OS3.2 | Linking marine plankton processes to meso- and fine-scale ocean variability through in situ observations
Linking marine plankton processes to meso- and fine-scale ocean variability through in situ observations
Convener: Gérald Grégori | Co-conveners: Wuchang Zhang, Jingyuan LI, Raffaella Casotti, Fang Zhang
Orals
| Wed, 06 May, 14:00–15:45 (CEST)
 
Room -2.92
Posters on site
| Attendance Wed, 06 May, 08:30–12:30 (CEST) | Display Wed, 06 May, 08:30–12:30
 
Hall X5
Orals |
Wed, 14:00
Posters on site
Wed, 08:30
Our current understanding of marine ecosystems has largely been shaped by observations at broad spatial and temporal scales. Cruise surveys typically provide only scattered transects, while fixed stations capture variability at a single location. Although these approaches have delivered valuable information on seasonal and interannual patterns, they are constrained by low spatial resolution and limited temporal coverage. As a result, the fine-scale dynamics of plankton and their interactions with environmental gradients have remained largely unresolved, leaving an incomplete picture of ecosystem functioning.

Over the past decade, rapid advances in ocean observing technologies have opened new opportunities to resolve fine-scale plankton dynamics. High-resolution satellites (e.g. SWOT), autonomous buoy networks equipped with sensors for temperature, salinity and fluorescence, and in situ plankton imaging systems (e.g. Imaging FlowCytobot, CytoSense/CytoSub, Imaging Plankton Probe, Video Plankton Recorder) now provide continuous, high-frequency records across multiple platforms. These innovations make it possible to observe plankton processes at unprecedented temporal and spatial resolutions, capturing short-lived events and small-scale environmental gradients that were previously undetectable. Together, these advances are reshaping our understanding of plankton variability and their ecological roles in marine ecosystems.

This session invites contributions on meso- to fine-scale plankton variability and its links to environmental gradients. We particularly welcome studies using in situ and high-resolution observing systems, as well as new developments in intensive monitoring technologies. The goal is to advance understanding of how fine-scale plankton–environment interactions shape ecosystem structure and function, and how they connect to larger-scale ocean and climate processes. A special issue in the Journal of Plankton Research will accompany this session.

Orals: Wed, 6 May, 14:00–15:45 | Room -2.92

Chairpersons: Gérald Grégori, Fang Zhang, Raffaella Casotti
14:00–14:05
5-minute convener introduction
14:05–14:15
|
EGU26-4652
|
On-site presentation
Zooplankton Growth Indicators as a Proxy for Marine Ecosystem Health 
Song Sun

Zooplankton play a crucial role in marine ecosystems. Changes in their species composition and population dynamics can lead to structural and functional shifts in the ecosystem. The growth condition and population recruitment rate of zooplankton act as a comprehensive proxy​ for the marine ecosystem. However, research on zooplankton species composition and population dynamics faces considerable challenges due to the complexity of their communities and the difficulties in observation, sampling, and analysis. This study proposes using key growth indicators of selected zooplankton species to assess the health condition of marine ecosystems. Specifically, we examine the seasonal and regional growth variations in Antarctic krill (Euphausia superba), such as the occurrence and extent of shrinking to infer the status of the Southern Ocean ecosystem. Additionally, we employ egg production rates and hatching success of key species of copepods as indicators to evaluate the health of coastal ecosystems. Through these approaches, we aim to establish zooplankton growth metrics as effective proxies for monitoring and indicating marine ecosystem health.

How to cite: Sun, S.: Zooplankton Growth Indicators as a Proxy for Marine Ecosystem Health, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4652, https://doi.org/10.5194/egusphere-egu26-4652, 2026.

