Month: June 2019

(the MOCNESS lonster!)

The daily commute is a necessary evil many humans participate in. You wake up at an ungodly hour, kiss your happily sleeping partner goodbye for the day, and hop in the car or on to a train to begin the journey to make ends meet. Now imagine that your commute was 130 miles each way and you had to time your commute perfectly to avoid your mortal enemies and certain death. That is the lifestyle of many smaller zooplankton in the ocean that are diel vertical migrators (DVM). Every night copepods, krill and even some small fishes, worms, and sea butterflies swim hundreds of meters from the deep ocean to the surface waters to graze on microscopic plant-like organisms called phytoplankton. Before the sun begins to rise, these diel vertical migrators swim back into the dark reaches of the ocean where their predators, fish with more evolved eyesight, cannot see them. *

Oceanographers can observe this migration using Acoustic Doppler Current Profiler (ADCP), an instrument traditionally used in physical oceanography to track water masses, and echosounders. Much like the fish-finders that recreational anglers use, the zooplankton scatter the acoustic signal and we can see them concentrating below 500 m during the day and swimming up to the surface at night. For the copepod Pleuromama sp., the distance traveled is roughly 120,000 times their body length! During this cruise we will be taking pictures of these DVMs in situ with the Scripps Plankton Camera (SPC), collecting these organisms with MOCNESS and ring net tows, and preserving samples for imaging with ZooScan.

Aside from being a fascinating natural wonder, Diel vertical migration has important implications for oxygen and carbon cycling in the ocean. DVMs are actively transporting carbon from the surface ocean to the deep ocean because they graze phytoplankton in the surface and then poop out this carbon at depth. At St. ALOHA, DVMs are estimated to contribute 10-30% of the carbon exported to depth, a process that allows the ocean to act as a sink for atmospheric CO₂.  Carbon transport would be less efficient and take 10s of days without the DVM. Diel vertical migration is energetically expensive for zooplankton and can draw down O₂ in the deep ocean where they reside during the day. It is hypothesized that this could further deter predators who tend to be more sensitive to O₂ depletion than small zooplankton. One of the goals of this cruise is to better understand the impact of DVM on ocean biogeochemistry by measuring how much oxygen they consume, their contribution to carbon export, as well as identifying who makes up this diverse group of organisms.

*Be sure to check out Tom Iwanicki’s forthcoming post detailing how diel vertical migrators use bioluminescence to camouflage themselves during their commute.

 

Images of the phytoplankton Trichodesmium taken by chief scientist trainee Eric Orenstein using the Scripps Plankton Camera developed by the Jaffe lab at Scripps Institution of Oceanography.

When I was kid I couldn’t read Judi Barrett’s book, Cloudy with a Chance of Meatballs, often enough. In the book all sorts of wild and sometimes delicious food came down from the sky. I just loved to imagine living in a world where my favorite foods magically rained down from above. Sadly though, a world like that is just too good to be true. Or is it?

In the ocean an important group of organisms, called phytoplankton, use the energy from sunlight to convert carbon dioxide from the atmosphere into sugars. Although it’s not magic, some might argue that the complex physiology required to perform photosynthesis is even more impressive! Phytoplankton then use these sugars to divide and grow, providing food to marine organisms and forming the basis of many marine food webs. As small organisms, called zooplankton, feed on phytoplankton they produce waste rich in carbon as well as other elements like nitrogen and phosphorus. This waste then sinks down into the dark ocean, moving important nutrients out of surface waters and providing food for bacteria. Phytoplankton can also die and aggregate, these sinking aggregates also move important elements out of the surface waters. These processes are integral the cycling of nutrients and carbon in the marine environment.

Different phytoplankton are found in different parts of the ocean depending on a variety of factors including nutrient and light availability as well as temperature. Here at station ALOHA one group of important primary producers are cyanobacteria [1, 2]. Cyanobacteria can be microscopic, free floating cells, such as Prochlorococcus, or long filaments of cells easily seen by the eye, such as Trichodesmium. Trichodesmium and some other cyanobacteria genera can perform the relatively unique task of nitrogen fixation, the conversion of nitrogen from the air into biologically available nitrogen compounds. This is especially important at station ALOHA where nitrogen concentrations are low. Some cyanobacteria are even specialized to live inside of species from another group of phytoplankton, diatoms. Diatoms containing these nitrogen fixers benefit from having an intracellular nitrogen source. Each year these diatoms have a boom of growth in the summer, accounting for up to 20% of annual productivity at station ALOHA.[3]

It may not be spaghetti and meatballs, but phytoplankton, zooplankton waste and dying cells suit many marine organisms just fine. Perhaps the world Judi Barrett inspired me to dream of wasn’t so unrealistic after all.

1. Karl DM, Bidigare RR, Letelier RM. Long-term changes in plankton community structure and productivity in the North Pacific Subtropical Gyre: The domain shift hypothesis. 2001. doi=10.1.1.500.283

2. Karl D, Letelier R, Tupas L, Dore J, Christian J, Hebel D. The role of nitrogen fixation in biogeochemical cycling in the subtropical North Pacific Ocean. Nature. 1997;388:533–8. doi:10.1038/41474.

