Modelling and mechanism analysis onspatio-temporal characteristics of phytoplankton overthe Ross Sea Shelf

The broad continental shelf of Ross Sea sustains high productivityand exhibits a typical seasonal species succession from Pheaocystis antarctica to diatoms with significant spatial variabilities. The low-trophic level ecosystem of the Ross Sea is the basis for maintaining a patchy distribution of habitats for higher trophic levels on the shelf. Therefore, the study on the spatio-temporal variation characteristics, driving mechanisms and ecological effectsof phytoplankton dynamics possesses a great guidance inecological and environmental protectionin the Southern Ocean. However, the scarcity of in-situ observation data in the Southern Ocean poses certain limitations to the further understanding of environment modulations on phytoplankton communities. Accordingly, we constructed a zero-dimensional(0-D) and a three-dimensional(3-D) low-trophic level ecosystem model of the Ross Sea. Combiningthe observational data, we studied the characteristicsand mechanisms of the spatial variabilities of blooms, the dominant factors of the seasonal speciessuccession overthe Ross Sea Shelf, and the seasonal variations in the low-trophic level energy transferbased on simulation results.In the process of constructing the 0-Decosystem model RISPEM, we designed a series of sensitivity experiments to screen out the optimal solutions regarding key ecological parameters such as growth, mortality and grazing processes. On this basis, a 3-D coupled ocean-sea ice-biogeochemicalmodel, ROSE, was constructed, and the simulation results were in good agreement with the observations and consistent with the basic characteristics of the Ross Sea low-trophic-level ecosystem.Based on RISPEM simulation results, the mechanism of phytoplankton succession in the Ross Ice Shelf Polynya (RISP) was clarified. Iron stress caused by growth depletion severelyinhibits thegrowth of P. antarctica. Small amounts of grazing by microzooplankton has increased thebiomass loss of P. antarctica. In addition, low-frequency external iron input is beneficial to trigger successionby promotingdiatom growth. In conclusion, the coupling of bottom-up and top-down processesplays a key role in regulating the seasonal succession.The multi-year average ofhindcast simulation of ROSE covering 2010-2020were used to analyze the spatial variabilities,mechanisms and ecological effectsof phytoplankton blooms. In winter and spring, the sufficient reservoir of iron in upper layer is a result of deep convection, which favors P. antarcticato bloom first, considering its higher light sensitivity at low irradiation. Meanwhile, sea ice controls the phytoplankton biorhythm. Thin ice improves the polar low-light environment significantly, which advances the start of the light season substantially. As a result, the permanentpolynyasbecome areas of earliest growth and highest biomass of P. antarctica. However, phytoplankton growth in summer-stratified waters is generally iron-limited due to iron deficiencyby extensive consumption. At this time,diatoms employ a low iron concentration adaptation strategy to rapidly and continuously dominate the shelf region with less spatial variabilitiesin the annual peak.Through excretionby microzooplankton, the iron fixed in P. antarcticais released and entered into iron bio-recyclein the upper layer, providing crucial replenishment to the stratified water column. Iron bio-recycle is regarded as a critical supplement to the stratified upper layerto support diatom blooms, when vertical input from deeper layer is difficult to cross the pycnocline. As the vertical gradient of dissolved iron increases in spring and summer, the supply of iron from deeper layer gradually increases. There are spatial differences in iron sources, with the adjacent slope region being supplied mainly by advecting the Circumpolar Deep Water, while the southern part of shelf is vertical mixing by upwelling the sedimentary iron. Thus, both bio-recycleand upward replenishment from deeper layer storage are indispensable for the maintenance of summer diatom blooms.The short-termspring P. antarcticablooms open short food chains, rapidly transfer energy to small zooplankton, and set the stage for further energy transfer in summer. The short-term spring blooms of P. antarcticarapidly transfer energy to microzooplankton, laying the foundation for summer. The long-term summer diatom bloom is the most important food source for mesozooplankton, and becomes a key source of energy to maintain high primary and secondary production, which is supportive to large and diverse populations of high-trophic level organisms.