Littoral periphyton responses to nitrogen and phosphorus: an experimental study in a subtropical lake

Aquatic Botany 63 (1999) 267±290

Littoral periphyton responses to nitrogen and phosphorus: an experimental study in a subtropical lake Karl E. Havens*, Therese L. East, Andrew J. Rodusky, Bruce Sharfstein Ecosystem Restoration Department, South Florida Water Management District, PO Box 24680, West Palm Beach, FL 33416-4680, USA Received 1 December 1997; accepted 23 November 1998

Abstract Portions of the natural littoral community in a subtropical lake were enclosed in mesocosms and subjected to high rates of nitrogen (N) and/or phosphorus (P) loading over a 28-day period. Changes in periphyton structure and function were compared with untreated controls to evaluate limiting resource effects. Mesocosms that received only P displayed minor increases in whole-community metabolism, measured by in situ oxygen evolution during mid-day hours, and small increases in the chlorophyll a (CHLA) content of surface periphyton mats. Mesocosms that received only N displayed significant increases in the CHLA content of epiphyton growing on submerged stems of Eleocharis. The most striking treatment effects were observed when both P and N were added. In that case, there were highly significant increases in the CHLA content of surface periphyton mats, epiphyton associated with submerged Utricularia, and periphyton growing on artificial substrata. There also were increases in the rates of phytoplankton carbon uptake and whole-community metabolism. These results indicate that periphyton and phytoplankton were co-limited by P and N, in contrast to previous findings of strong P limitation in more pristine regions of this marsh, and in the nearby Florida Everglades. The community responses in the present study may reflect early eutrophication effects, because the study site was located in close proximity to the lake's eutrophic pelagic region. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Periphyton; Littoral communities; Limiting nutrients; Nitrogen; Phosphorus; Subtropical lakes

* Corresponding author. Tel.: +1-561-687-6534; fax: +1-561-687-6442; e-mail: [email protected] 0304-3770/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 7 7 0 ( 9 8 ) 0 0 1 2 1 - 1

268

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

1. Introduction Research dealing with nutrient impacts on algae in lake ecosystems has a long history in aquatic ecology. Studies of the phytoplankton firmly established that phosphorus (P) was the nutrient most often limiting to that assemblage's growth in temperate freshwater lakes (Schindler, 1977). In tropical lakes, and lakes with long histories of excessive P loading, limitation by nitrogen (N) sometimes occurs (Henry et al., 1985; Schelske, 1984). Lakes with high concentrations of P also tend to have phytoplankton dominated by filamentous and colonial cyanobacteria (Horne, 1979; Smith, 1983). Early studies dealing with temperate lake periphyton also found growth increases due to P additions, and particularly strong responses by filamentous chlorophyte taxa, such as Cladophora (Herbst, 1969; Pitcairn and Hawkes, 1973; Bolas and Lund, 1974). Dodds and Gudder (1992) noted that the algae can ``reach nuisance levels as a result of cultural eutrophication.'' More recent observational studies have confirmed the notion that biomass of periphyton in temperate lakes is correlated with lake water total P concentration (Cattaneo, 1987), and that dramatic changes in biomass, taxonomic composition, and function sometimes ensue with enhanced nutrient inputs (Goldman, 1981; Hawes and Smith, 1993; Axler and Reuter, 1996). Experimental studies have shown that periphyton can sequester large amounts of N and P from the water column (Blumenshine et al., 1997), and that nutrient enrichment can produce dense growths of filamentous metaphyton (Howard-Williams, 1981). McDougal et al. (1997) performed nutrient addition experiments in open-water regions of an oligotrophic wetland in Canada. They found that additions of N and P to mesocosms led to dramatic changes in community structure, wherein filamentous chlorophytes (Cladophora) replaced native epiphyton as the dominant primary producers. We have performed similar research on the littoral periphyton in Lake Okeechobee, a shallow lake in Florida, USA. The pelagic region of this lake is highly eutrophic (Havens et al., 1996), but large areas of the littoral zone are oligotrophic, because they are hydrologically uncoupled from the pelagic region, except under periods of high water level (Richardson and Hamouda, 1995). In autumn 1996 we conducted a mesocosm experiment to evaluate the combined impacts of N and P (at pelagic concentrations) on the littoral periphyton. We observed a dramatic shift toward filamentous chlorophyte (Spirogyra) dominance (Havens, unpublished data). The added N and P both were rapidly sequestered from the water column, and the experimental design did not permit us to determine which nutrient was limiting to the periphyton community. The present study was a follow-up to the preliminary experiment of 1996, and included individual and combined additions of N and P. We predicted that the periphyton would respond primarily to additions of P on the basis of: 1. high total N : P ratios (>100 : 1) previously measured in the water column; 2. experimental research results from the Florida Everglades (McCormick and O'Dell, 1996); and 3. recently completed studies of algal P kinetics at a nearby site in Lake Okeechobee (Hwang et al., 1999). We predicted that N additions would have comparatively little effect on the periphyton.

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

269

To our knowledge, this study represents the first whole-community experiment to quantify the effects of N and P on a subtropical lake periphyton community. 2. Methods 2.1. Study site The experiment was performed in the shallow littoral region (Fig. 1) of Lake Okeechobee (latitude 268 580 N, Longitude 808 500 W). The littoral zone of this large lake has a surface area of 400 km2, and is comprised of a diverse mosaic of native and exotic plant assemblages (Richardson and Harris, 1995). Depending on lake stage and location, water depths in the littoral zone may vary from <0.1 to >2.0 m. The study site (known locally as `Moonshine Bay') was considered to be representative of one of the most pristine habitats of the littoral zone. Previous studies in this region documented water column total, and soluble, P concentrations of below 7 mg lÿ1 (Steinman et al., 1997), and nutrient-poor (<0.01 (g P) mÿ2) sand sediments (Richardson and Hamouda, 1995). The dominant macrophytes included an emergent sedge, Eleocharis cellulosa (spike rush) and two submerged bladderwort species, Utricularia vulgaris and U. purpurea.

Fig. 1. Map of Lake Okeechobee, showing the study site location within the littoral marsh zone (shaded area). Inset map shows location of lake in Florida, USA.

