APPENDIX A - ISSUE ANALYSIS AND TECHNICAL ASSESSMENTS

This section discusses issues leading to the need for restoration and conservation of the lake’s resources and considers much of the technical work that has been done on Lake Tarpon. Subjects covered include water quality, fisheries and aquatic vegetation. Additional information is contained in the referenced technical reports.

The Lake Tarpon Drainage Basin Management Plan or DBMP (PBS&J 1998) and the Lake Tarpon Groundwater Nutrient Study (ERM 1998) represent the most recent comprehensive studies of Lake Tarpon. The DBMP (PBS&J, 1998) identified the pollutant loading sources to the lake, the potential nutrient load reductions that were necessary to achieve the water quality goals of the LTMC and the estimated pollutant load reductions that could be achieved by implementation of various management strategies.

Much of the information contained in this appendix is taken verbatim from these reports and the 1994 Lake Tarpon SWIM Plan (SWFWMD 1994), however, references to the original reports are included for the readers information.

WATER QUALITY ISSUES

Trophic State

Trophic state can loosely be defined as an indication of the nutritional status of a lake or other waterbody. The blue-green algae bloom of 1987 was seen as an indication that the trophic state of Lake Tarpon was increasing. Nuisance algae blooms such as the one in 1987 occur when nitrogen and phosphorus are present in the water column at excessive concentrations.

Increases in trophic state can result in ecological changes in the lake. Increased algae concentrations result in higher turbidity values which impede light penetration to the lake bottom preventing the growth of rooted aquatic plants. Decomposition rates in the lake increase depleting oxygen in the water column. Depleted oxygen levels and changes in the algae community may then cause a shift in the fish population structure from a predominance of sportfish to a predominance of rough fish. This increase in trophic state is known as eutrophication and may occur naturally at very slow rates or may occur at accelerated rates due to human activity in the watershed. The classical lake succession sequence is usually depicted as a unidirectional progression through the following series of trophic states:

· Oligotrophy - nutrient poor, biologically un-productive, low turbidity.

· Mesotrophy - intermediate nutrients and biological productivity, moderate turbidity.

· Eutrophy - nutrient-rich, high biological productivity, high turbidity

· Hypereutrophy - turbidity and color similar to pea soup

Although trophic state concepts have been in existence for some time, much controversy has existed over the terminology, the precise definition of various trophic state classes, and the development of an ecologically meaningful and widely accepted quantitative procedure for determining trophic state. In general, the most widely accepted trophic state index for Florida lakes is that developed by Huber et al. (1983). This index is unique in that it was developed specifically for Florida lakes, and thus recognizes and assimilates various characteristics (e.g. well-mixed, nitrogen limiting conditions) generally not accommodated in trophic state indices developed for temperate lakes. The Florida lakes index is calculated differently for nitrogen limited, phosphorus limited, and nutrient balanced lakes; and involves the calculation of separate sub-indices for total nitrogen, total phosphorus, chlorophyll-a, and Secchi depth. The overall trophic state index (TSI) for a lake is determined by combining the appropriate sub-indices to obtain an average for the physical, chemical, and biological features of the trophic state.

To determine the current trophic state of Lake Tarpon, the most recent monitoring data available from Pinellas County, covering the period January 1999 through December l 1999, were used. The mean monthly concentrations of chlorophyll-a, TN, TP, and the mean monthly Secchi depth, for this time period are as follows:

· Chlorophyll-a (Chl-a) = 28.56 ug/l

· Total Nitrogen (TN) = 1.02 mg/l

· Total Phosphorus (TP) = 40.0 ug/l

· Secchi Depth (SD) = 0.87 m

As discussed by Huber et al. (1983), three classes of lakes can be described pursuant to the total nitrogen to total phosphorus ratio. They are as follows:

· Nitrogen-limited lakes = TN/TP < 10

· Nutrient-balanced lakes = 10 < TN/TP < 30

· Phosphorus-limited lakes = TN/TP > 30

Using the mean values shown above, the TN:TP ratio in Lake Tarpon is 25.5, making it a nutrient-balanced lake, at least under current conditions. Therefore, the TSI for nutrient balanced lakes is appropriate, and is defined as:

TSI(AVE) = 1/3 [TSI(Chl-a) + TSI(SD) + 0.5[TSI(TPB) + TSI(TNB)]]

