Phosphorus and nitrogen are important and required nutrients for terrestrial and aquatic plants, including algae. Under optimal conditions, nutrients can stimulate immediate algal growth. Alternatively, if absent, nutrients can limit algal abundance. If the nutrient concentrations in a system remain high for extended periods of time, eutrophic conditions may result, potentially changing the entire ecosystem by favoring the growth of some organisms and changing the optimal water quality conditions for other organisms. The term “eutrophic” generally signifies a nutrient-rich condition, resulting in a high concentration of phytoplankton (Naumann 1929). The more recent definition characterizes eutrophication as an increase in organic matter loading to a system (Nixon 1995). Eutrophication is a natural process, predominantly occurring in small, enclosed water bodies like ponds and lakes. However, anthropogenic (man-made) activities that increase the loading of nutrients into a waterway can greatly increase the level of eutrophication, even in rivers such as the Lower St. Johns River and its tributaries.
2.3.1. Description and Significance: Nitrogen
Forms of nitrogen typically found in water bodies include nitrate, ammonia and organic nitrogen. These different forms convert to each other in organisms and in the environment (Wright and Nebel 2008). While the atmosphere contains 78% nitrogen gas by volume, this form of nitrogen is unreactive and unavailable to most organisms. An exception is “nitrogen-fixers.” These bacteria take up nitrogen from the atmosphere, and convert it to forms usable by other organisms. Nitrogen-fixers can add significantly to the overall nitrogen loading to a system.
Nitrate is one of the most bioavailable forms of nitrogen and can be rapidly taken up by plants. Sources of nitrate in waterbodies include atmospheric deposition, stormwater runoff containing fertilizer from agriculture and residential areas, runoff from animal operations, and poorly treated sanitary wastewater. In particular, failing septic tanks contribute to nitrate contamination of shallow groundwater and surrounding water bodies (Harrington, et al. 2010). Nitrite and nitrate are converted from one to the other by microbes, depending on the availability of oxygen and the pH of the environment. Under typical environmental conditions nitrite concentrations are very low compared to nitrate. Generally, both nitrate and nitrite are measured together and the values reported as nitrate plus nitrite.
Ammonia is also taken up by phytoplankton (Dortch 1990) and is often converted to nitrate under the correct conditions. It is a waste product of aquatic organisms and naturally occurs in surface and wastewaters at concentrations ranging from 0.010 mg/L in some natural surface waters and groundwater, to 30 mg/L in some wastewaters (Clesceri 1989). Organic nitrogen such as proteins and urea, can decompose to ammonia (Hutchinson 1944; Wetzel 2001).
Total ammonia consists of two forms: un-ionized ammonia (NH3) and ammonium ion (NH4+). They interconvert depending on environmental pH, temperature and salinity. High pH, high temperature and low salinity promote formation of the more toxic form, un-ionized ammonia. It is more toxic to aquatic organisms because of its ability to cross biological membranes.
Other human sources of nitrogen compounds primarily include industrial fixation in the manufacturing of fertilizers, and the combustion of fossil fuels which liberates nitrogen oxides into the atmosphere. The form of nitrogen that enters a waterway can give an indication of its source. However, as noted above, in aquatic systems several abiotic and biotic processes can change the form of nitrogen, so the source may not be as easily identified. Abiotic processes include acid-base reactions and complexation; biotic processes include nitrification, denitrification, and nitrogen fixation. Sediments may act also as a major reservoir of nitrogen, just as they do for phosphorus (Levine and Schindler 1992).
Unbalanced total nitrogen levels in a system can have severe impacts on the distribution of phytoplankton and the zooplankton that eat it. Excess nitrogen can markedly increase some types of phytoplankton. Others, such as some cyanobacteria, thrive in low-nitrogen conditions because they can convert inert atmospheric nitrogen to reactive nitrogen, which allows them to grow rapidly and outcompete other species (Smith 1983).
