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.
Loving this stuff Marilyn!
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