For many of us living on the southern (Gulf) coast of the USA, it has become common to see numerous dead fishes floating in the bays and scattered across our beaches. Why is this happening? You may have heard on the news that there is a large harmful algal bloom (HAB) that began off the western coast of Florida and has slowly spread throughout the Gulf over the past few months. While the HABs occur somewhat regularly in Florida, it is quite rare for them to extend as far north as they have this year. The last major HAB of this particular dinoflagellate species (Karenia brevis) occurred ten years ago. The Mississippi Department of Marine Resources has been monitoring the bloom recently, and they have seen some of the negative effects of the ecosystem.

The lumpy greenish-transparent blob (near the center of the photo) is Karenia brevis. These images were captured with the FlowCam, which is an instrument that images algal sized particles in a water sample.

Thousands of dead Gulf Menhaden accumulate at a front off the coast of Mississippi. Fronts concentrate floating objects (dead fish and other debris) by providing a convergent flow (photo by Adam Greer taken on Dec. 15, 2015).

So how do these tiny plankton kill the fish that we see washing up onshore? After all, the individual phytoplankton within the HAB are only about 20 microns in size (that is 0.02 mm). Karenia brevis (a.k.a. “the red tide” derived from its brownish-red color at high concentrations) naturally produces a compound known as brevetoxin – a potent neurotoxin that inhibits sodium channels required for many neurological processes. Some of the most abundant fish species in the Gulf of Mexico are filter feeders such as Gulf Menhaden. These are commercially important fish that feed by opening their mouths and swimming to capture as much phytoplankton as they can. Karenia brevis is often present off the Florida coast, but in low abundances. Under these circumstances, Karenia brevis does not harm the fish that consume it. Only when the concentrations reach extreme levels do the fish experience the negative effects of the brevetoxin. Filter feeders like the menhaden are most directly impacted by the phytoplankton-produced toxin; however, the organisms that consume these fish are also susceptible to the harmful neurological effects. Large black drum have also been washing up onshore, presumably killed by eating too many menhaden and other fish species that had built up high levels of toxin in their tissues. Even dolphins and birds can become very sick from eating these suddenly toxic fish.

University of Southern Mississippi scientist Adam Boyette runs the FlowCam to characterize the phytoplankton community and monitor the HAB.

Why is this bloom so severe this year? While there are probably a variety of factors at play, most agree that an unusually strong El Nino is likely the primary culprit. Along the Gulf coast recently, it has been unseasonably hot and dry. The winds have also been calm. This provides ideal conditions for Karenia brevis to proliferate and form dense aggregations. With cooling temperatures, the growth rates of these phytoplankton should decrease. The increasing amounts of rain and wind recently should cause these HAB patches to break up, diluting the extreme concentrations, which will hopefully mitigate the harmful effects on Gulf of Mexico ecosystem.

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.

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.

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

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 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.

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!

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!

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!

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.

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