Published

2 years ago

February 28, 2024

Alice Rush

Anton and I have just brought CTD cast number 45 to light. While we are once again shaking freshly tapped bottles with great enthusiasm, I think I can make out question marks in Jamileh’s expression as she smiles good morning to us. That’s what everyone here seems to be thinking: 45 CTD casts already? And many of them in the same place? Why all this? We should know the water once we’ve “measured” it, right? Well, somehow we do.

1: Map of Madeira with all CTD stations performed so far (red dots)

“Our” CTD, which biologists prefer to call a “water sampler”, is moved out of the side of the hangar, lowered and in the basic version measures Conductivity (salinity), Temperature and pressure (Depth) quasi continuously (at 24 Hz), ideally down to the seafloor. In addition, oxygen and fluorescence are measured, which makes it possible to estimate biological productivity (see previous blog entries by Nicole and Manfred). As an addition, water samples can be taken at various depths using the 24 Niskin bottles (Manfred is by far our best customer in this respect). For oceanographers, however, the continuous measurements of temperature and salinity are of crucial importance, as they allow us to see how stable the water is stratified, for example, or to deduce the origin of the water masses and geostrophic currents. This is important information that forms the framework conditions that strongly influence the local ecosystem. In order to achieve maximum precision in the physical measurements, I take water samples myself “only” to calibrate the oxygen and salinity sensors later, but not to analyze the suspicious living beings in it.

2: This is where we “winkler”: The oxygen content of water samples is determined by titration. The results are used to calibrate the measurement results of the CTD sensors, which often have an offset.

Most of the deployments to date have been close to shore at a depth of about 1500 meters. The following figure shows one of our precious deeper profiles down to a depth of almost 3300 meters. Here, the top 100 meters form the so-called “mixed layer”, in which all measured variables are well mixed by the wind. We observe that the depth of this surface layer varies, but is generally comparatively thick – as is typical for the winter months at these latitudes. At our first station, the mixed layer depth was even around 200m! Temperature (red), salt (blue), oxygen (yellow) and chlorophyll (green) draw practically vertical lines in the diagram. Interestingly, a maximum of chlorophyll often forms exactly at or below the surface layer, which serves as an indicator for the presence of phytoplankton (see Nicole’s and Manfred’s blog entry on “Micro-Creatures”). Although phytoplankton is basically autotrophic, i.e. dependent on sunlight, it can survive in this rather deep layer with very little sunlight. One reason for this is the increased nutrient content in deeper layers.

3: An exemplary CTD profile from February 23rd south of Madeira, to a depth of about 3300m. Contains fluorescence (green), oxygen (yellow), salinity (blue) and temperature (red))

In addition, the pycnocline directly below the mixed layer forms a strong physical barrier to vertical mixing and can practically “trap” organisms that cannot actively swim themselves. The pycnocline is the layer in which the density of the water increases very rapidly with depth (here due to the temperature gradient). These layers contain a wide range of temperature and salt contents and are also called Central Waters. To identify water masses, temperatures and salinities are plotted against each other in a so-called “T-S diagram” (as shown in Figure 4). In our example, you can clearly see that the water around Madeira consists largely of Eastern North Atlantic Central Water (ENACW). This water mass dominates the pycnocline in the large North Atlantic Gyre and is significantly more saline than in the South Atlantic (see Eastern South Atlantic Central Water). In our profile number 41 (Figure 3), however, something else catches the eye. At around 1100m, there is a nose with a significantly higher salinity, which does not seem to match the linear Central Water. The influence of the Mediterranean Water (MW) is noticeable here, which has a particularly high salt content due to the predominantly high evaporation and low precipitation in the Mediterranean region.

Due to this high salt content, it manifests itself at greater depths, typically around 1100m to 1200m, despite the warm temperatures. However, we can also see in the T-S diagram that the Mediterranean water in the south of Madeira is already somewhat more mixed, i.e. less warm and saline than directly at the outflow of the Mediterranean. Even further down, which we can observe particularly well at our deeper CTD stations around 3000m, resides the famous North Atlantic Deep Water (NADW). This is formed by, for instance, deep convection in the North Atlantic and plays a central role in global thermohaline circulation and climate dynamics. Although constituting deep water, it is comparatively “young” and therefore rich in oxygen (we like to say “well ventilated”) and forms a contrast to the oxygen minimum, which we observe here around Madeira at around 800-900 meters. This minimum zone is formed by respiration of the sunken organic material, e.g. from the sunlight-dependent phytoplankton in the uppermost ~150 meters. Compared to the large known oxygen minimum zones in the subtropical eastern Atlantic and Pacific, however, there is still comparatively abundant oxygen.

