EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Natural Hazards Natural Hazards and Earth System and Earth System Sciences Sciences Discussions Open Access Open Access Atmospheric Atmospheric Chemistry Chemistry and Physics and Physics Discussions Open Access Open Access Atmospheric Atmospheric Measurement Measurement Techniques Techniques Discussions Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Open Access Biogeosciences Discuss., 10, 3891–3923, 2013 Open Access www.biogeosciences-discuss.net/10/3891/2013/ Biogeosciences Biogeosciences BGD doi:10.5194/bgd-10-3891-2013 Discussions © Author(s) 2013. CC Attribution 3.0 License. 10, 3891–3923, 2013 Open Access Open Access This discussion paper is/has been under review for the journal BiogeosciencesClimate (BG). Climate Inter-shelf nutrient Please refer to the correspondingof finalthe Past paper in BG if available. of the Past Discussions transport from the East China Sea Open Access Inter-shelf nutrient transport from the Open Access Earth System Earth System A. Han et al. East China SeaDynamics as a major nutrientDynamics source Discussions supporting winter primary production on Title Page Open Access Geoscientific Geoscientific Open Access the northeastInstrumentation South China SeaInstrumentation shelf Abstract Introduction Methods and Methods and Conclusions References A. Han1, M. Dai1, J. Gan2Data, S.-J. Systems Kao1, X. Zhao2, S. Jan3, Q. LiData1, H. Systems Lin1, 4 1 1 1 5 Discussions Open Access C.-T. A. Chen , L. Wang , J. Hu , L. WangOpen Access , and F. Gong Tables Figures Geoscientific Geoscientific 1State Key Laboratory of Marine Environmental Science, XiamenModel University, Development Xiamen, China 2 Model Development J I Division of Environment, Hong Kong University of Science and Technology,Discussions Kowloon, Hong Kong, China J I Open Access 3 Open Access Institute of Oceanography,Hydrology National Taiwan and University, Taipei, ChinaHydrology and 4 Back Close Institute of Marine GeologyEarth and Chemistry, System National Sun Yat-Sen University,Earth System Kaohsiung, China Sciences Sciences 5State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Full Screen / Esc Discussions Open Access Oceanography, State Oceanic Administration,Open Access Hangzhou, China Printer-friendly Version Ocean Science Received: 25 November 2012Ocean – Accepted: Science 9 January 2013 – Published: 28 February 2013 Discussions Correspondence to: M. Dai ([email protected]) Interactive Discussion Open Access Published by Copernicus Publications on behalfOpen Access of the European Geosciences Union. 3891 Solid Earth Solid Earth Discussions Open Access Open Access The Cryosphere The Cryosphere Discussions Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract BGD The East China Sea (ECS) and the South China Sea (SCS) are two major marginal seas of the north Pacific with distinct seasonal primary productivity. Based upon field 10, 3891–3923, 2013 observation in December 2008–January 2009 covering both the ECS and the northern 5 SCS (NSCS) in wintertime, we examined southward long-range nutrient-transport from Inter-shelf nutrient the ECS to the northeast SCS (NESCS) carried by the China Coastal Current (CCC) transport from the driven by the northeast prevailing monsoon. These nutrients escaped from the cold East China Sea ECS shelf to refuel the primary production on the NESCS shelf where river-sourced nutrients were limited yet water temperature remained favorable. By coupling the field A. Han et al. 10 observation of nitrate+nitrite (DIN) with the volume transport of the CCC, we derived a first order estimate of DIN flux of ∼ 1430 ± 260 mol s−1. This DIN flux was ∼ 7 times the wintertime DIN input from the Pearl River, a primary riverine nutrient source to Title Page the NSCS. By assuming DIN was the limiting nutrient, such southward DIN transport Abstract Introduction would have stimulated ∼ 8.8 ± 1.6 × 1011 gC of new production (NP), accounting for 15 ∼ > 58 ± 10 % of the total NP or ∼ 38 ± 7–24 ± 4 % of primary production on the NESCS Conclusions References shelf shallower than 100 m. Tables Figures 1 Introduction J I The continental shelf is well known to be characterized by high biological production, J I due to the abundant nutrient sourced from the land via river discharge and/or supplied Back Close 20 through coastal upwelling and shoreward cross-shelf transport (Wollast, 1991, 1993; Ladd et al., 2005; Whitney et al., 2005; Sugimoto et al., 2009). Another possible trans- Full Screen / Esc port pathway to redistribute dissolved and particulate nutrients and materials is through alongshore currents typically along the isotherms featured with