We’re moving further east and knocking down transects slowly but surely. The OPC will get it’s wet test in about 30 minutes, but the deck tests have been successful, so we’re [still] cautiously optimistic we’ve got the problem licked. Below you will see the data plots for all our completed transects. Not too much new; the hypoxia at the seaward end of the transects is still there, hugging the bottom. Chlorophyll and CDOM fluorescence are generally low, with some elevation near shore. There is definitely a different water mass in the deep part of the transect, as observed in the high salinity, lower temperature water.
Another way to look at this, and a way that the physical oceanographers often use is to look at a plot of the temperature vs. salinity, or TS plot, as shown in the graph below. Temperature and to an even larger degree Salinity are known as “conservative tracers”. This means that they do not change much as the water moves around. Things like plankton concentrations, chlorophyll, and oxygen change based on chemical and biological properties much faster than both salinity and temperature. Salinity is an estimate of the dissolved ions in seawater and the ratios of these ions are remarkably stable throughout the world oceans. Also, while some of these ions are used by organisms, the vast majority of ones that make up the salinity measurement: chlorine, sodium, and others, are not used widely by organisms and so their abundance in seawater is often diagnostic of a particular water mass. Temperature, which can change with atmospheric warming, actually does not change too much be cause water has such a high specific heat. This simply means water retains much of its heat, more so than things like metals which heat up much faster in the sun.
Two other things jump out from the TS plot below. You may notice I’ve color coded the
dots with the oxygen concentration, and that nearly all of the high values are in the high salinity, lower temperature water. This can be seen in the “curtain” plots like the one above, where the low oxygen blobs correspond with the high salinity, low temperature regions. The other thing is that there is a distinct inflection point in the upper right part of the data. This manifests as a quick turn in the data and suggests that there are likely three water water masses stacked up in the water column: one is low temp and high salinity, pulling the data to the upper left corner; one is at that inflection point – high salinity and moderate to high temperature; and one is low salinity and moderate temperature. They mix along the lines connecting those points, and so if you pinpoint the properties identifying each water mass, using simple vector algebra you can determine the amount of water from each water mass contributing to any given “parcel” of water on the TS diagram.
It took me a while in this business before I could really understand these plots, but now that I do they can really tell us a lot about the water in which we’re working. I like to think my contribution of color-coding the plots by oxygen help bridge the gap between physics and biology, though I’m sure I’m not the first one to do that.
** UPDATE: As I am writing this, the Scanfish / OPC is heading back into the water, here at 28° 54.504′ N, 92° 23.214′ W, and so far so good. We’ll be keeping an eye on it but we’re getting good data and we’re in the water. Our fingers will remain crossed for a while though, you can count on that.