Gulf of Mexico “Dead Zone”

Each year, scientists in the northern Gulf of Mexico measure the size of the “dead zone,” which is an area where the bottom waters contain oxygen levels below 2.0 mL oxygen per L of water. This is a commonly identified threshold below which many zooplankton and fishes cannot survive. The states along the Gulf of Mexico rely on productive coastal fisheries, and for bottom associated species like shrimp and crabs, a larger “dead zone” means less habitat. This year’s dead zone is exceptionally large (6,400 square miles), leading many people to believe that something or someone is to blame. In reality, there are many interacting factors that cause the large dead zone.

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To understand why the dead zone is so large, we must first understand why it forms. The dead zone has been forming in the Gulf of Mexico for centuries, but only in the last few decades has the dead zone dramatically increased in size and severity of hypoxia (low oxygen conditions). For a dead zone to form, two basic conditions have to be met: 1) there must be high freshwater runoff through river input (a supply of nutrients), and 2) there must be strong thermal stratification. During the summer months, the southeastern USA experiences high amounts of rainfall, which leads to increased river flow into the Gulf of Mexico. The summers are also extremely hot, so the sun’s radiation heats the surface layers of water more than the deeper layers. This thermal heating suppresses vertical mixing, which makes it difficult for the deeper waters near the bottom to re-equilibrate with the atmosphere.

When the freshwater enters the Gulf of Mexico, it supplies nutrients to the phytoplankton, leading to a bloom where they take up carbon dioxide for photosynthesis and produce oxygen. These phytoplankton, however, have short life spans (~2-4 weeks), so they die shortly after the bloom. When the phytoplankton die, they sink to the bottom of the Gulf of Mexico and are then broken down by bacteria. These bacteria use the available oxygen in the deeper waters to break down the dead phytoplankton, ultimately leading to hypoxic conditions and the formation of a “dead zone.” The intense summer sun suppresses the vertical mixing by keeping the surface waters much hotter than the low oxygen bottom waters. Large storms or wind events (i.e., hurricanes) can actually reduce the size of the dead zone because the winds promote mixing of the water column, allowing the low oxygen bottom waters to reach the air-sea interface and once again become saturated with oxygen.

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So why is the dead zone so large in 2015? For one, we are experiencing an extremely strong el Nino year. For the southeast USA, el Nino leads to high amounts of rainfall but poor conditions for hurricanes due to wind shear in the upper atmosphere. The combination of high rainfall but few hurricanes is favorable for a large dead zone. The consistently large dead zone documented in recent years appears to be predominantly due to farming practices that rely on large amounts of fertilizers, which ultimately end up in the rivers feeding into the Mississippi River and Gulf of Mexico. Because most of our gasoline now contains ~10% corn-derived ethanol, there is currently a huge incentive for farmers to grow corn. The problem is that corn is a crop that requires large amounts of nitrogen-based fertilizers that eventually end up in the Gulf of Mexico, providing fuel for the phytoplankton blooms that cause hypoxia. There is also some evidence that the Gulf of Mexico may have experienced a regime shift in the plankton and microbial communities that make the ecosystem more responsive to nutrient input changes (Dale et al. 2010).

The problem of the Gulf of Mexico dead zone is a perfect illustration of how oceanography is souch an interdisciplinary science. To understand why and how the dead zone forms, we must consider physics, climatology, biology, and even public policy which influences nutrient supply into our rivers. Fixing the problem of the Gulf of Mexico dead zone will require cooperation among many different interest groups, but some of the factors that influence the size of the dead zone are out of our control. Substantially reducing nutrient runoff into our rivers is an achievable goal that could help alleviate the stress on the ecosystem by reducing the size of the dead zone.

References:

Dale, V.H., Kling, C.L., Meyer, J.L, et al. (2010) Hypoxia in the Northern Gulf of Mexico. Springer. New York. 284 pp.

http://www.nola.com/environment/index.ssf/2015/08/2015_gulf_dead_zone_larger_tha.html#incart_river

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Announcing Plankton Portal 2.0!

There is a new dataset on PlanktonPortal!

In summer 2013, a group from the original science team behind PlanktonPortal (Bob Cowen, Cédric Guigand, and Jessica Luo) teamed up with French colleagues (Jean-Olivier Irisson, Robin Faillettaz and other members of the Laboratoire d’Océanographie de Villefranche) through a Partner University Fund-sponsored project. We roamed the Mediterranean sea, equipped with ISIIS, the instrument which takes the images seen on PlanktonPortal, and a collection of other sensors. Our aim was to understand how physical discontinuities in the ocean (such as the strong coastal current along the French Riviera) influence the plankton. These discontinuities often create conditions in which plankton thrives and this has important consequences down the rest of the food chain.