14:15–14:25
|
EGU26-6788
|
ECS
|
On-site presentation
|
Phytoplankton community structure responses to episodic summer storms in the eastern English Channel 
Harshal Chavan, Urania Christaki, Luis Felipe Artigas, and François G. Schmitt

Extreme events can rapidly alter the physical and biogeochemical environment, triggering pronounced shifts in phytoplankton community structure. In this study the impact of 10 well identified storms on phytoplankton communities was explored in a productive coastal temperate ecosystem, the eastern English Channel (EEC). Summer was selected as the focal season because it is typically nutrient‑poor, and major phytoplankton shifts or blooms are not expected. Low‑frequency (weekly to fortnightly) flow cytometry measurements of phytoplankton abundance were combined with high‑frequency meteorological (precipitation, wind) and hydrological records from 2012 to 2022. Additional biogeochemical and phytoplankton data were obtained from French National Observing Systems (SNOs): SNO SOMLIT and SNO PHYTOOBS (biweekly sampling) and SNO COAST‑HF (10–30 min resolution).

Storm impacts emerged in three distinct categories, illustrated by representative events: a high river inflow storm (02 July 2016), a high wind stress–low inflow storm (13 July 2015), and a low wind stress–low inflow storm (06 June 2022). High inflow storms, regardless of wind intensity, enhanced coastal advection of nutrient‑rich river plumes, sustaining diatom dominance and, under strong winds, promoting nano‑sized taxa such as nanophytoplankton and cryptophytes. Under low inflow conditions, limited riverine nutrient supply increased reliance on regenerated nutrients from vertical mixing. When paired with strong winds, these conditions favoured nanophytoplankton growth, whereas short, low‑wind storms supported pico‑sized phytoplankton, particularly Synechococcus spp. and picoeukaryotes.

Across years, storms repeatedly reset seasonal succession and maintained environmental heterogeneity, generating transient monospecific peaks (e.g., Leptocylindrus danicus) and shaping community trajectories throughout summer. These findings highlight storms as recurrent structuring forces in the EEC, mediating water-column structure (stratification versus mixing) and associated nutrient availability, thereby driving rapid shifts in phytoplankton composition.

How to cite: Chavan, H., Christaki, U., Artigas, L. F., and Schmitt, F. G.: Phytoplankton community structure responses to episodic summer storms in the eastern English Channel, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6788, https://doi.org/10.5194/egusphere-egu26-6788, 2026.

Discussion Post a comment
14:25–14:35
|
EGU26-16971
|
On-site presentation
Water-mass structuring of tintinnid communities within a mesoscale eddy in the Kuroshio Extension 
Yuan Zhao, Simeng Nan, Yi Dong, Li Zhao, and Wuchang Zhang

Mesoscale eddies create persistent hydrographic compartments that can reorganize microzooplankton assemblages, yet the extent to which tintinnid communities are structured by eddy-associated water masses remains unclear. We investigated tintinnid community organization across a mesoscale eddy in the Kuroshio Extension using two intersecting transects and a hydrography-based delineation of four water masses (Kuroshio Water, Eddy Center Water, Eddy Periphery Water, and Deep Water). Community composition showed clear water-mass structuring in ordination space and was supported by permutation-based tests. Importantly, compositional differentiation among water masses remained evident when comparisons were constrained to overlapping depth intervals, indicating that water-mass identity captured community variability beyond depth-related gradients. Alpha-diversity patterns were consistent with these shifts, with the eddy core exhibiting higher richness and diversity but lower evenness relative to surrounding waters. Species-level diagnostics further supported water-mass specificity: indicator-species analyses identified distinct assemblage signatures for each water mass, while SIMPER highlighted a recurrent subset of taxa contributing disproportionately to inter-water-mass dissimilarity. Decomposition of beta diversity suggested contrasting modes of within-water-mass variability across the eddy, characterized by stronger turnover in Kuroshio Water and greater richness-difference components within eddy waters. Collectively, these results identify eddy-associated water masses as a robust organizing framework for tintinnid community structure in the Kuroshio Extension.

How to cite: Zhao, Y., Nan, S., Dong, Y., Zhao, L., and Zhang, W.: Water-mass structuring of tintinnid communities within a mesoscale eddy in the Kuroshio Extension, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16971, https://doi.org/10.5194/egusphere-egu26-16971, 2026.

