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Le Fouest1, B. Zakardjian2,4, H. Xie3, P. Raimbault2,4, F. Joux5, and M. Babin6 Open Access 1Laboratoire d’Oceanographie´ de Villefranche, BP 8, CNRS UMR7093 & Universite´ Pierre etGeoscientific Marie Curie, 06238 Geoscientific Open Access Villefranche-sur-Mer Cedex, France Instrumentation Instrumentation 2 Universite´ du Sud Toulon-Var, CNRS/INSU, IRD, Mediterranean Institute of OceanographyMethods (MIO), UM and 110, 83957 La Methods and Garde, France 3Institut des sciences de la mer de Rimouski, Universite´ du Quebec´ a` Rimouski, 310, allee´ desData Ursulines, Systems C.P. 3300 Rimouski Data Systems (Quebec) G5L 3A1, Canada Discussions Open Access 4 Open Access Aix-Marseille Universite,´ CNRS/INSU, IRD, Mediterranean Institute of Oceanography (MIO), UM 110, 13288 Marseille, Geoscientific France Geoscientific 5Laboratoire d’Oceanographie´ Microbienne, CNRS UMR7621 & Universite´ Pierre et Marie Curie, 66650 Banyuls-sur-Mer, Model Development Model Development France Discussions 6Takuvik Joint International Laboratory, Universite´ Laval (Canada) & Centre National de la Recherche Scientifique (France), Departement´ de Biologie, 1045, Avenue de la Medecine,´ Quebec (Quebec), G1V 0A6, Canada Open Access Open Access Hydrology and Hydrology and Correspondence to: V. Le Fouest ([email protected]) Earth System Earth System Received: 28 September 2012 – Published in Biogeosciences Discuss.: 24 October 2012 Revised: 7 June 2013 – Accepted: 13 June 2013 – Published: 15 July 2013 Sciences Sciences Discussions Open Access Open Access Abstract. The Arctic Ocean (AO) undergoes profound rial production). The model also suggested that variable car- Ocean Science changes of its physical and biotic environments due to cli- bon to chlorophyll ratiosOcean were required Science to simulate the ob- mate change. In some areas of the Beaufort Sea, the stronger served herbivorous versus microbial food web competition Discussions haline stratification observed in summer alters the plankton and realistic nitrogen fluxes in the Beaufort Sea oligotrophic ecosystem structure, functioning and productivity, promot- waters. In face of accelerating Arctic warming, more atten- Open Access ing oligotrophy. A one-dimension (1-D) physical–biological tion should be paid in the future to the mechanisticOpen Access processes coupled model based on the large multiparametric database involved in food webs and functional group competition, nu- Solid Earth of the Malina project in the Beaufort Sea was used (i) to trient recycling and primary productionSolid Earth in poorly productive Discussions infer the plankton ecosystem functioning and related nitro- waters of the AO, as they are expected to expand rapidly. gen fluxes and (ii) to assess the model sensitivity to key light-driven processes involved in nutrient recycling and phy- Open Access Open Access toplankton growth. The coupled model suggested that am- monium photochemically produced from photosensitive dis- 1 Introduction The Cryosphere The Cryosphere solved organic nitrogen (i.e., photoammonification process) Discussions was a necessary nitrogen source to achieve the observed lev- The Arctic Ocean (AO) undergoes profound changes of its els of microbial biomass and production. Photoammonifi- physical and biotic environments due to climate change. cation directly and indirectly (by stimulating the microbial Overall net primary production (PP) is shown to have in- food web activity) contributed to 70 % and 18.5 % of the 0– creased in the last decades (Belanger´ et al., 2012; Arrigo and 10 m and whole water column, respectively, simulated pri- Djiken, 2011) and is expected to follow this trend in the fu- mary production (respectively 66 % and 16 % for the bacte- ture (Slagstad et al., 2011). Nevertheless, the PP response is not the same everywhere in the AO, with regions showing Published by Copernicus Publications on behalf of the European Geosciences Union. 