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Bob, and Jean-Olivier (in the background), ready to load ISIIS on the Tethys II oceanographic ship in Nice’s harbour

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Cédric setting his baby up with the crew of the NO Tethys II

After a rocky start (see our project blog), we eventually managed to get almost five full days of sampling. Five days; it seems short. And ISIIS was actually acquiring data of scientific interest during only 93 h within those five days. That is 3.8 full days, which seems even shorter. But that amounts to about 17.5TB of data (terabytes, as in 1024 gigabytes). In terms of images, that represents 19 trillion pixels, which could be divided into 34 million PlanktonPortal frames. So, clearly, we need your help!

We actually spent much of the last two years processing the images and classifying organisms in a small fraction of them, to be able to filter out most frames with no organisms using computer algorithms. Now we are ready. Ready with a few hundred thousands frames from the last two days of the cruise that no one has seen before and in which you can help us identify plankton.

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ISIIS in font of a Mediterranean sunset

Plankton featured in Science!

Many of you may have noticed a few popular news sites mentioning plankton (see the New Yorker and Time magazine for some examples). These are popping up everywhere because of a new special issue of Science magazine including 5 research articles relating to an unprecedented global study of marine plankton biodiversity.

The cover of a special issue in Science

The cover of a special issue in Science

The issue has scientific publications resulting from a four year study aboard the research schooner Tara, in which 210 sites were sampled in all major oceanic regions. The sampling primarily focused on the upper 200 m (the euphotic zone) where most plankton biomass is concentrated. They also used satellite imagery, which can detect circulation features and temperature changes often associated with eddies and fronts. These small scale features are considered “hotspots” of biological activity, so understanding the biodiversity in these areas is particularly interesting to scientists.

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The research schooner Tara was used to study plankton ecosystems around the world

There are two main reasons why these studies are garnering so much attention. First, the studies shed light onto the unknown genetic diversity in the microbial and viral ocean communities through the use of genome sequencing and analysis technology.
These microbial communities are the workhorses in the ocean, and understanding what causes microbial communities to change can help us better understand ocean ecosystem productivity and various components of carbon and nitrogen cycling. The viruses and microbes can also interact in complex ways to affect microbial community composition. The second major contribution is simply the impressive global extent of the study, which required international collaboration. Because of the spatial coverage of the study, the gene reference catalog and census of marine biodiversity across planktonic organisms will be useful for researchers all over the world. This highlights the importance of international and cross-disciplinary collaboration to study something as big and complex as the ocean.

Most of the zooplankton we have examined on this site (with the imaging system) has been what scientists call “mesozooplankton” – typically 500 microns to several centimeters in size. The Tara expedition made major contributions to our understanding of much smaller organisms, but it is important to remember that all of the ocean organisms are interconnected, and scientists are just scratching the surface of understanding these interactions and how they could affect everything from fisheries to global climate. It is an exciting time for any researcher when his or her study topic is featured in a journal as widely read as Science!

Appendicularians – putting mucus to good use

Appendicularians (also known as larvaceans) are unique animals that resemble tadpoles in shape. Unlike tadpoles, however, they do not metamorphose into a different form, but spend their entire lives as part of the zooplankton. Many marine animals use mucus for various purposes, but for appendicularians, mucus is necessary for them to feed and survive. They use mucus to build an intricate “house” that has a tunnel through the middle. The appendicularian beats its strong tail to create a current through the house, and the house collects the food particles in the water. It really is a special thing to be able to see an appendicularian inside of its house because most collection techniques destroy these houses. Even when someone does have access to an intact appendicularian house, it can be extremely difficult to view under a microscope, as documented here.

Appendicularians use a create a feeding current to filter food particles from the surrounding water. Source: http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artjan01/oiko.html

Appendicularians create a feeding current to filter food particles from the surrounding water. The dotted arrows indicate the direction of the current through the house.
Source: http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artjan01/oiko.html

If the housed appendicularian encounters waters that have food concentrations that are too high, the filter will clog, and the appendicularian must abandon its house. In order to keep feeding, it has to start from scratch and build a new house. Most appendicularians are thought to abandon and build several houses per day. For this reason, you see appendicularians in the ISIIS images that look as if they have no house (likely in the process of building a new one) or have fully formed mucous houses. Oftentimes you will see abandoned houses that appear slightly degraded, drifting slowly through the water column. Scientists think that these abandoned houses, which stick to all sorts of different particles causing them to increase in size and sinking speed, make up a large portion of organic material sinking through the water column (known as “marine snow”).