Where TSI(Chl-a), TSI(SD), TSI(TPB), and TSI(TNB) are sub-indices for chlorophyll-a, Secchi depth, TN nutrient-balanced, and TP nutrient-balanced, respectively. These sub-indices are given and solved as follows:

· TSI(Chl-a) = 16.8 + (14.4 ln Chl-a) = 63.72

· TSI(SD) = 10 [6.0 - (3.0 ln SD)] = 65.09

· TSI(TNB) = 10 [5.6 + (1.98 ln TN)] = 55.94

· TSI(TPB) = 10 [(1.89 ln TP) - 1.84] = 50.30

With the values of all sub-indices known, TSI(AVE) for Lake Tarpon can be solved as follows:

· TSI(AVE) = 1/3 [62.47 + 59.70 + 0.5 (57.15 + 52.24) = 60.64

Therefore, the calculated multi-parametric trophic state index for Lake Tarpon, for the period January 1999 through December 1999 is 60.64. A primary issue regarding the application of the TSI to the classification of Florida lakes for management purposes is the selection of a critical TSI value, or a value above which the lake is considered to have trophic related problems. Based upon a review of data from 573 Florida lakes, and the subsequent classification of each, Huber et al. (1983) determined the TSI value of 60 to be a generally applicable critical value defining eutrophy.

Previous studies on Lake Tarpon (Huber et al., 1983; KEA, 1992) have concluded that Lake Tarpon did not historically exhibit trophic related problems. Using the above described criteria, however, with a calculated current TSI of 60.64 Lake Tarpon is within the category of lakes with trophic related problems. Nutrient load reduction is recommended to meet the target TSI value of 55.

Surface Water

Until the algae bloom of 1987, water quality in Lake Tarpon was considered good and indicative of at most, mesotrophic conditions. Of 41 lakes sampled by the USEPA in 1973, Lake Tarpon was ranked fifth in overall trophic quality based on an analysis of nutrients, Secchi disk transparency, chlorophyll-a and dissolved oxygen data. Of the 41 lakes monitored, Tarpon exhibited the greatest Secchi transparency, 38 lakes (92 percent) had higher total nitrogen (TN) concentrations and 33 (80 percent) had higher total phosphorus (TP) concentrations (USEPA 1977).

Bartos (1976a) classified Lake Tarpon as oligo-mesotrophic based on an analysis of water quality data collected from 1970-1975 and using criteria proposed for Florida Lakes by Shannon and Brezonik (1972). Mean TP and total organic nitrogen (0.08 and 0.57 mg/l, respectively) fell in the oligotrophic to oligo-mesotrophic ranges proposed for colored lakes (Shannon and Brezonik 1972). Chlorophyll-a concentrations were an order of magnitude lower than that given for the oligotrophic range. Secchi transparency was consistent with an oligo-mesotrophic ranking. Bartos (1976b) summarized Lake Tarpon as a "colored circumneutral lake with good overall water quality except for relatively high chloride concentrations." A decline in nutrient concentrations was reported following enclosure of the Tarpon Sink, which was noted as a major nutrient source due to its connection to Spring Bayou. Following closure of the Tarpon Sink, nutrient concentrations were primarily influenced by Brooker Creek and most of the nitrogen from this source was entering Lake Tarpon as either organic or ammonia nitrogen (Bartos 1976a).

The blue green algae bloom (Anabaena circinalis) that occurred in 1987 covered 80 percent of the lake. The bloom persisted for much of the summer and significantly impacted recreational and aesthetic use of the lake during a peak recreational season. Extremely low dissolved oxygen concentrations were noted in the residential canals and minor fish kills were reported (SWFWMD 1994). Since this algae bloom, Pinellas County has been monitoring water quality. Unfortunately, due to inconsistencies in field sampling and laboratory techniques, only chlorophyll-a and TN concentrations can be reliably used to examine multi-year temporal trends in trophic state.

The DBMP (PBS&J, 1998) evaluated water quality data collected between 1988 and 1996. Mean annual chlorophyll-a concentrations in relation to mean annual TN concentrations and cumulative rainfall amounts are shown in Figure A-1. Figure A-2 shows mean annual chlorophyll-a concentrations in relation to mean annual pH and cumulative rainfall amounts. From these graphs PBS&J (1998) made the conclusions below.

