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沉水植物与水环境


Aquatic Botany 78 (2004) 67–82

Recovery of submerged plants from high water stress in a large subtropical lake in Florida, USA
Karl E. Havens? , Bruce Sharfstein, Mark A. Brady, Therese L. East, Matthew C. Harwell1 , Ryan P. Maki, Andrew J. Rodusky
Ecosystem Restoration Department, South Florida Water Management District, 3301 Gun Club Road, West Palm Beach, FL 33406, USA Received 5 November 2002; received in revised form 4 August 2003; accepted 15 September 2003

Abstract The spatial and temporal dynamics of submerged plants were examined in a large subtropical lake in Florida, USA. The objective was to characterize succession of the community following a natural experiment in 2000–2001, when release of water from the lake, followed by a severe drought, reduced water levels by 2 m, alleviating stress of multiple years of high water. A systematic survey of shoreline transects was used to compare attributes of submerged plants under pre-drought versus post-drought conditions. Initially, the plants did not respond to lower water because shoreline areas had high turbidity from resuspended sediments and, later algal blooms. In June 2000, approximately 2 months after the water level was lowered, Chara (a macro-alga) rapidly expanded across the near-shore landscape. For over 1 year, this plant strongly dominated the submerged plant community, with just scattered individuals or isolated beds of vascular plants, including Potamogeton, Vallisneria, and Hydrilla. This included a period when the lake reached a record low elevation, where much of the habitat became dry, and then subsequently re-?ooded in late summer 2001. However, in November 2001, Chara rapidly declined and vascular taxa (Hydrilla and Potamogeton) became dominant. They subsequently increased their biomass and spatial extent, and the previous Chara dominance did not return. Just prior to the loss of Chara, a frontal system passed over the lake, with wind velocities in excess of 30 km h?1 for 3 days. Concentrations of solids in the water more than doubled and uprooted Chara was observed ?oating in the water. In this large, wind-driven lake, Chara may only be an ephemeral pioneer because, lacking roots, it is probably more sensitive to excessive wind-related stress (e.g. wave energy and scouring) than vascular plants. ? 2003 Elsevier B.V. All rights reserved.
Keywords: Submerged plants; High water levels; Wind; Recovery; Subtropical lakes; Charophytes; Angiosperms

Corresponding author. Tel.: +1-561-682-6534; fax: +1-561-682-6442. E-mail address: khavens@sfwmd.gov (K.E. Havens). 1 Present address: A.R.M. Loxahatchee National Wildlife Refuge, 10216 Lee Road, Boynton Beach, FL 33437, USA. 0304-3770/$ – see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2003.09.005

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1. Introduction In shallow lakes and estuaries, submerged plants play a key role in controlling ecosystem dynamics (Dennison et al., 1993; Scheffer, 1998). The plants can stabilize sediments (Barko and James, 1998; Vermaat et al., 2000), support epiphytic algae that sequester nutrients from the water (Burkholder et al., 1990; Hansson, 1990; Vadeboncoeur and Steinman, 2002), reduce ?ow velocity, so that suspended particles are removed from the water (Schriver et al., 1995; Vermaat et al., 2000), facilitate phosphorus removal from the water column by co-precipitation with calcium at high pH (Murphy et al., 1983), and oxidize sediments, so that Fe3+ binds with sediment phosphorus, reducing ?uxes into the water column (Wigand et al., 1997). As a result, areas of dense submerged plants in shallow lakes typically have very clear water, and low concentrations of nutrients and phytoplankton (Scheffer et al., 1994; Jeppesen et al., 1998; Blindow et al., 2002). Water depth is a major factor in?uencing the biomass and spatial extent of submerged plants in shallow aquatic ecosystems. A variety of studies have described the wax and wane of submerged plant communities in temperate aquatic ecosystems with variable water depths (e.g. Wallsten and Forsgren, 1989; Kowalczewski and Ozimek, 1993; Gafny and Gasith, 1999). The recovery of temperate communities following high water stress (e.g. Carter and Rybicki, 1986) and reduced nutrient loading (van den Berg et al., 1999) also has been documented. Submerged plants can account for a substantial portion of total primary production in subtropical lakes (Can?eld et al., 1985), and as such, have potential to exert strong in?uences on ecosystem dynamics. However, it is uncertain whether information obtained in temperate lakes studies is directly applicable in the sub-tropics, where seasonal variation in water temperature, photoperiod, and other attributes are considerably different. To date, most studies of submerged plants in subtropical lakes have been regional lake

Fig. 1. Surface elevation of Lake Okeechobee in meters above sea level from 1998 to 2002, and the major events that impacted the submerged plant community. The period of data collection is also indicated. Surface elevation on a given day is determined as the mean from eight stations located around the lake’s perimeter.

