Particulate organic carbon and nitrogen in seawater
Particulate organic carbon and nitrogen in seawater
Source: Plymouth Marine Laboratory and ICES (2004)
Required:
25mm filters prepared as below, stored in acid-washed Petri plates.
Access to CHN analyzer
Note: it is necessary to remove all organics and keep free from contamination using gloves and covers, etc. The sample should be handled and transferred between containers as little as possible to avoid contamination during the steps between sampling and analysis (see Grasshoff et al., 1999 and ISO, 1999). It is important to obtain a representative sample, which under certain circumstances, e.g., during heavy algal blooms, can be achieved by shaking the water sampler immediately before taking the sub-sample. The homogeneity of the sample may be verified, for example, by separately analysing sub-samples from the upper and lower layers of the bottle.
Filter-preparation is as follows:
a. Ash at 450°C for >4h, laid out in foil tray with foil cover.
b. Cool in dessicator.
c. Store in box until required.
d. When using: filter material through, then wave filter in HCl vapour for 15 secs to destroy inorganic material (do not rinse filter), then place in numbered acid-washed petri-slide and oven dry at < 40 oC.
e. Upon removal from oven, seal petri-slide with cover.
Storage of samples
Filters containing particulate matter collected for POC analysis should be dried under vacuum for at least one day and stored dry in a desiccator with silica gel or, preferably, stored in a freezer and later dried in a drying oven at 60 °C for 30 min, cooled in a dessicator and weighed prior to analysis.
Sample processing
The amount of sample (volume of water) required will depend on water quality. Measures of chlorophyll a are very sensitive, so that one only needs enough sample on the filter such that one can see a change in colour. Measures of CHN require about 4 times more sample volume on filters of the same size.
However, the measure of POM and TPM requires as much sample as is reasonable on the filter, and certainly more than 2 mg TPM. Especially when filtering natural seawater, this means filtering until the filter is almost blocked. This may only require 500 ml in times of algal bloom or resuspension, but when seston levels are low, may need to filter up to 3 liters per sample (as has been done in eutrophic UK waters).
All filtering must be quantitative (i.e. we need to know the initial total volume of each sample, and the separate volumes of that sample filtered for separate determinations of Chl a, CHN and POM/TPM; thus allowing us to calculate the total of each within the sample as a whole).
Examples, for mesotrophic waters (e.g. ~ 8 ug/l chl)
for TPM, PIM, POM for CHN for Chl a
300ml on 47mm GF/F 50ml on 25mm GF/F 10ml on 25mm GF/F
(rinse with NH4 formate) (acid fumes) (no rinse)
2 duplicate filters 2 duplicate filters 2 duplicate filters
Therefore, for each sample, require: 2 47mm GFFs (washed, ashed, weighed)
2 25mm GFFs (ashed only)
2 25mm GFFs (no pretreatment)
After filtering, place all CHN and POM/TPM filters in the low temperature (< 40 oC) oven to dry (more than 40 oC will result in loss of lipids!!). Chl a samples (see below) must not be dried; instead, fold in half with material on inner side, and stored in freezer.
References
Grasshoff, K., Kremling, K., and Ehrhardt, M. (eds.) 1999. Methods of seawater analysis. VCH, Weinheim, New York
ICES. 2004. Chemical measurements in the Baltic Sea: Guidelines on quality assurance. Ed. E. Lysiak-Pastuszak and M. Krysell. ICES Techniques in Marine Environmental Sciences, No. 35. 149 pp
ISO. 1999. Water quality – Guidelines for the determination of total organic carbon (TOC) and dissolved organic carbon (DOC). ISO 8245. International Organization for Standardization, Geneva.
