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Pump and Dump

SO GasEx Blog - Fri, 03/28/2008 - 17:35

By Dale Hubbard, Oregon State University

There are approximately 60 people, including the crew and the science party, aboard the Ronald H. Brown. The good folks in the galley serve three meals each day, there are snacks available after hours, and there’s a seemingly unlimited supply of things to drink. This is a very good thing (it certainly beats going hungry and being dehydrated), yet it poses a quandary: once we’ve metabolized all of that food & drink, what to do with all of our waste?

There are basically two types of sewage generated aboard the vessel: grey water, which originates from sinks, showers, laundry, and the dishwasher; and black water, which originates from the toilets. (The engines also generate wastewater of a more industrial nature, e.g. oily waste, but this is contained separately and later pumped ashore when the ship is in port, as oily waste is illegal to discharge at sea.) Aboard the Ronald H. Brown gray water and black water are commingled and contained within a holding tank of approximately 5000 gallons capacity. Sewage aboard the Ronald H. Brown is not treated—it is mechanically ground up en route to the tank, then the contents are pumped overboard. Once the sewage tank has accumulated 4400 gallons, it’s time to break off from our study site and make a run for it.

Since our project involves continuously observing a relatively small patch of water in order to collect time-series measurements, we must move at least 3 nautical miles off-site before the ship evacuates the contents of its sewage tank. Sewage contains an assortment of compounds that serve as phytoplankton nutrients, so dumping the holding tank inside of our study area would drastically alter the biological and chemical processes within. Therefore, at least twice each day we must leave the patch to undertake what many aboard refer to as a “Pump and Dump.”

Unfortunately, several times during the course of a Pump & Dump, the ship’s underway seawater line entrained some of the sewage. This caused caused a great deal of excitement in the lab, as steaming through this particular hydrographic feature generates some of the most dramatic measurements observed during the cruise. This secondary “patch” is exemplified by elevated pCO2 and nitrate (both byproducts of the degradation of metabolic waste) and elevated temperature (see pictures below).

On at least one occasion we’ve been able to resolve a sewage signal in the data from our underway transmissometer, an instrument which essentially measures water clarity (or lack thereof) by passing a beam of light through a sample stream of water. Eeeewwww…

In order to pick these Pump and Dump events out when we’re processing the data long after the cruise is over, it’s important that we remember to log them carefully.

fig-1.png

Effect of “Pump & Dump” on pCO2 and temperature. Pump and dump signal, from approximately 12:00-12:30, manifested in approximately 20 ppm increase in pCO2 (white trace) and approximately 0.5o C increase in surface seawater temperature (red). “H2O” parameter is water vapor measured inside of shipboard pCO2 equilibrator and also reflects higher surface seawater temperature. The surface salinity values are highly variable (and rather irrelevant) because it had been raining.

fig-2.png

Effect of “Pump & Dump” on NO3 concentration. “Amplitude” values represent voltage generated by photodiode detector (note scale is reversed). The ~0.1 V decrease between approximately 12:00 -12:30 represents an approximately 3mM increase in NO3. The variation in amplitude at approximately 13:30 is a standard sequence—a blank, 20 mM, and 9.5 mM KNO3 solutions.

The Importance of Quenching

SO GasEx Blog - Thu, 03/27/2008 - 17:33

By Bob Vaillancourt, LDEO

The most common measurement to make for a biological oceanographer is for chlorophyll a (abbreviated “Chl a”). This is the pigment that is common to all phytoplankton, so is a convenient proxy for the size of the plant crop in the ocean at a given time. We have two ways of measuring Chl a: by direct measurement of its concentration on discrete water samples captured in a bottle, or indirectly, by measuring the amount of fluorescent light the plants emit.

Fluorescence is a phenomenon by which a material (in this case, Chl a) absorbs light energy, then re-emits a portion of this energy as light at a lower energy. In the case of phytoplankton, their Chl a (and other pigments) absorb energy in the more energetic blue-green portion of the spectrum, and re-emit (or “fluoresce”) in the less energetic red portion. This light is too weak to observe with the human eye, so we use special instrument called fluorometers to measure it.

Typically, we lower a submersible fluorometer over the side of the ship to measure the “vertical profile” of Chl a fluorescence. If we don’t have time to stop the ship, then we can make “underway” measurements by piping surface seawater into the ship’s lab and running it through benchtop fluorometers. We then look at this profile (or time-series) as a “proxy” or suitable substitute, for phytoplankton concentration. As chlorophyll concentration increases, generally so does the level of fluorescence- but not always. So it is important to realize the situations when chlorophyll fluorescence is not a good proxy for concentration. We most often see this in surface waters where light levels are highest.

Figure 1 shows a time series of surface sunlight (upper graph, blue line) measured on the deck of the ship, and Chl a (lower graph, green and black symbols) measured at a depth of 3 meters, near the surface. As sunlight increased throughout the day, we observe a suppression, or “quenching” of Chl a fluorescence by about a factor of nearly two between early morning and mid-afternoon (approx 15:00 GMT). We see this happening on rather short time scales too, as dips in sunlight, perhaps caused by the passing of clouds overhead, cause momentary relaxation of quenching in the fluorescence traces at approximately 1200 and 1700 hrs (red arrows). The Chl a concentration, meanwhile, showed a nearly constant level throughout the day. Quenching of Chl a fluorescence causes a huge departure from the normal co-variation between concentration and fluorescence. But does this happen at all depths?

The next figure shows similar data, but taken on two vertical profiles: one during mid-day (Station 7, left graph), and the other near mid-night (Station 4, right graph). You see that during the daytime, quenching of fluorescence occurs down to about 20 meters depth, and decreases fluorescence by a factor of about ten, when compared to Chl a concentration measurements made on captured samples. During night-time, however (right graph), the Chl a concentration data (green symbols) and Chl a fluorescence track each other nicely.

Another way of viewing these data is by property-property plots (see figure 3). Here we see that when plotted this way, the night-time data show a nice linear relationship (although over a limited range), but the day-time data, corrupted by fluorescence quenching show no such correlation. So, if one were to use the Chl a concentration data to calibrate their chlorophyll fluorometers, it makes sense to use nighttime data only.

image-1.png

Solar Irradiance (upper) and corresponding Chl a (lower) level changes through the day on March 16. Green symbols are concentration values (units of micrograms per liter) and black dots are fluorescence values (units of voltage). As sunlight increases through day, the fluorescence from chlorophyll is quenched, without a corresponding decrease in concentration.

image-2.png

Two vertical profiles of Chla fluorescence (black symbols) and corresponding concentration values (green symbols) for mid-day station 7 (left graph) and mid-night station 4 (right graph). During daytime, Chl a fluorescence in the upper 20 meters is quenched and does not track Chl a concentration. The quenching phenomenon disapears during the night-time.

image-3.png

Property-property plots of Chl a concentration (x-axis) versus Chl a fluorescence (y-axis) for mid-day (black symbols) and mid-night (red symbols) from Figure 2. Night-time fluorescence, devoid of quenching, shows linear relationship to Chl a concentration.

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