Northeast Pacific iron distributions in relation to phytoplankton productivity

Abstract

Dissolved and particulate Fe concentrations, measured at three deep ocean stations on a 1600 km inshore-offshore VERTEX transect, were compared with those found at four shallow California continental margin stations. The three VERTEX profiles shared similar features: very low dissolved Fe levels (<0.1 nmol kg−1) in surface waters, increasing amounts with depth, and maxima (∼1.0–1.3 nmol kg−1) in association with the oxygen minimum. In contrast, concentrations as high as 9 nmol kg−1 of dissolved Fe were found at the shallow margin stations, in association with elevated levels of Mn (17 nmol kg−1) and Co (200 pmol kg−1).Inshore and offshore Fe distributions were evaluated in relation to the phytoplankton's requirement for this essential element. Local shelf diagenetic Fe input appears to be adequate for phytoplankton growth even in environments where increased demand results from the upwelling of major nutrients. However, the Fe laterally mixed out into the ocean's interior within the oxygen minimum and supplied to the surface via vertical mixing processes provides only a few percent of open ocean phytoplankton demand; the other 95% must be provided by atmospheric input.We also consider environments in which Fe supplies may be limiting phytoplankton growth; i.e. surface waters of the Subarctic and Antarctic, where major nutrients are never depleted. We postulate that atmospheric Fe input rates are not high enough to meet the elevated phytoplankton demand resulting from offshore major nutrient upwelling. As a result, major nutrient depletion occurs only along continental margins and ice edges, where Fe supplies should be adequate.Atmospheric dust concentrations were one to two orders of magnitude higher in glacial times than those in the present and last interglacial periods. The lower glacial atmospheric CO2 levels, which resulted from the increased biological utilization of major nutrients at high latitudes, may have been stimulated by the enhanced availability of atmospheric Fe.