Satellites See Big Changes Since 1980s in Key Element of Ocean's Food Chain
Since the early 1980s, ocean phytoplankton concentrations that drive the marine food chain have declined substantially in many areas of open water in Northern oceans, according to a comparison of two datasets taken from satellites. At the same time, phytoplankton levels in open water areas near the equator have increased significantly. Since phytoplankton are especially concentrated in the North, the study found an overall annual decrease in phytoplankton globally.
The authors of the study are Watson Gregg, of NASA's Goddard Space Flight Center, Greenbelt, Md., and Margarita Conkright, a scientist at the National Oceanic and Atmospheric Administration's (NOAA) National Oceanographic Data Center, Silver Spring, Md. They discovered what appears to be an association between more recent regional climate changes, such as higher sea surface temperatures and reductions in surface winds, and areas where phytoplankton levels have dropped.
Phytoplankton consist of many diverse species of microscopic free-floating marine plants that serve as food to other ocean-living forms of life. "The whole marine food chain depends on the health and productivity of the phytoplankton," Gregg said.
The researchers compared two sets of satellite data -- one from 1979 to 1986 and the other from 1997 to 2000 -- that measured global ocean chlorophyll, the green pigment in plants that absorbs the Sun's rays for energy during photosynthesis. The earlier dataset came from the Coastal Zone Color Scanner (CZCS) aboard NASA's Nimbus-7 satellite, while the latter dataset was from the Sea-Viewing Wide Field of View Sensor (SeaWiFS) on the OrbView-2 satellite.
The researchers re-analyzed the CZCS data with the same processing methods used for the SeaWiFS data, and then blended both satellite measurements with surface observations of chlorophyll from ocean buoys and research vessels over corresponding time periods. By doing so, the researchers reduced errors and made the two records compatible.
Results indicated that phytoplankton in the North Pacific Ocean dropped by over 30 percent during summer from the mid-80s to the present. Phytoplankton fell by 14 percent in the North Atlantic Ocean over the same time period.
Also, summer plankton concentrations rose by over 50 percent in both the Northern Indian and the Equatorial Atlantic Oceans since the mid-80s. Large areas of the Indian Ocean showed substantial increases during all four seasons.
"This is the first time that we are really talking about the ocean chlorophyll and showing that the ocean's biology is changing, possibly as a result of climate change," said Conkright. The researchers add that it remains unclear whether the changes are due to a longer-term climate change or a shorter-term ocean cycle.
Phytoplankton thrive when sunlight is optimal and nutrients from lower layers of the ocean get mixed up to the surface. Higher sea surface temperatures can reduce the availability of nutrients by creating a warmer surface layer of water. A warmer ocean surface layer reduces mixing with cooler, deeper nutrient-rich waters. Throughout the year, winds can stir up surface waters, and create upwelling of nutrients from below, which also add to blooms. A reduction in winds can also limit the availability of nutrients.
For example, in the North Pacific, summer sea surface temperatures were 0.4 degrees Celsius (0.7 Fahrenheit) warmer from the early 1980s to 2000, and average spring wind stresses on the ocean decreased by about eight percent, which may have caused the declines in summer plankton levels in that region.
Phytoplankton currently account for half the transfer of carbon dioxide from the atmosphere back into the biosphere by photosynthesis, a process in which plants absorb carbon dioxide (CO2) from the air for growth. Since carbon dioxide acts as a heat-trapping gas in the atmosphere, the role phytoplankton play in removing carbon dioxide from the atmosphere helps reduce the rate at which CO2 accumulates in the atmosphere, and may help mitigate global warming.
The paper appears in the current issue of Geophysical Research Letters.
For more information, please see www.gsfc.nasa.gov/topstory/20020801plankton.html.
Study Finds Most Livestock Waste Lagoons Not Polluting Groundwater
Most Nebraska livestock waste lagoons are not polluting groundwater, a two-year University of Nebraska study indicates.
Researchers monitored 26 swine, dairy and beef cattle waste lagoons at 13 sites in central and eastern Nebraska. They tested nearby groundwater and lagoon water to assess the lagoons' potential threat to groundwater quality.
"The majority of the waste lagoons in the study have not adversely impacted groundwater," said Roy Spalding, a research hydrochemist and director of the university's Water Sciences Laboratory who headed this research. "This is particularly significant because of agriculture's importance to our state and to the public, considering roughly 85 percent of Nebraskans drink groundwater."
Water lab scientists and Mariappan Sadayappan, a University of Nebraska at Lincoln (UNL) agronomy and horticulture graduate student, recently completed the study that was designed to identify indicators of waste lagoon seepage and to evaluate these lagoons' effect on groundwater quality.
The size of lagoons and their environmental settings varied greatly; 12 of the 13 sites, were in use or active, Spalding said. Livestock producers who operated the lagoons volunteered for the monitoring project that ran from 1999 through 2001.
Ten of the 12 active sites sampled did not appear to pollute nearby groundwater, Spalding said.
Three of the lagoons "probably contributed to a reduction of high nitrate levels in the area around them." That's because beneath these lagoons, naturally occurring bacteria fed on the nitrates thereby reducing the nitrate in the sampling wells downgradient from the lagoon, Spalding said.
"This process tends to reduce the amount of nitrates flowing beneath the lagoons," he said.
Both these lagoons are in areas vulnerable to pollution because groundwater is less than 35 feet below the surface and the soils are coarse textured and easily permeable.
The Institute of Agriculture and Natural Resources team sampled each of the lagoons and the surrounding groundwater four times over the two-year study in both the spring and fall. Researchers took samples at different depths in the lagoons to help determine variations in sample content that might occur either vertically or laterally in lagoons or groundwater.
Samples were analyzed for levels of nitrate-nitrogen, ammonia-nitrogen, chloride, dissolved organic carbon and total organic carbon.
High concentrations of chloride and ammonia, are prime indicators of groundwater pollution from livestock waste lagoons, Spalding said.
Scientists also analyzed samples for nitrogen isotopes, which help distinguish the source of nitrogen, such as fertilizer, animal waste or naturally occurring in soils.
Spalding said that, overall, these findings show that most livestock lagoons don't harm groundwater quality. However, an individual lagoon's impact on groundwater quality depends on site conditions including depth to groundwater, geology, construction, soil type, extent of groundwater use in the immediate vicinity and how the lagoon is operated, such as how often and when it is pumped out.
"Though it appears a majority of the examined lagoons have not contributed to groundwater pollution, we can't be complacent since there are still documented cases where these facilities can have negative impacts on groundwater quality," he said. "That makes it important to carefully review each facility in order to better understand the possible effect it might have on groundwater pollution."
Researchers also sampled 11 of the 13 waste lagoon sites for antibiotics commonly added to livestock feed or water. These included oxytetracycline, tetracycline and chlortetracycline.
Groundwater samples collected at these sites showed no detectable levels of these antibiotics even though traces of at least one of the three antibiotics were found in the lagoon water in 23 of the 26 samples they collected.
"Although traces of antibiotics were measured in 23 of the 26 lagoon water samples, they were not detected in the groundwater beneath or near them," Spalding said.
The water lab studied the lagoons in cooperation with the Nebraska Department of Environmental Quality, which is responsible for developing and implementing programs to manage storage, handling and application of animal wastes associated with livestock operations.
The U.S. Environmental Protection Agency helped fund the research, which was conducted in cooperation with IANR's Agricultural Research Division.
This article originally appeared in the 09/01/2002 issue of Environmental Protection.