26 June 1995 MATERIAL FOR P10 CRUISE REPORT: NUTRIENT ANALYSES I. Analysts, Equipment and Techniques Nutrient analysts on P10 were Mar¡a Consuelo Carbonell-Moore and Joe C. Jennings, Jr. from L. I. Gordon's analytical group at Oregon State University. The continuous flow analyzer used was an Alpkem Rapid Flow Analyzer (RFA), model 300. A Keithley data acquisition system was used in parallel with analog stripchart recorders to acquire the absorbance data. The software used to process the nutrient data was developed at OSU. All of the reagent and standard materials were provided by OSU. The methods are described in Anonymous (1985) and in Gordon et. al. (a & b). II. Sampling Procedures Nutrient samples were drawn from all CTD/rosette casts at stations 1 through 94 and at several test stations which preceded station 1. High density polyethylene (HDPE) bottles of approximately 30 ml volume were used as sample containers, and these same bottles were positioned directly in the autosampler tray. These bottles were routinely rinsed at least 3 times with one third to one half of their volume of sample before filling, and were thoroughly cleaned with 10 % HCl every two or three days. The nutrient samples were drawn following those for gases: helium, tritium, dissolved oxygen and carbon dioxide. In some instances, the nutrient sampling procedure was not completed for almost 2 hours after the CTD arrived on deck. At most stations, the RFA was started before sampling was completed to reduce the delay and minimize possible changes in nutrient concentration due to biological processes. Analyses were typically completed within three to four hours of the end of the CTD/rosette casts except at Stns 21 and 24 where analytical problems resulted in a delay of about 5 hours. III. Calibration and Standardization The volumetric flasks and pipettors used to prepare standards were gravimetrically calibrated both prior to and after the cruise. The Eppendorf Maxipettor adjustable pipettors used to prepare mixed standards typically have a standard deviation of less than 0.002 ml on repeated deliveries of 10 ml volumes. High concentration mixed standards containing nitrate, phosphate, and silicic acid were prepared at intervals of 4 to 7 days and kept refrigerated in HDPE bottles. During the "deadhead" steam at the beginning of the cruise, duplicate high concentration standards were prepared for each nutrient and compared to ensure that both gave the same response. For almost every station, a fresh "working standard" was prepared by precise dilutions of 20 ml of the high concentration mixed standard with low nutrient seawater. This working standard has nutrient concentrations which are 75 - 85% of those found in Deep and Bottom waters. A separate nitrite standard solution was also added to these working standards. Corrections for the actual volumes of the flasks and pipettors were included in the preliminary data. The WOCE Operations Manual calls for nutrient concentrations to be reported in units of micromoles per kilogram (uM/kg). Because the salinity information required to compute density is not usually available at the time of initial computation of the nutrient concentrations, our concentrations are always originally computed and reported as micromoles per liter. This unit conversion will be made using the corrected salinity data when it is available. IV. Equipment and analytical problems There were no major problems with equipment. One failure of a power supply module was resolved quickly by replacement with a spare module. V. Measurement of Precision and Bias A. Short Term Precision and Bias Throughout the cruise, replicate samples drawn in different sample bottles from the same Niskin bottle were analyzed to assess the precision of the RFA analyses. These replicate samples were analyzed as adjacent samples (one after the other) at the beginning and again at the end of each sample runs to help monitor deterioration in the samples or uncompensated instrumental drift. Our estimates of short term precision based on these replicate analyses are given below. The values given are the absolute mean differences between replicate pairs from the beginning to the end of each sample run. (Units are reported in micromoles