270

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

Water depth averaged 60 cm during the experiment, which was conducted from 23 June to 21 July, 1997. This period corresponds to the peak growing season for E. cellulosa in the marsh (Harris et al., 1995), and a period when periphyton biomass also is high (Steinman et al., 1997). 2.2. Experimental design Experiments were conducted using 1.2 m diameter transparent Lexan1 cylinders, cut to a height of 1 m. Six mesocosms were located along each side of a wood boardwalk in an undisturbed area of the marsh. The mesocosms were pressed firmly into the sediments and fixed in place using PVC support poles attached with steel bolts. After a 48-h equilibration period, initial samples were collected (see below) and triplicate mesocosms were dosed with N and/or P at concentrations typical of those found in the nearby pelagic region (Table 1). Nutrients were added by mixing calculated doses of N and P with 1 l of filtered lake water, and then pouring the contents into a funnel connected to a PVC tube with numerous holes drilled below the water line. A 1-l volume of rinse water followed the addition of each nutrient spike. 2.3. Sampling methods Water temperatures and dissolved oxygen concentrations were measured using an Orbisphere microelectrode at mid-depth (30 cm), approximately three times weekly (just prior to adding nutrient spikes) between 0900 and 1000 h. Diel variations in water temperature (at 30 min intervals) also were measured, by installing Hobo1 electronic temperature loggers at mid-depth in the mesocosms and reference sites. We did not attempt to measure underwater irradiance, despite the fact that irradiance clearly is important in controlling rates of primary productivity. This reflected two concerns noted in the preliminary study: (i) the quantum sensor and its support arm caused considerable disturbance to the surface periphyton mat during sampling; and (ii) depending on the horizontal position of the sensor, dramatically different irradiances are measured due to shading by sections of floating mat material. For this same reason, we did not deploy Hobo1 light sensors in the mesocosms. Table 1 Design of the in situ mesocosm experiment performed in the littoral community of Lake Okeechobee, June±July 1997 Treatment

Description

Replicates

R C P N NP

open marsh sites adjacent to mesocosms - sampled to evaluate `enclosure effects' no nutrients added KH2PO4 added three times weekly, at a rate of 100 (mg P) lÿ1 per dose KNO3 added three times weekly, at a rate of 5000 (mg N) lÿ1 per dose both P and N added, as above

3 3 3 3 3

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

271

Weekly samples were collected for analysis of total P, soluble reactive P, total N, dissolved inorganic N, and plankton chlorophyll a (CHLA). The samples were collected using a peristaltic pump, with the inlet tube immersed to approximately mid-depth in each mesocosm. At the beginning (Day 0) and end (Day 28) of the experiment, we collected samples for determination of CHLA, ash-free dry mass (AFDM), and tissue P, N and C content of the various periphyton assemblages that comprised the community (Fig. 2), including: 1. a `surface mat' of epiphyton growing on the remains of decomposed Utricularia fronds; 2. epiphyton associated with submerged Utricularia; 3. epiphyton associated with submerged Eleocharis stems; and 4. a benthic community comprising diffuse metaphyton overlying an epipelon layer on the sediment surface. The surface mat and submersed Utricularia were sampled by randomly clipping five small whorls of plant material with scissors, and placing it into a plastic bag. Eleocharis stems were clipped at the water line and just above the sediment surface, and then were carefully placed into bags. We collected and analyzed living and dead stems separately, as the latter had distinctly more epiphyton growth. To sample benthic periphyton, we pushed a 4-cm diameter clear plastic core into the sediments at approximately the mid-point of each mesocosm, allowed the sand sediment material to exit the bottom, and then collected

Fig. 2. Diagrammatic representation of the littoral community, showing the various periphyton components considered in this study.

272

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

the overlying epipelon and metaphyton material into a plastic bag. We did not attempt to separate these two interspersed benthic algal fractions. In addition to sampling the natural components of the community, we placed artificial substrata into each mesocosm, in order to provide a consistent surface on which periphyton could develop under the various nutrient regimes. The substrata comprised 3 cm wide and 60 cm long strips of the material used to construct the mesocosms. Three such strips were attached with Velcro1 to the inside wall of each mesocosm, at equidistant locations around the perimeter. The strips were installed on Day 0 and removed on Day 28, at which time they were brushed clean of periphyton for the abovementioned analyses. The strip data were used to estimate nutrient uptake by communities growing on the mesocosm walls, for the purpose of N and P mass balances. Upon final collection, some strips were found lying at the bottom of mesocosms. These were not included in the analyses, however, all but one mesocosm had at least two intact strips. When sampling was completed on Day 28, several procedures were done to permit extrapolation of periphyton measurements to the community level. Entire surface mats were harvested from each mesocosm and squeezed to remove excess water. Wet weights were determined to permit extrapolation of nutrient, CHLA, and AFDM measurements, which were done on a per wet-weight basis. For the submerged Utricularia, we harvested material enclosed inside three replicate 26 cm diameter PVC tubes that were pushed though the water column and into the sediments of each mesocosm. The total quantity of this material in each mesocosm was determined by multiplying wet weight times total mesocosm area (11 309 cm2) divided by the area sampled (1593 cm2). The total numbers of living and dead Eleocharis stems were counted inside each mesocosm. The average surface area of a stem was calculated based on measurements of lengths and widths of 3±5 stems of each type, to permit extrapolation of data (below) that, in this case, were determined on an areal basis. Extrapolation of benthic algal data, from a per-cm2 basis to the entire community, was accomplished by multiplying by the total benthic surface area (11 309 cm2). On a weekly basis, we also obtained fixed-height photographs of each mesocosm's surface, using a wide-angle lens and a camera mounted on an L-shaped PVC arm. From these photographs, it was possible to approximate the areal coverage of the surface mats. Visual observations of mat coloration also were recorded. On days 0 and 28, phytoplankton carbon uptake rates were determined for each mesocosm and reference site, using the standard 14C light and dark bottle method (Vollenweider, 1974). Incubations were done in the laboratory at a fixed irradiance of 150 (mmol photons) mÿ2 sÿ1 and at a temperature that approximated the value measured at mid-depth in the field. The resulting data reflect photosynthetic potential of the phytoplankton per unit volume in the various treatments. On days 16 and 23, we also estimated whole-community metabolism. This was done by measuring dissolved oxygen at mid-depth in each mesocosm, at hourly intervals from 1000 to 1300 h. The slope of the regression line relating oxygen concentration to time was used as an approximation of net community metabolism. We assumed constant rates of air-water oxygen exchange in the mesocosms, which had nearly identical surface areas. On Day 28, after all of the sampling and harvesting procedures were completed, we treated each tank with an aqueous solution of Rotenone1, and collected mosquito fish

