PROBLEM TO BE SOLVED: To solve such a problem that although dispersion of iron sulfate into the surface of the sea is simple as an iron supplying method capable of effectively utilizing by-product of aggregate manufacturing and capable of maintaining a supply effect of ferrous ions into the sea, the influence of sulfate ions dissolved together with iron is feared, and when ferrous ions are dispersed into the sea and the ferrous ions are immediately oxidized to ferric ions due to oxygen dissolved in seawater, the ion supplying effect may be ended as a temporary effect and a problem of effect continuation is left.SOLUTION: A phytoplankton, seaweed and/or marine algae increasing aggregate is provided. The aggregate is a sintered compact obtained by reduction-firing by-product of aggregate manufacturing and containing 1 wt.% and more of the total amount of metal iron (calculated as Fe) and ferrous iron (calculated as Fe) in the aggregate and 25 wt.% and more in total iron (calculated as Fe).
