Sunday, August 4, 2019

A magnet for hurricanes: Atmospheric nitrogen deposition in the ocean

Experiments testing the effects of rain water on ocean plankton, Marilyn on the R/V Cape Hatteras, 1994
Marilyn as Drowned Rat, Hurricane waves, 1995
In 1989, out of the blue I received a phone call from Hans Paerl of the University of North Carolina’s Institute of Marine Science. Hans has devoted his career to studying the harmful effects that excess levels of inorganic nitrogen in estuarine and coastal waters can have on cyanobacteria and other algae. He had read the papers I’d published on the Delaware estuary and wondered if I would join him in a study on the effects of acid rain on the coastal zone. At that time, the U. S. Environmental Protection Agency was monitoring acid rain and its nitrate and ammonium concentrations across the entire continental United States. Their monitoring program did not extend past land, leaving a gap in our understanding of how important acid rain might be for altering coastal ecosystems. We brainstormed and concluded that stable isotopes might be able to distinguish sources of nitrogen from rainfall vs. fertilization or natural nitrogen fixation.
         What followed was nearly a decade of research, the exchange of graduate students and postdocs, and valuable shipboard and leadership experience for me. Our work started without government funding. Hans shipped liters of frozen rainwater to Washington, DC, where I isolated the ammonium and nitrate using methods worked out for the Delaware project. Hans was, and probably still is, a relentless collaborator. He thinks of ideas on the fly, likes to do his writing in a group, and always plots his next scientific move. We submitted three NSF proposals before one was finally funded in 1993 to carry out this work properly. In 1994, we published our first paper with measurements of the δ15N in ammonia and nitrate in rains collected in North Carolina. We discovered that the nitrogen isotopes of ammonium was more distinctly different from both the those of nitrate and dissolved organic nitrogen. We carried out mesocosm experiments in which four liters of estuarine waters were incubated with an aliquot of rainwater, then particulate organic matter—the algae and bacteria-- was filtered and analyzed. Our results showed that phytoplankton used the nitrogen from rainwater resulting in increased primary productivity (Paerl and Fogel, 1994).
         Nitrogen in rainfall in coastal areas could be considered as “new” nitrogen in areas of the ocean where phytoplankton production was nitrogen limited. Our work extended to the Sargasso Sea, a region in the central gyre of the North Atlantic Ocean. Over a period of three years, we conducted six cruises on the R/V Cape Hatteras from Beaufort, North Carolina, through the Gulf Stream, into the calm, warm waters of the Sargasso Sea. On our first cruise, Hans served as Chief Scientist, a position that requires 24 hour interaction with scientists, the crew, and most importantly the Captain. We sampled POM, nutrients, zooplankton, and floating Sargassum and Trichodesmium, while conducting onboard measurements of primary and bacterial productivity and dissolved organic matter concentrations. Postdoctoral researcher Carmen Aguilar and I filtered hundreds, if not thousands, of liters of seawater in a pressurized system using nitrogen gas.  In addition, Carmen and Hans’ students conducted mesocosm experiments on deck, as Hans and I had done previously.
          On our third cruise, I served as Chief Scientist. This particular cruise was filled with high adventure. In my first meeting with the Captain, I laid out my cruise plans to stop on station every 100 km to sample the water column. Captain XX looked at me with slight derision, then remarked that he used nautical miles, so what did 100 km mean in nautical miles? I was put in my place. With my tail between my legs, I retreated, converted units, then proposed we stop every 54 nautical miles until we reached the center of the Sargasso Sea. After only one day at sea, seriously rough weather began to affect our sampling. Some of the scientific party got sea sick. By late afternoon on the second day, our ship was required to “standby” to assist a sailboat with a snapped main mast, as the Coast Guard came to their rescue. By the end of that day, the Captain informed me that Hurricane Gordon, previously thought to have gone into the North Atlantic, had changed direction and was projected to intersect with our cruise track within 24 hours. We decided to head back to Beaufort rather than risk collision with the storm. I was disappointed.
         When we reached port, those who had been seasick were relieved to be off the rolling ship.  By next morning, the weather report showed that the storm had gone “safely out to sea” again, so we were cleared again for departure. Only half the scientific crew returned for the second leg of the voyage, but we left the dock in a hearty mood. Our strategy now was to compare the measurements we’d made before Hurricane Gordon to those taken after the storm had churned up surface waters. It wasn’t long before Gordon reversed course again, and turned back towards our ship. We made it out just to the edge of the Gulf Stream around 9 o’clock at night. Waves were crashing over the bridge--three stories up. The Captain, himself, was looking grim and said to me in his thick Southern accent, “Marilyn, we’ve got to turn around.” I said, “One more sample.”
         Lashed with a rope to the deck of the ship, I staggered outside in the wind and rain to collect that last sample, Station 17. As I came inside, I radioed to the bridge, “Ok, let’s go in.” Refrigerators, the scintillation counter, and freezers were flung back and forth like billiard balls. All of us were ordered to our staterooms, while the Captain was lashed to the ship’s controls. Years later, I can still feel the drama that made us a captive ship in a huge storm. Was it worth it?  Not many people have the chance to sample before and after a hurricane. Once we were safely back to port, we had the sense that Hurricane Gordon provided us with a unique scientific opportunity. Now, owing to global warming, hurricanes are more intense than they were 20 years ago. On one of our final cruises, as luck would have it Hurricane Marilyn intersected with our cruise track! Fortunately, we were able to dodge it without too much trouble.
         Subsequent cruises revealed that phytoplankton, which were primarily Prochlorococcus sp., were stimulated by both nitrogen and phosphorus, substantial constituents of continental rainwater source. On land, acid rain might be problematic, but in the open ocean, “fertilization” by atmospheric deposition could be a good thing (Paerl et al., 1999). Others followed us to measure the nitrogen isotopes of rainwater and found similar results (Felix et al., 2015; Alteri et al., 2014). The widespread occurrence of Prochlorococcus sp. was just being discovered (Partensky et al., 1999), and at that time, its importance in ocean productivity, particularly in oligotrophic areas, was unknown. Today our work is even more relevant for understanding the effects that major ocean storms may have on marine primary productivity as our climate changes.

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