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

273

(Gambusia holbrooki) and several other species of small forage fish from the tanks. The wet weight of each fish was recorded, and the total weight per mesocosm was calculated as the sum of individual fish weights. We found between 0 and 19 fish per mesocosm, with total wet weights from 0 to 11.0 g mÿ2. Differences in fish densities or weights among treatments were not statistically significant (p > 0.20). When ANOVAs (see below) were re-run using fish weights in each mesocosm as covariates, there were only slight changes in F values associated with the main treatment effects, and no changes in the final results regarding whether or not treatment effects were statistically significant. 2.4. Laboratory methods Total P (TP) concentrations in water samples were measured according to USEPA (1979) and total N (TN) was analyzed according to USEPA (1987), in both cases using a flow-injection autoanalyzer. Soluble nutrients were analyzed according to standard methods, on water samples that were field-filtered through 0.4 mm polycarbonate filters. Plankton CHLA was analyzed according to APHA (1995). All water column concentrations were expressed as mg lÿ1. Each periphyton sample was transferred from its collection bag into a white plastic tray. Epiphyton was removed from Eleocharis stems by hand, homogenized, and subdivided for analysis of CHLA and tissue P and N. It was impossible to separate periphyton from Utricularia whorls. Therefore, in the case of surface mats and submersed Utricularia, analyses were performed on the combined plant/epiphyton material. Benthic algae (metaphyton and epipelon) was homogenized and subdivided as indicated above. Periphyton CHLA was measured according to Steinman and Lamberti (1996). Tissue P was measured using standard methods (APHA, 1995), and tissue N was measured using a CNS analyzer. The CHLA contents of Eleocharis epiphyton and benthic algae were expressed in areal units (mg cmÿ2), while CHLA of surface periphyton and Utricularia epiphyton were expressed in terms of unit wet weight (mg (g ww)ÿ1). Concentrations of P and N were expressed in terms of unit dry weight (mg (g dw)ÿ1). Periphyton samples from certain components of the community (the surface mats and epiphyton) were observed microscopically on the final day of the experiment to determine dominant taxa. Quantitative processing of the samples, in order to determine species' biovolumes and numeric densities, was not performed. 2.5. Statistical analyses To test for significant treatment effects on the response variables (CHLA, AFDM, tissue P, N and C, phytoplankton carbon uptake, and community metabolism), we performed a one-way ANOVA, using Scheffe's test as the mean separation method. For the response variables measured only at the beginning and end of the experiment, ANOVAs were performed on Day 0 (to establish whether initial conditions varied among treatments) and Day 28 (to test for treatment effects). For variables measured on several occasions, we analyzed the data using repeated measures (RM) ANOVA. All statistical

274

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

analyses were performed using the general linear models (GLM) procedure in SAS (1996). When reviewing the statistical results, we found that high intra-treatment variability precluded detection of significant treatment effects on periphyton AFDM. Therefore, periphyton biomass responses are presented only in terms of chlorophyll a content.

3. Results 3.1. Physical and chemical parameters Water temperatures measured at the time of sampling (0900 to 1000 h) ranged from 288 to 318C (data not shown). Temperatures were similar among mesocosms and reference sites, and repeated measures (RM) ANOVA indicated no significant ( > 0.05) treatment, time, or interaction effects. Data from continuous temperature loggers indicated regular diel variations of 38 to 78C, depending on weather conditions during daytime hours. Nighttime low temperatures were always near 288C, and afternoon highs ranged from 308 to 378C. Diel (24 h) patterns were similar among mesocosms and reference sites. TP concentrations were initially between 10 and 14 mg lÿ1, and did not vary significantly among treatments (Fig. 3(a)). TP remained in this range during the experiment at the reference sites, controls, and N treatments. TP concentrations in the P and NP treatments increased to 30 and 40 mg lÿ1, respectively, by Day 28. RM-ANOVA indicated a highly significant treatment effect, a marginally significant effect of time (0.05 <  < 0.10), and no significant treatment±time interaction. By Day 28, the P and NP treatments had received cumulative P additions of 1100 mg lÿ1. Measured TP in the water column was <4% of the added amount. Soluble P concentrations (data not shown) always were near or below detection limits (4 mg lÿ1). TN concentrations (Fig. 3(b)) initially ranged from 2240 to 2990 mg lÿ1, and during the experiment they did not display systematic trends at the reference sites, controls, or P treatments (Fig. 3(c)). In contrast, there were dramatic increases in TN in the N and NP treatments as the experiment progressed. Final concentrations averaged 22 740 mg lÿ1 in the N treatment, and 16 790 mg lÿ1 in the NP treatment. RM-ANOVA indicated highly significant treatment, time, and treatment-time interaction effects. The observed increases in TN concentrations accounted for 40% and 30%, respectively, of the 55 000 mg lÿ1 that was added during the experiment. Dissolved inorganic nitrogen (DIN) concentrations (Fig. 3(c)) generally were below 40 mg lÿ1 in the treatments where N additions did not occur, except that higher values were observed on Day 28 (77±96 mg lÿ1). In the N and NP treatments, DIN increased to 19 190 and 13 240 mg lÿ1, respectively. This DIN accounted for approximately 80% of the observed increases in water column TN. The RM-ANOVA indicated highly significant treatment, time, and interaction effects for DIN. Ratios of TN : TP were close to 200 : 1 at the start of the experiment (Fig. 3(d)), and remained below this level at the reference sites, controls, and P treatments. The ratios

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

275

Fig. 3. Concentrations of total P (a); total N (b); dissolved inorganic N (c); and total N : P ratios (d) measured during the in situ mesocosm experiment. Vertical bars are 1 standard deviation above the means. Treatment symbols are defined in Table 1.

increased approximately twofold in the NP treatment, and tenfold in the N treatment by Day 28. The RM-ANOVA indicated significant treatment, time, and interaction effects. 3.2. Periphyton nutrient concentrations and ratios On Day 0 (data not shown), tissue nutrient contents varied little among the different components of the periphyton community, and there were no significant differences among treatments. Surface algal mats contained, on average, 1.9 mg (g dw)ÿ1 N and 0.05 mg (g dw)ÿ1 P. Tissue N : P ratios averaged 38 : 1. The epiphyton associated with submersed Utricularia displayed these same concentrations of N and P. Epiphyton growing on live Eleocharis stems initially had 3.0 mg (g dw)ÿ1 N and 0.05 mg (g dw)ÿ1 P, and a tissue N : P ratio of 60 : 1. The epiphyton growing on dead stems had 5.4 mg (g dw)ÿ1 N, 0.05 mg (g dw)ÿ1 P, and a tissue N : P ratio of 108 : 1. The benthic algal community contained 2.1 mg (g dw)ÿ1 N, 0.04 mg (g dw)ÿ1 P, and had a tissue N : P ratio of 53 : 1. On Day 28, the P content of surface algal mats in the P and NP treatments had increased by over threefold, relative to the reference site, control and N treatments

276

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

(Table 2), and the differences were significant. Increases of a similar magnitude were observed in the Utricularia and Eleocharis epiphyton assemblages, and in the periphyton that developed on artificial substrata. In contrast, the benthic algal community displayed no significant changes in P content with enrichment. Periphyton tissue N contents on Day 28 displayed no significant relationships with treatment. Periphyton tissue N : P ratios displayed significant treatment effects on Day 28. For surface periphyton mats (Fig. 4(a)), epiphyton on live and dead Eleocharis (Fig. 4(b±c)), epiphyton on submersed Utricularia (Fig. 4(d)), and periphyton growing on artificial Table 2 Nitrogen and phosphorus content (mg gÿ1) of periphyton tissues assayed at the end of the 28-day mesocosm experiment. Data are means with standard deviations in parentheses (nd implies no data, and nr ± replicates). The treatments are R (reference sites), C (controls), P (phosphorus additions), N (nitrogen additions) and NP (nitrogen and phosphorus additions) Component

