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Responses of marine primary production to a changing climate are determined by a concert of multiple environmental changes, for example in temperature, light, pCO 2 , nutrients, and grazing. To make robust projections of future global marine primary production, it is crucial to understand multiple driver effects on phytoplankton. This meta-analysis quantifies individual and interactive effects of dual driver combinations on marine phytoplankton growth rates. Almost 50% of the single-species laboratory studies were excluded because central data and metadata (growth rates, carbonate system, experimental treatments) were insufficiently reported. The remaining data (42 studies) allowed for the analysis of interactions of pCO 2 with temperature, light, and nutrients, respectively. Growth rates mostly respond non-additively, whereby the interaction with increased pCO 2 profusely dampens growth-enhancing effects of high temperature and high light. Multiple and single driver effects on coccolithophores differ from other phytoplankton groups, especially in their high sensitivity to increasing pCO 2. Polar species decrease their growth rate in response to high pCO 2 , while temperate and tropical species benefit under these conditions. Based on the observed interactions and projected changes, we anticipate primary productivity to: (a) first increase but eventually decrease in the Arctic Ocean once nutrient limitation outweighs the benefits of higher light availability; (b) decrease in the tropics and mid-latitudes due to intensifying nutrient limitation, possibly amplified by elevated pCO 2 ; and (c) increase in the Southern Ocean in view of higher nutrient availability and synergistic interaction with increasing pCO 2. Growth-enhancing effect of high light and warming to coccolithophores, mainly Emiliania huxleyi, might increase their relative abundance as long as not offset by acidification. Dinoflagellates are expected to increase their relative abundance due to their positive growth response to increasing pCO 2 and light levels. Our analysis reveals gaps in the knowledge on multiple driver responses and provides recommendations for future work on phytoplankton.

Atmospheric and oceanic CO2 concentrations are rising at an unprecedented rate. Laboratory studies indicate a positive effect of rising CO2 on phytoplankton growth until an optimum is reached, after which the negative impact of accompanying acidification dominates. Here, we implemented carbonate system sensitivities of phytoplankton growth into our global biogeochemical model FESOM-REcoM and accounted explicitly for coccolithophores as the group most sensitive to CO2. In idealized simulations in which solely the atmospheric CO2 mixing ratio was modified, changes in competitive fitness and biomass are not only caused by the direct effects of CO2, but also by indirect effects via nutrient and light limitation as well as grazing. These cascading effects can both amplify or dampen phytoplankton responses to changing ocean pCO2 levels. For example, coccolithophore growth is negatively affected both directly by future pCO2 and indirectly by changes in light limitation, but these effects are compensated by a weakened nutrient limitation resulting from the decrease in small-phytoplankton biomass. In the Southern Ocean, future pCO2 decreases small-phytoplankton biomass and hereby the preferred prey of zooplankton, which reduces the grazing pressure on diatoms and allows them to proliferate more strongly. In simulations that encompass CO2-driven warming and acidification, our model reveals that recent observed changes in North Atlantic coccolithophore biomass are driven primarily by warming and not by CO2. Our results highlight that CO2 can change the effects of other environmental drivers on phytoplankton growth, and that cascading effects may play an important role in projections of future net primary production.

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The collection of zooplankton swimmers and sinkers in time-series sediment traps provides unique insight into year-round and interannual trends in zooplankton population dynamics. These samples are particularly valuable in remote and difficult to access areas such as the Arctic Ocean, where samples from the ice-covered season are rare. In the present study, we investigated zooplankton composition based on swimmers and sinkers collected by sediment traps at water depths of 180-280, 800-1320, and 2320-2550 m, over a period of 16 yr (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016) at the Long-Term Ecological Research observatory HAUSGARTEN located in the eastern Fram Strait (79 N, 4 E). The time-series data showed seasonal and interannual trends within the dominant zooplankton groups including copepoda, foraminifera, ostracoda, amphipoda, pteropoda, and chaetognatha. Amphipoda and copepoda dominated the abundance of swimmers while pteropoda and foraminifera were the most important sinkers. Although the seasonal occurrence of these groups was relatively consistent between years, there were notable interannual variations in abundance, suggesting the influence of various environmental conditions such as sea-ice dynamic and lateral advection of water masses, for example, meltwater and Atlantic water. Statistical analyses revealed a correlation between the Arctic dipole climatic index and sea-ice dynamics (i.e., ice coverage and concentration), as well as the importance of the distance from the ice edge on swimmer composition patterns and carbon export.

