SIBER SPIS sept 2011.pdf - IMBER

More documents

Recommendations

Info

<strong>SIBER</strong> Science Plan and Implementation Strategy Co r e q u e s t i o ns 1) How do phytoplankton production and its controlling factors vary across the IO and with season First-order basinwide descriptive studies are needed to better characterize and understand the many unique aspects of the IO’s circulation features and their productivity signals, ecological responses and biogeochemical relationships. Indeed, major questions that emerged from the JGOFS expeditions remain unanswered. For example, crustacean zooplankton (e.g. Calanoides carinatus) migrate from hundreds of meters depth to the surface during the SWM in the western central AS (Idrisi et al., 2004). It has been suggested that grazing by this species prevents accumulation of phytoplankton biomass in response to upwelling of nutrients (Smith, 2001). This hypothesis might also explain why the phytoplankton bloom occurs toward the end of the SWM in this region, after the grazers migrate back to deeper waters. It has also been suggested that grazing, here and elsewhere in the AS, prevents complete utilization of the nitrate supplied via upwelling. By contrast, a new hypothesis posits that Fe and Si limit phytoplankton production during the SWM in the AS, despite large mineral fluxes from the surrounding dust source regions (Moffett et al., 2008; Wiggert and Murtugudde, 2007). That is, low Si concentrations in the initially upwelled waters limit diatom growth and promote blooms of other species like Phaeocystis, and low Fe concentrations limit phytoplankton growth in general and thus prevent complete exhaustion of nitrate. This effect appears to be most strongly manifested toward the end of the SWM in offshore waters where local dust deposition is greatly reduced because the Findlater Jet acts as a barrier to offshore transport (Moffett et al., 2008). There are clearly open questions even in the best-understood region of the IO regarding the relative influence of grazing and nutrient limitation. What are the temporal and spatial patterns of primary production in the IO What roles do micro- and mesozooplankton grazing versus nutrient limitation play in regulating primary production in different IO subregions, and how do these change seasonally Iron limitation is believed to occur throughout the Southern Ocean due to lack of input from continental sources (Boyd et al., 2007), presumably extending into the IO sector (30°–120°E). This supposition is supported by the results of a “natural” iron experiment (CROZEX, Pollard et al., 2007) in waters downstream of the Crozet Islands that demonstrate a sharp delineation in phytoplankton speciation associated with spatial variation in dissolved Fe availability (Planquette et al., 2007; Poulton et al., 2007). Despite this, the extent of Fe depletion and Fe limitation in the IO sector of the Southern Ocean and its northward extension into the southern IO basin remains unknown. Which nutrients limit phytoplankton production (N, P, Fe, Si) in different areas of the IO and at different times of the year It has been suggested that eastward aeolian Fe transport from South Africa alleviates Fe limitation in the subtropical South IO (Piketh et al., 2000). That in turn would stimulate carbon fixation and export in a broad swath across the southern IO. This appears to be borne out by the strong autotrophy and net CO 2 sink found in the 20-35°S zone (Bates et al., 2006a,b). By contrast, modeling suggests that vast areas of southern tropical waters (5°–20°S) are Fe limited, extending west and north along the western IO rim, particularly during the SWM (Fig. 18) (Wiggert et al., 2006). This lack of Fe may then play the opposite role in helping to form carbon source regions (ocean to atmosphere) in these tropical waters, along the western IO rim and further north in the AS. As discussed above, global distributions of fluorescence quantum yield obtained from a new ocean color algorithm indicate that phytoplankton in the SW IO are nutrient-stressed in precisely the region that model results suggest are Fe limited (Behrenfeld et al., 2009). Clearly, in situ Fe enrichment and Fe stress/physiological studies need to be carried out in the IO to determine conclusively the spatial and temporal extent of Fe limitation and its potential role in the carbon cycle. These studies should include measurements of dust deposition and Fe solubility in the dust, which will vary according to its source. 34
<strong>SIBER</strong> Science Plan and Implementation Strategy Barber et al. (2001) argued that phytoplankton production in the central AS is not generally light-limited. Modeling studies, however, suggest that during the NEM in particular, primary production can be sensitive to mixed layer depth (MLD) (McCreary et al., 2001; Wiggert et al., 2000). When and where the MLD becomes too deep, the average light may drop below the level needed by phytoplankton to cover metabolic and grazing losses. Is the magnitude of primary production per unit area in any part of the IO north of the southern subtropical convergence limited by light availability Finally, what role do the competing processes and denitrification and nitrogen fixation play in modulating nitrogen and phosphorus availability regionally and basinwide in the IO and how does this in turn influence phytoplankton productivity and species composition As discussed above, there is evidence that N 2 -fixation plays an important role in fueling phytoplankton blooms in the southern IO off Madagascar. Do N 2 -fixation supported blooms happen elsewhere in the IO What fraction of the N supply does N 2 -fixation account for in these regional blooms and also over larger tropical and subtropical areas of the IO How do nitrogen fixation and denitrification in the northern IO and especially the AS influence N:P ratios in surface waters and how does this in turn influence nutrient limitation and phytoplankton species composition. Indian Ocean: Surface Nutrient Limitation JAN a 20°N 10°N APR b 0° 10°S 40°E 60°E 80°E 100°E 120°E AUG c 20°S 20°N 10°N 40°E 60°E 80°E 100°E 120°E OCT d 0° 10°S 20°S -2 -1,5 -1 -0,5 0 0,5 1 1,5 2 Fi g u r e 18 Seasonal evolution of most limiting surface nutrient for netplankton, with blue (red) indicating Fe (N) limitation. Reproduced with permission from Wiggert et al. (2006). 35
Page 2 and 3: Additional copies of this document
Page 4 and 5: SIBER Science Plan and Implementati
Page 92 and 93:
SIBER Science Plan and Implementati
Page 94 and 95:
Page 96 and 97:
Page 98 and 99:
Page 100 and 101:
Page 102 and 103:
Page 104 and 105:
Page 106 and 107:
Page 108 and 109:
Page 110 and 111:
Page 112 and 113:
Page 114 and 115:
Page 116 and 117:
Page 118 and 119:
show all

SIBER SPIS sept 2011.pdf - IMBER

Create successful ePaper yourself

Delete template?

Save as template?