· Chlorophyll-a concentrations were relatively low and stable in 1988 and 1989 following the algae bloom of 1987.

· Chlorophyll-a concentrations decreased in 1990. It is hypothesized that this decrease was a lake response to the accidental release of water over the outfall structure in March 1990, which lowered the lake levels by approximately one foot. This release of water had the effect of flushing the lake of excess nutrients, and eventually diluting the lake volume with relatively nutrient-poor rainwater. In addition, it is hypothesized that groundwater seepage from the surficial aquifer

also resulted in a reduction in the lake pH which may have in turn suppressed algae growth during the summer of 1990.

· Chlorophyll-a concentrations increased to pre-drawdown levels in 1991. During the summer of 1991, pH levels in the lake returned to normal conditions.

· Chlorophyll-a concentrations decreased again in 1992. Although rainfall amounts were slightly higher in 1991 and 1992, it is hypothesized that the observed reduction in chlorophyll-a was in response to the dramatic expansion of hydrilla during this time period. The hydrilla expansion occurred during a hiatus in the chemical treatment of the lake by FDEP due to funding limitations. The expansion of hydrilla across the bottom of the lake may have reduced the rate of nutrient exchange between the sediments and the water column and the hydrilla and associated epiphytic algae may have been more effective at competing with the algae for the available nutrients.

· Chlorophyll-a concentrations increased substantially in 1993 and have remained relatively high since that time. It should be noted that the observed chlorophyll-a increases during 1993 and 1994 occurred during a period of reduced rainfall. Therefore, increased non-point source loadings cannot be attributed to this trend. The most plausible explanation for this trend involves the large scale chemical treatment of hydrilla. During late 1992 and early 1993, over 500 acres of dense hydrilla were chemically treated resulting in a major die-off. As this dead plant biomass decomposed, the nutrients contained within the plant tissue were released into the water column, thus stimulating algae growth.

· Like chlorophyll-a, TN concentrations have also increased in the lake since 1993. The cause of this increase is not known, however, the relationship of this trend to the 1993 hydrilla die-off is intuitive. It is also consistent with what has been observed in other Florida lakes where large scale hydrilla treatment has been implemented (e.g., Lake Seminole). Although, development has continued at a steady pace since 1993, especially in the East Lake area, no substantial land use changes and associated nutrient loadings have occurred in the study area during this time period to account for the observed trends.

PBS&J (1998) asserts that "Although, development has continued at a steady pace since 1993, especially in the East Lake area, no substantial land use changes and associated nutrient loadings have occurred in the study area during this time period to account for the observed trends." Therefore, the dramatic increase in chlorophyll-a is attributed solely to the hydrilla treatment in 1993. However, based on a review of land use changes from 1950 to 1990 conducted by ERM (1998) for the Lake Tarpon and Brooker Creek watersheds a dramatic change from natural areas to agricultural uses and ultimately urban land uses can be observed. ERM (1998) further states that "Given the long travel times of ground water in the Lake Tarpon Basin, all land uses that cause nutrients to be applied to the land surface or within the soil column, past and present, may have contributed to the nutrients found in the ground water in the Lake Tarpon Basin." Therefore, it would be expected that decades of changes in the watershed have had a cumulative effect on the trophic state of Lake Tarpon.

Due to complex interactions between nutrients, algae and aquatic macrophytes it is difficult to ascribe the increased trophic state of Lake Tarpon to any one event. In the case of Lake Tarpon, calculation of the TSI has been complicated by the existence of hydrilla, an exotic nuisance aquatic plant. Canfield and Hoyer (1992) cite several studies that show that aquatic macrophytes can inhibit algal growth and thus suppress measured chlorophyll-a. Since the multi-parametric TSI uses chlorophyll-a, Secchi depth and in-lake nutrient concentrations, then TSI values calculated using these parameters will underestimate the true trophic state of the lake if substantial amounts of aquatic macrophytes are present. Removal of these macrophytes without removal of the original source of nutrients may lead to increases in chlorophyll-a concentrations through two mechanisms. If the macrophytes are "killed in place" and allowed to decompose, they will release nutrients bound up in the plant tissue. Secondly, any continued nutrient inputs will be available for algal uptake. In both cases, TSI values based on chlorophyll-a will also increase. Thus, the expansion of hydrilla may be more a symptom of increasing eutrophication, rather than as a cause.