Treatment

Nitrogen

Phosphorus

Surface periphyton mat

R C P N NP

2.0 1.9 2.3 2.2 2.3

(0.4) (0.1) (0.2) (0.3) (0.5)

0.030 0.037 0.121 0.029 0.106

(0.008) (0.009) (0.044) (0.004) (0.011)

Utricularia epiphyton

R C P N NP

2.1 2.0 1.8 1.8 2.4

(0.3) (0.2) (0.2) (0.2) (0.2)

0.020 0.015 0.066 0.017 0.069

(0.003) (0.002) (0.009) (0.002) (0.009)

Live Eleocharis epiphyton

R C P N NP

10.3 9.6 10.5 11.8 25.8

(0.3) (0.8) (1.3) (3.0) (nr)

0.067 0.034 0.093 0.033 0.166

(0.006) (0.008) (0.057) (0.007) (nr)

Dead Eleocharis epiphyton

R C P N NP

2.7 2.0 2.6 2.3 2.9

(0.5) (0.3) (2.1) (0.1) (0.2)

0.053 0.031 0.126 0.033 0.119

(0.007) (0.007) (0.037) (0.003) (0.015)

Metaphyton and epipelon

R C P N NP

2.0 2.3 2.4 2.3 3.0

(0.3) (0.8) (0.5) (0.2) (0.2)

0.053 0.052 0.063 0.056 0.064

(0.014) (0.004) (0.008) (0.007) (0.008)

Artificial substrates

R C P N NP

nd 3.7 3.5 2.7 2.9

(1.8) (0.9) (0.8) (0.5)

nd 0.045 0.146 0.035 0.084

(0.011) (0.013) (0.006) (0.021)

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

277

Fig. 4. Periphyton tissue N : P ratios measured on Day 28 of the in situ mesocosm experiment for surface periphyton mats (a); epiphyton on live Eleocharis stems (b); epiphyton on dead Eleocharis stems (c); epiphyton on Utricularia (d); metaphyton and epipelon (e); and periphyton growing on artificial substrates (f). Vertical bars are 1 standard deviation above the means. Treatment symbols are defined in Table 1.

substrates (Fig. 4(f)), N : P ratios were significantly reduced in the treatments that included P additions, relative to the control and N treatments. Only the metaphyton and epipelon failed to display this pattern (Fig. 4(e)), consistent with the fact that P enrichment was not observed for that component of the community. 3.3. Nutrient mass balances Nutrient mass balances were constructed using the periphyton tissue P and N measurements from Day 28, and the estimates of total periphyton standing stocks determined by destructive harvesting. Mass balances were not determined for reference

278

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

sites, which were open systems, and therefore experienced influxes and effluxes of surface mat material and submerged Utricularia during the study. There was considerable intra-treatment variation in the P budget, and differences in periphyton P contents between treatments were not statistically significant. The largest amounts of P occurred in the benthos, which accounted for over 80% of the wholecommunity P balance in the control and N treatment (Table 3, Fig. 5(a)). Phosphorus additions enhanced the absolute and relative amounts of P contained in the surface mat, Utricularia periphyton, and water column. Thus, in the P and NP treatments, the relative contribution of benthic algae to the whole-community P balance was reduced to near 60%. By Day 28, a total of 746 mg of P had been added to each P and NP mesocosm. Only 36% of this added P could be accounted for by mass balance in the P treatment. Less than 10% of the added P could be accounted for in the NP treatment. The nitrogen mass balance (Table 4) indicated that the largest pools for N also were in the benthic community, which accounted for 70% of the total N (Fig. 5(b)), and the Utricularia epiphyton, which accounted for 20%. Inter-treatment differences in absolute quantities of N were not statistically significant for any periphyton component. There also were no noticeable differences in the relative amounts of N among treatments. It was not possible to account for any of the added N (37 322 mg total) by using the mass balance data. 3.4. Periphyton biomass and taxonomic structure Surface periphyton mats initially had 10 mg (g ww)ÿ1 CHLA, and there was little variation among treatments (Fig. 6(a)). On Day 28, the CHLA content of surface mats had increased in all treatments. At the reference sites and controls, CHLA averaged 30 mg (g ww)ÿ1, while the N treatment had a mean of 50 mg (g ww)ÿ1, and the N and NP treatments had 70 and 100 mg (g ww)ÿ1 of CHLA, respectively. The ANOVA on Day 28 Table 3 Phosphorus mass balance for the various treatments at the end of the 28-day mesocosm experiment. Values are mean total amounts of P (mg) contained by various periphyton components, with standard deviations in parentheses. `Wall' refers to the periphyton that grew on the inner tank walls during the study. The treatments are C (controls), P (phosphorus additions), N (nitrogen additions) and NP (nitrogen and phosphorus additions) Treatment

Surface Utricularia periphyton

Live Eleocharis

Dead Eleocharis

Water column

Metaphyton and epipelon

Wall

Total

C

34 (7)

57 (39)

<1 (<1)

<1 (<1)

10 (2)

545 (191)

<1 (<1)

646 (240)

P

117 (22)

276 (278)

1 (2)

<1 (<1)

22 (9)

634 (129)

<1 (<1)

1050 (440)

N

42 (24)

59 (26)

<1 (<1)

<1 (<1)

8 (1)

486 (119)

<1 (<1)

595 (170)

NP

121 (62)

137 (86)

1 (<1)

<1 (<1)

28 (14)

380 (94)

<1 (<1)

667 (256)

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

279

Fig. 5. Phosphorus (a) and nitrogen (b) budgets showing relative amounts of the nutrients accounted for by the various periphyton and water-column components of the communities. Treatment symbols are defined in Table 1.

indicated a highly significant treatment effect, with higher CHLA in the NP treatment than in the reference site and controls. We also observed distinct changes in the areal extent and visible appearance of surface periphyton mats during the experiment. They initially covered between 46% and 52% of the water surface, had a light brown color, and were comprised of a matrix of filamentous cyanobacteria (Microcoleus and Scytonema), with interspersed diatoms. There was considerable bleaching (perhaps due to photo-oxidation) of the upper mat surfaces. As the experiment progressed, surface mats in the control displayed little change in color, but they expanded to over 74% of the water surface by Day 28. Surface mats in the P treatment did not increase in areal extent, instead they developed a dark green coloration during the final week, new growth of the dominant algal taxa, and a proliferation of the chloromonad Gonyostomum. A change in color was also observed in the N treatment, where the mats expanded to cover 90% of the water surface by Day 28. These communities continued to display dominance by Microcoleus and Scytonema. We observed the most dramatic succession in the NP treatment. One week into the experiment, the entire mat became dark green in color, and disintegrated into small pieces

