234Th-derived particulate organic carbon export from an island-induced phytoplankton bloom in the Southern Ocean

It has long been recognised that some oceanic regions have persistently low-chlorophyll levels, even though there are abundant inorganic nutrients. Studies have shown that these high-nutrient low-chlorophyll (HNLC) areas are depleted in iron, an essential micronutrient. In these regions biological production can be enhanced with artificial mesoscale iron fertilisation. However, the ability of iron-induced blooms to efficiently sequester carbon to mesopelagic depths is still an open question. It is hypothesised that sub-Antarctic islands in the HNLC Southern Ocean are also a source of iron and thus fuel the natural phytoplankton blooms observed in their proximity, thereby enhancing levels of particulate organic carbon (POC) export. To test the third part of this hypothesis, POC export was measured in the Southern Ocean region of the Crozet Islands (52°E, 46°S) during the austral summer of 2004/2005 as part of the CROZEX project. Based on satellite imagery, a high-chlorophyll region (maximum concentration=4 μg l−1) north and downstream of the islands was distinguished from a low-chlorophyll region (typical concentration=0.3 μg l−1) south and upstream of the islands. POC export estimates were obtained by using the naturally occurring particle-reactive radionuclide tracer 234Th. POC export was initially 15 mmol C m−2 d−1 in the high-chlorophyll bloom region, compared with 5 mmol C m−2 d−1 in the low-chlorophyll, non-bloom region. After a moderately small bloom at the southern control stations (max concentration=0.7 μg l−1) the spatial variability in POC export was lost, resulting in equally high levels of POC export (ca. 20 mmol C m−2 d−1) throughout the study region. Comparison of 234Th-derived POC export with estimates of new production, calculated from nitrate budgets, revealed evidence for a decoupling of new and export production, with this effect most apparent within the northern bloom area. In addition to methodological issues this apparent decoupling of new and export production could be due to a buildup of dissolved organic nitrogen within the bloom region, thus reducing the amount of POC available for export to mesopelagic depths.