per liter and as percentages of typical deep water concentrations.) Phosphate: 0.022 (<1.0%) Nitrate + Nitrite : 0.09 (<0.3%) Silicic acid: 0.3 (<0.3%) Nitrite: 0.02 (<2.0%) B. Longer Term Precision: On most of the sample runs during P10, an "old" working standard from the previous station was run with the "new" working standard which had been freshly prepared. The "old" standards were kept refrigerated in plastic bottles. The average age of the "old" standards when reanalyzed was eight hours. We calculated the difference in absorbance (peak height) between the new standards and the old standard which were run immediately after them. These differences, with regard to sign, were tabulated and analyzed statistically. The results were converted to concentration units by multiplying the difference by the mean sensitivity factor for each nutrient (Table 1). Based on these statistics, it does not appear that significant degradation of the working standards occurred in the 3 to 8 hour time frame between stations. Table 1. Differences between working standards at adjacent stations. Differences are expressed as "new" standard minus "old", and are given in concentration units (şM). The number of comparisons used for these statistics was 87. Phosphate Nitrate Silicic acid Nitrite Mean, (uM/l) -0.008 -0.013 -0.09 -0.013 wrt sign: RMS dev : 0.009 0.095 0.30 0.032 VI. Comparison with other data. We made comparisons of the P10 nutrient data with data from several other cruises. Where possible, groups of several stations were selected where cruise tracks crossed or were parallel and the nutrients were then plotted against potential temperature (theta). The data we used came from the 1973-1974 GEOSECS cruise, the 1985 WEPOCS I cruise, and the 1989 WOCE section along 10 N. The nutrient data from these cruises was collected either with the Technicon AutoAnalyzer II (GEOSECS and WEPOCS) or the Alpkem RFA 300 (10 N and P10). A. Nitrate The deep and bottom water P10 nitrate concentrations tend to be somewhat lower than the historical data we used for this comparison. The difference is about 0.3 uM between the deepest P10 and WEPOCS I samples, 0.5 uM between the P10 and both the 10N and 24N data, and as much as 1.0 - 1.5 uM at the nutrient maximum (ca. 2300 db) between the P10 nitrates and GEOSECS stn 224. Below about 3500 db, the GEOSECS nitrates are only 0.5 to 0.75 uM higher than the P10 data. There is more overlap of the P10 nitrate/theta envelopes with all of the historic data in the upper water column. Relative to the deep water concentrations, the agreement between cruises is within 1 - 2 % except at the nutrient maximum in the GEOSECS stn, where the difference is as much as 3.5 %. B. Phosphate The deep phosphate/theta envelopes of the P10 data overlap with those of the WEPOCS I, 10N and 24N cruises. GEOSECS stn 224 plots mostly within the P10 envelope with the deepest GEOSECS samples about 0.03 uM lower than the P10 data. The 24N data envelope tends to be on the lower side of the P10 envelope, but they do overlap. Above about 1.5 C, the 10N phosphate data are somewhat higher (0.02 - 0.07 uM) than the P10 data. As a percentage of deep water concentrations, these cruises agree within 1 - 2 %. C. Silicic acid (silicate) The pattern here is similar to that with nitrate; good agreement with the WEPOCS data and overlapping, but slightly lower silicic acid/theta envelopes than the other reference cruises. In the deep and bottom waters, the P10 data is within 1.0 uM of the all of the other cruises. At the silicic acid maximum (2300 db), the GEOSECS data is higher by ca. 4 uM while the 10N and 24N cruise data is 1 - 2 uM higher than the maximum concentrations determined on P10. The agreement is within < 1 % in the bottom water and 1 - 3 % at the silicic acid maxima. VII. Nutrient QC Notes: P10 Cruise A first pass QC check on the nutrient data was carried out during the P10 cruise, primarily by comparing vertical profiles and nutrient/theta relationships. During the post-cruise quality control phase, all nutrient data were rechecked using log notes and the analog stripchart recordings made at sea and by examining parameter/parameter plots for outliers. Any correctable errors have been identified and corrected