Iron and biological production in the high-latitude Nort...( Southampton scientists have demonstr...)

Southampton scientists have demonstrated an unexpected role of iron in regulating biological production in the high-latitude North Atlantic. Their findings have important implications for our understanding of ocean-climate interactions.

Tiny plant-like organisms called phytoplankton dominate biological production in the sunlit surface waters of the world's oceans and, through the process of photosynthesis, sequester large amounts of atmospheric carbon dioxide. A proportion of the carbon is exported to the deep ocean, and because carbon dioxide is a greenhouse gas, this so-called 'biological carbon pump' helps prevent runaway global climate warming.

Iron is an essential micro nutrient for phytoplankton growth. In high-nutrient, low-chlorophyll (HNLC) oceanic regions, phytoplankton growth is limited by low iron availability. Classical HNLC regions, which account for about a third of the world's oceans, include the Southern Ocean and the subpolar North Pacific.

In contrast, it has been widely assumed that iron supply does not limit biological production of the high-latitude (>50 degrees N) North Atlantic Ocean. Here, winter cooling causes the sinking of nutrient-depleted surface waters and their replacement by deep nutrient-rich water. This winter 'overturning' replenishes surface water nutrients, and in the spring, when light intensities increase, a large phytoplankton bloom develops, leading to high rates of carbon export.

However, in many regions of the open North Atlantic, including the Iceland and Irminger Basins, residual amounts of nitrate persist into the summer period, after the spring bloom has ceased. This represents an inefficiency of the biological carbon pump that is potentially of global significance to the partitioning of carbon between the atmosphere and ocean.

Phytoplankton are grazed upon and some of the larger phytoplankton species such as diatoms with low grazing mortality are susceptible to sili
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