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Contribution of Riverine Nutrients to the Silicon Biogeochemistry

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Contribution of Riverine Nutrients to the Silicon Biogeochemistry Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Biogeosciences Discuss., 7, 4919–4951, 2010 Biogeosciences www.biogeosciences-discuss.net/7/4919/2010/ Discussions BGD doi:10.5194/bgd-7-4919-2010 7, 4919–4951, 2010 © Author(s) 2010. CC Attribution 3.0 License. Contribution of This discussion paper is/has been under review for the journal Biogeosciences (BG). riverine nutrients to Please refer to the corresponding final paper in BG if available. the silicon biogeochemistry Contribution of riverine nutrients to the C. Y. Bernard et al. silicon biogeochemistry of the global Title Page ocean – a model study Abstract Introduction C. Y. Bernard1, H. H. Durr¨ 2, C. Heinze1, J. Segschneider3, and E. Maier-Reimer3 Conclusions References 1Bjerknes Centre for Climate Research, Geophysical Institute, University of Bergen, Tables Figures Allegaten´ 70, 5007 Bergen, Norway 2 Department of Physical Geography, Faculty of Geosciences, Utrecht University, J I Utrecht, The Netherlands 3Max-Planck-Institute for Meteorology, 20146 Hamburg, Germany J I Received: 17 June 2010 – Accepted: 17 June 2010 – Published: 25 June 2010 Back Close Correspondence to: C. Y. Bernard ([email protected]) Full Screen / Esc Published by Copernicus Publications on behalf of the European Geosciences Union. Printer-friendly Version Interactive Discussion 4919 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract BGD Continental shelf seas are known to support a large fraction of the global primary pro- duction. Yet, continental shelf areas are mostly ignored or neglected in global biogeo- 7, 4919–4951, 2010 chemical models. A number of processes that control the transfer of dissolved nutrients 5 from rivers to the open ocean remain poorly understood. This applies in particular to Contribution of dissolved silica which drives the growth of diatoms that form a large part of the phyto- riverine nutrients to plankton biomass and are thus an important contributor to export production of carbon. the silicon Here, the representation of the biogeochemical cycling along continents is improved biogeochemistry by coupling a high resolution database of riverine fluxes of nutrients to the global bio- 10 geochemical ocean general circulation model HAMOCC5-OM. Focusing on silicon (Si), C. Y. Bernard et al. but including the whole suite of nutrients – carbon (C), nitrogen (N) and phosphorus (P) in various forms – inputs are implemented in the model at coastal coupling points using the COSCAT global database of 156 mega-river-ensemble catchments from Meybeck Title Page et al. (2006). The catchments connect to the ocean through coastal segments accord- Abstract Introduction 15 ing to three sets of criteria: natural limits, continental shelf topography, and geophysical dynamics. Conclusions References According to the model the largest effects on nutrient concentrations occur in hot Tables Figures spots such as the Amazon plume, the Arctic – with high nutrient inputs in relation to its total volume, and areas that encounter the largest increase in human activity, e.g., J I 20 Southern Asia. J I 1 Introduction Back Close Continental shelf seas play a crucial role in biogeochemical fluxes from the continents Full Screen / Esc to the open ocean; they are known to support a large part of the global primary produc- tion (Conley et al., 1993; Rabouille et al., 2001), they make up for only 8% of the ocean Printer-friendly Version 25 surface but 25% of the ocean primary production (Ver et al., 1999). They are also highly sensitive to human perturbations (Ver et al., 1999). The high level of production Interactive Discussion 4920 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | is supported by complex dynamics including interplay of riverine supply of nutrients with coastal processes such as tidal currents or upwelling, as well as riverine nutrient BGD inputs from the continents. Our perception of the functioning of the biogeochemical cy- 7, 4919–4951, 2010 cles in the global ocean requires a better understanding of the processes taking place 5 on the continental margins. Global biogeochemical models partly fail to integrate pro- cesses taking place on the continental margins such as benthic/pelagic coupling, tidal Contribution of currents mixing, coastal upwelling driven by wind forcing or general circulation and the riverine nutrients to land/ocean interaction; they reproduce only crudely the nutrient enrichment supported the silicon by coastal upwelling. biogeochemistry 10 The riverine contribution is in the best case represented by a couple of main rivers, in most cases riverine inputs are ignored, neglected or distributed