Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i at Manoa
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Dissolved Inorganic Nutrients
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. The average precisions during 2016 from duplicate analyses are given 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 28 years of the program. In addition to standard automated nutrient analyses, specialized 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.
Calibration, Data Reduction and Calculations
The calibration of dissolved inorganic nutrient determinations in the auto-analysis of seawater samples is performed using standard solutions containing dissolved N, P and Si salts. A nutrient stock solution is prepared by dissolving dried (50oC, 48 hr) analytical grade reagent chemicals with DIW in 1 L glass volumetric flasks containing 1 ml of chloroform. Once dissolved, this stock solution is immediately transferred into 1 L HDPE bottles and stored at room temperature in the dark. The reagent chemicals and concentrations are: KH2PO4 (1 mM), KNO3 (1 mM) and Na2SiF6 (1 mM).
Working standards are prepared daily in PMP volumetric flasks using gravimetric dilutions of the nutrient stocks in LNSW. The PMP flasks are thoroughly rinsed with DIW after use. The LNSW is 0.2 µm filtered open ocean surface seawater from Station ALOHA that is kept in the dark at room temperature for at least six months prior to use. This technique provides a mixed standard solution of N, P and Si that is matrix-matched with the seawater samples and any cross-nutrient interference effect should also be accounted for.
All seawater standard absorbance peaks are corrected for the absorbance of the seawater diluent (LNSW). All seawater sample peaks are corrected for the refractive index absorbance for each unique nutrient detection system. The refractive index corrections represent the increase in absorbance that is due strictly to the presence of dissolved salts in seawater when compared to the DIW baseline. These corrections are determined by running alternating seawater (LNSW) and DIW cups through the auto-analyzer with only non-color producing reagents online. DIW is run through the color producing reagent lines.
Wako CSK's and OSIL Nutrient Standards are measured in each channel as reference materials to validate sample measurements. The Wako CSK's are manufactured in 30.5 ‰ NaCl and are measured directly. The OSIL nutrient standards are manufactured in DIW and diluted using LNSW to the same concentration as the Wako CSK for direct comparison (40 µM for NO3, 2 µM for PO4, and 100 µM for Si). Due to the high price of the Wako CSK's, they are run only once per sample run. The OSIL check standards are run twice, once at the beginning and again at the end of each sample run.
Both the Wako and OSIL standards are used as checks of not only the sample analysis, but as checks of each other. Measured reference material values that are more than 2% from the expected concentration of the reference solutions are scrutinized and cross checked with the other reference material to determine if the analysis is correct. In most cases, both reference materials are within the accepted limits.
In the case of Phosphorus, literature shows that the Wako CSK provides low (~7%) return of the expected concentration value, therefore a measured value of 1.8 µM for a 2µM CSK is considered acceptable, and a higher than 2% difference from the expected 2 µM concentration is accepted. The use of a PO4 OSIL reference was introduced to have a reference material that produced a more reliable 2 µM concentration result.
In the case of NO3, the addition of a check standard containing only NO2 is also analyzed to check the cadmium column efficiency. If the CV of the NO2 check standard is more than 2% from the expected 40 µM value, the run is aborted and the cadmium column chips are regenerated.
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 about 200 and 600 dbar (25.75-27 kg/m3) (Figure 23). Most of the variations seen in these data are associated with vertical displacements 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, July-September 1999, mid-2002, late-2003, late-2007, mid-2008, late-2012, mid-2013, and late 2014 to early 2015. 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 (Nolan, 2008). 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 concentrations (Figure 31). A [nitrate + nitrite] decrease of similar magnitude was observed in 2013-2014, reaching record low levels by the end of 2014, with a corresponding increase in oxygen concentration, and followed by a sharp [nitrate + nitrite] increase throughout 2015 and 2016, and an oxygen decrease in early 2015.
Intermediate water SRP (between 27.0-27.8 kg/m3) reached low 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). The SRP maintained relatively low values throughout early 2001, when it increased sharply and maintained an increasing trend until 2005, to then start a decreasing trend ending in 2010 to values similar to those observed during 1997-2001. A sharp decrease was seen in 2013, corresponding to the [nitrate + nitrite] decrease mentioned above, but increased to 2012 values during 2014, and continued increasing during 2015, to stabilize in 2016. Decreases in phosphate in the deeper waters could persist for long 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.