Dissolved inorganic nutrients

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	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

Samples for the determination of dissolved inorganic nutrient concentrations (soluble reactive phosphorus, [nitrate+nitrite], and silicate) were collected as described in Tupas et al. (1993). Up until February 2000, analyses were conducted at room temperature on a four-channel Technicon Autoanalyzer II continuous flow system at the University of Hawaii Analytical Facility. Starting March 2000, samples have been run using a six-channel Bran Luebbe Autoanalyzer III. A summary of the precision of analyses for 2006 is shown in the Table below. Figure 20, Figure 21, & Figure 22 show the mean and 95% confidence limits of nutrient concentrations measured at three potential density horizons for the past 18 years of the program. In addition to standard automated nutrient analyses, specialized chemical methods are used to determine concentration of nutrients that are normally below the detection limits of autoanalyzer methods.

Between 2001 and 2004, the HOT nutrient program underwent substantial changes, including switching analysts twice, eventually establishing an analytical nutrient laboratory centered around a six-channel Bran Luebbe Autoanalyzer III. In an effort to continue to provide high-quality nutrient data to the scientific community during this transition period, we made the decision to ship nutrient samples to Oregon State University for nutrient analyses. The decision to send samples to OSU was reached after a blind nutrient analyses comparison was conducted among several oceanographic analytical laboratories (including UW, SIO, OSU and UH). Each laboratory received triplicate nutrient samples collected at 4 depths (750, 1200, 2200 and 4200 m) on HOT-163. Using our historical nutrient data as reference, we compared analyses of NO₂+NO₃, and PO₄ by these laboratories; analyses conducted by OSU were within our historical nutrient concentration climatology. As a result, samples from > 200 m depth from HOT 127-166 were shipped to OSU for analyses.

The OSU nutrient facility uses an AutoAnalyzer II manifold with 5 cm flow cell for PO₄ analyses, and an Alpkem RFA 300 system for analyses of NO₂+NO₃.

Results

Figure 23, Figure 27 and Figure 29 show [nitrate+nitrite], SRP, and silica at Station ALOHA plotted against pressure and Figure 24, Figure 28 and Figure 30 show them against potential density. The nitricline is located between 200 and 600 dbar (25.75-27 kg/m³) (Figure 23). Most of the variation seen in these data is associated with vertical displacement of the density structure, and when plotted versus potential density, most of the contours are level. Recurrent events with increasing [nitrate+nitrite] can be seen throughout the series between 25-26.25 kg/m³ (Figure 24). These events are accompanied by a decrease in the oxygen concentrations (Figure 14). The most obvious events occured in March-April 1990, January 1992, May 1992, February-March 1995, early 1996, mid- to late 1997, and July-September 1999, mid-2002, and late-2003. These events can likely be attributed to mesoscale features such as eddies. It is possible for eddies to transport water with different biogeochemical characteristics from distant sources into the region of Station ALOHA (Santiago-Mandujano and Lukas, in prep). The SRP variability is similar to the [nitrate+nitrite] in the upper water column (Figure 27).

During 1996, the intermediate waters between 27.0-27.8 kg/m³ recovered from anomalously low [nitrate+nitrite] with was observed during 1995 (Figure 25). This anomaly is apparent in a time-series of mean [nitrate+nitraite] between 27.0-27.8 kg/m³ (Figure 31). A decrease in [nitrate+nitraite] began in late 1994, with a comparable increase from mid-1995 through early 1996. The maximum decrease appears to be about 1 µmol/kg below 27.5 kg/m³ where nitrate concentrations are about 40 µmol/kg. This decrease appears to be real as it does have coherence over time. A precision estimate of 0.3% has been made for [nitrate+nitraite] measurements involving the high concentration samples associated with intermidiate water (Dore et al., 1995). This translates to a precision of roughly 0.12 µmol/kg for samples with a concentration of 40 µmol/kg. Hence, the 1 µmol/kg decrease seen during 1995 is well within the precision level for the concentrations observed. However, the amount of the decrease could be approaching the accuracy limits of [nitrate+nitraite] measurements. This low [nitrate+nitraite] episode is accompanied by an increase in oxygen concentrastions (Figure 31).

Intermidiate water SRP (between 27.0-27.8 kg/m³) reached lowest values in early 1997, after a decreasing trend established in early 1994 (Figure 26). A time-series of mean SRP in this layer shows this trend clearly (Figure 31). Decreases in phosphate in the deeper waters could persist for longer periods of time as the oceanic ecosystem associated with Station ALOHA has been hypothesized to be phosphorus limited in recent years (Karl, 1995). Oxygen concentrations between 27.0-27.8 kg/m³ vary during the decrease of phosphate from early 1994 through 1997 (Figure 31) without any apparent correlation.