Water Quality Research

Water Quality Research / Science

    Estuaries are places where salt water and fresh water meet and mix.

Estuaries are often defined as areas where fresh water mixes with salt water to create a distinctive brackish water ecosystem that has intermediate salinity levels. Many estuarine organisms thrive in water that is less saline than the open ocean, but most will tolerate short-term fluctuations in salinity levels. The salinity tolerance of estuarine organisms is significant because the salinity of the water within an estuary is not constant. Salinity is not extremely variable in the open ocean, but salinity in an estuary varies according to tidal fluctuations, the geographical position of the estuary, and the volume of fresh water entering the estuary. Fresh water may enter the estuary from rivers and streams, groundwater, or more directly from precipitation. Overall salinity levels in many estuaries decline seasonally when snow melt and/or heavy precipitation produces elevated freshwater inflow. The added fresh water “dilutes” the water in the estuary and lowers the salinity level. Freshwater inflow is critical to maintaining the lower salinity levels of estuarine ecosystems, but many of these ecosystems are currently at risk due to reduced freshwater inflow that can be attributed to various factors. The science collaborative research projects and student activities in this module highlight the importance of freshwater inflow to our nation’s estuaries.

Science Collaborative Research Projects

Researchers at the Mission-Aransas National Estuarine Research Reserve (NERR), in Texas, and the Rookery Bay NERR, in Florida, are investigating factors that affect the amount of fresh water that reaches estuaries. Balancing the water needs of human communities with those of the natural systems on which people depend has become an increasing challenge. The freshwater demands of growing human populations, changes in land use, water control structures, and a changing climate may all contribute to reduce the amount of freshwater that many estuaries receive. The NERR researchers are trying to better understand the level of freshwater inflow needed to maintain healthy estuaries. A main goal of these research projects is to develop recommendations regarding how much freshwater inflow is needed to support healthy human communities and the estuaries upon which they depend.

Understanding the ecological effects of fluctuating salinity levels is critical for developing appropriate freshwater inflow recommendations for estuaries. In Texas, coastal development and a growing human population are placing increasing demands on the freshwater supply, at a time when historic droughts have been recorded in the state. Limited freshwater inflow during drought years has caused the salinity levels of many Texas bays to rise above the average salinity of the open ocean. The Mission-Aransas NERR is conducting a research project that will help resource managers establish future freshwater inflow requirements in the face of changing climate and land use patterns.

Map showing the Mission-Aransas Estuary and the NERR boundary.

Mission-Aransas NERR researchers are studying how certain estuarine species respond to different salinity levels within the estuary. Researchers are collecting blue crab (Callinectes sapidus) larvae and environmental data at sites throughout the estuary, to assess whether salinity and other water quality and weather cues control the movement of larvae into the estuary from the open ocean. Blue crabs are an ecologically and economically important species that support a large commercial fishery in Texas and they are a critically important food source for the endangered Whooping Cranes (Grus americana) that winter in the Mission-Aransas Estuary.

Blue crab larva in the megalopal stage, collected in the Mission-Aransas Estuary.

In south Florida, coastal development and a growing human population have increased the need for advanced and effective stormwater management systems. Historically, south Florida and the famous Everglades ecosystem consisted of a large, flat wetland area where water moved slowly across the land as sheet flow. Today, much of the sheet flow is funneled into canals to make drier areas for roads, homes, and other developments. Canals bring stormwater into the estuary rapidly. This rapid delivery of freshwater to the estuaries in south Florida alters the amount, timing, and quality of water entering the estuary.

Map showing the location of the Rookery Bay NERR.

Rookery Bay NERR researchers are working with engineers to develop computer-based models that describe the historic watershed and water flows into the estuary as well as the local watershed under current conditions. Current conditions include canals and a variety of land uses. By understanding how the watershed works then and now, researchers, engineers, and natural resource managers can work together to find solutions that restore historical water flows and estuarine conditions while maintaining adequate drinking supply and flood control in south Florida.