In particular, iron (Fe) was considered of great importance as early biological oceanographers hypothesized that iron may also be a limiting factor for primary production in the ocean. As a result an extended Redfield ratio was developed to include this as part of this balance. This new stoichiometric ratio states that the ratio should be 106 C:16 N:1 P:0.1-0.001 Fe. The large variation for Fe is a result of the significant obstacle of ships and scientific equipment contaminating any samples collected at sea with excess Fe. It was this contamination that resulted in early evidence suggesting that iron concentrations were high and not a limiting factor in marine primary production.