Biology subjects

Decima, M.

Publications and source records attributed to Decima, M..

8 recordsLinked to original sources

bioRxiv2026

Comparative food-web analysis of bluefin tuna spawning habitats in the eastern Indian Ocean and Gulf of Mexico

Using linear inverse ecosystem modeling as a data assimilation tool, we compare spawning grounds of Atlantic and Southern Bluefin Tuna (ABT and SBT, respectively) based on results from field campaigns in the Gulf of Mexico (GoM) and eastern Indian Ocean off northwest Australia (Argo Basin). Both regions are warm, stratified, low-nutrient waters dominated by cyanobacteria (Prochlorococcus). Despite these similarities, the Argo Basin is more productive, with [~]1.5X higher net primary production and nearly 2X higher production of top trophic levels in the model (tuna larvae, planktivorous fish, and predatory gelatinous zooplankton). Higher primary production in the Argo Basin is mainly driven by higher N2 fixation and storm mixing of new nutrients in the upper and lower euphotic zone, respectively. Increased ecosystem efficiency (secondary production of top trophic levels / primary production) results from differences in plankton food web organization. In the GoM, protistan zooplankton are the direct consumers of nearly all phytoplankton production. In contrast, higher rates of herbivory by crustaceans feeding on nanophytoplankton combines with a higher impact of appendicularians on cyanobacteria to convert plankton production into larval tuna prey more efficiently in the Argo Basin. Despite similarities in the proportions of phytoplankton production mediated by cyanobacteria and other picoplankton in both systems, food web pathways to larval tuna and other planktivorous fish are substantially shorter in the Argo Basin. Our results highlight the impact of distinct zooplankton ecological niches on ecosystem efficiency and suggest a need for better inclusion of plankton food-web structure in models simulating climate impacts on fisheries production. HIGHLIGHTSO_LIDeveloped food web models of tuna spawning habitat (Indian Ocean & Gulf of Mexico) C_LIO_LISpawning habitats in the Argo Basin and Gulf of Mexico (GoM) are both oligotrophic C_LIO_LIArgo Basin had higher net primary production in part as a result of nitrogen fixation C_LIO_LIArgo Basin had higher rates of direct herbivory by metazoan zooplankton C_LIO_LIThis resulted in greater ecosystem efficiency in the Argo Basin. C_LI

ecology↗

bioRxiv2025

Exploring the roles of trophic mode and microbial prey size in grazing pathways of tropical oligotrophic waters of the eastern Indian Ocean

Prey removal incubations were conducted in the Argo Basin (eastern Indian Ocean) to investigate the trophic ecology of the zooplankton community supporting Southern Bluefin Tuna larvae. Grazing and selectivity were evaluated considering prey trophic mode and size as food quality descriptors in copepod diets and compared with microzooplankton grazing. Copepod ingestion of eukaryotes ranged from 3.4 to 138 ng carbon (C) ind-1 d-1. Diet was dominated by mixotrophic (5-89%) and heterotrophic (0-84%) prey, with autotrophs contributing 2-17%. Nanoplankton provided the highest C intake to copepods (62-99%) rather than more abundant picoplankton (0.8-38%). No measurable reduction in chlorophyll a (Chla) concentration occurred from copepod grazing through food removal, suggesting a possible trophic cascade, although gut content revealed ingestion of 0.8 {micro}g Chla ind-1 d-1. Copepods showed moderate selection for picoplankton (E=0.3) over nanoplankton, perhaps due to faster consumption of nanoplankton by microzooplankton or ingestion of picoplankton aggregates. Ingestion of 15N (nitrogen)-labelled prey indicated copepod consumption of protistan consumers or small metazoans. We found significantly greater ([~]5-fold) copepod N consumption when using 1-2 {micro}m 15N-Synechococcus, compared to <1 {micro}m sized 15N-Prochlorococcus. Microzooplankton grazing on eukaryotes (0.07-2.5 d-1) and prokaryotes (0.3-2.1 d-1) greatly exceeded copepod grazing. Microzooplankton diet consisted mainly of heterotrophs (25-59%) and mixotrophs (13-41%), with lower autotrophic contribution (12-33%) of more nano- (95-98%) than picophytoplankton (2-5%) ingested. Overall, microzooplankton removed most of daily production (111%) in contrast to 7% for copepods. Our findings indicate that mixotrophy, intraguild grazing and nutrient channeling support the food web in this oligotrophic region.

ecology↗

bioRxiv2025

Sinking particle export within and beneath the euphotic zone in the eastern Indian Ocean

