Dissolved oxygen (DO) is defined as the concentration of oxygen that is soluble in water at a given altitude and temperature (Mortimer 1981). The concentration of oxygen dissolved in water is far less than that in air; therefore, subtle changes may drastically impact the amount of oxygen available to support many aquatic plants and animals. The dynamics of oxygen distribution, particularly in inland waters, are essential to the distribution, growth, and behavior of aquatic organisms (Wetzel 2001). Many factors affect the DO in an aquatic system, several of them natural. Temperature, salinity, sediments and organic matter from erosion, runoff from agricultural and industrial sources, wastewater inputs, and excess nutrients from various sources may all potentially impact DO. In general, the more organic matter in a system, the less DO available. DO levels in a water body are dependent on physical, chemical, and biochemical characteristics (Clesceri 1989).

As discussed in Section 1, the St. Johns River is classified as a Class III water body by the State of Florida. Until 2013, the Class III Freshwater Quality Criterion (WQC) for DO has been 5.0 mg/L (62-302.530, F.A.C.; DEP 2013h), requiring that normal daily and seasonal fluctuations must be maintained above 5.0 mg/L to protect aquatic wildlife. The predominantly freshwater part of the LSJR extends north from the city of Palatka to the mouth of Julington Creek. The Florida DEP developed site specific alternative criteria (SSAC) for the predominantly marine portion of the LSJR between Julington Creek and the mouth of the river, which requires that DO concentrations not drop below 4.0 mg/L. DO concentrations between 4.0 and 5.0 mg/L are considered acceptable over short time periods extending up to 55 days. However, the DO average cannot be less than 5.0 mg/L in a 24-hour period (DEP 2010d).

In April 2013, the U.S. EPA approved new DO and nutrient-related water quality standards to be adopted by the Florida Environmental Regulation Commission (ERC). The revisions approved by the U.S. EPA include “revised statewide marine and freshwater DO criteria, anti-degradation considerations regarding any lowering of DO, protection from negative trends in DO levels, and the inclusion of total phosphorus, total nitrogen, and chlorophyll-a criteria, among other provisions relating to DO and nutrients.” The State’s revisions also require the protection of several federally listed threatened and endangered species, including three sturgeon and one mussel species.

Under the new revisions, in predominantly freshwaters of the SJR in the Peninsula bioregion, the DO should not be less than 38 percent saturation, which is equivalent to approximately 2.9 mg/L at 30°C and 3.5 mg/L at 20°C (DEP 2013b). Additionally, in the portions of the LSJR inhabited by Shortnose or Atlantic Sturgeon, the DO should not be below 53 percent saturation, which is equivalent to approximately 4.81 mg/L at 20°C, during the months of February and March. After much assessment, the FDEP supported that maintaining the 5.0 mg/L minimum DO criterion in the location where spawning would occur should “assure no adverse effects on the Atlantic and Shortnose sturgeon juveniles.”

For predominantly marine waters, minimum DO saturation levels shall be as follows:

DO reaches 100% saturation when an equilibrium between the air and water is reached.  However, many factors can influence DO saturation, namely temperature, salinity, biological activity, and vertical location in the water column.

As temperature increases, the solubility of oxygen decreases (Mortimer 1981). Biological activities, such as respiration, microbial decomposition, and photosynthesis also influence DO saturation and are also affected by temperature. Warmer temperatures increase respiration and microbial decomposition in aquatic organisms, which are processes that require oxygen, and thus lower the DO (Wetzel 2001). Warmer temperatures also increase metabolism and production of bacteria and phytoplankton which contribute to a higher BOD and a lower DO. Alternatively, the process of photosynthesis adds oxygen (as a waste product) into the water (Wetzel 2001).  Photosynthesis can contribute to supersaturation of a water body potentially bringing the DO above 100% saturation, particularly during daytime hours in photosynthetically active water bodies.

Shallow areas and tributaries of the LSJR that are without shade have particularly elevated temperatures in the summer months and can reach 100% saturation at a lower DO concentration. Therefore, DO concentration decreases during those times. The DO changes are compounded in waters with little movement, so turbulence is also a pertinent parameter in the system. Turbulence causes more water to contact the air and thus more oxygen mixes and diffuses into the water from the atmosphere.

Salinity is another factor that affects DO concentrations in the LSJRB. Increasing salinity reduces oxygen solubility causing lower DO in aquatic systems. At a constant temperature and pressure, normal seawater has about 20% less oxygen than freshwater (Green and Carritt 1967; Weiss 1970). Factors influencing DO, such as increasing temperatures and BOD, will be compounded in saltwater as compared to freshwater.

Furthermore, productivity and sediment type can also influence the DO concentration. DO usually exhibits a diurnal (24-hour) pattern in eutrophic or highly productive aquatic systems. This pattern is the result of plant photosynthesis during the day, which produces oxygen, such that the maximum DO concentration will be observed following peak productivity, often occurring just prior to sunset. Conversely, at night, plants respire and consume oxygen, resulting in an oxygen minimum, which often occurs just before sunrise (Laane et al. 1985; Wetzel and Likens 2000). The LSJR is highly productive; however, as discussed above, it is a blackwater river, and photosynthesis by submerged aquatic vegetation is limited. In addition to the diurnal DO cycle described, bacterial oxygen demand generally dominates following algal blooms due to decomposition processes and is present both during the day and the night.