280

Treatment

Surface Periphyton

Utricularia

Live Eleocharis

Dead Eleocharis

Water column

Metaphyton and epipelon

Wall

Total

C

1800 (260)

8090 (6570)

130 (60)

20 (10)

30 (3)

25500 (14300)

8 (4)

45600 (21200)

P

2520 (1050)

6950 (6300)

90 (110)

10 (10)

70 (30)

24000 (6000)

6 (6)

33600 (13500)

N

3050 (1740)

6270 (3050)

200 (70)

40 (20)

190 (90)

19900 (6100)

5 (1)

29600 (11000)

NP

2390 (670)

4710 (2520)

100 (nr)

20 (10)

510 (460)

18500 (7600)

7 (1)

26200 (13900)

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

Table 4 Nitrogen mass balance for the various treatments at the end of the 28-day in situ mesocosm experiment. Values are mean total amounts of N (mg) contained by various components of the community, with standard deviations in parentheses. `Wall' refers to the periphyton that grew on the inner tank walls during the study; and nr to no replication. The treatments are C (control), P (phosphorus additions), N (nitrogen additions) and NP (nitrogen and phosphorus additions)

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

281

Fig. 6. Periphyton chlorophyll a (CHLA) contents measured on Day 0 and Day 28 of the in situ mesocosm experiment for surface periphyton mats (a); epiphyton on live Eleocharis stems (b); epiphyton on dead Eleocharis stems (c); epiphyton on Utricularia (d); metaphyton and epipelon (e); and periphyton growing on artificial substrates (f). Vertical bars are 1 standard deviation above the means. Treatment symbols are defined in Table 1.

that spread across the water surface. The areal extent was never as great as that observed in the N treatment, and from Day 14 to Day 28, it actually declined, from 70% to 54%. The period of decline coincided with an observed decrease in mid-morning DO concentrations in the NP treatment (see below). Samples of the surface mat material from Day 28 showed that Microcoleus had proliferated and formed large macroscopic bundles of parallel filaments. At this point, Scytonema, a co-dominant in the controls, was not observed in the samples. The epiphyton growing on live Eleocharis stems did not display any significant treatment effect in terms of areal CHLA content (Fig. 6(b)). CHLA on Day 0 ranged from

282

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

0.7 to 1.0 mg cmÿ2, and on Day 28 it ranged from 0.3 to 1.3 mg cmÿ2. In contrast, the epiphyton associated with dead Eleocharis stems had a higher initial CHLA content and displayed significant responses to nutrient additions (Fig. 6(c)). On Day 28, CHLA in the N treatment averaged near 3 mg cmÿ2, and was significantly higher than the <1 mg cmÿ2 values observed in the control and P treatments. The epiphyton observed on Day 28 in the control, N, and P treatments were dominated by Microcoleus and Scytonema. In the NP treatment, we observed Microcoleus, an Anabaena-like cyanobacterium, and a proliferation of the diatoms Nitzschia and Cymbella. The epiphyton associated with submerged Utricularia responded to nutrients in a manner similar to the surface mat. On Day 0, the CHLA content averaged 5 (mg ww) gÿ1 across the range of treatments, while on Day 28 it ranged from near 20 (mg ww) gÿ1 in the reference sites and controls, to 80 (mg ww) gÿ1 in the NP treatment (Fig. 6(d)). The ANOVA on Day 28 was significant ( < 0.05), with greater amounts of CHLA in the NP treatment than in all others. The algal assemblage in the N treatment was similar to that in the control, and consisted of a matrix of Microcoleus with interspersed diatoms, primarily Synedra. The P treatment also had this general structure, but it included a large number of Aphanothece and Chroococcus colonies. These colonial cyanobacteria were the dominant taxa in NP treatment. The metaphyton and epipelon communities had initial areal CHLA contents of ca. 30 mg cmÿ2 (Fig. 6(d)). CHLA increased slightly from Day 0 to Day 28, but there was no significant treatment effect. Taxonomic composition of this periphyton component was not evaluated. The artificial substrata had developed a substantial layer of periphyton by Day 28. In the control, P, and N treatments, areal CHLA contents ranged from 0.3 to 0.6 mg cmÿ2 (Fig. 6(f)). No significant differences were found among these treatments. The periphyton had a brown color, suggesting dominance by diatoms. The periphyton from NP mesocosms had a bright green color, was noticeably thicker, and had an areal CHLA content in excess of 2 mg cmÿ2. This was significantly higher than that observed in any other treatment. The data presented here reflect only two of the three NP mesocosms. In the third mesocosm, apple snails (Pomacea paludosa) were found on the substrata, and over 50% of the surface had been cleared of periphyton. 3.5. Phytoplankton biomass and carbon uptake On Day 0, phytoplankton CHLA concentrations averaged ca. 4 mg lÿ1, and there were no significant differences among the treatments (Fig. 7(a)). On several of the subsequent sampling days, elevated concentrations were observed in the NP treatment, and RMANOVA indicated a highly significant overall treatment effect. Time and treatment±time interaction effects were not statistically significant. Given this statistical result, it was possible to combine the data from Day 8 to Day 28, and perform an overall ANOVA. This test indicated that CHLA was significantly elevated in the NP treatment relative to the control and P treatments during the experiment. Phytoplankton carbon uptake on Day 0 ranged from 20 to 30 (mg C) lÿ1 hÿ1, and did not differ significantly among treatments (Fig. 7(b)). On Day 28, rates in the NP treatment was significantly higher than in all other treatments.

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

283

Fig. 7. Phytoplankton chlorophyll a (CHLA), measured on a weekly basis during the in situ mesocosm experiment (a); and rates of carbon uptake, measured on Day 0 and Day 28 (b). Vertical bars are 1 standard deviation above the means. Treatment symbols are defined in Table 1.