2.3.2. Description and Significance: Phosphorus
Phosphorus predominately occurs in natural freshwater areas as organically bound phosphate, within aquatic biota, or adsorbed to particles and dead organic matter (Clesceri 1989; Wetzel 2001); whereas, the dominant inorganic species, orthophosphate, accounts for about 10% of the total phosphorus in the system (Clesceri 1989). Orthophosphate is released by the breakdown of rock and soils and is then quickly used by aquatic biota, particularly bacteria and algae, and incorporated as organic phosphate (Newbold 1992; Kenney, et al. 2002). Phosphorus can be released from biota by excretion and by the decaying of matter. Several other factors can influence the partitioning of phosphorus in aquatic systems. In oxygen-rich headwater streams of the LSJR, phosphorus may be bound to mobile particulate material; however, in the lakes and slower flowing freshwater parts of the river, phosphorus settles in sediments (Brenner, et al. 2001). Many factors, such as wind, turbulence, DO, water hardness and alkalinity, sulfide concentration, salinity, and benthic (bottom-dwelling) organisms may potentially re-mobilize phosphorus into the water column (Boström, et al. 1982; Boström, et al. 1988; Lamers, et al. 1998; Wetzel 1999; Smolders, et al. 2006). When reaching the mouth of the river, sulfur may release phosphorus bound to sediments, thus making it potentially available to aquatic organisms (Lamers, et al. 1998; Smolders, et al. 2006). This occurs more commonly in anoxic areas where bacteria reduce sulfate to sulfide as they decompose organic matter (Lamers, et al. 1998; Smolders, et al. 2006).
Humans add to the naturally occurring phosphorus in aquatic systems. In central Florida, phosphorus is mined quite extensively, and is used in fertilizers, commercial cleaners and detergents, animal feeds, and in water treatment, among other purposes. Runoff can result in the addition of phosphorus into local waterways (Clesceri 1989; Wright and Nebel 2008). In the past, phosphorus was also often used in laundry detergents. Orthophosphate generally averages 0.010 mg/L whereas total dissolved phosphorus averages about 0.025 mg/L in unpolluted rivers worldwide (Meybeck 1982). Orthophosphate concentrations in rivers can increase substantially following a rainwater event to as high as 0.050‑0.100 mg/L from agricultural runoff and over 1.0 mg/L from municipal sewage sources (Meybeck 1982; Meybeck 1993).
The drainage basin for the river consists of agricultural lands, golf courses, and urban areas, all of which add to the phosphorus loading in the river. Those inputs, plus effluents from municipal wastewater treatment plants and other point sources may contribute to eutrophic conditions in the LSJR.
2.3.3. Management of Nutrients
Nutrient excesses in the LSJR have led to algal overabundance and low dissolved oxygen levels throughout the river. To address the problems, a final TMDL report was drafted in 2008 by the DEP to reduce nutrient inputs into the LSJR so that algal blooms are reduced in the freshwater regions and healthy levels of dissolved oxygen are maintained in the marine portions of the river. A TMDL is a scientific determination of the maximum amount of a given pollutant (i.e. nutrients) that a surface water body can assimilate and still meet the water quality standards that protect human health and aquatic life (Magley and Joyner 2008; see Section 1). The nutrient TMDL indicates how much nutrients need to be reduced to meet water quality standards in the LSJR. Subsequent Basin Management Action Plans establish restoration strategies required to achieve the water quality standards. Government agencies are working with municipal and industrial wastewater treatment facilities and NPDES permitted facilities to reduce nutrient loadings from permitted discharges. Also, nutrient-rich waters coming from standard secondary water treatment plants may be recycled. These recycled waters can and have recently been used as a means for irrigation when nontoxic. This practice has been utilized in Clay County, within the LSJRB, as well as other areas of the U.S., mostly for irrigation of urban open spaces like parks, residential lawns and golf courses. A similar practice has been used in agriculture. Wastewater treatment improvements have been implemented in Palatka, Orange Park, Neptune Beach, Jacksonville Beach, Atlantic Beach and St. Johns County.
Local utilities and government agencies have worked to reduce nutrient discharges since 2000 including a large public outreach campaign to reduce fertilizer use in residential landscapes. Individual homeowners may also introduce excess nutrients into the LSJR through failing septic tanks; therefore, the replacement of these septic tanks is one of the actions designated to achieve the proposed TMDL. Government agencies have been working with farming and silviculture operations to implement best management practices to reduce and treat runoff of nutrients. The reduction and treatment of urban stormwater runoff by municipal stormwater programs, improvement of development design and construction by commercial developers and homebuilders, and restoration projects by federal, regional, and state agencies may all influence the attainment of projected future goals of the TMDL program. These methods among others have been included in the DEP Nutrient TMDL (Magley and Joyner 2008) and have widespread implications in reducing inputs of nutrients into the St. Johns River, provided government agencies, stakeholders, and the general public can meet this goal.
