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Southern Ocean

The Southern ocean is the largest high nutrient low chlorophyll (HNLC) region in the global ocean. The surface waters of the Southern Ocean have been widely identified as being replete in macronutrients (nitrogen and phosphorous) that phytoplankton are unable to utilize fully. [Chisholm and Morel, 1991]. The high macronutrient concentration in this ocean is attributed to large upwellings of nutrient rich deep water [Pollard et al., 2006]primarily due to ekman transport as a part of the global circulation Global ocean circulation But phytoplankton do not utilize the macro nutrients completely as their respiration is limited by micro nutrients like iron [de Baar et al., 2005; Martin, 1990; Martin et al., 1990] that are available only in low concentrations due to low dust inputs and low solubility of iron which drops the iron:nitrate ratio in the upwelled water below the threshold value required by the phytoplankton [Duce and Tindale, 1991; Jickells et al., 2005], thus restraining their growth, as made manifest by generally low concentrations of chlorophyll a (chl a) This leads to a high nutrient low chlorophyll situation in these regions.

But some parts of the Southern Ocean appear to have adequate available iron concentrations to support strong biological blooms. Major locations in the Southern Ocean exhibiting where this occurs are Crozet, Kerguelen islands and downstream of South Georgia. There have been numerous studies to understand the underlying processes behind this like CROZEX project ( CROZet natural iron bloom and Export) [Pollard et al. 2007] at the Crozet Islands, Project KEOPS 1 in the wake of the Kerguelen plateau [Blain et al. 2007] and explorations in the Southern Drake Passage region. The major feature that unites these regions of high biological productivity is that they are on or near the shelf regions that surround islands or the Antarctic continent, suggesting that the shelves themselves might serve as a source of natural iron fertilization. Ocean transport and mixing processes play a key role in delivering the iron-rich waters from these shelf regions to the HNLC areas deplete in micro nutrients. This off shelf transport rates are double during the winter when compared to summer and thus could play an important role in regulating the spring blooms in these shelf areas [Zhou et al. (2013)] [Hatta et al., 2013].



The major feature that unites these regions of high biological productivity is that they are on or near the shelf regions that surround islands or the Antarctic continent, suggesting that the shelves themselves might serve as a source of natural iron fertilization -   Main paper

Ocean transport and mixing processes are key in delivering iron-rich shelf/plateau waters to HNLC areas where this micro-nutrient is needed. Zhou et al. (2013) report that off shelf transport rates into the Drake Passage are two-fold higher in winter compared to summer. Coupled with higher wintertime Fe concentrations over the shelf (Hatta et al., 2013), this process could be key in regulating the timing, intensity and extension of the spring bloom downstream of the Antarctic Peninsula.

Also

Three papers set out to identify Fe sources, define seasonal changes in Fe distributions, and quantify Fe fluxes in and around the Antarctic Peninsula. Measures et al. (2013) used the relative concentrations of iron, manganese, and aluminum in austral summer to infer that water column Fe was ultimately derived from sediment diagenesis. Furthermore, ratios of Fe:Mn across a range of water masses were stable, which suggested that physical mixing processes were dominant in controlling their distribution; their distribution was used to explain to relatively large offshelf Fe fluxes compared with biological uptake over the shelf. During austral winter, Hatta et al. (2013) observed high concentrations of Fe, Mn,and Al associated with resuspended sediments over the shelf in regions of intense circulation. They noted that this was further evidence that the dissolved metals in this region are sourced from suboxic or anoxic sediment porewaters entrained into the water column during sediment resuspension.

Read further – and arrive at a conclusion

While availability of iron clearly regulates primary productivity rates in the Southern Ocean, it can also determine phytoplankton community structure. Selph et al. (2013) used flow cytometry to study austral summer Antarctic Peninsula biomass and distribution between two size classes of phytoplankton. They reported that, while elevated levels of dissolved Fe often led to increases in phytoplankton biomass, light limitation prevented significant biomass accumulation in certain areas where Fe levels remained high.

Quéguiner (2013) presents a synthesis of plankton community structure under both artificial and natural iron fertilized conditions. He separates Southern Ocean diatoms into two classes, fast growing/slightly silicified and slow growing/strongly silicified,and describes the physical and biogeochemical conditions under which each can thrive.

The results of this analysis are then transferred to HNLC areas to assess the potential importance of light limitation through the year. We conclude that light limitation does not significantly constrain the annual integrated standing stock of chl a in the HNLC Southern Ocean