3.6. Community metabolism Mid-day community metabolism was slightly elevated in the NP treatment on Day 16 (Fig. 8(a)); however, inter-treatment differences were not statistically significant. On Day 23, the rate of metabolism measured in the NP treatment was significantly greater than in the reference site, control, and N treatments. The lowest rate of metabolism was observed in the N treatment. Measurements of water-column dissolved oxygen (DO) at the time of sampling (0900± 1000 h) also suggested metabolic responses to the treatments. On Day 0, the concentrations (Fig. 8(b)) were ca. 5 mg lÿ1 and did not differ significantly among the various treatments. However, on subsequent sampling days, DO was consistently lower in the P treatment, and somewhat elevated in the N treatment, relative to the controls and reference sites. The NP treatment displayed elevated DO on Day 8, and then low DO

284

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

Fig. 8. Community net metabolism, measured on Day 16 and Day 23 of the in situ mesocosm experiment (a); and concentrations of dissolved oxygen, measured on a semi-weekly basis (b). Vertical bars are 1 standard deviation above the means. Treatment symbols are defined in Table 1.

values thereafter, possibly reflecting a succession in community structure, as evidenced by the periphyton taxonomic changes described above. The RM-ANOVA indicated highly significant ( < 0.01) treatment, time, and interaction effects on DO. Highest morning (sampling time) DO in the N treatment appears to contradict the finding of most rapid community metabolism in the NP treatment. One explanation is that larger diurnal changes (higher in the day and lower at night) in DO were occurring in the NP treatment, so that morning DO levels were still depressed at the time of sampling. 3.7. Summary of biomass and metabolic responses In summary (Table 5), we observed variable responses to the individual and combined additions of P and N. P additions generally had little effect, and resulted only in

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

285

Table 5 Summary of littoral community responses to additions of N (nitrogen), P (phosphorus) and N ‡ P in the mesocosm experiment. Statistically significant treatment effects (p < 0.05) are indicated by XX, effects that are marginally significant (p < 0.10) are indicated by X, and non-significant effects are indicated by 0. In the case of dissolved oxygen (DO) and plankton chlorophyll a (CHLA), results are given for a repeated measures ANOVA Response variable Surface periphyton CHLA Utricularia epiphyton CHLA Live Eleocharis epiphyton CHLA Dead Eleocharis epiphyton CHLA Metaphyton and epipelon CHLA Artificial substrate CHLA Phytoplankton CHLA Dissolved oxygen Phytoplankton C uptake Community net O2 metabolism

Units ÿ1

(mg g ww ) (mg g ÿ1) (mg cmÿ2) (mg cmÿ2) (mg cmÿ2) (mg cmÿ2) (mg lÿ1) (mg lÿ1) (mg lÿ1 hÿ1) (mg lÿ1 hÿ1)

P

N

NP

X 0 0 0 0 0 0 0 0 X

0 0 X XX 0 0 0 XX 0 0

XX XX 0 X 0 XX XX XX XX XX

marginally significant increases in the CHLA content of surface periphyton mats, and whole-community metabolism. N additions resulted in a marginally significant increase in the CHLA content of epiphyton associated with live Eleocharis stems, and significantly enhanced the CHLA content of epiphyton on dead stems of the same macrophyte. By far, the most dramatic changes in community structure and function occurred when N and P were added together. This treatment caused significant increases in the CHLA content of surface algal mats and Utricularia epiphyton, phytoplankton CHLA, dissolved oxygen concentrations, phytoplankton carbon uptake and whole community metabolism. 4. Discussion 4.1. Characteristics of the study site The study site in Lake Okeechobee was slightly enriched with P, as compared to sites deeper within the lake's marsh or the interior Florida Everglades. Water column TP averaged 16 mg lÿ1, and total N : P ratios averaged 146 : 1. In comparison, McCormick et al. (1998) recorded TP concentrations of ca. 10 mg lÿ1, and N : P ratios of 260 : 1 in oligotrophic Florida Everglades communities with a similar macrophyte composition. The TP concentrations observed here are relatively low by limnological standards (Wetzel, 1983), but they are within a range of TP concentrations (10±20 mg lÿ1) where changes in subtropical periphyton community structure have been documented (McCormick et al., 1996). It is well known that small differences in P availability (<1 mg lÿ1) can affect production : biomass ratios of periphytic algae (Bothwell, 1989). Thus, one might have expected different responses by the periphyton to nutrients at our site, as compared with more pristine locations. In fact, this is what we observed. Periphyton biomass is high in pristine regions of the Everglades and in the littoral zone of Lake Okeechoebee, in comparison with temperate wetlands of similar nutrient

286

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

status (Vymazal, 1995). McCormick et al. (1998) suggest that this may be due to the following: 1. the prolonged subtropical growing season; and 2. efficient P uptake and recycling due to ``a close aggregation of algal and heterotrophic microbial components'' in the community. Phosphorus cycling within the matrix of periphyton mats has been shown to be a substantial source of P, once biomass accrues to the point that boundary layers affect diffusion (Carlton and Wetzel, 1988; Mulholland, 1996). At a more oligotrophic marsh site in Lake Okeechobee, Hwang et al., 1999 found that littoral epiphyton had a very high affinity for P (Ks 0.5 mg lÿ1), and that this affinity was maximized during periods when soluble P concentrations in the water column are at their seasonal low. Similar results have been obtained at oligotrophic Everglades sites (Davis, 1982; Scinto, 1997). 4.2. Nutrient budgets Added P was rapidly sequestered from the water column of the mesocosms and concentrated in the tissues of several components of the periphyton community. After 28 days, a total of 1100 mg lÿ1 P had been added in the P and NP treatments, but <50 mg lÿ1 was found in the water column as TP. Soluble P always was below the analytical detection limit. Total P most likely represented organic P that was taken up by the community as soluble P, and subsequently processed through periphyton growth and microbial decomposition. Many previous studies have shown that wetland communities `transform' soluble nutrients into dissolved and particulate organic forms (Wetzel, 1996). The largest increases in periphyton tissue P content were observed in the surface algal mat, and the epiphyton associated with Utricularia. These two components represented floating and submerged portions, respectively, of a single community. The major difference, in addition to location, was that the Utricularia in the surface mats was decomposed nearly beyond recognition, while the submerged macrophytes were quite intact. Together, they dominated the biomass, and had a very large active surface area for P uptake. Epiphyton growing on Eleocharis stems, in contrast, had a relatively small total biomass and surface area, and a lesser uptake of added P. There was no measurable increase in the tissue P content of benthic algae, despite an active surface area of at least 11 000 cm2. Periphyton associated with the surface algal mats and Utricularia (taxonomically similar to communities at Everglades sites) may be best adapted to rapidly remove P from the water column, or they may be at a competitive advantage simply due to their proximity to the location of incoming nutrients. Browder et al. (1994) concluded that surface algal mats dominate both primary productivity and the sequestering of P from the water column in oligotrophic Everglades marsh communities. Of the total quantity of added P, we could account for only 36% in the P treatment, and <10% in the NP treatment. To a large extent, this may reflect the inherent difficulties in extrapolating from measurements of P content performed on small sections of tissue to entire communities. There was extreme variation in our final estimates of community P among the replicates of each treatment. Similar results have been obtained in previous

