Fig 1.
Sediment stratigraphic charts of Lake Arendsee and Lower Havel.
(a, d) phosphorus (P) and iron (Fe), (b, e) molar ratio of total sulphur to reactive iron (S:Fe) and sulphur (S), and (c, f) organic matter (org. matter) and calcium (Ca) from February 2014 (Lake Arendsee) and October 2013 (Lower Havel). According to sediment depth the age of the core is indicated. Bar charts in graph (a) and (d) indicate six different P forms in the correspondent sediment depth layers: (1) NH4Cl-TP: loosely adsorbed P, immediately available P, (2) BD-TP: redox sensitive P, mainly bound to Fe-(hydr)oxides, (3) NaOH-SRP: metal P, mainly bound to Fe- and Al-oxides, (4) NaOH-NRP: organic-bound P, (5) HCl-TP: P bound in calcium carbonates and apatite, and (6) Res-P: residual P determined after digestion of remaining sediment. P fractionation data originate from sediment cores taken in June 2007 (Lake Arendsee) and October 2011 (Lower Havel).
Fig 2.
Vertical distribution of inorganic sulphur (S) species in sediments of Lake Arendsee (October 2014) and Lower Havel (May 2012).
(a, c) CRS: chromium reducible S, AVS: acid volatile S, S0: elemental S and organic S (Sorg) in mg gdw−1. (b, d) Relative contribution of mono- and disulphidic bound Fe to the total Fe content (calculated). Note that the S peak was 10 cm further down-core here, than evident from Fig 1b. Data from Lower Havel are reprinted from [25] under a CC BY license, with permission from Springer, original copyright 2015.
Fig 3.
Analysis of high-density sediment samples of Lake Arendsee.
(a) Reflected-light microscope images of high-density (ρ > 2.3 g cm−3, H1–H3) and low density (ρ < 2.3 g cm−3, L1–L3) samples from different sediment depths (positive downward sampling depth) of Lake Arendsee (H1, L1: 27 cm; H2, L2: 35 cm; H3, L3: 41 cm). (b) Scanning electron micrographs of dark blue vivianite nodules enriched in high-density samples. (c) XRD patterns of bulk sediment (data in red) and high-density samples (data in blue) from 35 cm sediment depth of Lake Arendsee. The pattern confirm the presence of vivianite (Vi). Other minerals identified were calcite (Ca), quartz (Qz), pyrite (Py), mica (Mi) and plagioclase (Fp). For clarity main peaks of calcite and quartz are not shown in total.
Fig 4.
Analysis of high-density sediment samples of Lower Havel.
(a) Reflected-light microscope images of high-density (ρ > 2.3 g cm−3, H1–H3) and low density (ρ < 2.3 g cm−3, L1–L3) samples from different sediment depths (positive downward sampling depth) of Lower Havel (H1, L1: 0.5 cm; H2, L2: 5 cm; H3, L3: 9 cm). (b) Scanning electron micrographs of dark blue vivianite nodules enriched in high-density samples. (c) XRD patterns of bulk sediment (data in red) and high-density samples (data in blue) from 5 cm sediment depth of Lower Havel. The pattern confirm the presence of vivianite (Vi). Other minerals identified were calcite (Ca), quartz (Qz), pyrite (Py), mica (Mi) and plagioclase (Fp). For clarity main peaks of calcite and quartz are not shown in total.
Table 1.
Elemental composition and vivianite occurrence of high-density samples (ρ > 2.3 g cm−3) from different sediment depths of Lake Arendsee (February 2014) and Lower Havel (October 2013).
The P content of high-density samples (P) is also given as percentage of total P of bulk sediment (PSed).
Table 2.
Proportion of sedimentary phosphorus (P) forms according to [30] on total P of synthetic vivianite powder (Pviv, surface oxidised, blue appearance) and of a high-density sample (ρ > 2.3 g cm−3) naturally rich in sedimentary vivianite.
The first two steps of the P speciation according to [32] are given in brackets.
Fig 5.
Sedimentary total P content of Lake Arendsee and Lower Havel according to the corresponding molar ratio of total sulphur to reactive iron (S:Fe) of the sediment.
Vivianite occurs only at a lower S:Fe ratio.