S2P3-R (v1.0): a framework for efficient regional modelling of physical and biological structures and processes in shelf seas

An established one-dimensional (1-D) model of Shelf Sea Physics and Primary Production (S2P3) is adapted for flexible use in selected regional settings over selected periods of time. This Regional adaptation of S2P3, the S2P3-R framework (v1.0), can be efficiently used to investigate physical and biological phenomena in shelf seas that are strongly controlled by vertical processes. These include spring blooms that follow the onset of stratification, tidal mixing fronts that seasonally develop at boundaries between mixed and stratified water, and sub-surface chlorophyll maxima that persist throughout summer. While not representing 3-D processes, S2P3-R reveals the horizontal variation of the key 1-D (vertical) processes. S2P3-R should therefore only be used in regions where horizontal processes – including mean flows, eddy fluxes and internal tides – are known to exert a weak influence in comparison with vertical processes. In such cases, S2P3-R may be used as a highly versatile research tool, alongside more complex and computationally expensive models. In undergraduate oceanography modules and research projects, the model serves as an effective practical tool for linking theory and field observations. Three different regional configurations of S2P3-R are described, illustrating a range of diagnostics, evaluated where practical with observations. The model can be forced with daily meteorological variables for any selected year in the reanalysis era (1948 onwards). Example simulations illustrate the considerable extent of synoptic-to-interannual variability in the physics and biology of shelf seas. In discussion, the present limitations of S2P3-R are emphasised, and future developments are outlined

Microzooplankton regulation of surface ocean POC:PON ratios

The elemental composition of particulate organic matter in the surface ocean significantly affects the efficiency of the ocean's store of carbon. Though the elemental composition of primary producers is an important factor, recent observations from the western North Atlantic Ocean revealed that carbon-to-nitrogen ratios (C:N) of phytoplankton were significantly higher than the relatively homeostatic ratio of the total particulate pool (particulate organic carbon:particulate organic nitrogen; POC:PON). Here we use an idealized ecosystem model to show how interactions between primary and secondary producers maintain the mean composition of surface particulates and the difference between primary producers and bulk material. Idealized physiological models of phytoplankton and microzooplankton, constrained by laboratory data, reveal contrasting autotrophic and heterotrophic responses to nitrogen limitation: under nitrogen limitation, phytoplankton accumulate carbon in carbohydrates and lipids while microzooplankton deplete internal C reserves to fuel respiration. Global ecosystem simulations yield hypothetical global distributions of phytoplankton and microzooplankton C:N ratio predicting elevated phytoplankton C:N ratios in the high-light, low-nutrient regions of the ocean despite a lower, homeostatic POC:PON ratio due to respiration of excess carbon in systems subject to top-down control. The model qualitatively captures and provides a simple interpretation for, a global compilation of surface ocean POC:PON data

Cosmopolitan Early Jurassic Marine Gastropods from West-Central Patagonia, Argentina

A new, relatively diverse gastropod fauna is reported from the Chubut province of west−central Patagonia. The gastro− pod association at the “El Córdoba ” fossiliferous locality (Lower Toarcian of Osta Arena Formation) consists of three new species: the eucyclid Amberleya? espinosa sp. nov. and two procerithiids Cryptaulax damboreneae sp. nov. and Cryptaulax nulloi sp. nov. Other members of the association are the ataphrid Striatoconulus sp., discohelicid Colpom− phalus? sp., and an undetermined zygopleurid. Knowledge on Early Jurassic gastropods from South America and other southern continents is reviewed to show that the taxonomic composition of the El Cordoba association strongly resem− bles other gastropod associations of this age (even those from Europe), suggesting a wide distribution of cosmopolita

Modelling the effects of chromatic adaptation on phytoplankton community structure in the oligotrophic ocean

We explored the role of chromatic adaptation in shaping vertical phytoplankton community structures using a trait-based ecosystem model. The model included 1000 ‘phytoplankton types’ and was applied to the oligotrophic South Atlantic Gyre in a 1-dimensional framework, where ‘phytoplankton types’refers to the model phytoplankton that were stochastically assigned unique physiological characteristics. The model incorporates multi-spectral optics and light absorption properties for the different phytoplankton. The model successfully reproduced observed vertical gradients in the nitrate, bulk phytoplankton properties and community structure. Model phytoplankton types with Synechococcus-like spectral light absorption properties were outcompeted at depth, where eukaryote-like spectral properties were advantageous. In contrast, photoinhibition was important for vertical separation of high-light and low-light Prochlorococcus model analogues. In addition, temperature dependence was important for selection of phytoplanktontypes on the temperature gradient. The fittest, or successful, phytoplankton types were characterised by combinations of simultaneously optimal traits that suited them to a particular depth in the water column, reflecting the view that phytoplankton have co-evolved multiple traits that are advantageous in a particular environmental condition or niche

Iron–light interactions during the CROZet natural iron bloom and EXport experiment (CROZEX): II—Taxonomic responses and elemental stoichiometry

