The Impact of Global Commercial Harvest on the Ocean Iron Cycle

Although iron (Fe) is the fourth most abundant element in the Earth's crust, bioavailable Fe is established as a limiting factor in marine primary production. We hypothesize that removal of Fe due to the harvest of marine species is a significant loss term that is absent from current mass balance equations for oceanic Fe cycles. Total commercial catch data for 1950 to 2010 was obtained from the Food and Agriculture Organization of the United Nations (FAO), using FishStat software. The data was separated by taxa and fresh water species were excluded. High and low end values for elemental composition were obtained for each taxonomic category from the literature, and used to determine Fe per mass of total harvest over time using Matlab. The marine commercial catch is estimated to have removed 2-6x10^9 grams of Fe (0.4-1 x10^8 moles) in 1950, which constitutes the lowest values on record. There is an annual increase to 0.9-3x10^10 grams (2-5 x 10^8 moles) in 1996, and then a slight decline to 0.7-2x10^10 grams (1 - 4 x10^8 moles) in 2010. This Fe is in a bioavailable form. Much of this removal is permanent on biological time scales. The fraction that is returned to the ocean is accounted for by coastal and rivertine input terms in the oceanic Fe cycle models. To determine if this may have had a long term impact on Fe budgets, seawater data compiled by Moore and Braucher was averaged for each of the four ocean zones, and volumes were used to estimate total dissolved iron (Fe(d), that which passes a <0.4 micron filter). The total ocean Fe(d) for all zones is estimated to be in the range of 0.1-2 x10^13 grams (0.2 - 3 x10^11 moles) within the years 1978-2004. Fe in the top 3 zones (above 4000 m) is the most variable, in both time and space. The variation is due to seasonal inputs and a nutrient profile above 1000. However, the zones above 4000 m are the most relevant because this is where marine harvesting occurs. In spite of the variation, all available data above 4000 m (6516 points) was used without discrimination beyond which of the 3 zones it fell into. While these preliminary estimates are rough at best, the upper 4000 m of the ocean has experienced a drop in Fe(d) from 2x10^13 grams (3 x10^11 moles) in 1978 to 9x 10^12 grams (1 x10^11 moles) in 2004; a 50% reduction. These calculations support our hypothesis that ocean catch data is a term that has been neglected in global oceanic Fe cycles.