4: The Temperature-Salinity diagram belonging to profile 41 in Figure 3. The letters indicate the typical temperature and salinity values of known water masses (see text).

Now, we know the profile of a single CTD station a little better. Basically, this one is actually fairly representative of the other 44, so the question of why Anton and I keep “driving CTDs” like madmen remains unanswered. However, if we take a closer look, we can see that the temperature and salinity profiles are not completely “smooth”. In fact, we discover small wavelike deviations. Measurement inaccuracies? No. It is internal waves that bring “life” to the profiles. Internal waves can occur in any stratified medium, i.e. fluids in which the density is not constant. There are two restoring forces that act on internal waves in the ocean: Gravity and the Coriolis force. The main drivers of internal waves are the tides (such as ebb and flow), closely followed by wind. We know that internal waves play a crucial role in energy transport in the ocean. Like ordinary surface waves, internal waves can also break. When they do, mixing takes place. This in turn can transport nutrients and thereby influence biological productivity. The interaction of internal waves with topography (i.e. islands such as Madeira) and currents is very complex and not yet fully understood. By using a large number of stations at different times (and tidal stages), we obtain a better spatial and temporal resolution of the internal wave field and improve our understanding. That’s also why we are fans of so-called “yo-yo CTDs”. Just like a real yo-yo, we move the CTD up and down several times in direct succession at one and the same location.

5: A “yo-yo CTD”. Like Figure 3, but six CTD profiles plotted on top of each other

In the figure above, we have plotted six directly consecutive profiles of a “CTD yo-yo” on top of each other. You can see that the profiles deviate more from each other at some depths and not at others (nodal points). The most impressive influence is exerted by internal waves on the mixed layer depth, which can vary by several tens of meters within minutes.

There is a particular thrill when the “Eddy hunt” is called for. That sounds more martial than it is meant to be. Eddies are oceanic vortices that reach a diameter of about 50 km around Madeira, interact with topography (islands) and internal waves and are known to have an impact on biodiversity. They develop over a period of days/weeks and are unfortunately hardly predictable. Therefore, we check satellite and model data for the region daily to identify a possible feature and, if possible, sample in situ with Merian. Strong eddies can generate a signal in sea level, surface temperatures and chlorophyll, recognizable via satellites. Our colleagues from the Oceanographic Institute of Madeira are helping us on site by providing the regional satellite and model data (see https://oomdata.arditi.pt/msm126/). Overall, it is impressive how well the collaboration on board and beyond works! One “eddy hunt” has already taken place on the night of February 13-14. However, the satellite signal was weak, and accordingly we were unable to detect a strong, coherent eddy In Situ with our shipboard ADCP (Acoustic Doppler Current Profiler, which measures ocean currents down to a depth of almost 1000m). (Side note: However, another exciting feature (presumably a strong internal wave) was identified in the surface layer, which we are now analyzing.)

6: Eddy hunt! The rough plan for a “spontaneous” survey of the potential eddy. There was a weak satellite signal for the one negative anomaly in the sea level (blue contours). The red line indicates the planned track and the purple triangles indicate the planned CTD stations.

In one of the following contributions, we want to prove to you that our beloved CTD is something very special in purely “objective” terms thanks to sophisticated tuning, including high-resolution camera systems. Then we’ll explain why Anton, although he’s not a physical oceanographer, also likes to drive “CTD yo-yos” and there will finally be photos of aquatic animals again!

Greetings from on board RV MARIA S. MERIAN,

Marco Schulz und Anton Theileis

A physical oceanographer alone among biologists

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Alice Rush

Ocean Acidification

The Strata that Matta

Published

3 days ago

July 5, 2026

Alice Rush

From Desert to Seafloor

Fig. 1) team Strata That Matta: Victoria C., Maeghan D., Maddie B., Vale B. (from left to right)

The months leading up to OCEAN CORE Academy were filled with another type of adventure for me, surveying the badlands of New Mexico in search of dinosaur bones. Yet, my work in the Gulf Coast Repository consisted of examining ocean cores using a microscope. Although these experiences couldn’t be any more different, the two were similar in that each attempted to answer the same question: what did Earth look like in the past?

