Biology subjects

Selph, K. E.

Publications and source records attributed to Selph, K. E..

10 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

Primary productivity and N2-fixation in the eastern Indian Ocean: bottom-up support for an ecologically and economically important ecosystem

Oligotrophic regions of the global ocean are characterized by strong nutrient limitation, low standing phytoplankton biomass, and highly efficient nutrient recycling. We quantified nutrient inventories, primary productivity and N2 fixation during the BLOOFINZ-IO expedition (February 2022) in the Argo Basin located in the eastern Indian Ocean, the sole known spawning ground for Southern Bluefin Tuna. Surface nitrate concentrations were near depletion (<0.02 {micro}mol L-1), with low but persistent residual phosphate (P) concentrations suggesting nitrogen as the major limiting nutrient. Depth-integrated net primary production (NPP), from 14C-based in-situ incubations during 4 Lagrangian cycles, averaged [~]460 mg C m-2d-1, in good agreement with satellite-based NPP estimates. Nitrogen fixation provided a consistent new nitrogen source, contributing [~]16% to local NPP in the upper euphotic zone. Gross primary production (GPP), derived from fast-repetition-rate-fluorometry-based electron transport estimates, revealed significant autotrophic respiration losses, with GPP:NPP ratios averaging [~]1.8, consistent with metabolic costs under nutrient limitation. Net community production (NCP), estimated from O2/Ar ratios, remained positive across all cycles, averaging [~]20% of NPP in the upper 30 m. This result, in combination with N2 fixation measurement indicates that N2 fixation supports most of the export production in this region. Together, the multi-method approach revealed a recycling-dominated ecosystem affected by episodic mixing events, where primary productivity is maintained primarily through efficient nitrogen recycling and physiological photoacclimation. These results provide a comprehensive baseline of bottom-up support on ecosystem productivity for the Argo Basin for assessing future climate-driven changes in stratification, nutrient cycling, and food-web dynamics.

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

Microbial food web dynamics in tropical waters of the bluefin tuna spawning region off northwestern Australia

Whereas recruitment success for many fisheries depends on coincident timing of larvae with abundance peaks of their prey, less can be more in the tropical/subtropical spawning areas of bluefin tunas if lower but steady food resources are offset by reduced larval vulnerability to pelagic predators. To understand larval habitat characteristics for Southern Bluefin Tuna (SBT), we quantified microbial community carbon flows based on growth and grazing rates from depth profiles of dilution incubations and carbon biomass assessments from microscopy and flow cytometry (FCM) during their peak spawning off NW Australia (Indian Ocean) in February 2022. Two Chla-based estimates of phytoplankton production gave differing offsets due to cycling or mixotrophy, exceeding 14C net community production on average (677 {+/-} 98 versus 447 {+/-} 43 mg C m-2 d-1). Productivity was higher than in the Gulf of Mexico spawning area for Atlantic Bluefin Tuna but less than similar studies of oceanic upwelling regions. Microzooplankton grazing averaged 482 {+/-} 63 mg C m- 2 d-1 (71 {+/-} 13% of production). Two measurement variables for Prochlorococcus gave average production and grazing rates of 282 {+/-} 36 and 248 {+/-} 32 mg C m-2 d-1 (86 {+/-} 6% grazed). Prochlorococcus comprised almost half of production and grazing fluxes in the upper (0-25 m) euphotic zone where SBT larvae reside. Prochlorococcus declined and eukaryotic phytoplankton and heterotrophic bacteria increased in relative importance in the lower euphotic zone. These results describe relatively classic open-ocean oligotrophic conditions as the food web base for nutritional flows to SBT larvae.

ecology↗

bioRxiv2025

Acidic vacuole-containing organisms are a majority of the eukaryotic microbial community in oligotrophic Argo Basin waters (eastern Indian Ocean)

The Argo Basin of the eastern Indian Ocean in austral summer (February 2022) was characterized by warm (28.5-30.6{degrees}C), oligotrophic surface waters (nitrate and phosphate [≤]0.1 {micro}M), with relatively shallow mixed layers and deep chlorophyll biomass maxima. From euphotic zone depth-resolved samples analyzed by for DNA and acid vacuole staining (Hoechst and LysoTracker Green) by ship-board flow cytometry, we found that autotrophic populations were dominated by Prochlorococcus, followed by mixotrophs (58 and 28% of autotrophic community biomass, respectively), with only 14% obligate phototrophic phytoplankton (i.e., plastidic cells without acid vacuole fluorescence). Acid vacuole-containing microbes (mixotrophs and heterotrophs) were 34% of the microbial community, and 80% of the eukaryotic biomass. In shallow waters, the eukaryotic chlorophyll-containing community was comprised of pico-sized obligate phototrophs and mixotrophs (233-325 cells mL-1), nano-sized obligate phototrophs and mixotrophs (72 and 374 cells mL-1, respectively), with all groups increasing several-fold in the deep chlorophyll maxima. Mixotrophs were a higher proportion of the chlorophyll-containing community in the shallow nutrient-poor mixed layer, consistent with a nutrient-acquisition argument for their prevalence. Heterotrophic eukaryotes averaged 524 {+/-} 36 cells mL-1 in the euphotic zone, changing little with depth and showing a significant positive relationship with Prochlorococcus, but not any other group. In contrast, mixotrophs were positively correlated with heterotrophic bacteria, but not with Prochlorococcus. Overall, the high proportion of mixotrophs in the microbial community may channel more productivity to higher trophic levels than expected given the regions nutrient-poor status.

