (750 μatm) conditions. The N2 fixation rate (nitrogenase activity) was measured in the middle of the nighttime. The data are presented as the means ± SDs (n = 3), with dots representing individual data points from replicates. Different superscripted letters denote statistically significant differences (p < 0.05) among treatments (two‐way ANOVA followed by the least significant difference [LSD] post hoc test). In (a), the growth rate under P‐limited conditions (dashed line) was equal to the dilution rate of the chemostat (0.25 d⁻¹).
C fixation rate (a), chlorophyll a (Chl a) concentration (b), and percentage change (acidified normalized to ambient conditions) in gene transcription (c) of photosystem I (PSI), photosystem II (PSII), and Cytb6/f for Crocosphaera watsonii under four combinations of different Fe′ concentrations (Fe and P replete, 761 pM; Fe‐limited, 21 pM) and pCO2 levels (ambient, 400 μatm; acidified, 750 μatm). The data are presented as the means ± SDs (n = 3). The measurements were conducted in the middle of the light cycle. In (a) and (b), different superscripted letters denote statistically significant differences (p < 0.05) among treatments (two‐way ANOVA followed by the LSD post hoc test), with dots representing individual data points of replicates. In (c), asterisks denote statistically significant differences (p < 0.05) between the ambient and acidified treatments (two‐tailed paired Student's t‐test).
Gene expression trends of nitrogenase (nifH, nifD, and nifK) (a) and iron use efficiency for N2 fixation (IUE‐N2 fixation, mol of N fixed h⁻¹ mol Fe⁻¹) (b) for Crocosphaera watsonii under the four combinations of different Fe′ concentrations (Fe&P replete, 761 pM; Fe‐limited, 21 pM) and pCO2 levels (ambient, 400 μatm; acidified, 750 μatm). nifH, nitrogenase iron protein; nifD, nitrogenase molybdenum–iron protein alpha chain; nifK, nitrogenase molybdenum–iron protein alpha chain. In (a), black represents the ambient treatment, and red represents the acidified treatment. Transcripts per million is a normalization method used to represent gene expression levels. Gene expression was scaled down 100 times (100×) for visualization purposes. The data are presented as the means ± SDs (n = 3), and the asterisks indicate significant differences (p < 0.05) between the ambient and acidified treatments (two‐tailed paired Student's t‐test). In (b), different superscripted letters indicate significant differences (p < 0.05) among treatments (two‐way ANOVA followed by the LSD post hoc test).
(a) Particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP) of Crocosphaera watsonii under various phosphate concentrations (Fe‐ and P‐replete and P‐limited) and pCO2 conditions (ambient, 400 μatm; acidified, 750 μatm); (b) cellular polyphosphate (polyP) (femto‐equivalents of the standard per cell), phospholipids, DNA and RNA concentrations in P‐replete C. watsonii under ambient and acidified conditions; and (c) the same metrics under P‐limited conditions. The data are presented as the means ± SDs (n = 3), and the dots represent the individual data points of each replicate. Asterisks denote significant differences (p < 0.05) between the ambient and acidified treatments (two‐tailed paired Student's t‐test).
Iron and phosphorus limitations modulate the effects of carbon dioxide enrichment on a unicellular nitrogen‐fixing cyanobacterium

Iron (Fe) and phosphorus (P) availability constrain the growth and N2 fixation of diazotrophic cyanobacteria in the global ocean. However, how Fe and P limitation may modulate the effects of ocean acidification on the unicellular diazotrophic cyanobacterium Crocosphaera remains largely unknown. Here, we examined the physiological responses of Crocosphaera watsonii WH8501 to CO2 enrichment under both nutrient‐replete and steadily Fe‐ or P‐limited conditions. Increased CO2 (750 μatm vs. 400 μatm) reduced the growth and N2 fixation rates of Crocosphaera, with Fe limitation intensifying the negative effect, whereas CO2 enrichment had a minimal impact under P limitation. Mechanistically, the high CO2 treatment may have led to a reallocation of limited Fe to nitrogenase synthesis to compensate for the reduction in nitrogenase efficiency caused by low pH; consequently, other Fe‐requiring metabolic pathways, such as respiration and photosynthesis, were impaired, which in turn amplified the negative effects of acidification. Conversely, under P limitation, CO2 enrichment had little or no effect on cellular P allocation among major P‐containing molecules (polyphosphate, phospholipids, DNA, and RNA). Cell volumes were significantly reduced in P‐limited and high CO2 cultures, which increased the surface : volume ratio and could facilitate nutrient uptake, thereby alleviating some of the negative effect of acidification on N2 fixation. These findings highlight the distinct responses of Crocosphaera to high CO2 under different nutrient conditions, improving a predictive understanding of global N2 fixation in future acidified oceans.


Snow avalanche‐induced disturbances can resurrect extinct zooplankton and alter paleolimnological records

January 2025

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We present a detailed observational study of the effects of the impulse wave caused by a snow‐avalanche on an alpine lake (Lake Peñalara, Sierra de Guadarrama, Spain). The avalanche broke the lake's ice cover (> 50 cm thick) and caused the lake to overflow. The impulse wave altered the lake water column stratification and physicochemical properties (dissolved oxygen, conductivity) in the short (hours) and mid‐term (days and weeks). It also caused the mobilization of hundreds of cubic meters of sediment, changing the lake morphometry. The sediment reconfiguration is likely the cause of the observed increased sedimentation rate and changes in the zooplankton density and composition in the following 4 yr after the avalanche, including the resurrection of a cladoceran species (Daphnia pulicaria) that had disappeared from the lake decades ago. Events such as the one we present can have significant paleolimnological implications: in this case, 75 cm of the sediment sequence were lost. Given these results, we propose that past avalanches could be the explanation to the almost complete removal of sediment from the deepest part of the lake around 260 yr cal BCE.


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3.8 (2023)

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33%

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8.8 (2023)

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50 days

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