Evolutionary and ecological changes in zooplankton

An assistant professor at the University of Texas at Arlington has received a $220,000 National Science Foundation Long-Term Ecological Research grant to examine zooplankton in freshwater ecosystems in Alaska and Wisconsin. Dr. Matthew Walsh, a UT-Arlington biologist, will run various tests on zooplankton habitats in over 20 lakes to better document, understand and predict how these micro-invertebrate organisms respond to natural ecological changes and shifts that have been caused by global warming (and other environmental pollutants). 

Zooplankton are very important organisms in ecological systems, therefore their population sizes will impact life in their food webs. If they are evolving due to human impact as well as natural ecological succession, then this research will help scientists predict these changes for the future.

"We're really asking key questions about evolution and how predictable it is, and how consistent it is across diverse ecosystems," Walsh explained in a UT-Arlington press release.

The importance of zooplankton in marine ecosystems
Zooplankton are vital organisms for diverse oceanic food webs and ecosystems mainly because of their massive quantities. The most common group of zooplankton found in the oceans are copepods. According to the ICES Journal of Marine Science, copepods are the most abundant multicellular zooplankton on the planet, outnumbering insects by three orders of magnitude. 

"We're really asking key questions about evolution and how predictable it is."

Zooplankton are the grazers of ocean ecosystems. They are the first organisms to convert energy from the producers to the consumers at the higher trophic levels in food webs. Zooplankton are a food source for many different types of organisms, including various fish, marine mammals and turtles. Unlike terrestrial food webs, the largest animals in the ocean, such as baleen whales, act as secondary consumers when they eat zooplankton as their primary source of food, while the largest terrestrial animals tend to be herbivores or primary consumers. Presumably, this is because ocean ecosystems do not have the photosynthetic biomass to support large marine herbivores. 

Benthic organisms such as sponges, echinoderms, crabs and anemones also benefit from zooplankton. The nitrogen produced through zooplankton excretion helps support bacterial and phytoplankton reproduction. Bacteria colonize in the fecal matter produced by the zooplankton, making rich sources of organic carbon for detritivores. This food source is consistent for the diverse organisms found on the sea floor. 

Zooplankton's ability to survive in increased water temperatures
The impacts of global warming in aquatic ecosystems have more repercussions than terrestrial ecosystems because the temperature influences on the water column can affect nutrient abundance, reproduction rate, population density and sizes of organisms. In the ICES Journal of Marine Science, Dr. Anthony Richardson, a researcher of climate change and plankton ecology at the University of Queensland and CSIRO Marine and Atmospheric Research, said that temperature is the single most important physical variable structuring marine ecosystems. The production, abundance and trophic efficiency of zooplankton communities is directly linked to the temperature of the water. 

Both fresh and marine water that is warmed at the surface is a strong indicator for the amount of nutrients available in the water column. This results in an increase in nitrate, a principle organic compound that can limit the amount of phytoplankton growth in the water which, in turn, decreases the amount of zooplankton produced, effecting the rest of the trophic levels in the food web. Nutrient limitation is directly correlated to warm temperatures and can be used to predict long- and short-term effects on diverse fresh and marine ecosystems. As the global temperature continues to increase, the populations of zooplankton and other organisms in aquatic ecosystems will continue to decrease, which will cause a breakdown in food webs around the world.

Freshwater ecosystems depend on zooplankton for survival. Freshwater ecosystems depend on zooplankton for survival.

Impact of global warming on zooplankton in Atlantic ecosystems
Zooplankton are an integral part of many aquatic ecosystems and food webs, including organisms like whales, fish and seals. Although human fishing has caused a dramatic decrease in many of these populations, human-induced global warming has recently found to have caused a greater impact in the last 30 years. Calanus finmarchicus, a crustacean found in much of the Atlantic Ocean, is a key source of food for many marine organisms. This zooplankton species typically lives in dense patches where currents converge, or at a boundary of water with different densities. Changes in sea water temperature, salinity, winds and water currents can all effect zooplankton patch formation and density. 

According to New England Aquarium researchers, shifts in zooplankton species will affect various organisms that call the Atlantic Ocean home. During the late 1990s zooplankton populations decreased drastically, causing many animals, specifically whales, to struggle with reproduction. Atlantic right whale reproduction is directly correlated to the density of zooplankton; without zooplankton, female whales cannot bulk up before pregnancy, carry a pregnancy to term or produce enough milk post-pregnancy. 

"Shifts in zooplankton species will affect various organisms that call the Atlantic Ocean home."

In the last 50 years, scientists have mapped the densities of zooplankton patches using the North Atlantic Oscillation (NAO) index, which charts the changes in atmospheric pressure in the Atlantic Ocean. Areas that have a positive value on the NAO index have a greater population density of zooplankton compared to areas that have negative values on the index. The effect of the atmospheric pressure on the "great ocean conveyor belt" is where the mixing of the warm, salty waters from the Gulf Stream combine with the cold, less salty waters of the North. The resulting changes of temperature and salinity that occurs in the Mid-Atlantic Ocean determine zooplankton population density. 

As global warming continues to occur, the changes caused by the glacial ice melting will no doubt impact the NAO index, which has predicted a continued decrease in the zooplankton patch densities and formation. Many species of vertebrate organisms will be affected by this loss, and in turn, the human population. Endangered invertebrate species surveys have already been utilized to count different populations of the Atlantic right whale and will continue to be used as many fish populations, including cod, haddock, herring and mackerel, decline due to the loss of zooplankton patches in the Atlantic Ocean.