14:35–14:45
|
EGU26-18976
|
On-site presentation
Spatial heterogeneity of phytoplankton community structure across two distinct frontal systems in the NW Pacific: Insights from in situ high frequency observations  
Li Zhao, Yuan Zhao, Yi Dong, and Wuchang Zhang

Background: In the Northwestern Pacific, the complex marine environment drives rapid phytoplankton responses to drastic environmental fluctuations. However, how these primary producers are influenced by intricate physical processes remains poorly understood due to the paucity of targeted meso- and fine-scale observations.

Material and methods: To address this knowledge gap, we conducted a longitudinal transect survey in the northwestern Pacific during September–October 2024. Sampling stations were deployed at mesoscale eddy locations identified by the SWOT satellite. Two distinct frontal zones were delineated along the transect: the southern front (F1) from cold- warm eddy interaction, and the northern front (F2) from Kuroshio-Oyashio convergence. High-frequency sampling (30-min intervals) was performed using an in situ CytoSense flow cytometer to characterize phytoplankton abundance and community structure.

Results: Our results revealed 11 distinct phytoplankton groups across the survey region. Picophytoplankton dominated the entire transect, with OrgPicoPro as the most dominant group followed by RedPico. Phytoplankton abundance exhibited distinct spatial gradients: the lowest values were recorded in warm eddies, followed by cold eddies, and the highest in the two frontal zones—particularly at Front F1, where abundance surged to 20–40 times that in warm eddies. Notably, despite the low phytoplankton abundance in warm eddies, the Shannon diversity index was the highest there. Two subgroups of OrgPicoPro were exclusively detected in warm eddies, whereas only one subgroup was present in all other regions; in contrast, RedPicoPro was absent in warm eddies but distributed in other areas.

Regarding community composition, Front F1 showed significant differences from warm eddies but high similarity to cold eddies. Acting as a barrier separating warm eddies from others, F1 explains the pronounced differences in phytoplankton abundance and community composition between warm eddies and other areas. In contrast, Front F2 exhibited less pronounced differences in both phytoplankton abundance and community composition from the adjacent Kuroshio and Oyashio water masses on either side. Additionally, temperature and salinity exerted different regulatory effects on phytoplankton community among different water masses.

Conclusion: Our findings demonstrate that phytoplankton community structure changes drastically across frontal zones and responds differentially to the two frontal systems, driving significant spatial heterogeneity in the NW Pacific. This study highlights the value of in situ high-frequency observations for unraveling fine-scale physical-biological coupling mechanisms, providing critical insights into phytoplankton ecological dynamics in complex marine environments.

Keywords: Spatial heterogeneity, phytoplankton community structure, frontal zone, Northwestern Pacific

 

How to cite: Zhao, L., Zhao, Y., Dong, Y., and Zhang, W.: Spatial heterogeneity of phytoplankton community structure across two distinct frontal systems in the NW Pacific: Insights from in situ high frequency observations , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18976, https://doi.org/10.5194/egusphere-egu26-18976, 2026.

14:45–14:55
|
EGU26-23220
|
On-site presentation
Exploring the fine scale spatial distributions of carbon and biomass using shadowgraphy and direct collections of marine snow 
Alexander Bochdansky
Based on our shadowgraph data, gels numerically represent more than 85% of all particles in the size range from ~20 to several hundred micrometers in the ocean, and their relative contribution further increases below the euphotic zone. Gels also act as the “glue” in the formation of marine snow, resulting in small-scale heterogeneity that remains under-explored by conventional oceanographic sampling methods.
Here I present data on the relative enrichment of organisms and organic carbon using direct collections of individual marine snow particles combined with simultaneous imaging. After detailed image analysis and conversion of particle outlines to volumes, we found that both organic carbon and biomass (based on ATP) are substantially more enriched on particles relative to ambient seawater than previously assumed. Organic carbon is more strongly enriched than biomass in marine snow, likely reflecting two factors: (1) marine snow contains a higher proportion of refractory material such as detritus, dead cells, and fecal matter, and (2) gels themselves constitute a large fraction of the particle matrix.
Across all depths, marine snow exhibits a pronounced shift in community composition, with higher relative enrichment of eukaryotes compared to prokaryotes, including elevated contributions from eukaryotic microbes and even metazoans. In the bathypelagic ocean, the relative importance of fungi and the functionally related labyrinthulomycetes also increases on marine snow. These patterns on the spatial scale of particles mirror large-scale biogeographic trends, with increasing relative eukaryotic abundance from oligotrophic to eutrophic systems. However, in extremely eutrophic systems such as estuaries, biomass as a proportion of total carbon decreases again, likely due to enhanced allochthonous inputs of organic carbon, detritus, and gels.
The close spatial coupling of organisms and organic substrates on particles may allow biogeochemical processes to occur that would otherwise fall below functional thresholds. Collectively, our results indicate that particle-associated processes play a disproportionate role in regulating oceanic biogeochemical cycles.