3. Karl DM, Church MJ, Dore JE, Letelier RM, Mahaffey C. Predictable and efficient carbon sequestration in the North Pacific Ocean supported by symbiotic nitrogen fixation. Proc Natl Acad Sci. 2012;109:1842–9. doi:10.1073/pnas.1120312109.

 

(image: David Karl, UH)

The Earth’s oceans play integral roles in regulating global climate and supporting biodiversity. Our goal as marine scientists is to answer questions that broaden our understanding of the oceans role on Earth. An essential part of these efforts is providing context for our findings and communicating them to others. The goal of this blog is to not only share our excitement for the marine science, but provide insights into the inner workings of this far-out process of doing scientific research at sea. We may call it a cruise, but it’s nothing like a vacation with the Royal Caribbean. On this expedition, our sights are set on understanding the biological, chemical, and physical processes that ultimately control carbon production and export in the expansive North Pacific Subtropical Gyre.

The subtropical-gyre regions of the major ocean basins constitute the largest and most expansive ecosystems on earth, responsible for carbon fixation amounts on par with global rainforests. Gyre regions are nutrient-starved, but remain productive, complex, and richly diverse. There are hundreds of thousands of organisms in a droplet of seawater. Our understanding of what these organisms are doing, metabolically and interactively, is constantly being refined. At Station ALOHA we have a unique opportunity look to at how these organisms survive in a nutrient starved region of the ocean. We will use tools to measure the genes expressed, the organisms present, their size, and their chemical composition. This information helps assess the relative abundance of different organisms, what organisms can and are doing, and how their elemental composition scales with size. Moreover, these organisms are not self-sufficient, there are close relationships where organisms can trade resources to access elements critical to growth.

We can see the organisms in the water column using a number of methods, none of which involve sharks with laser beams on their heads, but many of which do involve lasers. Submersible instruments put in the water column shine lasers across a small window. As organisms and particles pass through that window and block the laser we can determine their size, shape, and depth (location). Traps, or open-floating trashcans, can also be used to collect dead, sinking particles and assess what amount of material from above falls into the abyss of the ocean and perhaps to the sediments. Together, these instruments assess the actively floating material and the sinking material. We can then measure the chemical compositions of the two and get further information about how and why different elements, like carbon, nitrogen, or phosphorus, move from the surface ocean to deeper waters when they aren’t recycled by nutrient-starved neighbors.

A number of approaches are used to help test the activity and speed at which organisms use elements in short-supply like nitrogen, phosphorus or iron. For instance, we will be using incubations to test how manipulating variables like: groups of organisms present, nutrient concentration(s), light, temperature, and carbon dioxide change the diversity of organisms present, their interactions, the elemental composition of the cells themselves, and ultimately the composition of the water. To trace the speed of elemental mobility, isotope additions to incubations track movement of elements between solution and particulate (living or dead organism) reservoirs given their unique character. Isotopes are elementally bigger and “easy” to measure as they move around. Combined, these approaches provide quantitative constraints on where and how fast elements channel between cells, the ocean, the atmosphere, and even the sediments.

Mobile and motile organisms, which are able to move without the help of ocean mixing, are an exciting and unique group that can move along and beyond physical, biological, and chemical barriers. For example, Station ALOHA is physically stratified with warm, nutrient-poor, clear water overlying cooler, nutrient-rich water with less available light. Organisms respond to this contrast by arranging themselves in the water according to what environment best suits their needs. Organisms which use photosynthesis to generate energy must remain in the light, while heterotrophs (organisms which eat others for energy) may have more flexibility. Mobile organisms, capable of swimming, may move into the deeper nutrient-rich waters to avoid nutrient stress and enhance their growth. This juxtaposition of environments, organismal needs, and traits presents a never-ending host of ecological and biological questions about how and why organisms live where they do.

The ocean’s role in regulating climate is closely tied to these systems. The living organisms we’re encountering and studying at sea are vessels to remove carbon from the atmosphere and lock it away in marine sediments, this in turn modulates climate. The delivery of biomass from the upper reaches of the ocean hinges upon physical, biological, and chemical reactions processes and interactions. Experiments at sea manipulate these variables and test responses to changes in things like nutrients and the subsequent biological interactions. Quantifying the biological and chemical processes present informs our understanding of how expansive gyre regions of the ocean sustain productive populations of organisms and how on a changing earth the ecology and biological productivity of this region might change.

Aloha!  The next NSF/UNOLS Chief Scientist Training Cruise is set to sail on the R/V Kilo Moana on June 15th.  We are excited to share our science with you!  While we gear up preparing for the cruise, please check out our scientist profile page (https://csw.unols.org/meet-the-scientists/), so that you can learn about our research team.

We don’t have any pictures from the cruise yet since it hasn’t begun.  However, for your viewing pleasure,  here is a sea turtle picture from one of the early career scientist’s last trip to Hawai’i.