Biology subjects

Shropshire, T. A.

Publications and source records attributed to Shropshire, T. A..

3 recordsLinked to original sources

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

Trade-offs between risks of predation and starvation in larvae make the shelf break an optimal spawning location for Atlantic Bluefin tuna

Atlantic Bluefin tuna (ABT) (Thunnus thynnus) travel long distances to spawn in oligotrophic regions of the Gulf of Mexico. To estimate regional larval ABT mortality, we developed a spatially-explicit, Lagrangian, individual-based model that simulates dispersal, growth, and mortality within realistic predator and prey fields during the spawning periods from 1993-2012. Modelled larval ABT experience high mortality in the first week of feeding with an average mortality rate of 0.53 {+/-} 0.26 d-1 prior to postflexion. Survival ranged from 0.12%-0.32% suggesting that recruitment may vary by a factor of 2.7 due to early life stage mortality alone. Starvation is the dominant source of mortality driven by the early critical period; however, survival is ultimately limited by predation on older individuals. As a result, first-feeding larvae survive better in the more food-rich areas on the shelf, while larger larvae survive better in the open ocean with fewer predators, making the shelf break an optimal spawning area. Our findings support the hypothesis that ABT spawn in oligotrophic regions to minimize predation on their larvae. Ocean modeling tools presented in this study may help facilitate an ecosystem-based management approach to improve future stock assessment models by better resolving the stock-recruitment relationship.

ecology↗

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↗