How to cite: Bochdansky, A.: Exploring the fine scale spatial distributions of carbon and biomass using shadowgraphy and direct collections of marine snow, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-23220, https://doi.org/10.5194/egusphere-egu26-23220, 2026.

14:55–15:05
|
EGU26-8071
|
ECS
|
On-site presentation
Reconstructing the Global Vertical Structure of Marine Phytoplankton Using a Two-Community Model 
Minyu Wang, Robert Brewin, Johannes Viljoen, Xuerong Sun, Sensen Wu, and Zhenhong Du

The vertical distribution of marine phytoplankton plays a crucial role in oceanic primary production and the carbon cycle. Satellite observations of surface chlorophyll (Chl) and particulate organic carbon (POC) capture large-scale surface patterns but are limited to roughly one diffuse attenuation depth (Kd), representing approximately the top one-fifth of the euphotic zone. Consequently, subsurface phytoplankton structures, including the deep chlorophyll maximum (DCM), remain poorly represented at the global scale, while in situ vertical profile observations are too sparse to provide continuous three-dimensional coverage. To address this, we employ a two-community parametric model that represents the vertical structure of phytoplankton as two functional groups. and examine their physiological characteristics, specifically their responses to light adaptation and nutrient availability. The model is tuned to global 4D fields of Chl and POC spanning 1998-2022 derived from satellite ocean color merged with BGC-Argo profiles. We reveal the relative contributions of the two communities to the total water column phytoplankton biomass, study their physiological dynamics, and their relationship with light and nutrient conditions, in different regions of the ocean. Globally, these findings clarify how environmental conditions shape phytoplankton vertical structure, particularly the seasonal dynamics of the DCM, advancing understanding of global subsurface phytoplankton patterns and their impact on the marine carbon cycle.

How to cite: Wang, M., Brewin, R., Viljoen, J., Sun, X., Wu, S., and Du, Z.: Reconstructing the Global Vertical Structure of Marine Phytoplankton Using a Two-Community Model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8071, https://doi.org/10.5194/egusphere-egu26-8071, 2026.

15:05–15:15
|
EGU26-12066
|
ECS
|
On-site presentation
Estimation of picophytoplankton and nanophytoplankton abundances in the Kuroshio extension using Sentinel-3/OLCI data
(withdrawn)
Yu Zhang
15:15–15:25
|
EGU26-5795
|
On-site presentation
Neuston transport across NW Romanian Black Sea shelf 
Mihaela Muresan and the Muresan Mihaela

The neuston community of the northwestern Black Sea is shaped by environmental conditions such as nutrient availability and temperature, and redistributed by physical drivers including surface currents, wind, wave-induced transport, and vertical turbulence. By combining physical and statistical modelling, we aimed to investigate the transport and variability of neustonic organisms, focusing on pontellid copepods along the Romanian shelf. We applied the OpenDrift Lagrangian modeling framework to simulate the probabilistic displacement patterns of neustonic organisms in the Black Sea (Romanian shelf). The model integrates hydrodynamic forcing (Copernicus-derived surface currents, wind fields, and wave parameters) and stochastic uncertainty to generate detailed drift patterns that reveal potential movement pathways.  The statistical results suggest that wind speed and direction, Danube River discharge, and primary productivity significantly influence neuston aggregation and dispersal. High wind speeds appeared to reduce accumulation, likely due to enhanced turbulence, while directional winds and plume dynamics structured transport pathways.These findings support the role of pontellids as bioindicators of near-surface hydrodynamic processes and offer a framework for tracing convergence and retention mechanisms in coastal systems.

