Description: 5 year means of 3D fields: nano-phytoplankton, diatom, small zooplankton and macrozooplankton carbon biomass, DIN, DSi fields, large and small detritus nitrogen and carbon. 5 year means of 2D fields: respiration, DOC production of small zooplankton and macrozooplankton, grazing of zooplankton groups and sinking particulate organic carbon production by phytoplankton aggregation and zooplankton processes, net primary production and chlorophyll concentrations. 5 year means of 2D depth-integrated macrozooplankton biomass.

Description: Data contains outputs of two global model simulations performed with ocean biogeochemical model FESOM-REcoM. Model results were averaged over the last five years of sixty years simulations. There are the three-dimensional outputs of biomass of plankton functional types (small phytoplankton, diatom, micro-, meso- and macrozooplankton). Also, two-dimensional files contain surface nitrate (DIN), silicate (DSi), iron (DFe), chlorophyll, zooplankton DON excretion, zooplankton grazing rate, grazing loss of phytoplankton, temperature and depth-integrated net primary production. In addition, there are daily outputs of phytoplankton carbon biomass, chlorophyll concentration, mixed layer depth, phytoplankton grazing and other loss rates in the Southern Ocean.

Description: The ability of global models in simulating the seasonality of biogeochemical cycles constrains their reliability for projections of primary production and ocean carbon uptake. In particular, the phasing and amplitude of the seasonal cycle of primary production affect the net flux of carbon between the ocean and the atmosphere. Models’ characterization of the seasonal cycle of primary production in high latitudes generally shows an amplitude and/or phasing bias of the spring-summer bloom. The question that we tackle in this study is to which extent model simulations of the seasonal cycle of primary production would benefit from a more mechanistic description of the links between phytoplankton physiology and environmental drivers. To explore that question we worked with the Regulated Ecosystem model version 2 (REcoM2) integrated within the Finite-Element Sea-Ice Ocean Model (FESOM). We included in the phytoplankton growth model a photodamage term that decreases the amount of active photosynthetic pigments when light becomes supersaturating. Eventually, the interplay between light-dependent photodamage and nutrient-dependent new synthesis of pigments determines the photosynthetic capacity of the cells. The immediate effect is that the model is able to simulate variations in the stoichiometry of phytoplankton with light, nutrients and temperature in better agreement with observations. Regarding the seasonal variations of primary production in polar regions, model simulations show a less steep increase of biomass and net primary production during the growing season and lower biomass concentrations at the peak of the bloom. However, the start of the bloom happens relatively early when compared to satellite observations. We suggest to further evaluate the role of other environmental factors interacting with the physiology of primary producers and driving both bottom-up (e.g. vertical mixing) and top-down (e.g. grazing) control of the spring bloom in polar regions.

Description: In this study, we investigate the role of macrozooplankton in the biogeochemistry of the Southern Ocean using a three-dimensional global ocean ecosystem model (FESOM- REcoM2). The macrozooplankton group was parameterized according to characteristics of Antarctic krill and a related fast-sinking detritus class (larger particles, e.g. fecal pellets) was introduced in the model. It was then analyzed how the ecosystem structure and major carbon export pathways in the Southern Ocean changed through this extension of the model. The spatial distribution of macrozooplankton biomass in the Southern Ocean was reproduced reasonably well. Preliminary results showed that the zooplankton proportion of living compartments (phytoplankton and zooplankton groups) in the model increased. Thus, zooplankton contribution to the particulate organic carbon (POC) flux increased. The contribution of macrozooplankton to POC export at 100 m depth was 0.12 Pg C per year or 15% of total export in the Southern Ocean. The transfer efficiency of organic carbon nearly doubled and reached up to 50% in regions with high macrozooplankton biomass. These results emphasize the important role of macrozooplankton in the Southern Ocean carbon cycle and have implications for studies of the biological carbon pump.