Phytoplankton growth is controlled by multiple environmental drivers, which are all modified by climate change. While numerous experimental studies identify interactive effects between drivers, large‐scale ocean biogeochemistry models mostly account for growth responses to each driver separately and leave the results of these experimental multiple‐driver studies largely unused. Here, we amend phytoplankton growth functions in a biogeochemical model by dual‐driver interactions (CO2 and temperature, CO2 and light), based on data of a published meta‐analysis on multiple‐driver laboratory experiments. The effect of this parametrization on phytoplankton biomass and community composition is tested using present‐day and future high‐emission (SSP5‐8.5) climate forcing. While the projected decrease in future total global phytoplankton biomass in simulations with driver interactions is similar to that in control simulations without driver interactions (5%–6%), interactive driver effects are group‐specific. Globally, diatom biomass decreases more with interactive effects compared with the control simulation (−8.1% with interactions vs. no change without interactions). Small‐phytoplankton biomass, by contrast, decreases less with on‐going climate change when the model accounts for driver interactions (−5.0% vs. −9.0%). The response of global coccolithophore biomass to future climate conditions is even reversed when interactions are considered (+33.2% instead of −10.8%). Regionally, the largest difference in the future phytoplankton community composition between the simulations with and without driver interactions is detected in the Southern Ocean, where diatom biomass decreases (−7.5%) instead of increases (+14.5%), raising the share of small phytoplankton and coccolithophores of total phytoplankton biomass. Hence, interactive effects impact the phytoplankton community structure and related biogeochemical fluxes in a future ocean. Our approach is a first step to integrate the mechanistic understanding of interacting driver effects on phytoplankton growth gained by numerous laboratory experiments into a global ocean biogeochemistry model, aiming toward more realistic future projections of phytoplankton biomass and community composition.

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<p>Phytoplankton growth is controlled by environmental drivers such as nutrients and light availability, temperature, and the carbonate system. Thereby, changes in one driver can modify the response towards another driver. These interactive effects are usually not considered in large-scale ocean biogeochemistry models, potentially leading to incomplete projections of future phytoplankton biomass. In the presented work, we first parameterized growth sensitivities to changes in the carbonate system. We then used the results of a meta-analysis on interactive driver effects in published phytoplankton laboratory studies to develop model parameterizations for dual driver interactions (carbonate system versus temperature, carbonate system versus light). The parameterizations were tested in the biogeochemistry and phytoplankton functional type model REcoM under present-day and future conditions. While future phytoplankton biomass decreases by a similar amount with and without driver interactions (5-6%), interactive driver effects become visible on a group-specific level. Once driver interactions are considered, the biomass of diatoms and small phytoplankton decreases by -8.1% and -5.0%, respectively, and the biomass of coccolithophores increases by +33.2% from present-day to future conditions on a global scale. In comparison, the biomass of diatoms, small phytoplankton, and coccolithophores changes by 0.0%, -9.0%, and -10.8%, respectively, in simulations without driver interactions. Hence, projections of the global future phytoplankton community shift towards a larger share of small phytoplankton and coccolithophores and a smaller share of diatoms if interactive driver effects are taken into account. Regionally, the effect of driver interactions is largest in the Southern Ocean, where diatom biomass decreases (-7.5%) instead of increases (+14.5%). In conclusion, our study reveals that model projections of future phytoplankton biomass may miss out important information on the future phytoplankton community composition and group-specific direction of change if driver interactions are not considered.</p>