How to cite: Muresan, M. and the Muresan Mihaela: Neuston transport across NW Romanian Black Sea shelf, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5795, https://doi.org/10.5194/egusphere-egu26-5795, 2026.

15:25–15:35
|
EGU26-6144
|
ECS
|
On-site presentation
Vertical Structure of Chlorophyll-a and Deep Chlorophyll-a Maximum Dynamics in the Southern Lombok Strait under Spring-Neap Tidal Forcing 
Suliskania Nurfitri, Rafli Yudha Asdana, Muhammad Fadli, Xu Tengfei, Saat Mubarrok, Rima Rachmayani, Adi Purwandana, Priyadi Dwi Santoso, Bayu Priyono, Teguh Agustiadi, Zexun Wei, Shujiang Li, and R. Dwi Susanto

The Deep Chlorophyll Maximum (DCM) is a defining feature of phytoplankton distribution in stratified tropical oceans, yet its response to episodic physical forcing remains poorly constrained. We investigate the vertical chlorophyll-a structure and associated physical–biogeochemical drivers in the southern Lombok Strait, Indonesia, a region influenced by strong internal wave activity and pronounced spring–neap tidal variability. Using 112 CTD profiles collected at three adjacent stations from 12 to 27 March 2021 (0–500 m), complemented by satellite-derived photosynthetically active radiation, biogeochemical model nutrients, and tidal observations, we characterize DCM depth, magnitude, and variability. The DCM consistently occurred at 50–100 m near the upper thermocline, but its intensity and vertical expression were modulated by tidal-driven mixing, with stronger spring-tide turbulence enhancing nutrient supply to the euphotic zone. Our results demonstrate that short-period variability in mixing exerts a first-order control on subsurface phytoplankton structure in this dynamically forced strait system, providing new insight into the coupling between physical processes and biogeochemical responses in tropical marginal seas.

How to cite: Nurfitri, S., Asdana, R. Y., Fadli, M., Tengfei, X., Mubarrok, S., Rachmayani, R., Purwandana, A., Santoso, P. D., Priyono, B., Agustiadi, T., Wei, Z., Li, S., and Susanto, R. D.: Vertical Structure of Chlorophyll-a and Deep Chlorophyll-a Maximum Dynamics in the Southern Lombok Strait under Spring-Neap Tidal Forcing, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6144, https://doi.org/10.5194/egusphere-egu26-6144, 2026.

15:35–15:45
|
EGU26-6228
|
ECS
|
On-site presentation
Spatial segregation of early life stages: fish eggs and larvae occupy distinct marine environments in the southern coastal waters of Korea 
Haeyoung Choi, Seong-Yong Moon, Mi-Hee Lee, Se-Ra Yoo, and Jeong-Ho Park

Fish eggs and larvae are key planktonic life stages influencing recruitment success and marine ecosystem functioning, yet their fine-scale habitat use remains insufficiently resolved. This study examines stage-specific spatio-temporal distributions of ichthyoplankton in the southern coastal waters of Korea, with a focus on differences between coastal and inner-bay environments. Ichthyoplankton were collected from April to September 2023 in southern coastal waters and a semi-enclosed embayment (Yeoja Bay). Species composition was identified using morphological analysis, DNA barcoding, and metabarcoding. A total of 126 species were detected, with higher richness in the coastal waters (121 species) than in the inner bay (57 species). Egg species richness increased from April and declined in early summer, while larval richness increased from May and remained relatively stable thereafter. Clear stage- and species-specific distribution patterns were observed. Jack mackerel (Trachurus japonicus) occurred exclusively in the coastal waters. Whereas silver croaker (Pennahia argentata), three-lined tongue sole (Cynoglossus abbreviatus), and fivespot flounder (Pseudorhombus pentophthalmus) were present in both regions, exhibiting spatial shifts between egg and larval stages. For example, P. argentata eggs were concentrated in the inner bay in spring, while larvae occurred sequentially in the inner bay and coastal waters during summer. Temperature and salinity analyses showed that eggs and larvae of C. abbreviatus were associated with relatively low-temperature and high-salinity conditions, whereas P. pentophthalmus larvae occurred in warmer waters. These results demonstrate stage-specific habitat partitioning of ichthyoplankton at fine spatial and temporal scales, highlighting the importance of resolving early life stage–environment relationships in coastal ecosystems.

