Science Results : Daily Update

Daily Update | Current SEA Research

July 11, 2010

By Giora Proskurowski

The end of the expedition is a day-and-a-wake-up away, meaning that we have a lot of loose ends to wrap up before tying the boat up in St. George's, Bermuda. One set of analysis that we typically do on SEA Semesters towards the end of the trip is chemical analysis of nutrients, namely nitrate and phosphate. These two dissolved molecules, the most common biologically available forms of nitrogen and phosphorous, are essential for the growth of phytoplankton in the ocean, and are often the main determinant of the productivity throughout the food chain.

Living plants use nitrate as a source of nitrogen for building proteins, and phosphate as a source of phosphorous that is the centerpiece of the energy transfer molecules ATP and ADP. One interesting phenomenon is that there is a fairly rigid 16:1 ratio between nitrogen and phosphorous in marine phytoplankton. Thus, by measuring the nitrate and phosphate in the water column we can learn a lot about the conditions for phytoplankton growth, as well as how the food chain responds to the physical movements of water with different nutrient compositions.

For the past 36 hours the lab has been analyzing samples from all our surface stations. Starting on Saturday night, we ran 102 phosphate samples plus standards, then rolled into chlorophyll-a analysis, and this morning started in on 102 nitrate samples plus standards. The nutrient analyses are done by colorimetric analysis, in which various chemical solutions react with our previously thawed samples. The resultant reactions produce a fluid that is bluish in the case of phosphates and pinkish in the case of nitrates. Using a spectrophotometer we can quantify exactly how blue or pink these solutions are, and – when compared with standards – the concentration of the nutrient of interest.

While at this point it is not known if there will be useful information in these analyses, it is extremely valuable to know as much as possible about the environmental conditions from where our plastic samples were collected. By placing each one of our surface net tows in an oceanographic context, we can begin to decipher the complex physical, chemical, and biological process associated with this system.