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

287

studies. McDougal et al. (1997) added P to large mesocosms in a Prairie wetland, and were able to account for 47% to 66% of the P in a mass balance. Davis (1982) added 32P to a natural Everglades sawgrass community, and quantified uptake by periphyton community components after a 10-day incubation. The periphyton and phytoplankton accounted for only 9% of the added P. The largest reservoir occurred in soils lying beneath the benthic algal mat (54%). Live macrophyte tissue contained only 4% of the added P. These pools were not quantified in the present study. Another possible pathway for P loss is by water exchange between the mesocosms and the surrounding water column, through the porous sand sediments. In contrast to the patterns observed for P, large quantities of N occurred in the water column as both TN and dissolved inorganic N. Combined levels of NO2, NO3, and NH4 exceeded 18 000 mg lÿ1 in the N treatment, and 12 000 mg lÿ1 in the NP treatment, where periphyton uptake presumably was responsible for the somewhat lower level. Despite the fact that the total amounts of added N (>37 000 mg) were of similar magnitude to the N content of the periphyton community (35 000 mg in the control), we observed no significant increases in community N content in the budgets for the N and NP treatments. Similar results were obtained in the preliminary study conducted in 1996, which included only control and combined NP treatments. One probable explanation is that much of the added N was lost from the community by the microbially mediated process of denitrification, which can occur at high rates in freshwater wetlands at high temperatures (Vymazal, 1995). Volatilization of ammonia may represent another loss process (Emerson et al., 1975). 4.3. Biomass and productivity responses to nutrient additions Experimental studies in the Florida Everglades indicated that periphyton taxonomic composition responds strongly to additions of P (McCormick and O'Dell, 1996). Taxonomic changes along anthropogenic P gradients mimic results observed in the experiments (McCormick et al., 1996), and increases in the primary productivity of Everglades periphyton also have been observed with increasing concentrations of P (McCormick, unpublished data). The study site in Lake Okeechobee was more P-enriched than pristine Everglades sites, in terms of water-column concentrations of P. The lake periphyton community also appeared to be less limited by P. None of the biomass or metabolism variables increased significantly in mesocosms where P was added alone. One biomass variable (Eleocharis epiphyton CHLA) responded to N, and seven responded to the combined addition of N and P. Two of the responses to N plus P (community metabolism and surface mat CHLA) may reflect a secondary limitation by N, but the other responses clearly indicated colimitation by the two nutrients. One explanation for these results is that our study site has experienced periodic advective inputs of water with a higher P content from the nearby pelagic region. One such episode occurred during 1995±1996, when water levels following a one-in-300-year rain event reached 5.5 m, nearly 1 m above the seasonal norm. At this water level, over 1 m of water covered the entire littoral marsh for several months, and there may have been considerable inputs of P from the nutrient-rich pelagic region (total P100 mg lÿ1).

288

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

Luxury consumption of P by periphyton at such times could provide a readily available reservoir of this nutrient, so that P limitation is reduced during times when inputs are low. The mesocosm experiment was performed at a time when lake levels had declined by over 1 m from their peak level. Under such conditions the interior marsh is hydrologically uncoupled from the open lake (Richardson and Hamouda, 1995), and periphyton uptake may have reduced water column P concentrations to the observed low levels (yet, still somewhat higher than observed at more oligotrophic sites). The data necessary to support this hypothesis (measurements of periphyton P content at various times after high water events) have not been collected. Another interesting result was observed for epiphyton growing on dead Eleocharis stems. On Day 28, the CHLA content was twofold higher in the N treatment than in the NP treatment. This pattern was exactly the opposite of that observed for the surface algal mats and submerged Utricularia epiphyton. Rapid growth by these other components may have resulted in greater competition for nutrients in the NP treatment, and thereby hindered the biomass accrual of Eleocharis epiphyton. Previous studies have indicated changes in relative biomass among various components of the periphyton. McDougal et al. (1997) documented that with N and P enrichment, prairie wetlands undergo a shift from an epiphyton-dominant state to a metaphyton (Cladophora)-dominant state. Goldsborough and Robinson (1996) developed a conceptual model of wetland periphyton community structure, and proposed that this kind of shift is a general response of wetlands to enhanced nutrient inputs. Hence, the finding that combined dosing with N and P reduced the biomass of one community component is not unusual. 5. Summary and conclusions Results of a month-long controlled mesocosm experiment in subtropical Lake Okeechobee indicated that components of the littoral periphyton were co-limited by N and P. This contrasts with results from oligotrophic areas of the nearby Florida Everglades, where periphyton is strongly limited by P (McCormick et al., 1996; Vymazal et al., 1994), and with results of P kinetics studies in more pristine areas of the lake, where P limitation also was observed (Hwang et al., 1999). The situation observed at the mesocosm study site may reflect enrichment effects of advective P inputs from the pelagic region during high water level events. Periphyton then may carry out luxury consumption of P, providing a readily-available reservoir of P during lower water periods with reduced external P inputs to the community. Acknowledgements The authors are grateful to Charles Hanlon and Mark Brady for their assistance with mesocosm and boardwalk construction and sample collection, and to Barry Rosen for identifying the dominant algal taxa. Comments from Charles Hanlon, Susan Gray, Paul McCormick, Garth Redfield, Alan Steinman, and two anonymous reviewers were helpful in improving an earlier version of the manuscript.