 

How to cite: Choi, H., Moon, S.-Y., Lee, M.-H., Yoo, S.-R., and Park, J.-H.: Spatial segregation of early life stages: fish eggs and larvae occupy distinct marine environments in the southern coastal waters of Korea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6228, https://doi.org/10.5194/egusphere-egu26-6228, 2026.

Posters on site: Wed, 6 May, 08:30–10:15 | Hall X5

Display time: Wed, 6 May, 08:30–12:30
Chairpersons: Wuchang Zhang, Jingyuan LI
X5.234
|
EGU26-21382
Frontal plankton communities: How fine-scale oceanic features shape « bacterial » community structure 
Gérald Gregori, Ludivine Grand, Andrea Doglioli, Elvira Pulido, Laurina Oms, Aude Barani, Marceau Dupin, Morgane Didry, and Francesco D'Ovidio

Fine-scale oceanic structures (1–100 km), though ephemeral (days-weeks), play a key role in shaping marine physical, chemical, and biological dynamics. Despite their ubiquity, their influence on plankton remains poorly documented due to the difficulty of targeting them in situ, particularly in the low energetic and oligotrophic regions constituting the majority of the ocean. The new «SWOT satellite era» makes possible to observe the fine-scale component of ocean dynamics even in low energetic and oligotrophic regions. In 2023, the BioSWOT-Med cruise (northwestern Mediterranean) employed an adaptive lagrangian strategy to target three distinct dynamical features: a cyclonic circulation to the north, an anticyclonic eddy to the south, too small to be clearly detected by conventional altimetry, and the front itself.

Heterotrophic prokaryotic communities are the main drivers of oceanic mineralization of the organic matter. By integrating hydrographic and cytometric data for bacterial abundance and diversity across the water column (0-500 m), we show that water masses act as ecological filters, shaping bacterial assemblages with distinct compositions, abundances, and functional traits. Functional statistics revealed that the composition of the bacterial community at the front was not similar to that of the adjacent waters, with higher surface concentrations of medium- to small-sized cells, suggesting that this type of nutrient-rich structure promotes bacterial growth. The open water mass was characterized by larger cells, while the eddy was distinguished by lower abundances and deeper bacterial signatures in the water column.

These results highlight the need for multi-scale and multidisciplinary observations to better understand the biological seascape and its broader implications for biogeochemical cycles and marine ecosystem functioning.

How to cite: Gregori, G., Grand, L., Doglioli, A., Pulido, E., Oms, L., Barani, A., Dupin, M., Didry, M., and D'Ovidio, F.: Frontal plankton communities: How fine-scale oceanic features shape « bacterial » community structure, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21382, https://doi.org/10.5194/egusphere-egu26-21382, 2026.

X5.235
|
EGU26-2383
Community-level phenological patterns of tintinnids in a coastal site with the largest annual temperature range 
Wuchang Zhang, Wenhua Bian, and Yuan Zhao

Surface seawater temperature varies seasonally across latitudes, shaping the temporal organization of plankton communities. However, the phenology of microzooplankton at the community level remains poorly understood, largely due to insufficient temporal resolution in most monitoring programs. In this study, we conducted a high-frequency (three times per week) year-round survey of tintinnids in Qingdao coastal waters, a site characterized by the largest annual temperature range among global coastal systems. Thirty tintinnid species were identified, all of which were neritic taxa. Based on their annual occurrence patterns, the community was classified into four phenological types: cold-water type, temperate type I, temperate type II, and warm-water type, each exhibiting distinct seasonal peaks and thermal performance ranges. These four phenological types constitute the community-level phenological pattern of tintinnids in Qingdao coastal waters. The effective breadths of thermal performance range for each type was 5–8 °C, and the number of phenological types increased with the site’s annual temperature range, from two in Hong Kong to three in Ningbo and four in Qingdao. Cold-adapted types exhibited positively skewed thermal performance curves, indicating higher sensitivity to temperature fluctuations at low temperatures, whereas warm-adapted types followed the generalized negatively skewed pattern. These results demonstrate that high-frequency observations provide a powerful framework for resolving community-level phenology and highlight the role of local thermal regimes in structuring tintinnid seasonal dynamics under a warming ocean.