along the continental C. Y. Bernard et al. limits as one uniform mean global value. However, riverine inputs to coastal zones show a high diversity and range over several orders of magnitude (Meybeck et al., Title Page 2006). Often, relatively small proportions of the continental area draining to the oceans 15 (i.e. exorheic parts) contribute major parts of material fluxes. As shown by Durr¨ et Abstract Introduction al. (2009) for the example case of dissolved silica (DSi), yields exceeding 6.6 t km−2 y−1, i.e. >2× the global average, correspond to 17.4% of the global continental ice-free Conclusions References exorheic area but contribute 56.0% of DSi fluxes from the continents to the coastal Tables Figures zones. Pacific catchments hold most of the hyper-active areas (>5× global average), 20 probably due to a close connection between tectonic activity and DSi fluxes resulting J I from silicate weathering. Taking into account this context, continental margins should be considered as a com- J I plex but essential interface between land and ocean, a contributor to global oceanic Back Close biogeochemical cycles and not just as passive boundaries. 25 In the open ocean, biogeochemical cycles follow a relatively simple scheme: plank- Full Screen / Esc ton takes up nutrients, dies and sinks to the deep ocean. The sinking organic matter is progressively remineralised and only a very small fraction reaches the sea floor and Printer-friendly Version is buried in the sediments. The euphotic layer where plankton growth takes place is then fuelled by the currents that bring back the nutrients to allow plankton growth. The Interactive Discussion 4921 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | system is more complex on the continental shelf: first the sinking process is highly modified by eddies, then interaction of bathymetry with waves and tidal currents can BGD lead to the resuspension of deposited organic matter, wind along the shore creates up- 7, 4919–4951, 2010 welling that brings nutrient rich waters rapidly back to the surface. Furthermore, many 5 coastal waters receive continuous fluxes of nutrients from the rivers. The riverine inputs are difficult to take into account in models because of the lack of data for thousands of Contribution of smaller rivers, which in total can contribute a significant amount of nutrients. A com- riverine nutrients to prehensive work of data integration is necessary to build up a database covering the the silicon entire world, and not only limited to the major rivers. This new generation of database biogeochemistry 10 is now made available: Durr¨ et al. (2009), Meybeck et al. (2006), the Global-NEWS project (Seitzinger et al., 2005). C. Y. Bernard et al. Detailed regional models succeed in describing the progressive transfer of nutrients from estuarine areas to the adjacent sea (Rabalais et al., 1996; Kroeze and Seitzinger, Title Page 1998), but only few efforts, mostly through global box modelling (Ver et al., 1999), 15 have been made to study the land/ocean coupling at the global scale (Da Cunha et Abstract Introduction al., 2007). Satellite-derived estimates of the chlorophyll concentration give evidence of high primary production in coastal areas, while global models still fail to reproduce Conclusions References the belts of high chlorophyll concentration along the continents (Longhurst et al., 1995; Tables Figures Yool and Fasham, 2001). Regional studies have tried to show the link between the 20 reduction/increase of primary production and changes in nutrient ratios as for example J I in the East China Sea after the Three Gorges Dam across the Yangtze River was built (Gong et al., 2006). An increasing number of regions of the world (e.g., Chesapeake J I Bay, Northern Adriatic Sea, Gulf of Mexico) have seen their jellyfish biomass increased, Back Close supported by an increasing riverine nutrient load due to human activity (crops fertiliza- 25 tion and urban sewage) (Graham, 2001; Purcell et al., 1999). Full Screen / Esc As shown by Humborg et al. (2000), changes in N:Si and P:Si alter biogeochemistry and aquatic food webs in coastal marine environments since diatoms need silica to Printer-friendly Version build up their siliceous frustules, and, in many coastal ecosystems, diatoms are the dominant phytoplankton species and the basic element of the food web. Interactive Discussion 4922 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | The insufficient integration of river contribution masks part of the potential of conti- nental shelf seas productivity. In this work we demonstrate the importance of riverine BGD inputs of DSi to the global biogeochemical cycles on the global scale by adding the 7, 4919–4951, 2010 riverine fluxes of nutrients to a global scale biogeochemical general circulation model
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