Performance of a recirculating wetland filter designed to remove particulate phosphorus for restoration of Lake Apopka (Florida, USA).

Operation of a 14-km2 wetland filter for removal of total phosphorus (TP) from lake water is part of the restoration program for hypereutrophic Lake Apopka, Florida. This system differs from most treatment wetlands because 1) water is recirculated back to the lake, and 2) the goal is removal of particulate phosphorus (P), the dominant form of P in Lake Apopka. The operational plan for the wetland is maximization of the rate rather than the efficiency of P removal. The St. Johns River Water Management District operated a 2-km2 pilot-scale wetland to examine the capacity of a wetland system to remove suspended solids and particulate nutrients from Lake Apopka. TP in the inflow from Lake Apopka ranged from about 0.12 to 0.23 mg l(-1), and hydraulic loading rate (HLR) varied from 6.5 to 42 m yr(-1). The performance of the pilot-scale wetland supported earlier predictions. Mass removal efficiencies for TP varied between about 30% and 67%. A first-order, area-based model indicated a rate constant for TP removal of 55 m yr(-1). We compared actual removal of P with model predictions and used modeled performance to examine optimal operational conditions. Correspondence between observed and modeled outflow TP was not good with constant variable values. Monte Carlo techniques used to introduce realistic stochastic variability improved the fit. The model was used to project a maximal rate of P removal of about 4 g P m(-2) yr(-1) at P loading 10-15 g P m(-2) yr(-1) (HLR 60-90 m yr(-1)). Data from the pilot wetland indicated that actual rates of P removal may prove to be higher. Further operation of the wetland at high hydraulic and P loading rates is necessary to verify or modify the application of the model.

Effects of nutrients on specific growth rate of bacterioplankton in oligotrophic lake water cultures.

The effects of organic and inorganic nutrient additions on the specific growth rates of bacterioplankton in oligotrophic lake water cultures were investigated. Lake water was first passed through 0.8-mum-pore-size filters (prescreening) to remove bacterivores and to minimize confounding effects of algae. Specific growth rates were calculated from changes in both bacterial cell numbers and biovolumes over 36 h. Gross specific growth rates in unmanipulated control samples were estimated through separate measurements of grazing losses by use of penicillin. The addition of mixed organic substrates alone to prescreened water did not significantly increase bacterioplankton specific growth rates. The addition of inorganic phosphorus alone significantly increased one or both specific growth rates in three of four experiments, and one experiment showed a secondary stimulation by organic substrates. The stimulatory effects of phosphorus addition were greatest concurrently with the highest alkaline phosphatase activity in the lake water. Because bacteria have been shown to dominate inorganic phosphorus uptake in other P-deficient systems, the demonstration that phosphorus, rather than organic carbon, can limit bacterioplankton growth suggests direct competition between phytoplankton and bacterioplankton for inorganic phosphorus.

Experimental evaluation of conversion factors for the [h]thymidine incorporation assay of bacterial secondary productivity.

The relationship between bacterial growth and incorporation of [methyl-H]thymidine in oligotrophic lake water cultures was investigated. Prescreening, dilution, and addition of organic and inorganic nutrients were treatments used to prevent bacterivory and stimulate bacterial growth. Growth in unmanipulated samples was estimated through separate measurements of grazing losses. Both bacterial number and biovolume growth responses were measured, and incorporation of [H]thymidine in both total macromolecules and nucleic acids was assayed. The treatments had significant effects on conversion factors used to relate thymidine incorporation to bacterial growth. Cell number-based factors ranged from 1.1 x 10 to 38 x 10 cells mol of total thymidine incorporation and varied with treatment up to 10-fold for the same initial bacterial assemblage. In contrast, cell biovolume-based conversion factors were similar for two treatment groups across a 16-fold range of [H]thymidine incorporation rates: 5.54 x 10 mum mol of total thymidine incorporation and 15.2 x 10 mum mol of nucleic acid incorporation. Much of the variation in cell number-based conversion factors was related to changes in apparent mean cell volume of produced bacteria. Phosphorus addition stimulated [H]thymidine incorporation more than it increased bacterial growth, which resulted in low conversion factors.

Improved double-vial radiorespirometric technique for mineralization of C-labeled substrates.

New procedures which simplify sample preparation and improve counting efficiency were developed for double-vial radiorespirometry. Under certain conditions, efficiency of fluor-NaOH-impregnated wicks was not stable, but was adversely affected by water vapor, NaOH concentration, and CO(2) loading. Glass fiber wick material treated with a methanolic fluor-NaOH solution showed improved C counting efficiency (58%) compared with previous methods.