How to cite: Zhang, W., Bian, W., and Zhao, Y.: Community-level phenological patterns of tintinnids in a coastal site with the largest annual temperature range, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2383, https://doi.org/10.5194/egusphere-egu26-2383, 2026.

X5.236
|
EGU26-6453
High-resolution in situ imaging reveals zooplankton continuous vertical distributions and eddy-driven community variations in the South China Sea 
Fang Zhang, Shuo Wang, Song Sun, and Hongsheng Bi

Mesoscale eddies are ubiquitous oceanographic features that play a pivotal role in regulating marine ecosystems by altering water column structure and redistributing biological resources. To examine their ecological effects on zooplankton communities in the South China Sea (SCS), we conducted in situ imaging observations using the PlanktonScope and Underwater Vision Profiler 6 systems within representative cyclonic eddy (CE) and anticyclonic eddy (AE) in April 2023. Integrating satellite remote sensing with in situ environmental measurements, we analyzed eddy-induced variations in zooplankton abundance, community composition, vertical distribution, and diel vertical migration (DVM) behavior. Zooplankton abundance within the upper 300 m was consistently higher in the CE, showing strong aggregation in the eddy core, whereas in the AE, abundance peaked at intermediate depths (50–100 m) near the periphery, forming a characteristic ring-shaped pattern. Seawater temperature and dissolved oxygen were identified as the dominant environmental drivers regulating zooplankton community structure, while nutrient enrichment at 100–150 m in the CE reflected upwelling-driven enhancement of bottom-up control. Zooplankton also exhibited more pronounced DVM amplitudes within the CE than in the AE, indicating that mesoscale hydrodynamic conditions strongly modulate vertical behavioral dynamics and trophic connectivity. Overall, mesoscale eddies exert a profound influence on zooplankton spatial ecology in the SCS by reshaping hydrographic structure, nutrient availability, and behavioral patterns, providing new insights into the coupling between mesoscale physical processes and biological responses in tropical ocean ecosystems.

How to cite: Zhang, F., Wang, S., Sun, S., and Bi, H.: High-resolution in situ imaging reveals zooplankton continuous vertical distributions and eddy-driven community variations in the South China Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6453, https://doi.org/10.5194/egusphere-egu26-6453, 2026.

Posters on site: Wed, 6 May, 10:45–12:30 | Hall X5

Chairpersons: Gérald Grégori, Wuchang Zhang, Raffaella Casotti
X5.237
|
EGU26-17719
|
ECS
Unraveling epipelagic microbial food web structure across water masses of the Kuroshio Extension based on meso- and fine-Scale in situ observation 
Jingyuan Li, Ludivine Grand, Li Zhao, Yuan Zhao, Wuchang Zhang, and Gerald Gregori

The microbial food web (MFW) forms the foundation of ecosystems and comprises unicellular organisms across multiple size classes. It is grouped into picoplankton (0.2–2 μm; PRO, SYN, PEUK, HP), nanoplankton (2–20 μm; NEUK), and microplankton (>20 μm; ciliates including tintinnids). Although individual microbial components have been studied in the North Pacific, integrated analyses of MFW structure across water masses are still scarce.

Here, we investigated the structure of epipelagic MFWs in the Kuroshio Extension region using in situ observations conducted at both meso- and fine-scale resolutions. Along a mesoscale transect (D transect), we examined variations in MFW structure within the 5–200 m water column across distinct water masses, including Kuroshio Water, Transition Zone (including a cold eddy), and Oyashio Water. Overall, the abundances and biomasses of three trophic levels increased from Kuroshio to transition waters and further to Oyashio waters in the upper layer (5–50 m), whereas an opposite pattern was observed in the 50–150 m layer. In terms of Pico, cold eddy exerted positive effects on SYN and HP, but negative effects on PRO and PEUK. Nano-sized organisms exhibited reduced abundance and biomass within cold eddy in the 50–200 m layer, while showing limited responses in the upper layer. At the micro-sized trophic level, cold eddy was unfavorable for tintinnids and suppressed ciliate abundance in the 5–50 m layer.

