In order to quantify PO4 production and removal in the individual sub-layers (Table 1), a mass balance was applied describing the temporal change in the PO4 concentrations for each time interval and in each SL (Table 1) by vertical mixing with the neighbouring SL and by PO4 production/removal, QPO4 (eq. (1)). QPO4 thus includes all PO4 related processes in the water column and PO4 exchange at the sediment surface of the individual SL. equation(1) ΔPO4Δt=An−1Fn−1+AnFn+1Vn+QPO4,where n – SL number; The PO4 gradients were obtained Torin 1 from the difference
between the mean PO4 concentrations in neighbouring SLs and division by the distance between the centres of the corresponding SLs. On the basis of these experimentally derived quantities, eq. (1) enables QPO4 to be calculated, which represents the PO4 release by organic matter mineralization and the Fe-P dissolution/precipitation for each time interval and each SL. The accumulation of QPO4 over time (accQPO4) for each SL and for the entire basin below 150 m were determined by the successive addition of QPO4 values (Table 4). A rapid increase in accQPO4 in SL1 occurred
during the selleck first year of the stagnation. This is a consequence of the fact that the bottom water had already become anoxic during the first time interval of the stagnation period (Figure 3b) and that previously deposited Fe-P was redissolved by reduction of Fe3+ to Fe2+. In SL2 to SL3 the
accQPO4 increase occurred during later stages of the Pyruvate dehydrogenase stagnation, coinciding with the upward migration of the redoxcline. The dependence of varying PO4 release rates on the redox conditions in the different SL is also reflected in the relationship between the accQPO4 and the accumulated carbon mineralization, accQCT (Figure 4). During the first phase of the stagnation, accQPO4 in SL1 and SL2 increased almost linearly with accQCT (Figures 4a, 4b). The slopes of the regression lines correspond to a C/P ratio of 40 and 45 respectively, and are thus far below the Redfield ratio of 106, which is assumed to characterize the organic matter composition. The decrease in the C/P ratio of the mineralization products may be due to the fact that the microbial decomposition of organic matter does not occur synchronously for the different elements and that organic P is the first to be mineralized. However, our observations refer to a time span of more than two years, and in the long term the elemental ratios of the mineralization products will correspond to the composition of the organic matter. Therefore, the low C/P ratios derived from the relationship between accQPO4 and accQCT during the early development of anoxic conditions in SL1 and SL2 are attributed to the dissolution of Fe-P.