2.2.1. Description and Significance: DO and BOD
DO is defined as the concentration of oxygen that is dissolved or solubilized 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 dissolved oxygen 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 2013j), requiring that normal daily and seasonal fluctuations must be maintained above 5.0 mg/L to protect aquatic wildlife. For regulatory purposes, the mouth of Black Creek divides the fresh and marine portions of the LSJR. Moving south from Black Creek (WBID I) to the city of Palatka (WBID N) is designated as the predominantly freshwater part of the LSJR. 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, provided that the DO average in a 24-hour period is not less than 5.0 mg/L (DEP 2010c).
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, the inclusion of total phosphorus, total nitrogen, and chlorophyll a criteria for the Tidal Peace River, 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:
“1. The daily average percent DO saturation shall not be below 42 percent saturation in more than 10 percent of the values; 2. The seven-day average DO percent saturation shall not be below 51 percent more than once in any twelve week period; and 3. The 30-day average DO percent saturation shall not be below 56 percent more than once per year.”
For more information, please refer to the U.S. EPA decision document (EPA 2013b).
Additionally, seasonal limits for Type 1 SSAC were implemented in February 2014 for certain areas of the LSJR, where the default criteria in Rule 62-302.530, F.A.C. would apply during the other times of the year. For the Amelia River, the segment between the northern mouth of the river and the A1A crossing, a SSAC for DO has been set to 3.2 mg/L as a minimum during low tide from July 1st through September 30th, and not below 4.0 mg/L during all other conditions. Likewise, Thomas Creek (including tributaries from its headwaters to the downstream predominantly marine portion) has a SSAC for DO of 2.6 mg/L, with no more than 10 percent of the individual DO measurements below 1.6 mg/L on an annual basis.
The 30-d average DO percent saturation value of 56%, which is the most conservative, was used as a reference value to compare to the data from the marine/estuarine portion of the LSJR. This value is equivalent to approximately 5.09 mg/L at 20°C and 4.28 mg/L at 30°C.
Biochemical oxygen demand (BOD) is an index of the biodegradable organics in a water body (Clesceri 1989). Simply, it is the amount of oxygen used by bacteria to break down detritus and other organic material at a specified temperature and duration. Higher BOD is generally accompanied by lower DO. The EPA suggests that the BOD not exceed values that cause DO to decrease below the criterion, nor should BOD be great enough to cause nuisance conditions (DEP 2013j).
Growth of bacteria and plankton requires nutrients such as carbon, nitrogen, phosphorus, and trace metals, in varying amounts. Nitrogen and phosphorus, in particular, may contribute to the overgrowth of phytoplankton, periphyton, and macrophytes, which then in turn senesce. Therefore, nutrient inputs into the river can increase the BOD, thereby decreasing the DO. Phytoplankton population responses to the increased nutrients in a system may be only temporary. However, if nutrient inputs are sustained for long periods, oxygen distribution will change, and the overall productivity of the water body can be altered (Wetzel 2001).
2.2.2. Factors that Affect DO and BOD
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 lowers 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 come in contact with 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.
Trophic state is an indicator of the productivity and balance of the food chain in an ecosystem. A good discussion of trophic state is found on the website of the Institute of Food and Agricultural Sciences at the University of Florida (IFAS 2009). High TSI values can indicate high primary (plant) productivity; however, these values can also be indicative of an unbalanced ecosystem, with increased nutrients and algal biomass, which can result in large fluctuations in DO.
2.2.3. Data Sources
All data used for the DO and BOD analyses were from the Florida DEP STOrage and RETrieval (STORET) database. STORET is a computerized environmental data system 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. Data designated with a matrix of “ground water,” “surface water sediment,” “stormwater,” or “unknown” were 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 are also presented as annual mean value ± standard deviation and compared to the designated reference values.
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.
2.2.5. Current Status and Trends
Figure 2.2 shows median DO concentrations and the range of the DO values measured in the LSJR. The median DO values have been fairly stable since 1999. In 2016 and 2017, the range of DO values decreased and the minimum DO concentrations increased, particularly in the marine/estuarine sections of the LSJR mainstem (Figure 2.2). Figure 2.3 shows annual mean DO values and trendlines of the same data set. Since 2013, there have been slight fluctuations, but mostly stable DO mean concentrations in the LSJR mainstem (Figure 2.3). Furthermore, both mean and minimum values in the mainstem have been above reference values and within acceptable limits (Figures 2.2, 2.3). In the tributaries, mean DO concentrations have been at or near the marine/estuarine reference value from 1997 to 2015; however, mean values have increased (improved) over the past three years (Figure 2.3). Annual mean DO values are now within acceptable limits in the tributaries while minimum DO concentrations are below or at 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.
A seasonal trend, with the lowest concentrations observed in the summer months, was observed in the data from the entire LSJR, but not in the data from the freshwater and the saltwater areas of the mainstem (Figure 2.4A-C). This suggests that the seasonal DO fluctuation could be most problematic in the tributaries, where the lowest DO concentrations were observed. It is likely that the aquatic life inhabiting these areas will be more affected by low DO events during the summer time. Water quality conditions in tributaries will be addressed separately in Section 2.8 because DO concentrations can vary among tributaries, depending on the surrounding land use, water flow, depth, and salinity.
Since 1997, there have been slight fluctuations in the median BOD values in the LSJR mainstem (Figure 2.5). In 2016, the BOD data range decreased, including the number of high BOD values, which may be reflective of the increased minimum DO concentrations reported (Figures 2.2 and 2.5). A seasonal pattern of increased BOD values was observed in the LSJR mainstem, particularly in the freshwater areas, with the highest values observed in summer months (Figure 2.6).
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 improving.
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 occurred intermittently during every month of every year prior to 2015. 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 St. Johns River 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 7th, 2016 and September 10-11, 2017, respectively, have resulted in changes in water quality in the LSJR over the past two years. In particular, massive flooding into the river has changed the salinity in some areas and could have contributed to changes in DO and other water chemistry parameters as well.