Change in the concentrations of iron in different size fractions during a phytoplankton bloom in controlled ecosystem...

To observe micronutrient dynamics in the plankton ecosystem, controlled ecosystem enclosure (CEE) experiments were conducted in Saanich Inlet, B.C., Canada. Two CEEs (2.5 m in diameter, 16 m in length, one for Fe studies and the other for biological studies) were launched for the period 22 July to 5 August 1996 and enriched with 10 µM nitrate and 5.2 nM Fe (13% of total Fe) on day 1. Sampling from three integrated depths, intervals 0-4, 4-8 and 8-12 m, was performed on days 0, 1, 2, 3, 4, 5, 7, 9 and 11. Iron concentrations were measured for five size fractions: >25 µm particles, 2-25 µm particles, 0.2-2 µm particles, 0.2 µm-200 kDa small colloidal particles and <200 kDa soluble species. The sediment in the Fe enclosure was also collected on every sampling day after day 2 and its Fe was determined. Size-fractionated particulate organic carbon and total chlorophyll-a were also analyzed.The Fe in small colloidal particles (200 kDa-0.2 µm) comprised 78% of the traditionally defined dissolved phase (<0.2 µm) on day 1. Of all the size fractions of Fe, the small colloidal particulate fraction decreased most significantly during the phytoplankton bloom. In the dissolved fraction (<0.2 µm), the small colloidal particle fraction comprised 79% of the decrease. The decrease in concentration of Fe in small colloidal particles was larger than that of total Fe from day 1 to day 4. In contrast, the >25 µm Fe particles increased over the same period. These results suggest that Fe in small colloidal particles changed to >25 µm Fe particles during phytoplankton growth. A large amount of Fe was kept in the surface layer with the phytoplankton, and transported to the deep layer by phytoplankton sedimentation, at the end of the bloom. From these results, the small colloidal particulate Fe seems to be the most dynamic size fraction and a high percentage of Fe in small colloidal particles changed to large particles due to chemical/physical aggregation and/or physical adsorption to suspended particles such as phytoplankton cells.