Marine life is controlled by multiple physical and chemical drivers and by diverse ecological processes. Many of these oceanic properties are being altered by climate change and other anthropogenic pressures. Hence, identifying the influences of multifaceted ocean change, from local to global scales, is a complex task. To guide policy-making and make projections of the future of the marine biosphere, it is essential to understand biological responses at physiological, evolutionary and ecological levels. Here, we contrast and compare different approaches to multiple driver experiments that aim to elucidate biological responses to a complex matrix of ocean

Diazotrophic (N2‐fixing) cyanobacteria provide the biological source of new nitrogen for large parts of the ocean. However, little is known about their sensitivity to global change. Here we show that the single most important nitrogen fixer in today's ocean, Trichodesmium, is strongly affected by changes in CO2 concentrations. Cell division rate doubled with rising CO2 (glacial to projected year 2100 levels) prompting lower carbon, nitrogen and phosphorus cellular contents, and reduced cell dimensions. N2 fixation rates per unit of phosphorus utilization as well as C:P and N:P ratios more than doubled at high CO2, with no change in C:N ratios. This could enhance the productivity of N‐limited oligotrophic oceans, drive some of these areas into P limitation, and increase biological carbon sequestration in the ocean. The observed CO2 sensitivity of Trichodesmium could thereby provide a strong negative feedback to atmospheric CO2 increase.

Abstract. Increasing atmospheric carbon dioxide (CO 2 ) concentrations due to anthropogenic fossil fuel combustion are currently changing the ocean's chemistry. Increasing oceanic [CO 2 ] and consequently decreasing seawater pH have the potential to significantly impact marine life. Here we describe and analyze the build-up and decline of a natural phytoplankton bloom initiated during the 2005 mesocosm Pelagic Ecosystem CO 2 Enrichment study (PeECE III). The draw-down of inorganic nutrients in the upper surface layer of the mesocosms was reflected by a concomitant increase of organic matter until day t 11 , the peak of the bloom. From then on, biomass standing stocks steadily decreased as more and more particulate organic matter was lost into the deeper layer of the mesocosms. We show that organic carbon export to the deeper layer was significantly enhanced at elevated CO 2 . This phenomenon might have impacted organic matter remineralization leading to decreased oxygen concentrations in the deeper layer of the high CO 2 mesocosms as indicated by deep water ammonium concentrations. This would have important implications for our understanding of pelagic ecosystem functioning and future carbon cycling.

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The Sundarban mangrove forest (4,264 km 2 ) constitutes about 3% of the total area of the world mangrove. We measured diurnal and seasonal variations of air-water CO 2 exchange in relation to the occurrence of phytoplankton during January-December 2001. Diurnal variations of airflows showed that the minimum and maximum CO 2 flux of Ϫ16.2 mol m Ϫ2 h Ϫ1 and 49.9 mol m Ϫ2 h Ϫ1, respectively, occurred during the higher sea breeze. The average ratio of dissolved inorganic nitrogen (DIN ϭ 13.85 Ϯ 7.19 mol L Ϫ1 ) to dissolved inorganic phosphorus (DIP ϭ 1.23 Ϯ 0.57 mol L Ϫ1 ) was 11 Ϯ 4 and the surface water was undersaturated with respect to dissolved oxygen. The mean value of 0.1 Ϯ 0.08 for the ratio of phytoplankton production (P) to community respiration (R) indicated that the ecosystem was heterotrophic. The saturation of dissolved carbon dioxide with respect to the atmosphere varied seasonally between 59% and 156%, with minimum levels in postmonsoon and maximum levels in premonsoon/early monsoon (June/July). Out of the 36 genera of diatoms, 1 blue green alga, and 3 dinoflagellates that occurred throughout the year, only 6 reached bloom proportions in postmonsoon, when mangrove water was a sink of atmospheric CO 2 . Although 59.3% of the emitted CO 2 was removed from the atmosphere by biological processes, on an annual basis, the Sundarban mangrove forest supplies 13.8 kg C ha Ϫ1 yr Ϫ1 of CO 2 from water surface to the atmosphere. Even though it is important to compare all in and out fluxes, there is no direct link between CO 2 emission and the later CO 2 removal by biological processes.

Inter-annual variations of phytoplankton abundance and community organization were observed over a two-decade period along with the ancillary parameters at the land-ocean boundary associated with the Sundarban mangrove forest (21°32′ and 22°40′ N and 88°05′ and 89°E), along the NE Coast of the Bay of Bengal. The number of definable Bacillariophyceae species exceeded Dinophyceae taxa, and the total number of bloom-forming species declined from a maximum of ten in 2000 and a minimum of two in 2007. Blooms of the diatom Coscinodiscus radiatus were common in 2000 and 2007. Tide cycles and the onset of the monsoon season played important roles in diurnal and seasonal variability of phytoplankton. Phytoplankton biovolume showed seasonality, with the highest levels during post-monsoon periods and lowest levels during the monsoon period. Phytoplankton abundance was correlated to rainfall patterns, which may be altered by long-term changes in climate.

Monthly variation of CO2 fugacity (fCO2) in surface water and related atmospheric exchanges were measured in the Hooghly estuary which is one of the most important estuaries, since it is fed by one of the world's largest rivers, the Ganges with a flow of 15,646 m3 s-1 (1.6% of the world's combined river flow). Carbon dioxide fluxes averaged over the entire estuary are in the range of -2.78 to 84.4 mmol m-2 d-1. This estuary acts as a sink for CO2 during monsoon months and seasonal variation of its flux is controlled by dilution of seawater by river water. Since the solubility of CO2 and the disassociation of carbonic acid in estuarine water are controlled by temperature and salinity, the observed variations of CO2 fluxes are compared with those predicted from seasonal changes in temperature, salinity and the ratio of gross primary production to community respiration using empirical equations with an explained variability of 55%.