Salinity samples from HOT cruises are generally measured over a 3-5 day period during the week after the cruise. Prior to starting a measurement run, the salinometer is standardized using IAPSO standard seawater ampoules. After standardization, two or more substandard seawater samples are measured until a value is repeated within 0.00002 twice-the-conductivity-ratio units from the previous measurement. The result serves as a baseline against which to compare subsequent measurements of the substandard over the period during which the salinity samples are measured. The salinity of the substandard does not change significantly over the course of a week (see Kennan, 1992 analysis below), so that changes in its measured salinity from day to day represent drift or jumps in the Autosal's electronics, which may need to be corrected. Corrections to the Autosal measurements are usually applied when the substandards differ from the substandard session's mean by more than +/-1 mpsu.

In his "Summary of Lab-Standard Salinity Measurements from 12/27/90 to 8/11/92", Sean Kennan (1992, Appendix R) compared mean substandard salinities from substandard batches #1-5. He concluded that the standard deviation of substandard measurements during a session was always less than 1 mpsu, and after July 1991, less than or equal to 0.5 mpsu. His results showed that substandard water was indeed adequate to monitor drift of the Autosal during our measurement sessions. His calculations of substandard drift over time showed that substandard water has an insignificant drift of -0.4 0.8 mpsu over 100 days. He attibuted the 0.8 mpsu scatter to variations in the Autosal's electronics, or to differences between IAPSO standards.

Figure 5 shows the mean salinities of substandard seawater batches for each salinity measurement run over time (one run per cruise). A line connects mean salinities of the same batch, and error bars (in Figure 5B) indicate the standard deviation of each substandard run.

Figure 5. Mean salinities of the Substandard batches 1 through 31 used during the HOT project (a), and after removing batches 1-3, 7a, 11, and 13 (b). The circles indicate the mean of each salinity measurement run, and the bars in (b) are plus minus one standard deviation of the mean. Solid lines connect mean salinities from the same batch.

Most of the substandard batches have similar salinities between 34.4 and 34.5. Seawater used for batches #1-3, #7a, #11, and #13 had mean salinity values that were very different from the rest of the substandard batches. For substandard batches #1-3, mercuric chloride was used to prevent biological growth and as it precipitated out of the seawater, the salinity of the substandard changed (Kennan 1992, Appendix R). Batch #7a was made as an emergency substandard for HOT-52 because batch #7 was running low. Batch #7a was prepared from the remaining seawater used to make batch #7, and thus had a higher salinity due to evaporation in the plastic carboy over time. The cause of lower salinities for batches #11 and #13 are likely due to mixed seawater from depths other than 1000 m. Batch #11 had a lower salinity than other batches because 10 of the 60 liters used to make the batch came from 500 m (salinity minimum) instead of 1000 m. Batch #13 also had a lower salinity than other batches. Batch #13 was collected during HOT-76, but only five Niskin bottles (12 L) were fired at 1000 m, providing an insufficient amount of seawater for the substandard batch. We hypothesize that batch #13's low salinity is due to mixing seawater collected from a different depth, but there is no documentation regarding the preparation for this substandard batch.

Batches #1-3, #7a, #11, and #13 have been removed as outliers in Figure 5B, which shows a closer look at the range of mean substandard salinity variability. The range of the mean substandard salinity variability is about 0.04 psu. For comparison, Figure 6 shows the mean CTD salinity variability at Station ALOHA from HOT 1-140 at 1000 dbar, the depth from which substandard water is sampled.

Figure 6. Mean CTD salinity at 1000 dbar during HOT cruises 1 through 140.

Although most of the substandard batches' salinities correspond to the salinities in Figure 6, many of them have higher salinties than those observed at 1000 dbar (e.g. batches 5, 6, 9, 10, 20). This discrepancy may be due to evaporation of the sample in the plastic carbuoy before preparing the substandard, or due to water of a higher salinity level being included in the substandard. However, no documentation was found in this regard.

Though the salinity of the substandard does not change significantly during the course of a salinity measurement session, the mean salinity of substandard seawater from the same batch sometimes changed between sessions. For many of the batches, the salinity tends to decrease towards the end of the substandard batch lifetime. We hypothesized that because saltier water tends to settle on the bottom of the carboy and the water is drawn out from the bottom, what remains at the end of the substandard batch lifetime is fresher water that settled higher up at the beginning of the substandard batch lifetime. Some of the error bars are very large for some of the cruises, which could be because bad substandard measurements were included in the data set. We suggest looking carefully at the data with large error bars to eliminate outliers before using these data for further analysis.

Biological growth in substandard water was noted in batch #20 in the HOT-108 Salinity Measurement Report (see Don Wright's update report, Appendix S). Biological growth did not seem to interfere with the substandard salinity during HOT-108 measurements. There was concern for HOT-109, however, and thus the group decided to take nine samples of the affected substandard and store them away from light. They also filtered the remaining substandard with a 142 mm Gelman 0.2 um pore size membrane filter, drew six samples and stored them away from light. A comparison between the filtered and unfiltered samples showed a difference of approximately 1 mpsu higher for unfiltered samples. It is important to learn from this experiment the gravity of keeping the substandard batch covered with dark plastic bags and away from light, for biological growth may affect the adequacy of the substandard water as a means to monitor drift of the Autosal.

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