As mentioned above, there are two hypotheses to explain the observed decrease in algal productivity during the summer of 1990. In an attempt to determine the cause of the decrease, PBS&J (1998) plotted the relationship between mean annual chlorophyll-a and TN and chlorophyll-a and pH. These plots are shown in Figures A-3 and A-4, respectively. Both plots show that there is a direct relationship between chlorophyll-a concentrations and pH and TN. Increased algal productivity can lead to increased pH due to the removal of carbon dioxide and the production of carbonate in the water column during active photosynthesis. Although the low pH values in Lake Tarpon during the summer of 1990 may have been a result of decreased algal productivity, they are more likely explained by the seepage of acidic groundwater from the surficial aquifer into the lake following the accidental drawdown that occurred in March 1990. This decreased pH may have stunted the algal productivity during this time, thus contributing to the observed reduction in chlorophyll-a.

The accidental drawdown in March 1990 occurred while the Lake Tarpon Outfall Structure was being operated in automatic mode. Strong winds from the north had pushed water into the southern end of the lake and in the Lake Tarpon Outfall Canal, in effect "piling it up" against the Outfall Structure. Sensors on the Outfall Structure read this increased water level elevation as a flood condition and opened the gates to release water. The actual release of water was from 3.1' mean sea level (msl) to about 2.4' msl (0.7' drop), at which time the malfunction was realized the gates were closed. From the time the gates were closed, the lake continued to drop for almost a two month period to a low elevation of 1.73' msl on May 22, 1990. Because the structure was closed, this continued decline in water levels can only be attributed to evapotranspiration and seepage. Figure A-5 shows a hydrograph of this event.

Due to the observed decrease in chlorophyll-a after the accidental release, PBS&J (1998) recommended that the Outfall Structure be operated in a similar manner to reduce in-lake retention time of nutrients and to dilute in-lake nutrient concentrations. Based on mean annual TN and TP concentrations from 1995 Lake Tarpon water quality data it is estimated that the discharge of 1.0' of water through the Outfall Structure would result in a nutrient mass discharge of 4.41 tons of TN and 0.25 tons of TP. Although lake retention time would be somewhat reduced, the discharged nutrient mass would be replaced by nutrients in the inflowing precipitation, runoff and groundwater. Therefore, dilution would occur only if the inflow waters had lower nutrient concentrations than in-lake concentrations.

Since the accidental release of 1990, another release of a similar magnitude occurred in 1998 (Personal Communication: Scott Stevens, District). In February 1998, an accidental release resulted in a 1.3' lowering of the lake elevation. This drawdown occurred during the El Nino event and heavy frequent rains raised the lake within 36 hours to pre-release levels. This release actually resulted in a larger volume of water being discharged over the structure than the March 1990 event, however, the lake did not continue to drop and therefore, rainfall not groundwater was the primary inflow to the lake. Figure A-6 shows a hydrograph covering this event. Although the lake was discharging, annual average chlorophyll-a values for 1998 appear to have slightly increased (Figure A-7).

Groundwater

The most recent groundwater investigation related to Lake Tarpon was the Lake Tarpon Groundwater Nutrient Study prepared by ERM (1998). In this report, ERM provides a comprehensive review of the numerous groundwater studies that have been conducted in and around the Lake Tarpon watershed. These studies were grouped according to their objectives and included studies of regional groundwater quality, Lake Tarpon groundwater quality, potential nutrient sources, water and nutrient flux to Lake Tarpon, photolinear analysis and aquifer vulnerability mapping. The following discussion was adapted from ERM (1998).

Regional groundwater quality studies focused on defining conditions in the Floridan Aquifer, including chloride concentrations and potentiometric mapping (Cherry 1966; Hutchinson 1983; Corral 1983; Maddox et al. 1992; SWFWMD 1996). The study areas of the Lake Tarpon groundwater quality studies were centered around the lake and its connection to the Floridan Aquifer and did not specifically look at potential nutrient sources (Hunn 1974; Spechler 1983).

