Vertical gradients in particle distribution and its elemental composition under oxic and anoxic conditions in a eutrophic lake, Scharmutzelsee, NE Germany

Two vertical profiles of soluble and suspended components were registered a t the deepest location of the eutrophic Lake Scharmutzel (29.5m) - one in M ay under oxic conditions, and one under anoxic conditions during stratifica tion in September 1997. The sequential accumulation of soluble components f rom May to September was illustrated by decreasing partition coefficients, i.e, by the quotients of elemental concentration of particulate and water p hase for Al, Si, Ca, Cu, Mn, Fe, P, and N. Particle counting and in-situ ph otographs neither showed significant differences between May and September nor in the vertical percentual distribution of the four size classes. All s ize classes increased with depth in the epilimnetic layer. The aggregates a re formed by very small particles. They are not effectively removed from an y lake compartment, and they accumulate at steady-state to high concentrati ons in the thermocline and in the hypolimnion in transient maxima at middep th under anoxic conditions. In September the coincident particulate P and F e transient maxima at 15m depth were evidence for a cosedimentation of Fe a nd P even under anoxic conditions, which, can be substantially less than pa rticle removal. FeS precipitation was responsible for efficient Fe removal below Is m water depth at an apparent pK of 2.76. As a result, the partitio n coefficient for S and particulate S increased. The Fe:P ratio decreased, contributing to a more rapid release and recycling of P already in the wate r column.