The eastern Indian Ocean is substantially under sampled with respect to the biological carbon pump - the suite of processes that transport the carbon fixed by phytoplankton into the deeper ocean. Using sediment traps and other ecosystem measurements, we quantified sinking organic matter flux and investigated the characteristics of sinking particles in waters overlying the Argo Abyssal Plain directly downstream of the Indonesian Throughflow off northwest Australia. Carbon export from the euphotic zone averaged 7.0 mmol C m-2 d-1, which equated to an average export efficiency (export / net primary production) of 0.17. Sinking particle flux within the euphotic zone (beneath the mixed layer, but above the deep chlorophyll maximum) averaged slightly higher than flux at the base of the euphotic zone, suggesting that the deep euphotic zone was a depth stratum of net particle remineralization. Carbon flux attenuation continued into the twilight zone with a transfer efficiency (export at euphotic depth + 100m / export at euphotic depth) of 0.62 and an average Martins b-value of 1.1. Within the euphotic zone, fresh phytoplankton (chlorophyll associated with sinking particles, possibly contained within appendicularian houses) were an important component of sinking particles, but beneath the euphotic zone the fecal pellets of herbivorous zooplankton (phaeopigments) were more important. Changes in carbon and nitrogen isotopic composition with depth further reflected remineralization processes occurring as particles sank. We show similarities with biological carbon pump functioning in a similar semi-enclosed oligotrophic marginal sea, the Gulf of Mexico, including net remineralization across the deep chlorophyll maximum. Submitted to: Deep-sea Research II HighlightsDespite low productivity, export efficiency was 17% of primary production Flux attenuation beneath the euphotic zone (EZ) was low for a tropical region Sinking particle flux from the upper to lower EZ exceeded export from lower EZ The deep EZ was a stratum of net particle remineralization (and net heterotrophy)

ecology↗

bioRxiv2025

Export dynamics of protists across the southern subtropical frontal zone reveal taxon-specific patterns

Gravitational particle sinking is the main mechanism for carbon export in the biological carbon pump. However, the export dynamics of the particle-associated protist community are not fully understood. We used 18S rRNA gene metabarcoding to characterise the exported protist community within sinking particles and bathypelagic surficial sediments in oligotrophic subtropical and high-nutrient, low-chlorophyll subantarctic waters. Sinking particles were collected with formalin-fixed and preservative-free particle interceptor traps (fixed and live traps, respectively) to identify the community involved in particle export (fixed) and protist loss from remineralisation (live). We paired this with community analysis of the upper and lower water column (mixed layer and below mixed layer to mesopelagic, respectively) to compare the relative sources of exported protists. Amplicon sequences variants (ASVs) from upper water column samples accounted for 2 to 4-fold higher proportion of reads and ASV rich-ness compared to lower water column samples in fixed trap and sediment samples, suggesting low influence of the suspended protist community from the lower water column on export. We further traced the export patterns of upper water column protist taxa by analysing the change in taxa relative abundance across the mixed layer to mesopelagic depths. Export patterns differed between taxa, which is similarly suggested by taxa-specific loss of ASV richness between fixed and live traps, but remained the same across biogeochemically-contrasting water masses. This could imply that the drivers for protist loss during export are related to characteristics consistent across environmental conditions, such as specific microbial interactions or inherent cell properties.

microbiology↗

bioRxiv2024

Phago-mixotrophy of small eukaryotic phytoplankton might alleviate iron limitation in HNLC Southern Ocean

Small phytoplankton, consisting of pico and nano size fractions, are diverse in size and taxonomy. Yet, the differences in their productivity and taxonomic diversity are poorly described. Here, we measured the cell-specific carbon fixation rates of picocyanobacteria Synechococcus, picoeukaryote and nanoeukaryote populations while unveiling their taxonomic composition in oligotrophic subtropical (ST) and high-nutrient low-chlorophyll subantarctic (SA) waters. We coupled 24 h in-situ radiolabelled 14C incubations to flow cytometry sorting (FCM-sorting) and DNA metabarcoding from the same incubated samples, offering a direct account of the community associated with the carbon fixation rates measured. In both water masses, nanoeukaryotes had the highest cell-specific carbon fixation rate, followed by picoeukaryotes and Synechococcus (2.24 {+/-} 29.99, 2.18 {+/-} 2.08 and 0.78 {+/-} 0.55 fgC cell-1 h-1, respectively). The cell-specific carbon fixation rates and growth rates of Synechococcus were 3-fold higher in ST compared to SA waters, while the rates of picoeukaryotes and nanoeukaryotes had no significant difference between the biogeochemically-contrasting water masses. Despite significant differences in their taxonomic composition, the FCM-sorted picoeukaryote and nanoeukaryote populations in SA waters were dominated by taxa with reported phago-mixotrophic strategies (Chrysophyceae, Dinophyceae and Prymnesiophyceae), suggesting phago-mixotrophy might alleviate nutrient stress in iron-limited conditions for discrete small photosynthetic eukaryote populations.

