Effects of phytoplankton blooms on the cycling of manganese and iron in coastal waters

Abstract

The development of intense plankton blooms during spring controls the concentrations of dissolved Mn and Fe in eutrophic shallow coastal waters of the North Sea. Proliferation of diatoms, Phaeocystis colonies, and Noctiluca was accompanied by changes of particulate and dissolved Mn and Fe in the water column. The latter parameters were measured simultaneously with relevant physical, chemical, and biological variables at two sites in the coastal waters of the Southem North Sea, in 1993 at one site and in 1994 at both sites. Removal of dissolved Fe and particulate Fe, Mn, and Al occurred during the inception of the spring bloom. This could be due to biological uptake, adsorption of abiotic particles onto settling phytoplankton cells, or ingestion of abiotic particles by zooplankton during grazing, with subsequent settling of fecal pellets. We also observed a seasonal increase of dissolved Mn and Fe and progressive changes in the composition of the particulate Mn and Fe in the water column after peaks in spring diatom and Phaeocystis blooms. This can be explained by the following sequence of events. The organic matter produced during the spring phytoplankton blooms is degraded by the heterotrophic organisms in the water column and the sediment. This leads to anoxic conditions near the surficial layer in the shallow coastal sediments. First, Mn oxides and then Fe oxides are used as oxidants for the degradation of organic matter, resulting in the reduction and dissolution of Mn and Fe. Dissolved Mn and Fe diffuse to the oxic overlying water and are partly adsorbed or precipitated on the suspended particulate matter. The other part remains in solution longer because of a decrease in the oxidation rate. This decrease results from a lowering of both the pH from 8.6 to 8.1 and the oxygen concentrations from 378 to 204 μM in the water column due to the activity of heterotrophs in the water column and sediment. Noctiluca could also be responsible for the reduction of Mn and Fe associated with bacteria and phytoplankton. Overall, the seasonal evolution of dissolved and particulate Mn and Fe is consistent with the successive autotrophic and heterotrophic activities.