SIBER SPIS sept 2011.pdf - IMBER

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SIBER Science Plan and Implementation Strategy Direct and indirect effects of global warming and ocean acidification, such as enhanced stratification, climatic/regime shifts and reduced calcification of key plankton species, may have significant impacts on community structure and elemental cycling in the IO (Hood et al., 2006). If these shifts occur in the absence of established regional climatologies and baseline knowledge of the lower food web, it will be difficult to differentiate them from natural interannual variability. Moreover, if simultaneous changes occur in fishing effort, with possible cascading effects through the food web, identifying climate-induced change will be even more challenging. In areas like the AS that have some historical databases of integrated stock and process measurements, one might therefore ask whether underlying food web relationships have already changed significantly in response to climate forcing. Are there indicator processes, species or food web effects that are showing signs of long-term changes due to anthropogenic impacts The answer appears to be yes, at least for some zooplankton species, as discussed above. In addition, in contrast to trophic studies in the euphotic zone, the IO provides a unique opportunity to investigate the regulation of food webs and biogeochemical cycles under varying conditions of oxygen content and organic fluxes in the OMZ. 2) At intermediate and higher trophic levels, what are the dynamics, impacts and vulnerabilities of dominant stocks/populations like myctophids, and how do their biomass variations affect lower trophic levels and vice versa Net sampling and acoustic data suggest that myctophids are extremely abundant in the IO. However, survey data on these and other mesopelagic species are very limited. First-order descriptive sampling and survey work is needed to determine the seasonal and interannual variability of biomass and composition of the mesopelagic stocks. In addition, elemental compositions should be measured so that the stocks can be related to biogeochemical budgets and cycles, as well as to the physiological requirements of the organisms and their potential contributions as food resources to higher trophic levels. Basic taxonomic work needs to be done so that the IO stocks can be referenced to the literature for other ocean basins. The apparent diminished role of euphausiid crustaceans (krill) is an interesting basic science problem for IO ecosystems. It is tempting, for example, to suggest that myctophids “replace” euphausiids in the AS, or that myctophids occupy the euphausiid niche. A careful study of euphausiid biology and ecology in conjunction with the myctophids is necessary. If myctophids are the shelf break/upper slope dominants, this may be very different to similar habitats elsewhere. In the upwelling system off NW Africa, for example, euphausiids at the shelf break support a large hake fishery. For assessing ecological relationships and fluxes among mesopelagic system dominants, we need to know major trophic pathways and rates. Gut content analyses, for example, can tell us about dietary compositions, as well as where in the water column and approximately when they feed. Gut throughput, defecation and metabolic rates (via shipboard and lab studies) are also needed so that consumption rates and food requirements can be estimated. They would also allow the impact of diel vertical migrants on biogeochemical cycles to be assessed, i.e. how much C, N and P is transferred between the epipelagic and mesopelagic zones, and are these fluxes significant at basin scales To what extent do mesopelagics respond to interannual fluctuations in surface production and export Finally, studies of the C and N stable isotope compositions of key consumers, like myctophids, are needed in order to quantify their position in the pelagic food web. For ecosystem dominants, emphasis needs to be focused on species-level investigations, which should in turn allow development of appropriate fisheries and biogeochemical models for synthesis and prediction. How do species’ distributional patterns relate to variables like temperature, salinity, O 2 and food availability (for deeper layers, the supply of food from export fluxes) Recruitment, growth and mortality rates (including predation) need to be determined for most mesopelagic stocks, particularly myctophids. The role of behavior also needs to be 46
SIBER Science Plan and Implementation Strategy investigated, e.g. foraging mechanisms, extent of vertical migration, and species differences in depth of OMZ penetration. If diel migratory behavior is a strategy for predator avoidance, does it vary as a function of predator density or predation pressure Global warming may impact mesopelagic species by influencing the extent and intensity of the OMZ (see Themes 3 and 5). If many pelagic stocks depend on the OMZ as a refuge and for food, will global warming lead to the expansion of this zone (Stramma et al., 2008), and what will be the likely consequences for food web relationships and biogeochemical cycling 3) At the top of the trophic hierarchy, what are the climate and human (fisheries) influences on major predator stocks (such as tuna) that could exert top-down pressures on the functioning of food webs As discussed in Theme 2, natural climate variability associated with phenomena such as the MJO and the IOD in particular, have influences that manifest strongly not only at the equator but also into the AS and BoB basins, changing patterns of phytoplankton surface productivity (Wiggert et al., 2009) as well as distribution of commercially important top predators such as tuna (Marsac et al., 2006). One overarching question is how such changes propagate upwards and downwards through food webs What are the likely consequences of such trophic cascades with future climate changes Human harvesting of fish and squid can be expected to have an overriding top-down effect in the open sea. A gross decline in the open IO of catch per unit effort has been observed, and the lessons from the warm parts of the north Atlantic cannot be ignored for long. Stock assessments of tunas and other large pelagics are beyond the scope of SIBER. However, many aspects of the biogeochemical and ecological impacts of these organisms need to be studied. For example, is fishing on tuna stocks in equatorial waters in the IO sustainable Will commercial harvest of myctophids, as recently begun by Iran, be large enough to impact the stocks If profitable uses are found for the fish oil and protein products, will this fishery expand to other countries and regions in the IO, possibly through fleets from outside the region What are the potential biogeochemical and ecological impacts of such a fishery expansion Anthropogenic impacts are also apt to increase during the next decades due to increasing population density, especially around the AS and the BoB (see discussions above and in Theme 5). The influence of pollution due to eutrophication and aquaculture on higher trophic levels in coastal waters and marginal seas needs to be investigated with regard to causes and extent of anthropogenically-induced fish kills, and the potential impacts of overfishing need to be assessed. What are the human impacts of these losses and their potential feedbacks to pelagic and benthic food webs 47
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