microbiology↗

bioRxiv2022

Prey Size Spectra and Predator:Prey ratios of 7 Species of New Zealand Salps

Salp grazing is important in shaping planktonic food-web structure. However, little is known about the size ranges of their prey in the field or how grazing impacts size structure. This study investigated the feeding habits of 7 different species of salps, representing a variety of sizes and life stages across subtropical and subantarctic waters east of New Zealand. Scanning electron microscopy was used to examine the gut contents of 58 salps, which were then compared to water column plankton communities characterized via epifluorescence microscopy, FlowCam, and flow cytometry. While most of the gut contents resembled ambient waters, substantial differences were found amongst some co-occurring species, such as increased retention of submicron bacteria amongst smaller salps like Thalia democratica. We found that even for those salps capable of feeding on bacteria efficiently, nanoplankton and small microplankton still made up the majority of gut biomass. Larger microplankton were rarer in the guts than in the water column, potentially suggesting an upper size-threshold in addition to the lower size-threshold that has been the focus of most previous work. Salp carbon-weighted predator to prey size ratios were variable, with the majority falling between 1,000:1 and 10,000:1 depending largely on the size of the salp. Taken together our results indicate that despite being able to feed on submicron particles, picoplankton make up at most 26.4% (mean = 6.4%) of salp gut carbon and are relatively unimportant to the energetics of most salps in this region compared to nanoplankton such as small dinoflagellates and diatoms.

ecology↗

bioRxiv2022

Salp blooms increase carbon export 5-fold in the Southern Ocean

The Southern Ocean (SO) contributes substantially to the global biological carbon pump (BCP). Salps in the SO, in particular Salpa thompsoni, are keystone grazers that produce large, fast-sinking fecal pellets with high export potential. In a first study of this kind, we conducted Lagrangian experiments to quantify the salp bloom impacts on export pathways by contrasting locations differing in salp bloom presence/absence. We show that blooms increased particle export by ~5-fold, and exported up to 46% of net primary production out of the euphotic zone. BCP efficiency increased from 5% in non-salp areas to up to 28% in salp areas, which is among the highest recorded in the global ocean. Using SO salp abundances from KRILLBASE, we estimate they can consume ~ 13% of regional production, mediating 13-40% of the SO BCP. Consideration in models forecasting the SO BCP is recommended considering long-term increases in SO salp abundances.

ecology↗

bioRxiv2022

Quantifying biological carbon pump pathways with a data-constrained mechanistic model ensemble approach

The ability to constrain the mechanisms that transport organic carbon into the deep ocean is complicated by the multiple physical, chemical, and ecological processes that intersect to create, transform, and transport particles in the ocean. In this manuscript we develop and parameterize a data-assimilative model of the multiple pathways of the biological carbon pump (NEMUROBCP). The mechanistic model is designed to represent sinking particle flux, active transport by vertically migrating zooplankton, and passive transport by subduction and vertical mixing, while also explicitly representing multiple biological and chemical properties measured directly in the field (including nutrients, phytoplankton and zooplankton taxa, carbon dioxide and oxygen, nitrogen isotopes, and 234Thorium). Using 30 different data types (including standing stock and rate measurements related to nutrients, phytoplankton, zooplankton, and non-living organic matter) from Lagrangian experiments conducted on 11 cruises from four ocean regions, we conduct an objective statistical parameterization of the model and generate one million different potential parameter sets that are used for ensemble model simulations. The model simulates in situ parameters that were assimilated (net primary production and gravitational particle flux) and parameters that were withheld (234Thorium and nitrogen isotopes) with reasonable accuracy. Model results show that gravitational flux of sinking particles and vertical mixing of organic matter from the surface ocean are more important biological pump pathways than active transport by vertically-migrating zooplankton. However, these processes are regionally variable, with sinking particles most important in oligotrophic areas of the Gulf of Mexico and California, sinking particles and vertical mixing roughly equivalent in productive regions of the CCE and the subtropical front in the Southern Ocean, and active transport an important contributor in the Eastern Tropical Pacific. We further find that mortality at depth is an important component of active transport when mesozooplankton biomasses are high, but that it is negligible in regions with low mesozooplankton biomass. Our results also highlight the high degree of uncertainty, particularly amongst mesozooplankton functional groups, that is derived from uncertainty in model parameters, with important implications from results that rely on non-ensemble model outputs. We also discuss the implications of our results for other data assimilation approaches.

ecology↗