2.2.3. Data Sources

All data used for the DO and BOD analyses were from the Florida DEP STOrage and RETrieval (STORET) database through June 2017, and data were retrieved from the EPA Water Information Network (WIN) database from July 2017 to the present. STORET and WIN are computerized environmental data systems containing water quality, biological, and physical data. From the data sets, negative values were removed.  Values designated as present below the quantitation limit (QL) were replaced with the “actual value” if provided or replaced with the average of the method detection limit (MDL) and practical quantitation limit (PQL) if the “actual value” was not provided. For “non-detect” values, half the MDL was used; and, for values designated as “zero” the MDL was used. All samples with qualifier codes K, L, O, Q, or Y, which indicate different data quality issues, were eliminated from the STORET data file. Data designated with a matrix of “ground water,” “surface water sediment,” “stormwater,” or “unknown” were also removed. Records with no analytical procedure listed were also removed. This section examines the data from the freshwater part of the mainstem (WBID 2213I-N), the predominantly saltwater part of the mainstem (WBID 2213A-H), the entire LSJRB (Figure 2.1) and the tributaries (discussed more in Section 2.8).

Data are presented in box and whisker plots, which consist of a five-number summary including: a minimum value; value at the first quartile; the median value; the value at the third quartile; and the maximum value. The size of the box is a measure of the spread of the data with the minimum and maximum values indicated by the whiskers. The median value is the value of the data that splits the data in half and is indicated by the horizontal blue line in the center of the boxes.  The data also are presented as annual mean value ± standard deviation and compared to the designated reference values.

2.2.4. Limitations

The time of day in which water quality is measured can strongly influence the result due to the diurnal pattern of DO. Additionally, some of the more historic data lacks pertinent corresponding water quality characteristics, such as tides, which may have impacted the measurements.

Figure 2.2 shows median DO concentrations and the range of the DO values measured in the LSJR from 2005 to 2022. Median DO values have fluctuated between 60 and 85 percent of saturation in the LSJR during that time, however, median values have been stable and above the water quality criteria values (within acceptable limits) since 2016 (Figure 2.2). However, minimum DO concentrations have been below the criteria since 2005, particularly in the tributaries (Figure 2.2). Figure 2.3 shows annual mean DO values and trendlines of the same data set. Since 2016, there have been slight fluctuations, but DO mean concentrations in the LSJR have been mostly stable (Figure 2.3). Mean values in the mainstem have been above reference values and within acceptable limits (Figure 2.3). In the tributaries, mean DO concentrations were at or near the marine/estuarine reference value from 2005 to 2015, then increased (improved) slightly in 2016 and have been stable since then (Figure 2.3). Annual mean DO values are now within acceptable limits in the tributaries (Figure 2.3) while minimum DO concentrations remain below the freshwater and marine/estuarine reference values (Figure 2.2). Resident aquatic life may be at risk during low DO events in the tributaries of the LSJR, particularly in the marine/estuarine areas. Water quality conditions in each tributary will be addressed separately in Section 2.8 because DO concentrations can vary among tributaries, depending on the time of day, surrounding land use, water flow, depth, and salinity.

A seasonal trend, with the lowest concentrations observed in the summer months (June-September), was observed in the data from the entire LSJR (Figure 2.4). It is likely that the aquatic life inhabiting these areas will be more affected by low DO events during the summertime.

Since 2005, there have been fluctuations in the median BOD values in the LSJR mainstem (Figure 2.5). In 2017, median BOD values increased to approximately 2 mg/L and have been stable at this concentration to the current date (Figure 2.5). The median and upper quartile BOD values have increased substantially in the LSJR tributaries since 2016 (Figure 2.5B). A seasonal pattern of increased BOD values was observed in the LSJR mainstem, especially in the marine/estuarine areas, with the highest values observed in summer months (Figure 2.6), concurrent with the reported decreased DO concentrations.

Taking everything into account, the current overall STATUS for DO in the LSJR mainstem is satisfactory, and the TREND is unchanged. However, the STATUS for DO in the LSJR tributaries is unsatisfactory (dependent on location, time of day, and season), and the TREND is unchanged.

2.2.6. Future Outlook

Analysis of available data indicates that the average DO levels in the LSJRB are generally within acceptable limits; however, unacceptable DO concentrations still occur intermittently. Low DO was most problematic during summer months with many of the lowest measurements occurring in tributaries. Certain areas of the LSJR that experience DO concentrations below 5.0 mg/L for prolonged periods may be too low to support the many aquatic animals that require oxygen (EPA 2002a; EPA 2002b). Maintenance above minimum DO levels is critical to the health of the LSJR and organisms that depend on it. Nutrient reduction strategies, discussed in the next section, have recently been devised by government agencies and may combat the low DO concentrations observed in the LSJR to some extent. Additionally, monitoring agencies are now making efforts to collect data that better represent the variable DO conditions and to concurrently document other important water quality characteristics for an improved assessment of the river’s health. It is also important to note that hurricane Mathew and hurricane Irma, which impacted Jacksonville October 7, 2016, and September 10-11, 2017, respectively, resulted in changes in water quality in the LSJR which persisted through 2018 (Bielmyer-Fraser et al. 2020). Massive flooding into the river changed the salinity in some areas and could have contributed to changes in DO, BOD, and other water chemistry parameters. Other severe storms in recent years have had similar effects, although to lesser degree.