This appendicularian has a house that appears to be accumulating many particle on the outside. It will likely abandon this one soon and start building a new house.

This appendicularian has a house that appears to be accumulating many particles on the outside. It will likely abandon this one soon and start building a new house.

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The faded mucous material below this copepod is what remains of an appendicularian house. Other zooplankton might scavenge the food particles that collect on abandoned mucous houses as they sink into the deep ocean.

Not only are appendicularians responsible for creating marine snow that exports carbon to deeper waters, but they are also important food sources for the early life stages of some commercially important fishes. In fact, research in the Cowen Lab demonstrated that many tropical tunas (little tunny, yellowfin tuna, skipjack tuna, etc.) feed almost exclusively on appendicularians as larvae, before beginning to consume other larval fishes once they reach a certain size (Llopiz et al. 2010).

Many people also also surprised to learn that humans and appendicularians are classified under the same phylum (Chordata). This means that a few hundred million years ago, there was an animal living in the ocean that became the first organism to have a notochord as part of its developmental process. This ancestor gave rise to the fishes, which proliferated during the Devonian, and later all other animals that have backbones. Humans, modern reptiles, and appendicularians are all different end points on the tree of life that emerged from this common ancestor.

Reference:

Llopiz, J.K., Richardson, D.E., Shiroza, A., Smith, S.L., Cowen, R.K.,2010. Distinctions in the diets and distributions of larval tunas and the important role of appendicularians. Limnol. Oceanogr. 55, 983-996.

Plankton Pioneers

A substantial part of being a scientist, especially when you are relatively new, is learning about the research that has been done over the years. The purpose of this is to critically examine the published scientific work and identify the knowledge gaps that still remain. On this blog, we talk a lot about plankton images and all of the interesting data that you are helping us analyze. But what got us started down this path? How did scientists realize that there was a completely different world other plankton samplers (nets) were missing?

With improvements in SCUBA diving gear and underwater photography, the 1970s were an exciting time to be a marine biologist. A handful of young scientists, who would later become some of the leading experts in their field, pioneered the use of “blue water diving.” This involves divers going out to the open ocean, descending to a particular depth, and observing the life around them. Blue water diving is little bit more difficult than your typical dive on a coral reef because there is no reference point for your eye to detect if you are ascending or descending in the water. They had to be very good at controlling their buoyancy!

Dr. Alice Alldrege, now a professor at UC Santa Barbara, counts plankton within a fixed volume to estimate their concentrations

Alice Alldredge, a research professor at UC Santa Barbara, counts plankton within a fixed volume to estimate their concentrations

The amazing things the scientists witnessed, and that many of you get to see in the ISIIS images, were documented in an article in National Geographic in 1974, written by Dr. William Hamner. The article effectively captures the sense of wonder and amazement that they experienced observing these fragile animals up close. They departed from Bimini, Bahamas and would dive on the side of the island protected from the strong Gulf Stream current. Each researcher focused on a different plankton group, capturing a variety of specimens and estimating their abundance. During their research, they were the first people to image larvaceans swimming freely in the ocean, as well as discovering abundant pteropods once thought to be very rare. They even occasionally saw curious sharks passing by!

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Salp reproducing a new chain asexually – photo credit LP Madin

Ctenophore ocyropsis maculata with its lobes open.. The scientists discovered that this species uses its lobes primarily for locomotion

Ctenophore Ocyropsis maculata with its lobes open. The scientists discovered that this species uses its lobes primarily for locomotion.

The data gathered by these “Plankton Pioneers” was so valuable that later, when computer technology advanced, their work provided justification for developing imaging systems, such as the Video Plankton Recorder (Davis et al. 1992) and ISIIS, that could collect data at a much faster rate than divers. No doubt that as technology advances even more, there will be plenty of discoveries made about the secret lives of plankton. So if you ever have a chance to be in the ocean, take a minute to ignore the pretty corals and fish to look just a few inches in front of your mask. You just might see a whole world most overlooked for so long.

All photos are from National Geographic. Head to your local library and read the entire article! It is definitely worth the trip!

References:

Davis CS, Gallager SM, Solow AR (1992) Microaggregations of oceanic plankton observed by towed video microscopy. Science 257: 230-232

Hamner, WM (Oct. 1974) Ghosts of the Gulf Stream: Blue-water plankton. National Geographic Magazine 146: 530-545

OSTRICH cruise leg 2 almost done!

