Low-level nutrients

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Mixed layer nutrient concentrations at Station ALOHA are at or well below the detection limits of the autoanalyzer methods. Alternative high-sensitivity analytical techniques were used to measure the nanomolar levels of [nitrate+nitrite] and SRP in the upper water column.

Low-level Nitrate

The chemiluminescent method of Cox (1980) as modified for seawater by Garside (1982) was used to determine the [nitrate+nitrite] content of near surface (0-200m interval) water samples (Tupas et al., 1993). The limit of detection for [nitrate+nitrite] was approximately 2 nM with a precision and accuracy of ± 1 nM (Dore et al. 1996). Figure 32 shows the profiles obtained from our low level [nitrate+nitrite] analyses at Station ALOHA during 2006. The upper 100m is generally depleted in [nitrate+nitrite] with values usually not exceeding 5 nmol/kg. A contour plot from 0-100 dbar during the 1989-2006 time period is shown in Figure 33.

Low-level Phosphate

Dissolved inorganic P (DIP) was analysed using the MAGnesium Induced Coprecipitation (MAGIC) method (Karl and Tien 1992). MAGIC improves both the sensitivity (detection limit ~ 1 nmol P l^-1) and the precision of the low-level P (LLP) determination in oligotrophic waters. Typical precision estimates for triplicate determinations of SRP are from 1-3%. The MAGIC SRP measurement is also corrected for arsenate interference of the molybdenum blue colorimetric procedure (Johnson 1971), unlike the standard autoanalytical method. Figure 34A and Figure 34B presents the low-level SRP data from 2006. At depths shallower than 100m, SRP is typically less than 100 nmol/kg. A contour plot of LLP from 0-100 dbar during the period 1989-2006 is shown in Figure 35. Several trends are evident, including a general reduction in DIP concentrations from >90 nmol/kg in 1989-1990 to <30 nmol/kg in 2001. The 0-100m depth integrated inventory was reduced from a high of >10 mmol P m^-2 to a low of <2.5 mmol P m^-2. It has been suggested that this long-term, decadal-scale reduction in DIP is a result of selection for N₂ fixation microorganisms with an attendant shift from a N-controlled to a P-controlled ecosystem (Karl et al. 2001). Despite this general reduction in DIP concentration, there appear to be aperiodic injections of DIP (for example in early 1995 and less dramatic increases in 1998, 2000 and 2001). The mechanism(s) controlling these inventory enhancements is not well resolved in the HOT field data.