Seasonal variations in the cycling of aluminium, cadmium and manganese in a Scottish sea loch: biogeochemical processes involving suspended particles

Concentrations of Al, Cd and Mn were determined in water and particles collected from water bottles over one year during 19 surveys of a silled fjord. Nutrient and hydrographic data were also collected. Particle fluxes were determined at one site using a sediment trap array. Concentrations of dissolved Al showed the strongest (negative) relationship to salinity (r = 0.91). This correlation decreased with the onset of the diatom bloom and was insignificant immediately post bloom. Removal of dissolved Al was coincident with an increase in particulate Al concentrations. The degree of removal could be predicted from previously reported Al/Si uptake ratios. Concentrations of leachable P in particulate material from the sediment traps increase at the onset of the bloom and decrease coincident with rapid regeneration as the bloom crashed. Particulate leachable Cd showed a lag in apparent uptake and regeneration relative to particulate leachable P. A substantial amount (ca. 53%) of the annual particle flux of Mn to the deepest trap is material recycled at the sediment surface and is not accumulated in the bottom sediment. Dissolved and particulate Mn levels in the loch have a strong seasonal pattern with low values prior to the spring phytoplankton bloom and increased values after the bloom crash. Maximum concentrations were observed in late autumn. Levels decreased to near pre-bloom values early the following year. This suggests bacterial decay of deposited organic carbon associated with the spring bloom-maintained suboxic conditions at or near the seawater-sediment interface throughout the majority of the year. After exhaustion of this organic matter pool recycling ceased for a brief (2-3 months) winter period. (C) 1999 Elsevier Science Ltd. All rights reserved.