The CROZet natural iron bloom and EXport experiment (CROZEX) investigated the annual phytoplankton bloom that occurs in the vicinity of the Crozet plateau in the Polar Frontal Zone (PFZ) of the Southern Ocean. Shipboard manipulation experiments designed to investigate potential responses of phytoplankton community structure and elemental stoichiometry to iron (Fe) and/or light perturbation are compared to in situ data collected during CROZEX. The outcome of individual experiments was strongly influenced by initial phytoplankton community structure. For example Fe amendment of high (&gt;8 ?M) silicic acid waters resulted in a strong response by medium-sized diatoms, including Eucampia antarctica. In contrast Phaeocystis antarctica dominated the community response to increased Fe within an experiment initiated during the early declining stage of the bloom. Conversely, small diatoms responded more strongly to increased irradiance, while the population of very large diatoms remained relatively static. Consistent with experimental results, the intense natural blooms north of the Crozet plateau were observed to be dominated by either P. antarctica or medium-sized diatoms while small phytoplankton and large diatoms dominated lower-chlorophyll waters in the south. In situ data and results from experiments supported previous observations of lower nitrate to phosphate removal ratios for diatoms compared to P. antarctica. Experimental and in situ data also supported previous work showing that silicic acid to nitrate removal ratios can be reduced under conditions of enhanced Fe availability. Higher irradiance decreased this ratio still further. Interactions between Fe and light were thus likely to have contributed to the observed decoupling of the major element cycles between the intense bloom that occurs north of Crozet and more typical PFZ conditions to the South. Specifically, silicic acid became exhausted in both systems, with drawdown in the south potentially resulting from co-limitation of diatom growth by Fe and light. In contrast, increased Fe supply in the north enhanced nitrogen, phosphorous and carbon drawdown in ratios that were dependent on the community composition of the resultant bloom. <br/

Phytoplankton photoacclimation and photoadaptation in response to environmental gradients in a shelf sea

Variability in the photosynthetic performance of natural phytoplankton communities, due to both taxonomic composition and the physiological acclimation of these taxa to environmental conditions, was assessed at contrasting sites within a temperate shelf sea region. Physiological parameters relating to the structure of the photosystem II (PSII) antenna and processes downstream from PSII were evaluated using a combination of fast repetition rate fluorescence, oxygen flash yields, spectral fluorescence, and 14C photosynthesis versus irradiance measurements. Parameters relating to PSII antenna structure, specifically the functional absorption cross-section (sPSII) and the chlorophyll to PSII reaction center ratio, varied principally as a result of spatial (horizontal) taxonomic differences. Phenotypic plasticity in the size of the PSII light-harvesting antenna appeared to be limited. In contrast, parameters related to electron transport rates (ETRs) downstream of PSII, including the maximum ETR (1/tPSII), the chlorophyll-specific maximum rate of carbon fixation (P*max), and the light-saturation intensity (Ek), all decreased from the surface to the subsurface chlorophyll maximum (SCM) in stratified waters. The primary photoacclimation response to the vertical light gradient thus resulted in decreasing light-saturated carbon fixation per reaction center with increasing depth. Increases in the ratio of PSII reaction centers to carbon fixation capacity thus dominated the phenotypic response to decreased irradiance within the SCM. Perhaps counterintuitively, phytoplankton populations within fully mixed water columns, characterized by low mean irradiance, were acclimated or adapted to relatively high irradiance. <br/

Iron–light interactions during the CROZet natural iron bloom and EXport experiment (CROZEX) I: Phytoplankton growth and photophysiology

The CROZet natural iron bloom and EXport experiment (CROZEX) investigated phytoplankton blooms in the vicinity of the Crozet Plateau in the polar frontal zone (PFZ) of the Southern Ocean. Peak chlorophyll concentrations reached during an intense bloom within naturally iron (Fe)-fertilised regions north of the plateau were an order of magnitude higher than those observed in deeper mixed layers and low-Fe waters to the south. To establish the factors influencing phytoplankton dynamics, a suite of in situ phytoplankton physiological measurements and shipboard Fe–light perturbation experiments was performed. Addition of Fe in experiments performed during bloom decline north of the plateau resulted in increased accumulation of phytoplankton biomass and changes in a number of phytoplankton physiological characteristics. In particular photosystem II (PSII) photochemical efficiencies (Fv/Fm) measured by fast repetition rate fluorometry increased above in situ values within 24 h of Fe amendment, suggesting that Fe stress had contributed to bloom termination. In contrast, responses to Fe amendment were minor within an experiment initiated in low-silicic acid, post-bloom waters south of the Plateau. Within the intense bloom in the north, light limitation due to self-shading may have constrained the peak phytoplankton standing stock. However, in the absence of Fe amendment, incubation at higher than in situ irradiance levels had little influence on phytoplankton biomass accumulation for declining bloom populations. Instead reduced Fv/Fm, reflecting increased photoinhibitory damage to PSII, was observed in high-light incubations and was also apparent in situ. Interactions between Fe and light availability thus influenced phytoplankton physiology and growth and potentially contributed to bloom longevity during CROZEX. <br/