ecology↗

bioRxiv2025

Phytoplankton community composition in the oligotrophic Argo Basin of the eastern Indian Ocean

Phytoplankton community composition during austral summer 2022 in the Argo Abyssal Plain (Argo Basin), a 5000-m deep area northwest of the Australian continent in the eastern Indian Ocean, is described in detail, including phytoplankton abundance, biomass, size structure, taxonomic identifications through DNA and pigment analyses, as well as the percent of functional mixotrophs. The region was characterized by warm (up to 30.5{degrees}C), stratified, oligotrophic (nitrogen-limited) waters, with integrated euphotic zone (EZ) chlorophyll a (CHLa) of 13 mg m-2. The EZ mean CHLa was low in the upper layer (0.085 {micro}g L-1) and 0.32 {micro}g L-1 at the pronounced deep CHLa maxima. EZ-integrated phytoplankton carbon averaged 1229 mg C m-2. Prochlorococcus was the dominant taxon throughout the EZ, but the lower EZ had [~]4-times more eukaryotic carbon biomass than the upper EZ, along with a distinct community. In the upper EZ, prymnesiophytes, dinoflagellates and prasinophyte taxa without prasinoxanthin had the highest contributions to monovinyl chlorophyll a (MV-CHLa). In the lower EZ the community was more diverse, with prymnesiophytes, dinoflagellates, prasinophyte taxa with prasinoxanthin, pelagophytes, and cryptophytes all comprising significant contributions to MV-CHLa. Diatoms were a minor part of the community. In the upper EZ, a higher percent of the community showed mixotrophy (35-84%) relative to the lower EZ (30-51%). Although a low abundance, nitrogen-fixing organisms (symbionts of diatoms and cyanobacteria taxa) were ubiquitous. Overall, the community was similar to that found at the Hawaii Ocean Time-series site and the central Gulf of Mexico.

ecology↗

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↗

bioRxiv2021

Taxon-specific phytoplankton growth, nutrient utilization, and light limitation in the oligotrophic Gulf of Mexico

The highly stratified, oligotrophic regions of the oceans are predominantly nitrogen limited in the surface ocean and light limited at the deep chlorophyll maximum (DCM). Hence, determining light and nitrogen co-limitation patterns for diverse phytoplankton taxa is crucial to understanding marine primary production throughout the euphotic zone. During two cruises in the deep-water Gulf of Mexico, we measured primary productivity (H13CO3-), nitrate uptake (15NO3-), and ammonium uptake (15NH4+) throughout the water column. Primary productivity declined with depth from the mixed-layer to the DCM, averaging 27.1 mmol C m-2 d-1. The fraction of growth supported by NO3- was consistently low, with upper euphotic zone values ranging from 0.01 to 0.14 and lower euphotic zone values ranging from 0.03 to 0.44. Nitrate uptake showed strong diel patterns (maximum during the day), while ammonium uptake exhibited no diel variability. To parameterize taxon-specific phytoplankton nutrient and light utilization, we used a data assimilation approach (Bayesian Markov Chain Monte Carlo) including primary productivity, nutrient uptake, and taxon-specific growth rate measurements. Parameters derived from this analysis define distinct niches for five phytoplankton taxa (Prochlorococcus, Synechococcus, diatoms, dinoflagellates, and prymnesiophytes) and may be useful for constraining biogeochemical models of oligotrophic open-ocean systems.

plant biology↗

bioRxiv2020

Plankton food webs of the Gulf of Mexico spawning grounds of Atlantic Bluefin tuna

We used linear inverse ecosystem modeling techniques to assimilate data from extensive Lagrangian field experiments into a mass-balance constrained food web for the Gulf of Mexico open-ocean ecosystem. This region is highly oligotrophic, yet Atlantic Bluefin Tuna (ABT) travel long distances from feeding grounds in the North Atlantic to spawn there. Our results show that the food web is dominated by the microbial loop (>80% of net primary productivity is respired by heterotrophic bacteria and protists that feed on them). In contrast, herbivorous food web pathways from phytoplankton to metazoan zooplankton process <4% of net primary production in the mixed layer. Nevertheless, ABT larvae feed preferentially on calanoid copepods and other suspension-feeding zooplankton that in turn derive much of their nutrition from diatoms and mixotrophic flagellates. This allows ABT larvae to maintain a comparatively low trophic level ([~]4.0 for pre-flexion larvae; [~]4.2 for post-flexion larvae) that increases trophic transfer from phytoplankton to larval fish.

ecology↗