Research methods
The Daily Water Sample program was started in 1978 to moniter the
daily nutrient cycling within the North Inlet Estuary System. The
original sampling site was at Town Creek. Later samples were taken
at Clambank and Oyster Landing as well. Samples were taken 365 days
a year at the same time every day, approximately 10:00 AM EST. The
samples were filtered at the Lab to separate the particulates from
the water. The filtered water was then run through a colorimeter and
the nutrient value peak heights derived were recorded on a ticker
tape type printout. These values were recorded into a master
nutrient note book, and entered onto a CMS file by the Data
Management Department at the lab. The filters that were used in the
filtering process were first weighted, burnt off and cleaned, then
they were weighted with the particulates dried onto them. These
filters were then cooked again for 24 hours at 450 degrees. This was
done to determine the inorganic particulates that were left. All
data files have since been stored on a SUN workstation and backed up
on 8mm tapes at both the field lab and the data management office at
the University of South Carolina.
Analysis methods for the individual variables are described
below:
Ortho-Phosphate
The basic method was the Technicon Industrial Method No. 155-71W
(1973), which was a modification of the Murphy and Riley (1962)
single solution method. The method depended on the formation of a
phospomolybdate blue complex, the color of which is read at a
wavelength of 880 mm.
Sulfate
For this automated procedure for sulfate (Technicon Industrial
Method No. 118-71W/B), the sample was first passed through a
cation-exchange column to remove interferences. The sample
containing sulfate was then reacted with barium chloride at a pH of
2.5-3.0 to form barium sulfate. Excess barium reacted with
methylthmol blue to form a blue-colored was gray; if it was all
chelated with barium, the color was blue. Initially, the barium
chloride and methylthymol blue were equimolar and equivalent to the
highest concentration of sulfate ion expected; thus the amount of
uncomplexed methylthymol blue, measured at 460 mm, was equal to the
sulfate present.
Nitrate/Nitrite
The basic method was Technicon Industrial Method No. 158-71W/B,
which utilized the reaction in which nitrate was reduced to nitrite
by a copper- cadmium reductor column. The nitrite ion then reacted
with sulfanilamide under acidic conditions to form a diazo compound.
This compound then coupled with N-1-napthylethylenediamine
dihydrochloride to form a reddish-purple azo dye.
Chlorides
The basic method was Technicon Industrial Method No. 99-70W/B.
This automated procedure for the determination of chloride depended
upon the the liberation of thiocyanate ion from mercuric thiocyanate
by the formation of unionized but soluble mercuric chloride. In the
presence of ferric ion, the liberated thiocyanate formed a highly
colored ferric thiocyanate proportional to the original chloride
concentration.
Ammonia
This method was from "The Automated Analysis of Nutrients in
Seawater: A Manual of Techniques" by Gilbert and Loder (1977), a
Woods Hole Oceanographic Institution publication No. WHO-77-47. This
method was dependent upon the Berthelot Reaction, during which the
formation of a blue colored compound closely related to indophenol
occurred when the solution of an ammonium salt was added to sodium
phenoxide, followed by the addition of sodium hypochlorite. A
solution of potassium sodium tartrate and sodium citrate was added
to the sample stream to eliminate the precipitation of the
hydroxides of calcium and magnesium. Total Nitrogen - Total
Phosphorus The procedure consisted of an alkaline persulfate
oxidation followed by automated analysis for nitrogen (as nitrate)
and phosphorus (as phosphate) on a Technicon Auto Analyzer. With
alkaline persulfate digestion, all nitrogen in the sample was
oxidized to nitrate, which was then reduced by cadmium and analyzed
as nitrite. The precision and recovery of this method compared
favorably with a Kjeldahl procedure (D'Elia et al., 1977). The main
advantage of the persulfate oxidation was the speed and convenience
with which it was accomplished. Unlike the Kjeldahl method, which
recovered only organic and NH4 - nitrogen, the persulfate oxidation
also included NO3 and NO2 nitrogen. The procedure described by
D'Elia (1977) has been so the digest could be analyzed using an
autoanalyzer (Gilbert et al., 1977). This semi- automated procedure
combined the persulfate oxidation with automated nitrate and
phosphate analysis to provide simultaneous analysis for total
persulfate nitrogen (TPN) and total persulfate phosphorus (TPP).
Gilbert's procedure was further modified to use a smaller sample
size so that the digestion flask could double as the sampler tray
cup (Loder, 1978). Using this method, many samples could be run
quickly with a minimum of handling. Carbon (prior to September 1989)
Particulate Carbon 20 ml of daily water sample was filtered through
a precombusted (24 hr @ 450 C) Whatman GFF 0.7 micron glass filter.
The filter was waved over concentrated HCl to eliminate inorganic
carbon, then placed in a precombusted ampule and frozen. Batches
were dried at 60 C for 48 hrs. 0.1 gr of precombusted (48 hr @ 750
C) reagent grade CuO was added to each ampule, the ampule was then
purged with pure oxygen and sealed, using an Oceanigraphics
International Purging and Sealing Unit #524PS. Ampules were etched,
then heated to 580 C for 4 hrs. Sorted ampules were then analyzed on
an Oceanigraphics International #524C TOC Analyzer by comparing
infrared absorption peak heights (Horiba PIR 2000) on an LDC strip
chart recorder with those of KHP (Potassium Hydrogen Phthalate)
standards. Standards were prepared by microsyringing a series of
volumes of a KHP solution into ampules and processing them as
samples. Reagent blanks were subtracted and average concentration of
duplicate samples was calculated for each DWS site. Dissolved
Organic Carbon The daily water sample was filtered through a
precombusted (24 hr @ 450 C) Whatman GFF 0.7 micron glass filter. An
aliquot of filtrate was refrigerated. Samples were purged of
inorganic carbon by adding 25percent phosphoric acid to lower the pH
to 2 and bubbling for 10 minutes with pure oxygen. The pH was then
neutralized using ammonium hydroxide to protect the oxidizing
catalyst. Every 5 minutes, using a Hamilton CR-700-200 spring loaded
syringe, a 100 microliter sample was injected into a Beckman 915A
TOC Analyzer and infrared absorption peak heights were compared to
those of KHP standard solutions to calculate carbon concentration.
Carbon (after September 1989) Dissolved Organic Carbon Water samples
were filtered through a precombusted Whatman GFF 7.0 micron glass
filter. Samples were purged of inorganic carbon by adding 10 percent
HCl and sparged with ultra zero grade air. A 40 microliter aliquot
was injected, via a autosampler, into a Shimadzu TOC-500 organic
carbon analyzer. A non-dispersive infrared gas analyzer was utilized
to measure the carbon dioxide produced and samples were run in
triplicate with means reported. ***NOTE*** A Whatman GFF 7.0 micron
glass fiber filter was used throughout the LTER Daily Water Sample
analysis.