In August 2013, the FDEP and the Division of Environmental Assessment and Restoration reported to the Governor and Florida legislature on the status of efforts to establish numeric nutrient standards from narrative criteria (DEP 2013j). In August 2013, the FDEP also submitted a plan to EPA to implement numeric nutrient standards in Florida’s waters. The FDEP discussed how it developed numeric interpretations of existing State narrative criteria (DEP 2013f).
In this document, the site-specific numeric standards for the LSJR, including marine tributaries, were expressed as TMDL loading per year, 1,376,855 kg TN/year and 412,720 kg TP/year. The numeric interpretation for chlorophyll-a is that the long-term annual averages will not exceed 5.4 µg/L. For streams without site-specific interpretations required by TMDL stipulations, numeric thresholds and biological benchmarks were developed to assess nutrient status. The nutrient thresholds for peninsular Florida, based on analysis of reference streams, were 0.12 mg TP/L and 1.54 mg TN/L. These values are not to be exceeded more than once in a three year period and are based on annual geometric means. Annual geometric means are similar to medians in that outliers (i.e., extremely high or extremely low values) influence the result less than they influence arithmetic means. Extensive biological assessment accompanies the numeric thresholds.
Progress towards meeting the TMDL goals for the LSJR mainstem has been reviewed in the 2013 LSJR Mainstem Basin Management Action Plan progress report (DEP 2014a) and most recently in the River Accord Status Report. In late 2014, the FDEP Environmental Regulation Commission approved slightly different numeric criteria for the LSJR (DEP 2015c).
2.3.4. Data Analysis
Because of the variability in the characteristics of the river extending from the mouth to the freshwater lakes, it is useful to examine the differences in nutrient profiles in different river regions. The section we refer to as the marine/estuarine reach spans from the mouth at WBID 2213A to WBID 2213H which contains Doctors Lake (Figure 2.1). The section we refer to as the freshwater region extends from WBID 2213I upstream to WBID 2213N at the confluence of the Ocklawaha River.
The nutrients assessed include total nitrogen (TN), total phosphorus (TP), nitrate plus nitrite (NO3-NO2), ammonia, and orthophosphate (OP). The TN and TP parameters reflect total loading of nutrients into the system including different forms that are readily transformed and those that decay slowly. The sums of the dissolved and particle-bound forms are included in the TN and TP assessments. Orthophosphate, nitrate-nitrite, and ammonia are inorganic nutrients that are considered reactive because they can be taken up rapidly by biota and readily undergo chemical reactions in the environment. Chlorophyll-a is an indirect measure of biological responses to nutrient enrichment and is included in some discussions below. More detail about chlorophyll-a and its relationship to phytoplankton growth is provided in the following section on harmful algal blooms.
In this report, the numeric standards for nutrients in peninsular Florida (0.12 mg TP/L and 1.54 mg TN/L; DEP 2013f), described above in Section 2.3.3, are compared to LSJR data to generally assess the status of the LSJR. However, the water body is not regulated under those standards; numeric criteria consist of total nutrient loading rates (1,376,855 kg TN/year and 412,720 kg TP/year) that cannot be compared to actual water concentrations.
Additionally, while nitrate is regulated for springs and drinking water, neither application is appropriate for the LSJR. There is no Florida orthophosphate criterion.
In the following analyses, the current status and time trends of the four nutrients are examined in different ways. Data are displayed in annual box and whisker plots, which show the distribution of the high and low concentrations each year. These plots 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. Data are also displayed as annual means. In these graphs, the peninsular Florida numeric nutrient thresholds for streams, described above, are overlaid on the charts as a general reference point to assess the status of the LSJR.
Trends over time in annual average concentrations are identified by using the Spearman Rank 1-tailed test at p < 0.05.