4786 V. Le Fouest et al.: Modeling plankton ecosystem functioning and nitrogen fluxes stable or even decreasing PP (Arrigo et al., 2011; Slagstad biological coupled model of the water column was set up et al., 2011). In the Hudson Bay, Foxe Basin, Baffin Sea, off based on the extensive use of physical and biogeochemi- the coasts of Greenland, in the Kara Sea and around Novaya cal variables and rates measured during the Malina cruise. Zemlya, earlier blooms are observed in response to earlier Steady-state runs were analyzed to budget the system and light exposure caused by sea ice retreat (Kahru et al., 2011). to gain a better understanding of the plankton ecosystem In some areas of the Beaufort Sea, the stronger haline strati- functioning in the most oligotrophic shelf waters of the AO. fication recently observed mediates the growing contribution The objectives of this study are, on one hand, to infer the of small phytoplankton cells to the planktonic community in functioning and nitrogen fluxes within the summer plankton summer (Li et al., 2009) suggesting oligotrophy is expand- ecosystem and, on the other hand, to assess the model sen- ing in this part of the AO. In the Barents Sea, the 40 % pro- sitivity to key light-associated processes involved in nutrient jected widening of the productive time period will probably recycling and phytoplankton growth. allow heterotrophic organisms to optimize grazing through growth and reproduction on phytoplankton, and hence alter the carbon quality and quantity exported to the benthic realm 2 Material and methods (Wassmann and Reigstad, 2011). In this context of acceler- ating Arctic sea ice decline (Comiso et al., 2008), a better 2.1 Observations knowledge of the mechanistic processes and biogenic fluxes mediating PP is required, with particular attention paid to the The large multiparametric dataset of physical, chemical and oligotrophic season when biogenic fluxes are complex and so biological measurements collected during the Malina cruise far are poorly quantified. (18 July–24 August, 2009) in the Beaufort Sea was used (i) In the AO, more than 80 % of the PP takes place in shelf to initiate and constrain the model runs, (ii) to set param- seas (Sakshaug, 2004). The Beaufort Sea exhibits the lowest eters and transfer functions and (iii) to compare with the annual production rate (8 Tg C; Sakshaug, 2004) with respect model outputs. We provide here a summary of the data used to its surface area (∼ 476 000 km2), which makes it the most along with their respective reference in the Malina special oligotrophic shelf sea in summer (Ardyna et al., 2013). After issue, where the detailed methodology for each measure- the bloom occurring in June, a deep chlorophyll (Chl) max- ment can be found. Measurements were taken in the Beau- imum (DCM) forms as a result of relatively low nitrate con- fort Sea at the continental edge slope and ice-edge station centrations in the surface layer at the end of spring (Tremblay 345 (71.33◦ N, 132.56◦ W), sampled on 14–16 August, 2009 et al., 2008). Over the growth season, the DCM progressively (Fig. 1). Temperature, salinity and fluorescence were mea- lowers the nitracline down to 60 m depth, where light be- sured using a conductivity-temperature-depth (CTD) sensor. comes the limiting factor (Martin et al., 2010). On the slope Temperature and salinity data were used to compute potential of the Mackenzie Shelf, where waters were amongst the most density, which were in turn used to compute Brunt-Vais¨ al¨ a¨ oligotrophic sampled during the Malina cruise (Tremblay frequencies (N). The latter were calculated in a leap-frog et al., 2013), picoplankton (Micromonas ecotype) and phy- fashion, with the potential density from the previous and fol- toplankton < 5 µm dominated respectively the surface and lowing depths (i.e., N at 5 m is computed with the data at DCM autotrophic community (Balzano et al., 2012; Claustre 4 m and 6 m) (Gratton and Prieur, unpublished data). Surface and Ras, unpublished data) whose role is central in mediating and vertical profiles of downwelling photosynthetic available carbon fluxes in summer (Li et al., 2009). radiation (PAR) were respectively measured by an on-deck The ability of coupled physical–biogeochemical models sensor and a Compact-Optical Profiling System (C-OPS) applied to the AO to simulate realistic plankton dynamics profiler (Hooker et al., 2012). With respect to photosynthesis and production rates relies on both the simulated physics parameters, photosynthesis initial slopes (α) and light sat- (e.g., Popova et al., 2010) and elemental biogeochemical uration parameters (Ek) were taken from Huot