We have been zigzagging up and down the Florida Straits for the last two weeks. This photo, from two days ago, shows our ship track (in blue) and drifter track (in red).

RV Walton Smith Tracks

Our first study was the Spatial Study, where we deployed a drifter at the beginning of the day, and then followed it, sampling with ISIIS and MOCNESS in a zig-zag track for the whole day. We started our cruise sampling near Miami and Bimini, Bahamas, and finished up the Spatial Study in the FL Keys.

Photo by Cedric Guigand, taken with a Phantom DJI drone

Research Vessel F.G. Walton Smith during the OSTRICH Cruise, June 2014. Photo by Cedric Guigand, taken with a Phantom DJI drone.

 

In the second leg of our cruise (which concludes tonight), we have been sampling in the Lagrangian Study, which lasted a total of 4 days. We deployed a drifter in the Lower Keys at the beginning of the study and have been following it continuously, sampling with ISIIS and MOCNESS.

The idea is that by following a drifter, we will be continuously sampling the same body of water. Consider this analogy: you are sampling a fast moving stream. If you stand at a fixed point on land and continuously sample, you sample different parts of the stream because it is constantly flowing in front of you. However, if you build a raft and float down the stream with it, sampling along the way, then you are actually sampling the same part of the stream at different points in time. The latter case is what we are doing. The Gulf Stream / Florida Current is our fast moving “stream” and we are drifting with the stream in a fancy raft, sampling along the way.

R/V F.G. Walton Smith, aerial view of the back deck, with ISIIS on board. Photo by Cedric Guigand

Continue following our updates on the ISIIS Facebook page as we conclude our 18-day cruise this week.

ISIIS in the field: OSTRICH cruise in progress

Hi Plankton Portal!

The Science Team is currently out in the field in the Straits of Florida, on the R/V Walton Smith, sampling with both ISIIS and MOCNESS (Multiple Opening Closing Net and Environmental Sampling System), on an 18-day cruise titled OSTRICH (Observations on Subtropical TRophodynamics of ICHthyoplankton).

OSTRICH logo

The overall goal of this NSF-sponsored project is to quantify the patterns and consequences of the fine-scale to sub-mesoscale distributions of larval fishes, their prey, and their predators near and across a major western boundary current passing through the Straits of Florida. By sampling a series of water masses at very high resolution, this study addresses specific hypotheses concerning: i) the drivers of aggregations and patchiness, and ii) the biological consequences of predator-prey interactions at fine scales.

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Sample ISIIS images showing diversity of plankton from multiple coastal sites (including the Southern California Bight!)

Sampling involves a novel combination of detailed in situ sampling of the horizontal and vertical distributions of plankton, targeted fine-scale net sampling, and analyses of individual-level recent daily larval growth to enable the identification of the biological and physical processes driving fine-scale plankton distributions.

Follow along on the ISIIS facebook page as we periodically post updates (via our terrible internet connection at sea!) and also check out this cool video made by one of our cruise participants, Chris Muiña:

 

 

Plankton blooms: Causes and Consequences

As organisms that cannot swim against the currents, plankton are intimately connected to their physical environment. Many species are quite sensitive to the temperature, salinity, and nutrient levels that either lead to their proliferation or demise. Physical conditions and nutrient levels can lead to high abundances of particular plankton types. These plankton “blooms” are common throughout the world’s oceans and can be composed of phytoplankton, zooplankton, or gelatinous zooplankton, depending on the environmental conditions.

Generally phytoplankton (plankton that use photosynthesis like plants) need nutrients and light to grow at very high rates. Since light is readily available in the surface ocean, nutrient availability is the most important driver of phytoplankton blooms. Phytoplankton blooms of most concern to environmental monitoring groups are often described as Harmful Algal Blooms (HABs). Some HABs composed of diatom species Pseudo-nitzschia spp. or the dinoflagellate Karenia brevis can produce toxins harmful to copepods, fish, and higher trophic levels like dolphins and humans. “Red tides” are actually blooms of Karenia brevis that sometimes lead to massive fish dieoffs. Other phytoplankton blooms are harmful not because of the toxins that they produce, but because of the processes that happen when the blooms die off: massive amounts of phytoplankton die and sink to the bottom where they are decomposed by bacteria. These bacteria use oxygen to consume the dead phytoplankton, creating large portions of the water column that are low in oxygen. Fishes and some zooplankton avoid these low oxygen zones, but gelatinous zooplankton seem to be able to withstand low oxygen conditions. These low oxygen regions are often referred to as “dead zones” because very few animals can live there. A dead zone occurs regularly in the summertime in the northern Gulf of Mexico and has been expanding in recent years. Reducing nutrient/fertilizer runoff from farmlands and cities is therefore crucial to limiting the growth of phytoplankton and maintaining healthy coastal ecosystems.