All data were obtained from the FDEP STORET. STORET is a statewide computerized environmental data system containing water quality, biological, and physical data. EPA methods 365.4 and 365.1 were used to measure total phosphorus in surface waters. Total Kjeldahl nitrogen (organic nitrogen plus ammonia), total ammonia, and nitrate plus nitrite were measured using EPA methods 351.2, 350.1 or 4500-G, and 353.2, respectively. Total nitrogen was represented by the sum of the Kjeldahl nitrogen and nitrate-nitrites in each sample. Data for the entire LSJRB and tributaries were collected from FDEP STORET and culled for applicability to this study. Data were reviewed for quality and data points were discarded when samples appeared analytically compromised (contaminated blanks, poor recovery, poor replication, etc.) or were missing important information. Records with no analytical procedure listed were also removed. 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.
2.3.5. General Characteristics
Nutrient profiles vary with the region of the river and depend on proximity to the mouth, rainfall, local sources, and upstream and tributary sources, as well as biological activity (Figure 2.7). The dilution of river water with lower-nutrient ocean water is evident for most nutrients because annual average concentrations sharply decrease as the river reaches the mouth in WBIDs 2213A-2213C. In most years, both forms of phosphorus and nitrate-nitrite concentrations increase as the fresh water moves downstream to estuarine areas, where it becomes diluted by ocean water. By contrast, TN and pheophyton-corrected chlorophyll-a gradually decrease as the river moves from freshwater to estuarine conditions (Figure 2.7). As a consequence of the different ratios of nitrogen to phosphorus, the downstream, saltier section is generally more susceptible to nitrogen pollution, and the upstream, more riverine section is more susceptible to phosphorus pollution.
2.3.6. Current Status and Trends: Total Nitrogen
The median mainstem total nitrogen concentrations have been below the TN water quality reference concentration (used only for the purpose of this report) of 1.54 mg N/L in both freshwater and marine/estuarine sections of the river since 1997. In 2015, the TN median was 0.8 mg N/L in the freshwater section and 0.6 mg N/L in the marine/estuarine section (Figure 2.7). The maximum values in the LSJR, particularly due to values reported in the tributaries have continued to be well above the TN reference values (Figure 2.7).
2.3.7. Current Status and Trends: Total Phosphorus
The median, minimum, and maximum TP concentrations in the LSJR are presented in Figure 2.10. The maximum TP concentrations have decreased in 2016, particularly in the tributaries (Figure 2.10). The yearly mean TP concentrations are presented in Figure 2.11. Yearly mean values have been below the TP reference concentration of 0.12 mg P/L (used only for the purpose of this report) since 1997 in the freshwater sections of the mainstem (Figure 2.11B); and below the reference concentration in the marine/estuarine areas of the mainstem, with a significant decrease in mean TP values, since 2010 (Figure 2.11C). Mean TP values in the entire LSJR, particularly the tributaries, have fluctuated around the reference value, and maximum reported values in the tributaries have been above the reference TP value (Figures 2.10, 2.11).
2.3.8. Current Status and Trends: Nitrate, Ammonia and Phosphate
The reactive inorganic nutrients ammonia, nitrate-nitrite, and orthophosphate are readily taken up by various organisms and released back into the environment. Concentrations of the nutrients vary widely with environmental conditions such as rainfall and phytoplankton growth. Maximum and median ammonia concentrations have been slightly reduced since 2014 in both the freshwater and marine/estuarine portions of the LSJR mainstem (with the exception of the maximum value in 2015 in the marine/estuarine mainstem) and to some extent the entire LSJR (Figure 2.13). The median nitrate-nitrite concentrations in the LSJR have remained stable in the entire LSJR (Figure 2.14); however, since 2015, the median nitrate-nitrite concentration in marine/estuarine section of the mainstem has significantly increased and the median concentration in the freshwater section of the mainstem has significantly decreased (Figure 2.14). There is also a seasonal trend in the levels of nitrate-nitrite, with the highest concentrations occurring in the winter (Figure 2.15). This may be the result of limited uptake of nitrate for phytoplankton growth in winter months. Along with nitrate, orthophosphate tends to be higher in the marine/estuarine section than in the freshwater section (Figure 2.16, 2.17). The median orthophosphate concentration in the marine/estuarine sections in 2016 was over two times higher than the median in the freshwater regions of the river (Figure 2.16). Despite the variability over time in the concentrations of the reactive inorganic nutrients, there was still a statistically meaningful downward trend in the mean nitrate-nitrite and orthophosphate concentrations in the marine/estuarine sections of the LSJR (Figure 2.17). No trends in freshwater nitrate-nitrite and orthophosphate concentrations were evident. An interesting feature of both time series is the low concentrations in 2010-2011 corresponding to times of intense algal blooms. Significant phytoplankton growth and die-off contribute to the fluctuations as nutrients are consumed and released.
