UGA Skidaway Institute team studies nutrient levels in Georgia’s coastal estuaries

How much of a nutrient load is too much for Georgia’s coastal rivers and estuaries? A research team from University of Georgia Skidaway Institute of Oceanography is helping Georgia’s Environmental Protection Division answer that question. Their primary focus is on the estuary at the mouth of the Ogeechee River, where the researchers are measuring nutrient concentrations and other water properties to determine how they change as they flow through the estuary.

The nutrients are chemicals like nitrates and phosphates typically introduced into the rivers by agricultural runoff, storm water or sewage effluents, and the natural decay of organic matter in the river. When present in high concentrations, the nutrients act as fertilizer, promoting excessive growth of marine plants, especially microscopic marine plants called phytoplankton.

Researcher Kate Doyle lowers a sensor package into the water to measure salinity, temperature and depth.

Elsewhere on the East Coast, excessive nutrients in estuaries have been linked to toxic algal blooms that can cause fish kills or shellfish closures. Death and decay of algal blooms by bacteria can drive oxygen concentrations down to levels that are unhealthy for other marine life. These are not presently known to be significant problems in Georgia’s waters, but scientists and regulators do not know what the thresholds are for developing water quality problems.

“The Georgia EPD wants to know how much nitrogen is coming down the river and whether it has any consequences when it gets to the estuary,” said UGA Skidaway Institute scientist William Savidge. “It doesn’t really matter if you have high nutrient concentrations if it is not having a harmful effect.”

The EPD is interested in these issues because they are mandated by the Environmental Protection Agency to set limits on nutrient levels for Georgia’s estuaries. Savidge describes the mandate as a difficult problem for several reasons.

“There is not any current and systematic information on nutrient conditions in most of the estuaries,” he said, “nor is there much information on the consequences of nutrient availability in the estuaries, and it’s those consequences that are the most important.”

They are currently mapping the biological and chemical properties of the Ogeechee River estuary each season to assess the nutrient changes throughout the year and to see what effects can be seen in the river and the estuary. Twice every quarter for the last year, the researchers have followed the incoming tide and sampled the river continuously as they moved upstream from the mouth of the estuary to fresh water. They used an onboard set of sensors to obtain continuous surface measurements of temperature, salinity, dissolved oxygen, chlorophyll (indicative of phytoplankton), turbidity and colored dissolved organic matter. In addition to the continuous surface measurements, the team stopped periodically and collected water samples from the bottom and throughout the water column. The product of each of these expeditions was a detailed map of conditions on the river, and when and where they are changing.

Researcher Lixin Zhu filters larger-volume surface water samples collected from the flow-through system to analyze for dissolved organic carbon.

As they expected, Savidge and his team observed a wide range of conditions depending on the season. Nutrient inputs tend to be highest in the spring when agricultural fields are fertilized.

“Nutrient delivery is high in the spring, but we don’t have a high chlorophyll concentration in the Ogeechee River because, presumably, the nutrients are being washed off into the coastal ocean before any effect is noticed,” Savidge said

On the other hand, chlorophyll levels — which indicate phytoplankton population — are highest in the summer. Low summer river flow means water remains in the system longer. When combined with more sunlight and warmer temperatures, this slow flow this allows more time for the microscopic plants to grow.

In addition to sampling the Ogeechee River, the team is also conducting a smaller sampling project in the Altamaha River for comparison purposes.

Field work on the project will end in June, and Savidge expects to report the team’s findings to Georgia EPD by mid-summer.

“The Georgia EPD is going to have to balance the potential negative risks of nutrient loading versus the economic consequences of restricting nutrient additions,” Savidge said. “If, for example, most of the nutrient additions are agricultural, and that is creating problems downstream, the Georgia EPD may be forced by EPA to regulate nutrient additions, either by restricting how much fertilizer is placed on fields or mandating larger buffer zones around rivers and creeks.”

In addition to Savidge, the research team includes UGA Skidaway Institute scientists Jay Brandes and Aron Stubbins, research associate Kate Doyle and graduate student Lixin Zhu. UGA researchers Brock Woodson and Mandy Joye are also contributing.

Blogging on a Pacific Ocean cruise

Skidaway Institute scientist Clifton Buck has just begun a 57-day research cruise that will take him from Ecuador to Tahiti. He will be updating this blog with accounts of his trip.

Ahoy There!

I am will be spending the next two months aboard the University of Washington’s scientific Research Vessel Thomas Thompson  My shipmates and I are travelling from Ecuador to Tahiti as part of an international effort called GEOTRACES to better characterize the sources, sinks, and biogeochemical cycles of trace elements and isotopes (TEIs) in the oceans of the world. Trace elements are present in seawater at concentrations that are often far less than a part per billion but often play important roles in the ocean as nutrients, contaminants, and process tracers. The research implications will help us understand areas of study including climate change, the carbon cycle, ocean ecosystems, and environmental contamination.

Our route will take us through three distinct regions of the eastern Pacific Ocean, each with their own distinct chemical characteristics. We begin the highly productive waters off the coast of Peru. This region is known as one of the finest fisheries in the world due ocean currents that bring nutrient rich waters to the surface by a process called upwelling. These nutrients support a great deal of biological production in depths that light can reach otherwise known as the photic zone. However, when the microscopic plants and animals living in the photic zone die they sink towards the bottom. As they sink, their organic matter is remineralized by respiration which uses the dissolved oxygen in the surrounding waters. The result is an area within the interior of the ocean that is very low in oxygen called the oxygen minimum zone or OMZ. Similar areas can be found off the coasts of Oregon and Louisiana in the United States. The low oxygen concentrations not only impact the plants and animals in the area but also affect the cycling of the trace elements as well.

Next we will study the hydrothermal zone at the East Pacific Rise. The rise is an area of active volcanism on the sea floor and is a source for both particulate and dissolved metals to the interior of the ocean. At a hydrothermal zone, water within the ocean bottom can come into contact with magma and become incredibly hot. In addition to heat, the magma releases trace elements into the water. This super-heated water is then released through fissures in the crust creating features like “black smokers” which creates a plume of seawater enriched in TEIs. The magnitude of these sources is poorly understood and it is hoped that work on this project will help provide insights into their importance. We also hope to characterize the processes responsible for supply and removal of TEIs with the plume.

Finally, we will traverse the northern edge of the central South Pacific gyre. A central gyre is a system of currents which flow in a circular pattern over thousands of miles. The gyre interior is one of the most nutrient poor (oligotrophic) regions in all of the world’s oceans. In fact, this area contains the bluest water in the world because there is so little living in it.

My role on this project is to collect atmospheric samples. The atmosphere is an important source for trace elements to the surface ocean particularly in areas that are distant from the continental shelf and rivers. In future posts, I will describe the processes and equipment that we use to do this work. I will also try to give a sense of what life is like on a ship at sea for 57 days. Please check back often and leave questions for me in the comments section. I will do my best to answer them.

Thanks for reading!