A bloom of Karenia brevis viewed from the air. This "crimson tide" is only composed of small dinoflagellates, but it can have devastating consequences for a coastal ecosystem. Source: http://microbewiki.kenyon.edu/index.php/File:Red_tide_genera.jpeg

A bloom of Karenia brevis viewed from the air. This “crimson tide” is only composed of small dinoflagellates, but it can have devastating consequences for a coastal ecosystem.
Source:
http://microbewiki.kenyon.edu/index.php/File:Red_tide_genera.jpeg

Blooms of zooplankton can form via two different mechanisms 1) currents from different water masses merge to create a dense patch of organisms, or 2) consistently favorable conditions allow the zooplankton to reproduce faster than their predators can consume them. These two mechanisms are distinguished in the scientific literature as “apparent blooms” and “true blooms” (Graham et al. 2001). Apparent blooms can result from converging currents such as fronts, or the behavior of zooplankton aggregating along some kind of physical discontinuity, such as a thermocline. True blooms are typically the result of high food concentrations, high survival of larvae/juveniles, or a combination of multiple factors. For scientists, it is sometimes difficult to know what mechanism led to a zooplankton bloom, and they need to consider the history of the water masses where the zooplankton are found to figure out how the bloom formed (Greer et al. 2013). Zooplankton blooms associated with copepods are generally considered to be healthy for the ecosystem. The timing of copepod peak abundances with the first feeding of larval fishes is thought to be an important factor contributing to the variation in fish population abundances (Cushing 1975). Blooms of jellyfish, on the other hand, are often associated with ecosystems that are environmentally degraded through high nutrient input or consistent overfishing (Jackson et al. 2001), but there are some scientists who think jelly blooms are simply a characteristic of their life histories (Condon et al. 2013). More data on jellyfish abundances over longer time periods will help scientists understand relationships between the environment and the frequency of jellyfish blooms. You can also report jellyfish sightings at http://www.jellywatch.org.

Here is avideo of one ISIIS downcast through the water column of near a frontal feature offshore of San Diego, CA, USA. It shows a bloom of Solmaris spp. jellies concentrated near the surface. Convergent currents that are commonly seen at fronts likely contributed to the formation of this bloom.

References:

Condon RH, Duarte CM, Pitt KA, Robinson KL, Lucas CH, Sutherland KR, Mianzan HW, Bogeberg M, Purcell JE, Decker MB, and others (2013) Recurrent jellyfish blooms are a consequence of global oscillations. Proc Natl Acad Sci U S A 110:1000-1005

Cushing DH (1975) Marine ecology and fisheries. Cambridge University Press, London

Graham WM, Pagès F, Hamner WM (2001) A physical context for gelatinous zooplankton aggregations: A review. Hydrobiologia 451:199-212

Greer AT, Cowen RK, Guigand CM, McManus MA, Sevadjian JC, Timmerman AHV (2013) Relationships between phytoplankton thin layers and the fine-scale vertical distributions of two trophic levels of zooplankton. Journal of Plankton Research 35:939-956

Jackson JBC, Kirby MX, Berger WH, Bjorndal KA, Botsford LW, Bourque BJ, Bradbury RH, Cooke R, Erlandson J, Estes JA, and others (2001) Historical overfishing and the recent collapse of coastal ecosystems. Science 293:629-637

Great plankton pictures taken offshore Miami

Here is a nice collection of plankton underwater shot  by Dr. Evan D’alessandro while conducting research offshore Miami

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the Portuguese Man of War is not a Jellyfish but a Siphonophore!

The Portuguese Man of War or Physalia is a siphonophore floating at the surface of the ocean like a drifting balloon with deadly tentacles waiting to capture careless preys. Its body looks like a sail that catches the ocean wind to propel itself.

More Information on Physalia

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Portuguese Man of War Fish

The Portuguese Man of War Fish or Nomeid seems to be immune to the powerful sting of the Physalia. It is actually a very agile swimmer that can avoid the stinging tentacles.It uses this deadly siphonophore for shelter against predators and can also feed on some of the smaller tentacles that do not seems to have a strong sting.

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Nice ctenophore

Our favorite, the Venus Belt! Nice to see this one in color!

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salp chain

If you click on the this amazing picture you will see a small fish larvae seeking shelter among the salps. very neat!

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ctenophore bloom

Beautiful bloom of lobate ctenohphores. The wall of death some smaller plankters:)