» Home » HOT » Analytical Methods & Results

Dissolved organic matter

HOT-STUFF FTP View Data
To assist in the interpretation of the data, it can be displayed using the Hawaii Ocean Time-series Data Organization & Graphical System (HOT-DOGS©).

Analytical Method

Dissolved organic carbon (DOC) was determined by the high temperature catalytic oxidation method using a Shimadzu Total Organic Carbon Analyzer. Prior to HOT-125 (March 2001) TOC concentrations had been measured on a commercially available MQ model 1001 TOC analyser equipped with a LICOR infrared detector. Beginning in 1997, certified TOC reference materials were obtained from J. Sharp (University of Delaware) and D. Hansell (University of Miami) and run each time TOC concentrations were analyzed. UV-oxidation distilled water is used to determine the instrument blank. The average precisions during 2005 from duplicate TOC analyses are given in Table 1 below.


Table 1: Precision of Dissolved organic carbon analyses during 2005

DOC
Cruise mean CV
(%) 		mean SD
(µM) 		N
167 2.1 0.76 4
168 1.5 0.56 4
169 4.5 1.76 3
170 2.5 0.98 4
171 1.3 0.53 4
172 1.5 0.56 4
173 5.8 2.39 3
174 2.0 0.74 3
175 4.1 1.51 4
176 1.5 0.53 4
Mean 2.7 1.03 10

Dissolved organic nitrogen (DON) was calculated as the difference between total dissolved nitrogen (TDN) and [nitrate+nitrite] concentrations. DON by this definition also includes ammonium, however, ammonium concentrations in these waters are below the detection limit of standard nutrient analysis (~ 50 nM). Dissolved organic phosphorus (DOP) was calculated as the difference between total dissolved phosphorus (TDP) and SRP concentrations. DOP, by this definition includes inorganic polyphosphates. TDN and TDP were determined by the UV oxidation method as described in Tupas et al. (1997). The average precisions during 2000 from duplicate analyses are presented in Table 2 below.


Table 2: Precision of Dissolved organic nitrogen and phosphorus analyses during 2000

DON DOP
Cruise mean CV
(%) 		mean SD
(µM) 		N mean CV
(%) 		mean SD
(µM) 		N
111 2.4 0.03 2 7.1 0.014 1
112 4.4 0.10 5 16.3 0.018 4
113 3.4 0.16 5 14.4 0.011 4
114 6.7 0.20 5 21.3 0.012 4
115 8.1 0.33 5 37.1 0.035 4
116 9.1 0.27 5 30.4 0.034 4
117 10.1 0.34 5 4.2 0.007 3
118 6.1 0.58 5 16.0 0.012 4
119 4.8 0.17 6 8.6 0.011 4
120 3.5 0.11 6 12.2 0.008 6
121 9.7 0.55 6 15.3 0.051 5
Mean 6.2 0.26 11 16.6 0.019 11


Results

A contour plot of dissolved organic carbon (DOC) from 0 to 1000 dbar over the 1993-2005 time period is presented in Figure 36 and contour plots of dissolved organic nitrogen (DON) and phosphorus (DOP) from 0 to 1000 dbar over the 1988-2000 time period are presented in Figure 37 and Figure 38. DOC concentrations are typically about 70-110 µmol/kg at the surface and decreases to about 40-50 µmol/kg at 800 m. DON is typically 5-6 µmol/kg at the surface, decreasing to about 2 µmol/kg at 800 meters. DOP is about 0.2-0.3 µmol/kg at the surface and decreases to <0.05 µmol/kg at 800 m. All three organic nutrients exhibit substatial interannual variability.