I focus much of my research on vertebrate paleontological and geological fieldwork, such as prospecting for fossils, measuring strata, or describing ancient paleoenvironments and faunal assemblages. While I knew about microfossils, I had not fully grasped how much geological history is present in them.

Fig. 2) fieldwork, NM (May 2026)

History Through a Microscope

This leads me to one of the most memorable parts of OCEAN CORE Academy, learning to prepare smear slides and identify what existed within the ocean cores. Ocean sediments are fairly recent in that they have not yet been lithified, each layer represents tens to hundreds of years of depositions onto the seafloor. What I looked at was much deeper!

It was a momentous occasion when I first saw a radiolarian beneath the microscope! These tiny fossilized organisms provide surprisingly detailed insights into ancient environments. The conditions in which different groups of microfossils thrive vary, but by tracking how they fluctuate between layers, we can reconstruct climatic shifts over geologic time.

Team Strata That Matta correlated a transition from calcareous to siliceous ooze layers with a cooling climate!

Fig. 3) my first time seeing microfossils

Fig. 4) radiolarian Fig. 5) coccolithophores Fig. 6) sponge spiccules

Bringing OCA Back to AZ

Upon my return to Arizona, I will carry this new perspective with me. As I move forward with future projects and field seasons in New Mexico, volunteer at the Arizona Museum of Natural History, and pursue my degree, the skills I developed here will prove to be invaluable for strengthening my own research.

Prior to attending OCEAN CORE Academy I viewed microfossils as existing, yet somewhat separate from my projects. This place has challenged that perspective. I came to understand that many of the most detailed records of Earth’s past are the microfossils hidden within a single grain of sediment!

Fig. 7) class of OCA 2026

Written by OCA 2026 student, Maddie Baare

The Strata that Matta

Ocean Acidification

Earth’s History at Every Scale

Published

3 days ago

July 4, 2026

Alice Rush

From Desert to Seafloor

Fig. 1) team Strata That Matta: Victoria C., Maeghan D., Maddie B., Vale B. (from left to right)

Fig. 2) fieldwork, NM (May 2026)

History Through a Microscope

This leads me to one of the most memorable parts of OCEAN CORE Academy, learning to prepare smear slides and identify what existed within the ocean cores. It was a momentous occasion when I first saw a radiolarian beneath the microscope!

Before, I had been hunting for fossils measured in centimeters/meters, but now I am studying those measured in micrometers. These tiny fossilized organisms provide surprisingly detailed insights into ancient environments. The conditions in which different groups of microfossils thrive vary, but by tracking how they fluctuate between layers, we can reconstruct climatic shifts over geologic time.

Using these changing microfossil assemblages, my team correlated a transition from calcareous to siliceous ooze layers with a cooling climate!

Fig. 3) my first time seeing microfossils

Fig. 4) radiolarian Fig. 5) coccolithophores Fig. 6) sponge spiccules

Bringing OCA Back to AZ

Fig. 7) class of OCA 2026

Written by OCA 2026 student, Maddie Baare

Earth’s History at Every Scale

Ocean Acidification

Microplastic Pollution Research at Sea

Published

5 days ago

July 2, 2026

Alice Rush

I have been studying plastic pollution for more than a decade. I’ve analyzed hundreds of samples in labs, pored over data and spent years thinking hard about where plastics go once they leave our hands and enter the environment. I love doing work on the water—this was a big part of my previous professional roles in Alaska and in Saipan, Northern Mariana Islands.

And here’s where it took me! I was thrilled to have the opportunity to join the first leg of eXXpedition’s voyage in the South Pacific this past spring, trading my lab coat for a lifejacket to study microplastics at sea. Sailing from Auckland, New Zealand, to the Bay of Islands aboard the 70-foot research vessel Wind Shift over 10 days, our crew of 12 women conducted ocean water-surface sampling via manta tow nets (a long cone-shaped mesh net), cleaned up debris on remote beaches and examined city streets with measuring tapes and field equipment. Our purpose? To collect key data to help us better understand the flow of plastics from land to sea.

Our all-female guest crew—hence the XX in “eXXpedition”—brought aboard expertise from the fields of structural engineering, circular economy strategy, sustainable fashion, plastics research, robotics and more. Together, we represented a remarkable cross-section of disciplines united around a shared concern for the health of our ocean.