The other surface F transect, a high-frequency, fine-scale transect crossing the Kuroshio Extension front, was used to conduct targeted in situ surface biological observations to examine microbial food web structure. Satellite remote sensing provided complementary information on sea surface frontal dynamics, enabling interpretation of fine-scale biological variability in this highly dynamic region.

Through combined mesoscale and fine-scale observations, our results demonstrate how microbial food web structure in the Kuroshio Extension responds to both distinct water masses and highly dynamic frontal zones, offering rare in situ data in this complex and previously under-sampled region and highlighting the importance of multi-scale physical–biological interactions.

How to cite: Li, J., Grand, L., Zhao, L., Zhao, Y., Zhang, W., and Gregori, G.: Unraveling epipelagic microbial food web structure across water masses of the Kuroshio Extension based on meso- and fine-Scale in situ observation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17719, https://doi.org/10.5194/egusphere-egu26-17719, 2026.

X5.238
|
EGU26-543
Preliminary results on the influence of seamount topography on hydrography, macronutrient and biomass distribution in the Southern Tropical Atlantic 
Marius N. Müller, Nana Hocke, Luiz Gustavo de Sales Jannuzzi, Mayza Pompeu, Frederico Brandini, Gilvan Takeshi Yogui, Pedro Augusto Mendes de Castro Melo, Ralf Schwamborn, Moacyr Araujo, Tim Fischer, and Rebecca Hummels

Seamounts (commonly defined as submarine elevations rising >1000 m above the surrounding seafloor) are found in all ocean basins, including the Southern Atlantic. Depending on seamount topography, they may create distinct hydrographic regimes that provide ecological habitats for elevated primary production and biodiversity due to the generation of internal waves that promote vertical mixing and redistribution of energy and nutrients from deep to surface waters. Here, we present preliminary results from an oceanographic expedition (RV Maria S. Merian – n° 117) in the Brazilian EEZ across two seamounts (i.e., Toninha and Aracati seamounts at 4°S, 36°W and 3.5°S, 37.5°W, respectively). Multiple datasets were acquired to describe the physical-chemical conditions around and above the seamounts, including hydrography (CTD, UCTD, ADCP, velocity shear and temperature variabilities), macronutrients (NO2- + NO3-, PO43-, SiO42-), and chlorophyll distribution. The different topography of the two seamounts created specific hydrographic patterns at the two sites, resulting in varying macronutrient and chlorophyll distribution. The chlorophyll maximum was aligned with the pycnocline at Aracati, however, a substantial deviation between the chlorophyll maximum and the pycnocline was observed at Toninha. This discrepancy is likely attributable to hydrographic variations, stemming from the shallower depth of Toninha (approx. 80 m), compared to the 250 m of Aracati, influencing internal waves and energy dissipation above the seamount plateau. Surface waters at both seamount sites were characterized by low nitrogen (NO2- + NO3-) compared to phosphate (PO43-) availability with a N:P ratio of <10, whereas below the chlorophyll maximum the N:P ratios were close to the Redfield ratio of 16. The partial discrepancy between the two seamounts in terms of hydrographic dynamics, nutrient availability and chlorophyll maximum represents an interesting basis for evaluating biological dynamics and phytoplankton community structure. It is speculated and discussed that the ecological dynamics (in terms of phyto- and zooplankton abundance, diversity and function) are distinctively different at the two investigated seamounts, representing a case study for the biological diversity at tropical South Atlantic seamounts.

How to cite: Müller, M. N., Hocke, N., de Sales Jannuzzi, L. G., Pompeu, M., Brandini, F., Yogui, G. T., Mendes de Castro Melo, P. A., Schwamborn, R., Araujo, M., Fischer, T., and Hummels, R.: Preliminary results on the influence of seamount topography on hydrography, macronutrient and biomass distribution in the Southern Tropical Atlantic, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-543, https://doi.org/10.5194/egusphere-egu26-543, 2026.