Richard Lowrance

Measurement of Water Residence Time, Flowpath and Sediment Oxygen Demand in Seasonally Inundated Floodplain Swamps of the Georgia Coastal Plain

Blackwater streams and rivers are found throughout the Coastal Plain of the southeastern United States. These streams are characterized by low slopes, high summertime temperatures, large inputs of dissolved organic material, and extensive inundation of surrounding floodplains. Typically lasting from winter to early spring, the long inundation period creates a multitude of instream swamps and floodplain wetlands that play a vital role in overall water quality. Over 90% of the blackwater streams listed as impaired on the Coastal Plain of Georgia are listed because of violation of the state’s dissolved oxygen (DO) standard. Streams are listed as impaired if DO falls below a 4 mg L-1 minimum or 5 mg L-1 24-hour average. Generally assumed to be a consequence of increased biological activity from nitrogen and phosphorus enrichment, lowered DO may instead be a natural phenomenon within this system. In an effort to measure the magnitude and influence of floodplain swamps on levels of DO in the river channel, we investigated the residence time, flowpath and sediment oxygen demand (SOD) within a large instream swamp in a representative blackwater river system. Water within one swamp (1550 m in length) had a travel time between 15.5 and 27 hours, with the degree of dispersion highly dependent on flow. SOD is often a critical and dominant sink of oxygen in river systems and despite its importance, is often poorly investigated or estimated in oxygen budgets. Results show SOD rates between 0.87 - 15.84 g O2 m-2 day-1, which is generally higher than values reported in the literature for southeastern sandy-bottomed streams. Coupled with the long residence time, SOD may play a central role in determining DO levels within these instream swamps and for the river system as a whole.

Nutrient Enrichment and Stream Periphyton Growth in the Southern Coastal Plain of Georgia

Blackwater rivers are common throughout the Atlantic coastal plain and water quality is heavily influenced by the flat topography, sandy soils and floodplain swamp forests. In the southern coastal plain of Georgia, streams regularly violate dissolved oxygen (DO) standards established by the Georgia Department of Natural Resources. Total Maximum Daily Load (TMDL) management plans must be developed for watersheds that are drained by DO-impaired streams but previous studies suggest DO may be naturally low. At nine sites throughout the region, eighteen passive nutrient diffusion periphytometers were deployed to determine if algal growth was nutrient and/or light limited. Periphyton biomass for treatments in the sun, measured as chlorophyll a, was significantly (p < 0.05) greater than corresponding treatments in the shade and algal growth was nutrient-limited at several sites where DO concentrations were well below regulatory standards. Factors other than algae may be responsible for low DO concentrations during summer.

Sediment Oxygen Demand of Coastal Plain Blackwater Streams

Sediment Oxygen Demand (SOD) has become and integral part of modeling dissolved oxygen within surface water bodies. Because very few data on SOD are available, it is common for modelers today to take SOD values from the literature for use with dissolved oxygen (DO) models. SOD is such an important parameter in modeling DO that this approach may lead to erroneous results. This paper reports on an extensive study to quantify SOD in blackwater streams of the Georgia coastal plain. In-situ SOD measurements are made in the Upper Suwannee, Alapaha, Little River, and Withlacoochee river basins. The subwatersheds within which SOD measurements are taken are chosen to vary from 3000-7000 ha in area and are classified as predominantly forested or predominantly agricultural. SOD is measured using four in-situ chambers. In addition to SOD measurements, a particle size analysis is completed on the sediment and water flow is measured at each site. The result of this paper connects SOD values to specific sediment composition and land use properties. By recording percent sand, silt, clay, organics, and flow, SOD values recorded in one region may be applied to similar conditions in another region. Results from this study will be used by the Georgia Department of Natural Resources – Environmental Protection Division as input data to their Georgia DO Sag model which is used to develop DO TMDLs or evaluate already developed DO TMDLs in the Georgia coastal plain.

Long-Term Nitrogen Load from the Little River Experimental Watershed on the Coastal Plain of Southwest Georgia

The USDA-Agricultural Research Service, Southeast Watershed Research Laboratory initiated flow measurement of the Little River in a 334 km2 area near Tifton, Georgia in the late 1960’s. Monitoring of stream nitrogen concentrations began in 1974 for seven of the eight nested subwatersheds in the area, known as the Little River Experimental Watershed. This paper summarizes the first 30 years of the stream nitrogen record, from 1974-2003. The calculated nitrogen loading data provides insight into the effects of changing land use and agricultural practices, stream-side riparian zones, and climate cycles on nitrogen cycling over the long term in the Southeast Coastal Plain.

Tree species effects on microbial respiration and leaf breakdown in a coastal plain blackwater stream: implications for dissolved oxygen TMDLs

Dissolved oxygen (DO) levels in the Little River, a tributary within the Suwannee River basin of southern Georgia, regularly drop below 1 mg L-1. This requires the development of total maximum daily load (TMDL) management and implementation plans for watersheds drained by these water body segments. However, potential site-specific factors influencing oxygen demand are not fully understood. We are developing a budget of coarse particulate organic matter (CPOM) for third- and fifth-order reaches of the Little River. Our CPOM budget will examine the dynamics of riparian inputs, standing stocks of various types of CPOM (e.g. leaf litter, woody debris), leaf litter breakdown, and transport and settling of CPOM and the fine particulate organic matter that results from its processing. In addition, we are examining the oxygen demand generated by the metabolic activities (respiration) of litter-associated microbial assemblages. Preliminary findings indicate that rates of litter mass loss and respiration of attached microbes differs significantly among the tree species examined. As a consequence, our budget of CPOM inputs and standing stocks will be sub-divided by tree species. Once developed, this budget will enhance the understanding of dissolved oxygen dynamics within coastal plain blackwater streams. Furthermore, this budget may provide important reference information for policy makers interested in management plans for these watersheds.

Low dissolved oxygen levels in Georgia floodplain swamps: Effect of flowpath and sediment oxygen demand

Blackwater streams and rivers are found throughout the Coastal Plain of the southeastern United States. These streams are characterized by low slopes, high summertime temperatures, large inputs of dissolved organic material, and extensive inundation of surrounding floodplains. Typically lasting from winter to early spring, the long inundation period creates a multitude of instream swamps and floodplain wetlands that play a vital role in overall water quality. Over 90% of the blackwater streams listed as impaired on the Coastal Plain of Georgia are listed because of violation of the state’s dissolved oxygen (DO) standard. Streams are listed as impaired if DO falls below a 4 mg L-1 minimum or 5 mg L-1 24-hour average. Generally assumed to be a consequence of increased biological activity from nitrogen and phosphorus enrichment, lowered DO may instead be a natural phenomenon within this system. In an effort to measure the magnitude and influence of floodplain swamps on levels of DO in the river channel, we investigated the residence time, flowpath and sediment oxygen demand (SOD) within a large instream swamp in a representative blackwater river system. Water within one swamp (1550 m in length) had a travel time between 15.5 and 27 hours, with the degree of dispersion highly dependent on flow. SOD is often a critical and dominant sink of oxygen in river systems and despite its importance, is often poorly investigated or estimated in oxygen budgets. Results show SOD rates between 0.87 – 15.84 g O2 m-2 day-1, which is generally higher than values reported in the literature for southeastern sandy-bottomed streams. Coupled with the long residence time, SOD may play a central role in determining DO levels within these instream swamps and for the river system as a whole.