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

289

References APHA, 1995. Standard Methods for the Analysis of Water and Waste Water. American Public Health Association, Washington, D.C. Axler, R.P., Reuter, J.E., 1996. Nitrate uptake by phytoplankton and periphyton: whole-lake enrichments and mesocosm 15 Nexperimentsinanoligotrophiclake:Limnol:Oceanogr:41; 659 ÿ ÿ671: Blumenshine, S.C., Vadenboncoeur, Y., Lodge, D.M., Cottingham, K.L., Knight, S.E., 1997. Benthic-pelagic links: responses of benthos to water-column nutrient enrichment. J.N. Am. Benthol. Soc. 16, 466±479. Bolas, P.M., Lund, J.W.G., 1974. Some factors affecting the growth of Cladophora glomerata in the Kentish Stour. Water Treat. Exam. 23, 25±51. Bothwell, M.L., 1989. Phosphorus-limited growth dynamics of lotic periphyton diatom communities: areal biomass and cellular growth rate responses. Can. J. Fish. Aquat. Sci. 46, 1293±1301. Browder, J.A., Gleason, P.J., Swift, D.R., 1994. Periphyton in the Everglades: spatial variation, environmental correlates, and ecological implications. In: Davis, S.M., Ogden, J.C. (Eds.), Everglades:The Ecosystem and its Restoration. St. Lucie Press, Delray Beach, FL., pp. 379±418. Carlton, R.G., Wetzel, R.G., 1988. Phosphorus flux from lake sediments: effect of epipelic algal oxygen production. Limnol. Oceanogr. 33, 562±570. Cattaneo, A., 1987. Periphyton in lakes of different trophy. Can. J. Fish. Aquat. Sci. 44, 296±303. Davis, S.M., 1982. Patterns of Radiophosphorus Accumulation in the Everglades after its Introduction into Surface Water. Technical Publication 82-2, South Florida Water Management District, West Palm Beach, FL, 33416-4680, USA. Dodds, W.K., Gudder, D.A., 1992. The ecology of Cladophora. J. Phycol. 28, 415±422. Emerson, K., Russo, R.C., Lund, R.E., Thurston, R.V., 1975. Aqueous ammonia equilibrium calculations: effect of pH and temperature. J. Fish. Res. Bd. Can. 32, 2379±2383. Goldman, C.R., 1981. Lake Tahoe: two decades of change in a nitrogen deficient oligotrophic lake. Verh. Int. Ver. Limnol. 21, 45±70. Goldsborough, G.L., Robinson, G.G.C., 1996. Patterns in wetlands. In: Stevenson, R.J., Bothwell, M.L., Lowe, R.L. (Eds.), Algal Ecology. Academic Press, San Diego, CA, pp. 77±117. Havens, K.E., Aumen, N.G., James, R.T., Smith, V.H., 1996. Rapid ecological changes in a large subtropical lake undergoing cultural eutrophication. Ambio 25, 150±155. Harris, T.T., Williges, K.A., Zimba, P.V., 1995. Primary productivity and decomposition of five emergent macrophyte communities in the Lake Okeechobee marsh ecosystem. Arch. Hydrobiol., Adv. Limnol. 45, 63±78. Hawes, I., Smith, R., 1993. Effect of localized nutrient enrichment on the shallow epilithic periphyton of oligotrophic Lake Taupo, New Zealand. N.Z. J. Mar. Freshwat. Res. 27, 365±372. Henry, R., Hino, K., Tundisi, J.G., Riberio, J.S.B., 1985. Responses of phytoplankton in Lake Jacaretinga to enrichment with nitrogen and phosphorus in concentrations similar to those in the River Solimoes (Amazon, Brazil). Arch. Hydrobiol. 103, 453±477. Herbst, R.P., 1969. Ecological factors and the distribution of Cladophora glomerata in the Great Lakes. Am. Midl. Nat. 82, 90±98. Horne, A.K., 1979. Management of lakes containing nitrogen-fixing blue-green algae. Arch. Hydrobiol. Ergebn. Limnol. 13, 133±144. Howard-Williams, C., 1981. Studies on the ability of a Potamogeton pectinatus community to remove dissolved nitrogen and phosphorus compounds from lake water. J. App. Ecol. 18, 619±637. Hwang, S.J., Steinman, A.D., Havens, K.E., 1999. Phosphorus kinetics of planktonic and benthic assemblages in a shallow subtropical lake. Fresh water Biol., in press. McCormick, P.V., O'Dell, M.B., 1996. Quantifying periphyton responses to phosphorus in the Florida Everglades: a synoptic±experimental approach. J.N. Am. Benthol. Soc. 15, 450±468. McCormick, P.V., Rawlik, P.S., Lurding, K., Smith, E.P., Sklar, F.H., 1996. Periphyton±water quality relationships along a nutrient gradient in the northern Florida Everglades. J.N. Am. Benthol. Soc. 15, 433± 449. McCormick, P.V., Shuford, R.B.E., Backus, J.G., Kennedy, W.C., 1998. Spatial and temporal patterns of periphyton biomass and productivity in the northern Everglades, Florida, USA. Hydrobiologia, in press.

290

K.E. Havens et al. / Aquatic Botany 63 (1999) 267±290

McDougal, R.L., Goldsborough, G.L., Hann, B.J., 1997. Responses of a prairie wetland to press and pulse additions of inorganic nitrogen and phosphorus: production by planktonic and benthic algae. Arch. Hydrobiol. 140, 145±167. Mulholland, P.J., 1996. Role in nutrient cycling in streams. In: Stevenson, R.J., Bothwell, M.L., Lowe, R.L. (Eds.), Algal Ecology: Freshwater Benthic Ecosystems. Academic Press, San Diego, CA, pp. 609±639. Pitcairn, E.R., Hawkes, H.A., 1973. The role of phosphorus in the growth of Cladophora. Water Res. 7, 159± 171. Richardson, J.R., Hamouda, E., 1995. GIS modeling of hydroperiod, vegetation, and soil nutrient relationships in the Lake Okeechobee marsh ecosystem. Arch. Hydrobiol. Spec. Issues Advanc. Limnol. 45, 95±115. Richardson, J.R., Harris, T.T., 1995. Vegetation mapping and change detection in the Lake Okeechobee marsh ecosystem. Arch. Hydrobiol. Spec. Issues Advanc. Limnol. 45, 17±40. SAS, 1996. Statistical Analysis System, Version 6.12. SAS Institute, Cary, NC. Schelske, C.L., 1984. In situ and natural phytoplankton assemblage bioassays. In: Shubert, L.E. (Ed.), Algae as Ecological Indicators. Academic Press, London, pp. 15±47. Schindler, D.W., 1977. Evolution of phosphorus limitation in lakes. Science 195, 260±262. Scinto, L.J., 1997. Phosphorus cycling in a periphyton dominated freshwater wetland. Ph.D. Dissertation, University of Florida, Gainesville, FL, USA. Smith, V.H., 1983. Low nitrogen to phosphorus ratios favor dominance by blue-green algae in lake phytoplankton. Science 221, 669±671. Steinman, A.D., Havens, K.E., Louda, J.W., Winfree, N.M., Baker, E.W., 1997. Spatial and temporal distribution of algal biomass in a large, subtropical lake. Arch. Hydrobiol. 139, 29±50. Steinman, A.D., Lamberti, G.A., 1996. Biomass and pigments of benthic algae. In: Hauer, R., Lamberti, G.A. (Eds.): Methods in Stream Ecology. Academic Press, NY, pp. 295±313. USEPA, 1979. Methods for Chemical Analysis of Water and Waste Water. United States Environmental Protection Agency, Washington, D.C. USEPA, 1987. Handbook of Methods for Acid Deposition Studies. United States Environmental Protection Agency, Washington, D.C. Vollenweider, R.A., 1974. A Manual on Methods for Measuring Primary Production in Aquatic Environments. IBP handbook No. 12., Blackwell Scientific, Oxford. Vymazal, J., 1995. Algae and Element Cycling in Wetlands. CRC Press, Boca Raton, FL. Vymazal, J., Craft, C.B., Richardson, C.J., 1994. Periphyton response to nitrogen and phosphorus additions in Florida Everglades. Algol. Stud. 73, 75±97. Wetzel, R.G., 1983. Limnology. Saunders College Publishing, Philadelphia, PA, USA. Wetzel, R.G., 1996. Benthic algae and nutrient cycling in lentic freshwater ecosystems. In: Stevenson, R.J., Bothwell, M.L., Lowe, R.L. (Eds.), Algal Ecology. Academic Press, San Diego, CA, pp. 641±753.