Sediment and pore water chemistry analyses
The remaining sediment in the frozen 215 ml containers were thawed,
homogenized, and 15 ml sediment from each sample was dried at 60ºC for
seven days for C/N analysis. In addition, 20 ml of sediment from each
sample was centrifuged at 2200 × g to extract the pore water for
ammonium (NH4+) and phosphate
(PO43-) analyses. The dried sediment
was ground, homogenized, and 1 ml dry weight sediment per sample stored
in a desiccator prior to freeze drying, re-grinding, re-homogenizaton
and treated with HCl to remove inorganic carbon. Samples were
subsequently weighed into tin capsules. Concentrations of total OC and
total nitrogen were determined on an elemental analyser (Flash 2000,
Thermo Scientific). The pore water was collected after centrifugation by
filtering 10 ml of the supernatant through a 0.45 µm polyethersulfone
membrane filter (Filtropur S 0.45, Sarstedt).
NH4+ and
PO43- were determined colorimetrically
(Multiskan GO spectrophotometer, Thermo Scientific) and
NH4+ analysis followed the modified
salicylate-hypochlorite method by Bower and Holm-Hansen (1980), and
PO43- analysis followed the standard
methods for seawater analyses (Grasshoff, Kremling, & Ehrhardt, 2009).