We talk a lot about the health of marine life in terms of fish, oysters, crabs, and other large organisms that we can observe with the naked eye, but we often look past the microorganisms that, in some cases, can be better indicators of water quality, productivity, and overall ecosystem health. [1] The microorganisms in question often fall under the umbrella term of “plankton”. Directly translated from Greek, the root word “planktos” means wandering and describes how these organisms move through ecosystems. There are many varieties of plankton, including producers (phytoplankton) and predators (zooplankton).

One key characteristic that all plankton share is that they do not move very far on their own; they rely on currents and other movement in the water column for most of their migration. While some planktonic organisms can move with a flagellum (tail made of proteins), most of the movement for all plankton comes from external forces. [2] Currents and mixing often occur in the pelagic (uppermost open water) layer because of wind, temperature variation, and a variety of other factors. Phytoplankton thrive in the shallower layers of the pelagic zone where abundant sunlight and gas exchange are available. Higher concentrations of carbon dioxide at the surface allow phytoplankton to photosynthesize, removing carbon dioxide from the atmosphere and giving off the oxygen we breathe. Phytoplankton produce about half of the oxygen in our atmosphere by some estimates, which makes them just as important as our rainforests and other terrestrial ecosystems. [3]

Plankton do not all fit into one taxonomic group; there are plankton from each kingdom. [4] Our coastal waters contain species from each, including microcrustaceans like copepods, bacteria like blue-green algae (cyanobacteria), and juvenile life stages of larger animals like oysters and fish. Jellyfish in their most recognizable form, the medusa, are technically plankton, too: they have no locomotion so they just drift. As easy, plentiful prey, plankton often form the base of marine food webs. Unfortunately, having too many phytoplankton is a very real, very dangerous issue in coastal Louisiana where excess nutrients coming down our waterways provide lots of food for phytoplankton. An overgrowth of phytoplankton can cause an algal bloom [5] and associated hypoxic or dead zone. See our post ‘Stress pt. II: Flooding and Hypoxia’ for more information on how too many producers can decrease the amount of oxygen.

CWPPRA projects help to decrease the area of the Gulf of Mexico dead zone by restoring wetlands. The more wetlands we have in our Mississippi river watershed, the more filtration of excess nutrients we have. Filtration is a major benefit of wetlands and can prevent phytoplankton from accessing these excess nutrients. Hopefully one day nutrient filtration and other pollution reducing practices will allow the gulf to return to its former glory.







Featured image from



Stress Part II: Flooding and Hypoxia

Wetland inhabitants must also deal with flooding stress. All parts of a plant must have oxygen, which causes problems when a plant is rooted in hypoxic soils and it is flooded. Gases diffuse about 10,000 times more slowly through water than through air, and wetland soils are often inundated and hypoxic. This poses an issue for supplying roots with enough oxygen since they don’t have any around them. Some root systems will have adventitious roots, which means they extend above the surface of the water or soil to allow gas exchange with the atmosphere.[1] Red mangroves have prop roots, black mangroves have pneumatophores, and both supply oxygen directly to the root system rather than relying on transport all the way from the leaves to the roots.[2]

Hypoxia can be caused by eutrophication and decomposition. Hypoxia and anoxia are dangerous to most plants and animals because most cannot live only with anaerobic (without oxygen) respiration. Bacteria can sometimes live in anoxic conditions by using different electron receptors that are more plentiful in wetland soils like sulfates. Plants can sometimes cope with hypoxia thanks to adaptations like aerenchyma development in their roots. Aerenchymous tissues are much more porous to allow gases to diffuse up to 30 times more easily through a plant! In animals, lungs can allow some fish, mammals, and aquatic gastropods (snails) to live in hypoxic waters, but many fish have gills that are not adapted to hypoxia. The Gulf of Mexico along Louisiana’s coast boasts one of the largest hypoxic zones in the world with a peak area of over 8,500 square miles in 2017, where many commercial fisheries have seen a large decline in fish catch. [3]

PHOTO- dead zone map-NOAA-700x345-Landscape
Photo from NOAA, Dead Zone 2017

Works Cited:

[1] Gilman, Sharon. “Plant Adaptations.”

[2] “Adaptations.” Adaptations :: Florida Museum of Natural History,

[3] “Gulf of Mexico ‘Dead Zone’ Is the Largest Ever Measured.” Gulf of Mexico ‘Dead Zone’ Is the Largest Ever Measured | National Oceanic and Atmospheric Administration,

Featured image is of Rhizophora mangle (red mangrove) from Flickr by barloventomagico



Hypoxia is the lack of oxygen in the water column. In the Gulf of Mexico’s Texas-Louisiana Shelf, hypoxia is defined as seasonally low oxygen levels (less than 2 milligrams/liter). This Gulf of Mexico “dead zone” is caused by input of excess nutrient pollution, primarily nitrogen, to the gulf from the Mississippi River. Due to an overabundance of nutrients, excessive algal growth (eutrophication) may result and demand large quantities of oxygen which decreases both dissolved oxygen in the water and available aquatic habitat in the water column. The significant decrease in levels of dissolved oxygen in the water column results in the death of fish and shellfish and/or in their migration away from the hypoxic zone. The northern Gulf of Mexico adjacent to the Mississippi River is the site of the largest hypoxic zone in the United States (8.5 million acres) and the second largest hypoxic zone worldwide. Freshwater and sediment diversions from the Mississippi and Atchafalaya Rivers may help reduce the hypoxic zone off Louisiana’s coast by channeling nutrient-rich waters into coastal wetlands, where the nutrients can be used or trapped by marsh and aquatic vegetation.