as appropriate, and the data quality flags have been edited to conform to the definitions in the WOCE Operations Manual (WOCE Report No. 67/91). A detailed list of flagged data is given below. ---------------------------------------------------------------------------------------------------------- STN NUTRIENTS HYDRO PROBLEM FLAG # AFFECTED BTL # NOTED ASSIGNED ---------------------------------------------------------------------------------------------------------- 003 ALL 14 empty hydro bottle 9 007 ALL 18 empty hydro bottle 9 015 ALL 11 empty hydro bottle 9 015 ALL 21 empty hydro bottle 9 016 N+N, PO4 4 Low; oxygen and Salt flagged; bad bottle? 3 016 N+N, PO4 14 Low; oxygen and Salt flagged; bad bottle? 3 017 ALL 1 empty hydro bottle 9 017 ALL 20 empty hydro bottle; row missing in file. It should be flagged with 9's and not deleted 9 019 ALL 24 empty hydro bottle 9 020 ALL 5 empty hydro bottle 9 021 ALL 35 Noted as leaker 4 022 N+N 2,4,6-8,12,13,15 Out of profile 3 023 N+N 1,2,6,8-16 Cd coil dying, crummy peaks 3 025 ALL 17 empty hydro bottle 9 025 ALL 5 Bad bottle 4 025 ALL 11 Noted as leaker 4 025 ALL 3 empty hydro bottle 9 025 ALL 29 Noted as leaker 4 026 ALL 3 empty hydro bottle 9 026 ALL 11 empty hydro bottle 9 026 ALL 29 Leaker 4 026 ALL 1 empty hydro bottle 9 026 ALL 20 Leaker? 3 027 ALL 3 empty hydro bottle 9 027 ALL 13 empty hydro bottle 9 028 ALL 1 didn't sample, leaking badly 9 028 ALL 21 didn't sample, leaking badly 9 028 PO4 3 Too high 3 029 ALL 29 didn't sample, leaking badly 9 029 ALL 13 didn't sample, leaking badly 9 029 ALL 11 didn't sample, leaking badly 9 030 ALL 11 too low, Salt flagged, O2 suspicious 9 030 ALL 26 out of water, did not sample 9 030 ALL 31 didn't sample, leaking badly 9 031 ALL 13 Noted as leaker 4 031 ALL 11 empty hydro bottle 9 032 N+N 11,14 Low on theta plot, no obvious problems 3 033 N+N 13 Low in theta plot, no obvious problems 3 033 PO4 8-17 Possible shift; can't be corrected 3 034 ALL 7 didn't sample, leaking badly 9 035 ALL 11 didn't sample, leaking badly 9 036 ALL 14 empty hydro bottle 9 036 ALL 5 Noted as leaker 4 041 ALL 21 Noted as leaker 4 042 ALL 22 High? Salt bad 3 043 ALL 21 didn't sample, leaking badly 9 043 ALL 33 didn't sample, leaking badly 9 043 N+N 22 High? Salt bad 3 044 ALL 13 Noted as leaker;no notes in logsheet 4 045 N+N 19 High 3 047 ALL 25 didn't sample, leaking badly 9 048 ALL 33 didn't sample, leaking badly 9 050 ALL 11 Noted as leaker 4 050 ALL 5 Noted as leaker 4 051 ALL 3 Bad bottle, petcock open 4 051 ALL 5 Bad bottle, petcock open 4 052 ALL 8 Leaker? 3 058 ALL 17 Noted as leaker 4 058 ALL 5 Noted as leaker 4 059 ALL 5 Noted as leaker 4 061 ALL 4 didn't sample, leaking badly 9 062 ALL 27 Leaker, low 4 065 ALL 1 Noted as leaker 4 069 ALL 27 Noted as leaker, high 4 070 ALL 9 Noted as leaker 4 071 ALL 9 Noted as leaker 4 071 Si(OH)4 16-18 Low 3 071 ALL 15 Noted as leaker 4 072 ALL 21 Noted as leaker 4 074 ALL 29 Noted as leaker 4 077 N+N 6-21 High; apparent baseline shift 3 079 PO4 18-23 Very high, no obvious reason 3 079 ALL 24 Leaker 4 080 Si(OH)4 16,17 Low 3 081 ALL 4 Noted as leaker 4 082 ALL 33 High, no reason, oxygen flagged 3 082 ALL 15 Noted as leaker 4 086 ALL 11 Noted as leaker 4 088 ALL 21 Noted as leaker 4 Note: "Noted as leaker" generally refers to samples which were drawn and analyzed, but were noted in the Small Volume Sample Log as suspected of leaking. This data is reported, but is considered to be "bad". By contrast, "didn't sample" generally refers to hydro bottles which were clearly identified as leaking early in the process of drawing samples and which were therefore not sampled. VIII. References Anonymous. 1985. RFA-300 Rapid Flow Analyzer Operation Manual. Preliminary. Alpkem Corporation, Clackamas, Oregon. Looseleaf binder, unnumbered pages. Gordon, L.I., J.C. Jennings, Jr., A.A. Ross and J.M. Krest., A suggested protocol for continuous flow automated analysis of seawater nutrients (phosphate, nitrate, nitrite and silicic acid) in the WOCE Hydrographic Program and the Joint Global Ocean Fluxes Study. Available from the US WHP Office or the authors. Gordon, L. I., J. Krest, and A. Ross, b. (in preparation), Reducing temperature sensitivity in continuous flow analysis of silicic acid in seawater.