Studies of potential nutrient sources to the aquifer were conducted by Fernandez and Hutchinson (1993) and Brown (1982). Fernandez and Hutchinson (1993) studied water quality in stormwater ponds in north-central Pinellas County. Although the ponds were not located in the Lake Tarpon watershed, this study did demonstrate the potential for stormwater to contribute nutrients to the surficial aquifer. Brown (1982) investigated the effects of spray irrigation of wastewater effluent just west of Lake Tarpon. These two studies are consistent with the findings of Jones and Upchurch (1993, 1994) and Jones et al. (1996, 1997) which point to many potential sources of nutrients to groundwater including fertilization of citrus, lawns and pasture, feed lots, stormwater and septic tanks. All of these potential sources either currently exist or have historically existed in the Lake Tarpon watershed (ERM 1998).

Several studies have been conducted to determine water and nutrient flux to Lake Tarpon (CCI 1990; N.S. Nettles & Associates, Inc. 1991; KEA 1992 and Robison 1994). The 1994 revision of the Lake Tarpon SWIM Plan (SWFWMD 1994) used the estimated groundwater flux prepared by KEA (1992) and the nutrient flux estimates prepared by Robison (1994). The ERM study provided updated groundwater nutrient flux estimates that were used in the DBMP (PBS&J, 1998).

The objectives of the ERM study (1998) were to: 1)estimate the flux of nutrients, especially nitrate, into Lake Tarpon via groundwater; 2) determine the origins of nutrient rich groundwater in the Lake Tarpon watershed; and 3) identify the potential for future flux of nutrients in the lake and Brooker Creek from groundwater.

ERM (1998) concluded that water quality in the surficial aquifer reflects a combination of processes including 1) rapid recharge in the eastern half of the Lake Tarpon watershed and along the eastern side of Lake Tarpon; 2) mixing of rainfall-derived water with water from the Florida Aquifer by irrigation; and 3) mixing of somewhat saline Floridan Aquifer water as a result of up-coning or irrigation. Floridan Aquifer water in the eastern and middle thirds of the Lake Tarpon basin is dominated by water quality developed through interaction with the limestone aquifer. The western third of the basin, including the immediate vicinity of the lake, is characterized by the presence of the salt-water/fresh-water transition zone.

Potential sources for groundwater contamination have changed with changing land use in the last 50 years. Although, some land uses may no longer occur in the Lake Tarpon watershed, the nutrients they contributed to land surface and soils are likely still influencing groundwater quality. These are coupled with the more recent nutrient sources such as spray irrigation or wastewater effluent and lawn and golf-course fertilization.

ERM (1998) found ammonia to be widespread in low concentrations throughout the surficial aquifer in the basin. High ammonia concentrations were found at two locations in the Brooker Creek watershed. Both these reflect local wastewater sources (septic tanks or animals). Ammonia concentrations are high in the vicinity of the lake, which reflects application of fertilizers and wastewater. The distribution of ammonia in the Floridan Aquifer is more uniform than in the surficial and concentrations are typically low.

Nitrate concentrations in the surficial aquifer are highly variable. High concentrations, which reflect septic tanks or animal wastes, were found in isolated plumes in the eastern half of the basin (Brooker Creek watershed). Areas of elevated nitrate were also found near the lake, where golf courses, wastewater reuse facilities and suburban development are predominant. Virtually no nitrate was detected in the Floridan Aquifer, probably owing to reducing conditions in the Floridan. Nitrogen compounds in the isolated plumes in the Lake Keystone area are a threat to surface waters, especially lakes. The surficial aquifer in this area can drain to the surface waters in this part of the Brooker Creek watershed and then be transported to the lake through the Brooker Creek system. Similar threats exist nearer to Lake Tarpon.

The ERM study (1998) provided a comprehensive and detailed investigation of the nutrient sources in the Lake Tarpon and Brooker Creek watersheds and the potential for these sources to lead to increased nutrient inflows to Lake Tarpon. However, ERM concluded that additional groundwater wells were needed to refine the estimates of nutrient flux to the lake. Without these wells and additional data, it is difficult to point to any one land use or waste disposal practice as being the most significant source of nutrients to the lake. This is especially true in the northwest corner of the lake. This area of the lake is developed in residential land uses and is served by septic tanks. There is concern that these septic tanks are a significant source of nutrients to Lake Tarpon and that they should be abandoned and the residences connected to municipal wastewater treatment. However, the results of the ERM study (1998) were inconclusive as to the amount of nutrients from this source that actually entered the lake. ERM has recommended that additional wells be installed in this area to evaluate the need for installing central sewer facilities in this area and to further refine the nutrient budget for the lake.