Edgar F. Lowe

NUTRIENT REMOVAL FROM EUTROPHIC LAKE WATER BY WETLAND FILTRATION

Abstract Lake Apopka is a large (125 km 2 ), shallow (mean depth 1.6 m) lake in Florida, USA. The lake was made hypereutrophic by phosphorus loading from floodplain farms and has high levels of nutrients, phytoplankton (Chl a 80 μg l −1 ), and suspended matter. The restoration plan developed by the St. Johns River Water Management District encompasses the biomanipulation concept in which the critical step for large shallow lakes is increasing the transparency of the water to allow the re-establishment of submerged macrophytes. Restoration includes operation of a treatment wetland, reduction in external P loading, harvest of fish, fluctuation of lake levels, and littoral planting. The District constructed a 2-km 2 pilot-scale treatment wetland to test nutrient-removal and hydraulic performance. Lake water was recirculated for 29 months, and the removal of suspended solids and particle-bound nutrients was assessed. Hydraulic loading rate varied from 6.5 to 65 m year −l with a mean hydraulic residence time of about 7 days. The inflow contained 40–180 mg l −l TSS, 80–380 μg l −l TP (mostly particulate organic), and 3–9 mg l −l TN (mostly dissolved and particulate organic). Overall, particulate matter was removed (> 90%) by the wetland, and soluble organic compounds were unaffected. Soluble inorganic compounds such as nitrate, ammonia, and soluble reactive phosphate (SRP) were low in the lake water but increased during passage through the wetland. Particulate matter at the outlet was enriched in both N (2-fold) and P (5-fold) compared to particles in the inflow. Mass removal efficiencies were 89–99 (TSS), 30–67 (TP), and 30–52% (TN), but efficiency fell when hydraulic short-circuiting occurred. First-order removal coefficients were 107 (TSS), 63 m year −1 (TP) and 98 m year −l (particulate N). Areal particulate removal rates were 5.4 g dry matter m −2 day −l , 0.18 g PON m −2 day −l , and 0.006 g POP m −2 day −l . The ratio of N:P removal was 28:1. Total sedimentation rate was 0.4 mm day −l of very light matter (4.4 g dw l −l ). About 40% of the dry matter and nitrogen removed and about 80% of the phosphorus was found in the new sediments. Relative to the inflow of lake water, evapotranspiration (4.3%), seepage (2.6%), and rainfall (2.8%) were low. Major problems were initial leaching of SRP, but not ammonia, from native organic soils and vegetation when this former farmland was flooded; hydraulic short-circuiting via former drainage ditches; and low inflows under drought conditions. After 6 months SRP release declined, and initial SRP leaching could be prevented with soil treatment. Hydraulic short-circuiting occurred only after modifications were made. Low gravity flows were augmented with pumped inflows. With these improvements P-removal should increase from the measured 0.48 to at least 3 g P m −2 year −l . Based on the pilot project results, the first phase of an improved 14-km 2 wetland filter has been constructed. This project should accelerate improvements in the water quality of Lake Apopka and, ultimately, create a new, large wildlife-rich marsh.

Setting seagrass depth, coverage, and light targets for the Indian River Lagoon system, Florida

Seagrass protection and restoration in Florida’s Indian River Lagoon system (IRLS) is a mutual goal of state and federal programs. These programs require, the establishment of management targets indicative of seagrass recovery and health. We used three metrics related to seagrass distribution: areal coverage, depth limit, and light requirement. In order to account for the IRLS’s spatial heterogeneity and temporal variability, we developed coverage and depth limit targets for each of its 19 segments. Our method consisted of two steps: mapping the union of seagrass coverages from all availabe mapping years (1943, 1986, 1989, 1992, 1994, 1996, and 1999) to delineate wherever seagrass had been mapped and determining the distribution of depth limits based on 5,615 depth measurements collected on or very near the deep-edge boundary of the union coverage. The frequency distribution of depth limits derived from the union coverage, along with the median (50th percentile) and maximum (95th percentile) depth limits, serve as the seagrass depth targets for each segment. The median and maximum depth targets for the IRLS vary among segments from 0.8 to 1.8 and 1.2 to 2.8 m, respectively.Halodule wrightii is typically the dominant seagrass species at the deep-edge of IRLS grass beds. We set light requirement targets by using a 10-yr record of light data (1990–1999) and the union coverage depth limit distributions from the most temporally stable seagrass segments. The average annual light requirement, based on the medians of the depth limit distributions, is 33 ± 17% of the subsurface light. The minimum annual light requirement, based on of the 95th percentile of the depth distributions, is 20 ± 14%; the minimum growing season light requirement (March to mid September) is essentially the same (20 ± 13%). Variation in depth limits and light requirements, is probably due to factors other than light that influence the depth limit of seagrasses (e.g., competition, physical disturbance). The methods used in this study are robust when applied to large or long-term data sets and can be applied to other estuaries where grass beds are routinely monitored and mapped.

