Posts Tagged ‘rivers’

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

June 6, 2016

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.

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.

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.

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Scientists work to predict 22nd century look of the Georgia coast

August 27, 2015

University of Georgia Skidaway Institute of Oceanography scientist Clark Alexander is working on a project to predict how the Georgia coast—characterized by a complex system of barrier islands, salt marshes, estuaries, tidal creeks and rivers—may look 25, 50 and 100 years from now. As sea level rises over the next century, that picture is changing.

Predictions of sea level rise over the next century vary from the current rate of roughly 30 centimeters—about a foot—to as much as two meters—about 6 feet. Although scientists disagree on the ultimate height of the rise, they all agree that salty water is moving inland and will continue to do so for the foreseeable future, Alexander said. Here on the Georgia coast, islands will become smaller or disappear entirely; salt marshes will be inundated by the rising waters and migrate towards the uplands; and some low-lying uplands will become salt marshes.

To predict the extent of these changes, scientists are using the predictive Sea Level Affecting Marshes Model, or SLAMM, which was originally developed for the U.S. Fish and Wildlife Service.

SLAMM predicts the effects of future sea level rise based on two key inputs: an elevation mapping of the coastal zone and salinity profiles up the rivers and waterways. Salinity and elevation are two key factors that determine the type of plants, and thus habitat, that will be present at any particular location.

“As sea level rises, the fresh water in rivers will be pushed further upstream,” Alexander said. “The brackish and salty water will also move up, and the salt marshes will expand.”

Researcher Mike Robinson adjusts the salinity monitoring equipment while LeAnn DeLeo drives the boat.

Researcher Mike Robinson adjusts the salinity monitoring equipment while LeAnn DeLeo drives the boat.

Funded by a Coastal Incentive Grant from the Georgia Department of Natural Resources Coastal Management Program, Alexander and his team have been studying the five key river systems along the coast and numerous salt marsh estuaries. Salinity along the coast is dominantly affected by river discharge into the estuaries, so the team has been conducting its surveys during both winter—high river flow—and the summer—low river flow—conditions.

“We start at the mouth of a river about an hour before high tide and then we follow that high tide up the river, mapping the surface salinity along the way,” Alexander said. “We find the maximum inshore intrusion of salinity at high tide during a spring tide. That is the location that defines the boundary between the brackish marshes and the freshwater marshes.”

In addition to tracking surface salinity, the researchers also stop periodically and measure the salinity throughout the water column to determine if what they measure at the surface is similar to what is present near the bottom. They lower a device that measures the water conductivity (which is related to salinity), temperature and depth from the surface to the bottom. Also equipped with GPS capability, the device automatically captures the location of every water column profile.

Researcher LeeAnn DeLeo lowers the sensor to measure conductivity, temperature and depth from the surface to the bottom.

Researcher LeeAnn DeLeo lowers the sensor to measure conductivity, temperature and depth from the surface to the bottom.

In many coastal regions, denser, saltier water tends to sink to the bottom and the lighter, fresh water remains near the surface. However, because of the energy produced by Georgia’s wide tidal range, the team found that most of the water on the Georgia coast is well mixed and doesn’t show up as layers.

The second part of the project is to fine-tune existing elevation data. Scientists have an extensive set of elevation information from airplane-mounted Light Detection And Ranging systems. LIDAR is usually very accurate, except in marshes, because it cannot see through the vegetation to the actual ground surface.

“You might be off by 30 centimeters or more, and in a low-lying, flat area like our coastal zone, that can make a big difference in predicting where the water will flood,” Alexander said.

The Skidaway Institute team is working with Georgia Southern University scientist Christine Hladik on a fix. By comparing LIDAR data with the true elevation in a particular area, Hladik observed that the LIDAR error varied according to the type of plants growing there. For example, if the area contained the dense, tall spartina, the error was large and, on average, a consistent number of centimeters. If the region was covered with a different, less-dense-growing salt marsh plant, like short spartina, the error was smaller but also consistent.

“She discovered that if you know what type of vegetation is covering a section of marshland, you can plug in the correction and come back with an accurate measure of the elevation,” Alexander said.

The research team observed the vegetation and measured the true ground level at 400 randomly selected points throughout coastal brackish and salt marshes in Georgia. That information and knowledge of plant types is being used to correct the existing marsh elevations.

The research team will complete one more set of river surveys before the project ends in September. Alexander hopes to obtain continued funding to use this newly acquired elevation and salinity data in a fresh SLAMM model run for the Georgia coast, using all the high-resolution data developed in this project.

“We should be able to look out as much as 100 years in the future and see where the different wetlands will be by then,” he said. “That way we can plan for marsh sustainability, retreat and sea level rise.”