2.3.9. Summary and Outlook
The yearly mean TN concentrations have declined gradually but significantly in both freshwater and marine/estuarine sections of the river, however, maximum values in the LSJR, particularly due to values reported in the tributaries and in freshwater areas of the mainstem, have continued to be above the TN reference value. Significant reductions in nitrogen loading are likely to be the primary reason for the observed decline of TN in the LSJR (DEP 2013c).
Mean and maximum TP concentrations are below the reference value in the LSJR mainstem; however, maximum TP in the tributaries continues to be above the reference value. The reactive inorganic nutrients, nitrate and orthophosphate, are higher in the marine/estuarine section than in the freshwater section; however, the concentrations have been decreasing over time.
For these reasons, the overall STATUS of nitrogen in the mainstem and the tributaries is unsatisfactory, and the TREND is improving. The STATUS of phosphorus in the mainstem is satisfactory, and the TREND is unchanged in the freshwater portion of the LSJR and improving in the marine/estuarine areas of the LSJR. The STATUS of phosphorus in the tributaries is unsatisfactory, and the TREND in the tributaries is improving.
There are wide fluctuations of these and other nutrients due to phytoplankton growth and die-off as well as weather conditions. Reduced nutrient loading may be lowering concentrations of some forms of nutrients in the mainstem. Changes in nutrient concentrations typically correlates with changes in algal growth. Algal growth, as indicated by average annual chlorophyll-a levels, is discussed separately in Section 2.4.
The complex ecology of the LSJR and its highly variable characteristics and weather patterns make it difficult to assess its overall status. As a result, assessments can differ when different methods of analysis are used. It is reported in the 2013 LSJR BMAP progress report that total nitrogen is decreasing at benchmark sites in marine and freshwater areas of the river (DEP 2014a). Total phosphorus is unchanged at the freshwater site but could be increasing at the marine site. To date, wastewater treatment facilities in the freshwater portion of the LSJR have achieved their total nitrogen and total phosophorus TMDL-required reductions; and wastewater treatment facilities in the marine portion of the LSJR have achieved their TMDL-required total nitrogen reductions. The next few years will be critical in definitively determining when the considerable effort and expenditures to reduce nutrients in the LSJR have been successful. Robust data sets are particularly critical for assessing trends.
Numerous projects have been carried out by multiple counties and agencies in the last several years to reduce nonpoint sources of nutrients from stormwater runoff, agricultural runoff, landscape fertilizer and septic tanks, as well as point sources such as wastewater treatment plants. Projects include wastewater treatment plant upgrades, reclaimed water projects, general drainage improvement, septic tank phase-outs, and the construction of regional stormwater treatment facilities. These efforts are detailed in the 2013 LSJR Mainstem Basin Management Action Plan progress report (DEP 2014a). In addition, nongovernmental NPDES permit holders have also reduced the discharge of nutrients in their effluents to meet TMDL load reduction allocations. In an interesting, cost-effective restoration project in Lake Apopka, the SJRWMD is reducing the mobilization of phosphorus from sediments by harvesting gizzard shad, which disturb the sediments and release the phosphorus for uptake by algae (DEP 2014a). As a consequence of all of these efforts, the lower basin stakeholders have made substantial progress in meeting their targeted nutrient load reductions required by the LSJR TMDL limits, a very positive development for the river. Additionally, a similar, larger scale project has been implemented in Lake George since 2013.
To determine whether load reductions and numeric criteria have achieved a real environmental benefit, reliable and consistent data is essential. There is a very clear need for continued and increased monitoring to assess the effectiveness of the nutrient TMDLs that have been implemented for the LSJR mainstem. Responses to TMDL efforts of other water bodies in the entire St. Johns River basin, particularly upstream and tributaries, also need to be monitored if benefits are to be accurately assessed. It is critical to maintain adequate monitoring capacity for nutrients, chlorophyll-a and other water quality parameters in the LSJR mainstem so that information that is essential for effective management is available.