Abrupt Biological Response to Hydrologic and Land-use Changes in Lake Apopka, Florida, USA

Abstract Lake Apopka is a shallow, hypereutrophic lake in north-central Florida that experienced an abrupt shift in primary producer community structure (PPCS) in 1947. The PPCS shift was so abrupt anecdotal accounts report that dominant, submersed aquatic vegetation was uprooted by a hurricane in 1947 and replaced by phytoplankton within weeks. Here we propose two hypotheses to explain the sudden shift to phytoplankton. First, hydrologic modification of the drainage basin in the late 1800s lowered the lake level ca. 1.0 m, allowing the ecosystem to accommodate moderate, anthropogenic nutrient enrichment through enhanced production in the macrophyte community. Second, additional hydrologic changes and large-scale agricultural development of floodplain wetlands began in 1942 and altered the pattern and scale of phosphorus loading to the lake that triggered the rapid shift to phytoplankton dominance in 1947. Historic land-use changes and paleolimnological data on biological responses to nutrient loading support these hypotheses.

Setting Water Quality Goals for Restoration of Lake Apopka: Inferring Past Conditions

ABSTRACT Lake Apopka is a large (12,500 ha), hypertrophic lake in central Florida which is the subject of a state-sponsored restoration program. We used three quantitative methods in concert with an analysis of the history and general character of the lake and drainage basin to infer the past conditions. We specifically examined two past conditions: 1) pristine (before any major anthropogenic disturbance) and 2) antecedent (before a specific, major anthropogenic disturbance). For Lake Apopka the pristine condition ended in the 1890s when a canal was dug which lowered the elevation for surface water outflow. The antecedent condition ended in the late 1940s when most of the lakes 8,900 ha of floodplain marsh was drained for farming. History, general lake and basin characteristics, and the quantitative analysis indicate that Lake Apopka was mesotrophic; with clear-water and native, submersed macrophyte beds; in both the pristine and antecedent conditions. The three quantitative methods (reference lakes, emp...

Particulate phosphorus removal via wetland filtration: An examination of potential for hypertrophic lake restoration

Lake Apopka in Florida, USA, is a large (area=124 km2), hypertrophic (mean total phosphorus=0.220 g/m3; mean chlorophylla=60 mg/m3) lake, with a large sedimentary store of available P (1635 × 106 g P). Phosphorus loading from floodplain farms (132 × 106 g P/yr) has been the primary cause of eutrophication. Assuming elimination of farm P loading, the Vollenweider model predicts a decline in equilibrium P concentration from 0.270 to 0.024 g/m3, if the P sedimentation coefficient (σ) remains constant. It is likely, however, that the value for σ will fall with the elimination of farm loading due to unabated internal P loading from the sediments. Under a worst-case scenario (σ=0), the model predicts that exportation of P from the lake via wetland filtration will greatly accelerate the lakes recovery. Recirculation of lake water through a 21-km2, created wetland and elimination of farm P loading is projected to result in a negative P balance for the lake (−23 × 106 g P/yr) leading to depletion of P stores in the lake in about 60 yr. The estimated cost of the project,

The restoration of Lake Apopka in relation to alternative stable states: an alternative view to that of Bachmann et al. (1999)

20 million, is less than 3% of the estimated cost of dredging. A 3.65-km2 demonstration project is underway to test and refine the wetland filtration technique. We believe the technique could be cost-effective for other hypertrophic lakes.

Contrasting Relationships Between Nutrients, Chlorophyll a and Secchi Transparency in Two Shallow Subtropical Lakes: Lakes Okeechobee and Apopka (Florida, USA)

