SIBER SPIS sept 2011.pdf - IMBER

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SIBER Science Plan and Implementation Strategy There are very few biogeochemical modeling studies that have addressed basinwide variability (Kawamiya and Oschlies, 2001; Kawamiya and Oschlies, 2003; Wiggert and Murtugudde, 2007; Wiggert et al., 2006). To date, only Wiggert et al. (2006) and Koné et al., (2009) have attempted to provide holistic insight into all sub-regions and their interconnections. It is nevertheless clear from these initial efforts that regional interconnectivity plays a critical role in defining the basin’s biological and biogeochemical variability. The impact of the IOD on surface chlorophyll concentrations and productivity (Fi gs. 12 a n d 13) during the prominent occurrences of the 1990s has been simulated successfully (J. Wiggert, personal communication), but applications of retrospective biogeochemical modeling to study weaker IOD events (Annamalai et al., 2005; Meyers et al., 2007) are generally lacking. Forcing data are available to extend modeling simulations back to the late 1940s (Kistler et al., 2001). Retrospective studies along these lines could be used to investigate how IOD events affected biogeochemical variability pre-SeaWiFS. Similarly, long-term retrospective and climate forecast model simulations can be used to identify climate change impacts on both the physical and biogeochemical dynamics of the IO. Model projections focused specifically on the IO and its sub-regions should be run out to the year 2050 and beyond. Existing earth system models that include processes like river run-off (and associated nutrients supply), atmospheric deposition (Fe, Si, N, C), ocean biogeochemical processes and ecosystem dynamics can and should be used to develop these projections. Downscaling techniques should also be applied, i.e. using larger scale climate models to force higher resolution regional simulations that can capture local variability and change with greater realism. In this context it is important to emphasize the need to engage scientists from outside the earth system modeling community who have specific expertise in IO basin-scale and regional modeling. Emphasis should also be placed on combining retrospective modeling (coupled physical-biological models) with satellite observations (SST, SSH, SSS and ocean color etc.) to investigate climate change. Mod e l i n g th e im p a c t s of ri v e r i n e an d at m o s p h e r i c in p u t s As with satellites, models can be applied to study the sources, fate and impacts of freshwater and nutrient inputs in the AS and the BoB. Model simulations can be run with nutrient and freshwater fluxes that can be turned on and off to quantify their impact on physical structure and biogeochemical cycles in both basins. These would help to answer fundamental questions such as the degree to which freshwater inputs are responsible for the observed physical and biogeochemical differences between the AS and the BoB. Similarly, model simulations can and should be applied to study the influences of marginal seas, e.g. the spreading and impact of Persian Gulf and Red Sea water in the AS and the exchange of deep water between the Andaman Sea and the BoB. With the advent of the SSS remote sensing missions (SMOS and Aquarius), a major additional component will be available for assessing the veracity of models that include both terrigenous and atmospheric freshwater sources and highly saline inputs from marginal seas. Modeling studies of atmospheric dust and pollution fluxes should also be undertaken because of the importance of dust to particle fluxes and sedimentation in the AS and other IO regions. Also, retrospective watershed modeling simulations should be motivated to simulate how riverine nutrient loads have changed in the past and that project how they are likely to change in the future with increasing population density and associated changes in land use. These simulations could then be used to force coastal circulation and biogeochemical models to project these changes in the watershed onto coastal biogeochemical cycles and ecosystem dynamics. As for modeling the river inputs themselves, there are major gaps in existing knowledge for the river basins draining to the IO. The global dataset of Meybeck and Ragu (1995) of river discharge and carbon and nutrient loads does not include many river basins and available data are scarce. For understanding the impact of river carbon and nutrient discharge on coastal and 54
SIBER Science Plan and Implementation Strategy open ocean biology, more long-term measurements are required of both river discharge and loadings of nutrient and organic matter. The Global NEWS modeling project (Seitzinger et al., 2005) made a major effort to collect these data on a global scale, including the IO. However, for the purpose of investigating changes in primary production, export and ecosystem impacts, more regionally-oriented efforts focused on processes specific to the IO should be motivated. Hig h e r tr o p h i c le v e l mo d e l i n g Applying coupled models to study ecosystem dynamics and higher trophic levels is still a significant research challenge. At present these models are primarily used to simulate and understand lower trophic level dynamics (e.g. NPZD-type dynamics and interactions). New modeling approaches for simulating higher trophic levels that emerged from GLOBEC and other programs, can and should be leveraged. SIBER should encourage the development and use of new, alternative end-to-end modeling approaches; especially model structures that are adaptive and/or generate emergent behavior. Such models will be needed to investigate the sensitivity of marine biogeochemical cycles and ecosystems to long-term global change (i.e. decades and longer) as environmental changes on these timescales are likely to give rise to shifts in marine ecosystem structure that are due to evolution and therefore cannot be anticipated. There are several recently developed alternative modeling approaches that are based on more fundamental ecological principles (e.g. Follows et al., 2007; Laws et al., 2000; Maury et al., 2007) that can capture such adaptive and/or emergent behaviors. Modeling efforts of this type, with specific application to the IO, should be undertaken. As discussed above, “offline” individual-based modeling (IBM) approaches can be applied to study higher trophic levels, for example, fish behavior and migration in simulated physical environments. Significant opportunities exist for applying such models to study behavioral responses of commercially important fish (e.g. tuna) to physical and biogeochemical variability in equatorial waters (as applied to the Pacific Ocean by Lehodey et al., 1998). More traditional food web modeling approaches may also be productively applied to study the dominant pathways of trophic interactions in ecological processes in the IO, and their potential vulnerabilities to climate change. Such models include the Ecopath with Ecosim package (EwE, see http://www.ecopath.org), which can be used to provide a static, massbalanced snapshot of the ecosystem, as well as temporally and spatially dynamic simulations. Although the data requirements of these models will likely exceed the available information, efforts towards constructing them would provide a good overarching goal for food web studies and help to define data needs. Unlike tuna, SST and ocean color data cannot be used with IBM models to simulate the distribution of myctophid stocks in the IO because their behavior is not keyed to oceanic fronts observable by remote sensing platforms. However, idealized modeling studies should be undertaken to obtain insight into the behavior of this species. For example, simple 1-D and 2-D models can be constructed to investigate predator-prey interactions under idealized scenarios where food supplies are restricted to surface waters and refuge areas employed by myctophids (i.e. the OMZ) that occur only in deep waters. Diurnal variations in light and visual predator success can be imposed, along with differences in O 2 tolerance between predator and prey, in simulations that seek to reveal how OMZ regions may function as a refuge from predation. These kinds of modeling studies, which have not yet been undertaken, could provide important insight into why myctophid species migrate the way they do and their biogeochemical and ecosystem impacts. Such modeling investigations could also provide a means of synthesizing existing information on metabolic characteristics, low O 2 tolerance and other behaviors of these fish. 55
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