Seeing it with my own eyes

We found plastics of all shapes and sizes everywhere we went—in the city streets of Auckland, while crossing the Hauraki Gulf and even at Aotea Great Barrier Island (one of the most remote and protected stretches of New Zealand’s coastline). Our ocean is vast and some of these places felt far removed from the centers of human activity, but this eXXpedition was a good reminder that plastic doesn’t respect remoteness. It moves, accumulates and shows up where we least expect.

Working alongside local NGO Sustainable Coastlines, we arrived on a remote stretch of beach on Aotea Great Barrier Island to audit and clean up any plastics we came across. What we found there told the same story our Auckland street surveys did: We found bottle caps, food packaging, fragments, plastic pellets and fishing debris. The everyday materials of modern life—but weathered, broken and scattered.

Science at sea

One of my favorite parts of the voyage (which was also one of the most challenging, if I’m being honest!) was the sea-surface manta trawl analyses we did onboard. I found out quickly that sorting microplastics from krill-laden seawater samples under a microscope while sailing is not for the faint of stomach.

The most common plastic culprit we found in those samples? Microplastic fibers. This type of microplastic is no wider than a human hair and is the most common type of microplastic found in the environment. Microplastic fibers can come from a variety of sources like cigarette butts, weathered ropes or wet wipes, but actually, most microplastic fibers shed from synthetic clothing and textiles. Laundering is a major source— shockingly, a single load of laundry can generate up to 18 million microfibers.

And yet, we found these tiny plastic fibers floating in the ocean many miles away from the nearest washing machine.

In my lab research, I have found microplastic fibers time and time again, but there’s something even more sobering about hand-picking them out of a seawater sample collected from pristine-looking waters. It was a good reminder of why understanding where plastic comes from, how it moves and where it ends up is so critical to addressing the problem at its roots.

Filter Out NSFW Microplastics

Tell your elected officials to take action against plastic pollution by requiring microplastic fiber filters! Adding your name takes less than two minutes, and goes a long way in protecting our ocean, forever and for everyone.

TAKE ACTION

What I’m bringing back

Studying plastic pollution from the deck of a boat in some of the most remote waters in the Southern Hemisphere made me appreciate the work I do even more. It also made me appreciate how important people are in this giant puzzle of plastic pollution solutions. The plastic pollution crisis is a human problem, and solving it requires all of us. The courage and dedication of the women I shared those 10 days with is something I won’t forget. Going to sea, doing the science and pushing through discomfort to collect data that matters was not easy. We were seasick some days and exhilarated others. Despite that fact, we showed up for it fully, every day.

The plastic is out there, even in far-flung corners of the ocean. And the answer is not to be paralyzed by that fact, but to use it as fuel. Every sample we collected is now a data point in a larger story about where plastic comes from and where it goes. Every cleanup, every surface trawl, every street block walked and every hour spent at a microscope are parts of building the evidence base that informs policies, regulations and systems-level changes that can actually turn this crisis around.

Cleaning up beaches and coastlines is valuable and necessary work. But we also must stop plastic from entering the ocean in the first place—through stronger policy, better product design and real investment in waste management infrastructure everywhere. Luckily, when it comes to the most common microplastics in the ocean— microplastic fibers—there is already an effective, affordable solution to immediately reduce microplastics coming from our laundry by roughly 90%: washing machine filters. These filters act just like laundry lint filters in our dryers, capturing fibers in tightly-woven mesh and effectively preventing them from leaving our homes and leaking into the environment.

What can you do?

There’s no better time to tackle plastic pollution than right now, during Plastic Free July! Take two minutes to add your name and call on your elected leaders to combat those pesky, dangerous microfibers that are pouring into our ocean daily—like the ones I found from my samples at sea. Together, we can stop plastic pollution at the source and protect our ocean forever and for everyone.

My biggest takeaways from this experience? People are remarkable. Our ocean is remarkable. And our ocean is worth fighting for, including from 70 feet of sailing vessel in the South Pacific, staring down a microscope with a pair of tweezers and a queasy stomach.

The eXXpedition South Pacific I voyage ran from April 27 to May 6, 2026, sailing from Auckland to the Bay of Islands. Learn more about the research team and our itinerary at https://exxpedition.com/voyage/auckland-to-bay-of-islands/.