Bachmann et al. (1999) postulated that wind energy initiated, and has maintained, high turbidity in hypertrophic (mean chlorophyll a = 92 μg l−1) Lake Apopka, Florida (mean depth = 1.6 m; area = 12 500 ha). They asserted that the turbid condition was initiated by a hurricane in late 1947 that destroyed submersed plant beds and that high turbidity has since been maintained by wind-driven resuspension of fluid sediments. In their view, there has been sufficient light for re-establishment of submersed plants over about 38% of the lake bottom, but plant growth has been precluded by the fluid character of the sediments. They concluded that the restoration program of the St. Johns River Water Management District, which includes reduction of the phosphorus (P) loading rate, will not restore water clarity or submersed vegetation. An alternative explanation for Lake Apopkas turbid state is that it was initiated, and has been maintained, by excessive P loading that led to algal blooms and elimination of submersed vegetation through light limitation. The transition to the turbid state was contemporaneous with drainage of 7300 ha of the floodplain wetland to create polders for farming, beginning in the early 1940s. Lake P budgets indicate that drainage of the farms caused a seven-fold increase in the P loading rate (0.08 g TP m−2 yr−1 to 0.55 g TP m−2 yr−1). Paleolimnological analysis of lake sediments also indicates an increase in the P loading rate in mid-century, concomitant with the decline in submersed vegetation and the increase in phytoplankton abundance. After the increase in P loading, wind disturbance may have accelerated the transition to the turbid state; but, before the increase in P loading, wind disturbance was insufficient to elicit the turbid state, as evidenced by the stability of the clear-water state in the face of 14 hurricanes and 41 tropical storms from 1881 to 1946. Measurements of photosynthetically active radiation (PAR) indicate that light limitation has inhibited submersed plant growth except on the shallowest 5% of the lake bottom. Further, the correlation between the diffuse attentuation coefficient (KPAR) and chlorophyll a (CHLA) indicates that light limitation would be removed over about 82% of the lake bottom with a reduction in CHLA from 92 μg l−1 to 25 μg l−1. Recently, following a 40% reduction in the P loading rate, the mean total P (TP) concentration, mean CHLA, and total suspended solids fell by about 30% while mean Secchi depth increased by more than 20%. Submersed plant beds appeared in areas devoid of macrophytes for nearly 50 years. These improvements, during a period with no change in mean wind speeds measured at Lake Apopka, provide the strongest evidence that the turbid state has been maintained by excessive P loading and that the current restoration program, which combines P load reduction with planting and removal of planktivorous fish, will be effective.

Managing phosphorus-based, cultural eutrophication in wetlands: a conceptual approach

ABSTRACT Ten years of monthly water quality data were compared from two large shallow lakes in Florida, USA – Lakes Okeechobee and Apopka. Seasonal changes in trophic state index (TSI) values and log-log regression models relating total phosphorus (TP), total nitrogen (TN), chlorophyll a (CHLA), and Secchi transparency (SD) were considered. The objective was to quantify the extent to which empirical models might vary due to the heterogeneous nature of benthic-pelagic coupling that can occur in shallow lakes. In the offshore region of Lake Okeechobee, TP and SD-based TSI values increase dramatically during winter, while CHLA-based TSI declines. These changes coincide with the windy season in south Florida, when average wind velocities can exceed 20 km h−1. Resuspension of bottom sediments occurs and this reduces light penetration to the extent that the growth of phytoplankton is inhibited. During summer, winds are calm, and these conditions occur less often. Log-log regression models of CHLA vs. TP have a ...

Minimum Flows and Levels Method of the St. Johns River Water Management District, Florida, USA

Abstract The literature relevant to phosphorus-based eutrophication in wetlands has focused on the effects of wetlands on inflowing phosphorus rather than the effects of inflowing phosphorus on the wetlands. Simple conceptual models of wetland eutrophication are needed to manage phosphorus loading to wetlands. The continuously stirred tank reactor (CSTR) model used for lakes, which predicts effects which are patternless and generalized, does not aptly describe eutrophication effects in wetlands, which are patterned and localized. We propose a simple conceptual model for wetland eutrophication, the thin leaky sponge (TLS), and use a first-order, plug flow reactor model to describe the steady-state pattern of decrease in the phosphorus concentration of inflows as a function of time. The model predicts the area affected by phosphorus loading above the background level. Two threshold concentrations of phosphorus, Ps and Pw, are used to predict the areas of four zones: the strong effects, weak effects, total effects, and no effects zones. We propose that eutrophication of wetlands can appropriately be managed through management of the phosphorus load in order to limit the sizes of these zones. By predicting the areas of these zones, the TLS model predicts the area of wetland at risk from the modeled inflow.

Improvements in water quality following biomanipulation of gizzard shad (Dorosoma cepedianum) in Lake Denham, Florida

The St. Johns River Water Management District (SJRWMD) has developed a minimum flows and levels (MFLs) method that has been applied to rivers, lakes, wetlands, and springs. The method is primarily focused on ecological protection to ensure systems meet or exceed minimum eco-hydrologic requirements. MFLs are not calculated from past hydrology. Information from elevation transects is typically used to determine MFLs. Multiple MFLs define a minimum hydrologic regime to ensure that high, intermediate, and low hydrologic conditions are protected. MFLs are often expressed as statistics of long-term hydrology incorporating magnitude (flow and/or level), duration (days), and return interval (years). Timing and rates of change, the two other critical hydrologic components, should be sufficiently natural. The method is an event-based, non-equilibrium approach. The method is used in a regulatory water management framework to ensure that surface and groundwater withdrawals do not cause significant harm to the water resources and ecology of the above referenced system types. MFLs are implemented with hydrologic water budget models that simulate long-term system hydrology. The method enables a priori hydrologic assessments that include the cumulative effects of water withdrawals. Additionally, the method can be used to evaluate management options for systems that may be over-allocated or for eco-hydrologic restoration projects. The method can be used outside of the SJRWMD. However, the goals, criteria, and indicators of protection used to establish MFLs are system-dependent. Development of regionally important criteria and indicators of protection may be required prior to use elsewhere.