Description: In this study, a three-dimensional, coupled ocean ecosystem model (FESOM- REcoM2) is used to investigate the effect of krill on the biogeochemistry of the Southern Ocean. The implementation of Antarctic krill in the model was done in three steps. 1) A second zooplankton group was implemented, which grazes on diatoms, mesozooplankton and nanophytoplankton (in order of descending preference). 2) A new detritus group was added to the model, which represents faster-sinking krill fecal pellets. 3) The grazing impact of both zooplankton groups on detritus was described. Afterward, four different simulations (control and three krill simulations for previously described steps) were conducted to evaluate, how the implementation of the new zooplankton group and additional features affected biogeochemical processes in the Southern Ocean. In our krill simulation, the spatial distribution of krill biomass in the Southern Ocean was reasonably reproduced. Preliminary results showed that the proportion of living compartments (phytoplankton and zooplankton groups) in the model changed, which led to different POC (particulate organic carbon) flux pathways to the deep ocean. Zooplankton biomass contribution to total carbon biomass increased from 2.4% to 10% in our model in the Southern Ocean. The contribution of zooplankton to POC production doubled. The implementation of krill in the ecosystem model enhanced nutrient recycling in the upper ocean layer. Therefore, our novel krill implementation improved the bias between model and observations in surface spatial distributions of the macronutrients silicic acid and nitrate.

Description: Macrozooplankton and its grazing pressure shape ecosystem structures and carbon pathways in the Southern Ocean. Here, we present the implementation of “polar macrozooplankton” as a plankton functional type and a related fast-sinking detritus class (fecal pellets) into the biogeochemical model REcoM-2. We use the model to assess major carbon pathways and ecosystem structure in the Southern Ocean south of 50°S. The model represents zooplankton biomass and its spatial distribution in the Southern Ocean reasonably well in comparison to available biomass data. A distinct difference of our model from previous versions is the seasonal pattern of particle formation processes and ecosystem structures in the Southern Ocean. REcoM-2 now captures high zooplankton biomass and a typical shift from a dominance of phytodetrital aggregates in spring to zooplankton fecal pellets later in the year. At sites with high biomass of macrozooplankton, the transfer efficiency of particulate organic carbon can be as high as 50%, and the carbon content of the exported material increases. In our simulations, macrozooplankton is an important component of the Southern Ocean plankton community, contributing up to 0.12 Pg C per year (14%) to total modeled carbon export across 100 m depth. Macrozooplankton changes the phytoplankton composition and supports the recycling of macronutrients. These results support the important role of macrozooplankton such as krill in the Southern Ocean and have implications for the representation of Southern Ocean biogeochemical cycles in models.

Description: Zooplankton plays a notable role in ocean biogeochemical cycles. However, it is often simulated as one generic group and top closure term in ocean biogeochemical models. This study presents the description of three zooplankton functional types (zPFTs, micro-, meso- and macrozooplankton) in the ocean biogeochemical model FESOM-REcoM. In the presented model, microzooplankton is a fast-growing herbivore group, mesozooplankton is another major consumer of phytoplankton, and macrozooplankton is a slow-growing group with a low temperature optimum. Meso- and macrozooplankton produce fast-sinking fecal pellets. With three zPFTs, the annual mean zooplankton biomass increases threefold to 210 Tg C. The new food web structure leads to a 25% increase in net primary production and a 10% decrease in export production globally. Consequently, the export ratio decreases from 17% to 12% in the model. The description of three zPFTs reduces model mismatches with observed dissolved inorganic nitrogen and chlorophyll concentrations in the South Pacific and the Arctic Ocean, respectively. Representation of three zPFTs also strongly affects phytoplankton phenology: Fast nutrient recycling by zooplankton sustains higher chlorophyll concentrations in summer and autumn. Additional zooplankton grazing delays the start of the phytoplankton bloom by 3 weeks and controls the magnitude of the bloom peak in the Southern Ocean. As a result, the system switches from a light-controlled Sverdrup system to a dilution-controlled Behrenfeld system. Overall, the results suggest that representation of multiple zPFTs is important to capture underlying processes that may shape the response of ecosystems and ecosystem services to on-going and future environmental change in model projections.