long distance volume Printer-friendly Version transport (Liu et al., 2000; Kao et al., 2003; Keafer et al., 2005; Whitney et al., 2005; Interactive Discussion 25 Liu et al., 2007; Chen, 2008; Guo et al., 2012). However, the role of such alongshore currents in the transport of nutrients and the subsequent biological effects has rarely 3892 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | been examined, probably because it is commonly believed that the largest gradients both chemically and biologically are only important in the cross-shelf dimension. BGD Two major continental shelves of the western North Pacific are those of the East 10, 3891–3923, 2013 China Sea (ECS) and the northern South China Sea (NSCS). The ECS shelf and 5 the NSCS shelf are connected by the Taiwan Strait (TWS) (Fig. 1a) (Chen, 2003, 2008). Hydrographic data characterizes the China Coastal Current (CCC), which is driven by Inter-shelf nutrient the northeast winter monsoon, to be one of the major water masses in the TWS (Jan transport from the et al., 2006, 2010), with relatively low SST (< 18.0 ◦C) and salinity (< 33.0) (Fig. 1a). East China Sea Chen (2008) reviews the winter nutrient distribution pattern both in the ECS and the A. Han et al. 10 TWS and points out the potentially high nutrient fluxes carried by the CCC through the TWS. Based on our new observations together with the available literature data of nu- trients and volume transport in the TWS aided by a numerical model, this study sought Title Page to quantify such southward nutrient flux and examine its role in sustaining wintertime primary production (PP) on the shelf of the northeast South China Sea (NESCS). We Abstract Introduction 15 intend to demonstrate that such inter-shelf nutrient flux is a critically important nutri- ent source sustaining PP on the NESCS shelf in winter, which would be otherwise Conclusions References oligotrophic because of the limited river discharge. Tables Figures 2 Materials and methods J I 2.1 Study area J I ◦ Back Close 20 The ECS has a broad shelf located in the temperate zone, which is warm (22–28 C) in summer and cold (∼ 9 to 21 ◦C, Fig. 1a) in winter, especially the inner shelf. Full Screen / Esc The Changjiang River is the largest river emptying into the ECS, with a peak 3 −1 3 −1 discharge of 50 000 m s in summer and a minimum of 13 000 m s in winter Printer-friendly Version (http://xxfb.hydroinfo.gov.cn/). The ECS is influenced by nutrient enriched Changjiang Interactive Discussion 25 discharge and Kuroshio subsurface water, and has a moderately high PP in summer, ranging ∼ 0.2–1.0 gC m−2 d−1 (Chen, 1996; Chen and Wang, 1999; Chen et al., 2001; 3893 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Gong et al., 2003, 2006). However, PP in winter is only one tenth of that in summer probably due to low water temperature and light availability (Gong et al., 2003). BGD The NSCS has a northeastward widened shelf located in the sub-tropical climate 10, 3891–3923, 2013 zone with a complicated coastline and topography variations. The water temperature ◦ 5 on the shelf is warm in summer (27–28 C, Han et al., 2012). In winter, the shelf water temperature is lower (18–24 ◦C, Fig. 1a), but much higher than that in the ECS. The Inter-shelf nutrient Pearl River is the largest riverine nutrient source to the NSCS shelf with a discharge transport from the of 15 500 m3 s−1 in summer and 1800 m3 s−1 in winter (http://xxfb.hydroinfo.gov.cn/). East China Sea In summer, the coastal upwelling also plays an important role in supplying nutrients A. Han et al. 10 (Han et al., 2012). Noteworthy, the PP in the NSCS in winter maintains at similar level as that in summer (∼ 0.8–1.0 gC m−2 d−1, Chen and Chen, 2006; Wang et al., 2012). During this time period, the warm and oligotrophic Kuroshio surface water intrudes and Title Page occupies a large area of the NSCS basin (Hu et al., 2000). The TWS is ∼ 180 km wide with an average depth of ∼ 60 m (Hong et al., 2011). In Abstract Introduction 15 winter, the northeast prevailing wind drives the CCC flowing southward. The current is confined within the narrow inner shelf with water depth < 50 m due to the Ekman Conclusions References effect. This coastal current, which originates from the ECS around the Changjiang es- Tables Figures tuary (Chang and Isobe, 2003; Chen, 2003; Lin et al., 2005; Guan and Fang, 2006), is featured by high concentrations of nutrients as compared with the ambient seawa- J I 20 ter (Chen, 2003; Gong et al., 2003), primarily owing to the large amount of nutrient input from the Changjiang and wind induced resuspension. The CCC starts in mid- J I September, peaks from October to January