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surveys (e.g. Can?eld et al., 1985; Bachmann et al., 2002). We recently have been examining in detail the seasonal and year-to-year variation in submerged plant communities in subtropical Lake Okeechobee (e.g. Havens et al., 2001; Steinman et al., 2002a; Havens, 2003), as well as conducting small-scale controlled experiments to evaluate plant responses to modi?ed environmental conditions (Grimshaw et al., 2002). Here, we report on the results of a “natural experiment” at the whole ecosystem level, where a planned drawdown of the large lake was followed by a severe drought, which resulted in water levels dropping by nearly 2 m (Fig. 1). This event had dramatic impacts on the submerged plant community, which previously had been decimated by 6 years of sustained high water levels. It also provided a unique opportunity to quantify the recovery sequence of plants in a large subtropical lake following a major disturbance, and identify possible environmental forcing functions.

2. Methods 2.1. Study site Lake Okeechobee has a surface area of 1730 km2 , a mean depth of 2.7 m, and it is classi?ed as eutrophic based on concentrations of total phosphorus, chlorophyll a, and Secchi transparency (Havens et al., 1996). The lake is located at 27? 00 N Latitude, 80? 50 W Longitude in south-central Florida, USA. The lake is a natural feature of the regional landscape, but in the mid 20th century, it was enclosed with a ?ood control dike, with locks and gates that can be used to regulate water levels (Steinman et al., 2002b). The lake serves as a regional water resource for agriculture and urban areas, a water supply for downstream natural ecosystems, a recreational and commercial ?shery, and a habitat for wildlife, including federally-endangered species (Aumen, 1995). The lake contains three distinct zones. In a central pelagic zone, phytoplankton is the single contributor to primary production. In a western littoral zone, primary production is by a diverse community of emergent, ?oating-leaved, and submerged vascular plants, charophytes, periphyton, and benthic algae. A third zone consists of a near-shore shoal that lies between the pelagic and littoral zones. Here, primary production is dominated either by vascular plants and periphyton, when lake level is low, or phytoplankton, when lake level is high. Depths in the pelagic zone average near 4 m, and Secchi transparencies typically are between 10 and 20 cm. This explains the lack of vascular plants or benthic algae in this zone. Depths in the littoral zone average less than 1 m and there is a high degree of light penetration to the lake bottom. Depths in the near-shore zone average 1 m, and Secchi transparency varies from 20 cm to over 1 m. The amount of underwater irradiance in the near-shore zone, the primary habitat for submerged plants, is largely determined by the degree of transport of ?ne mud sediments from the pelagic zone. This transport is enhanced under high wind/high water level conditions (Jin et al., 2000; Havens et al., 2001). The zone is underlain by sand and peat sediments (Fisher et al., 2001). The dominant submerged species are Chara spp., Hydrilla verticillata (L.F.) Royle, Vallisneria americana Michx., Najas guadalupensis (Spreng) Morong, Ceratophyllum demersum L., Utricularia spp., and Potamogeton illinoensis Morong. Benthic

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mats of Lyngbya spp. sometime cover large portions of the lake bottom at the south end of the lake. Among the Chara species found in the lake, C. zeylanica Klein ex Willd. is most common. It is a tropical species with cosmopolitan occurrence in the United States, Central and South America, Africa, southern Asia, and northern Australia (Proctor et al., 1971). It is monoecious, with oospores that are quite resistant to stress such as passage through guts of migratory birds (Proctor, 1962), and which germinate independent of light at a high rate compared to other Chara species under greenhouse conditions (Proctor, 1960). 2.2. Sampling methods Submerged plants were sampled along 15 transects that were established perpendicular to the shoreline in the south, west, and north near-shore zone. No transects were established on the eastern side of the lake, which is deeper and has a shoreline of large rocks that reinforce the levee; no plants occur in that location. Sampling began in May 1999 and is ongoing. For this analysis, we present data from 22 events (May, July, and October 1999; February, April–August, and October 2000; January, April–June, August–November 2001; January, March–May 2002). After May 2002, the sampling network was recon?gured, so data beyond that point are not included. From 1999 to January 2001, and after June 2001, sampling was done at three ?xed stations along each transect. The most distant point offshore on each transect corresponded to the maximal lakeward extent of submerged plants during a period of low water in the late 1980s, when the transects were established (Zimba et al., 1995). The other two stations were located at approximately 50 m from the shoreline, and at the mid-point between the terminal stations. The distance between stations along any given transect varied from 200 to 2100 m. In April, May, and June 2001, when water levels were low and certain shoreline stations dry, additional stations were added to the lakeward end of transects because plants expanded out into deeper water. In some cases, as many as six stations were sampled along a transect, while in other cases, only one station was used. At each station, water depth was measured with a calibrated plastic rod and Secchi transparency was measured with a 20-cm black and white disk. Underwater irradiances also were measured, with a LiCor spherical quantum sensor. Water samples were collected for measurement of light-attenuating materials, including chlorophyll a, color, total suspended solids, and volatile suspended solids (for methods see below). Non-volatile solids, an indicator of inorganic solids from sediment resuspension (Phlips et al., 1995), were calculated as the difference between total and volatile solids concentrations. From 1999 to mid-summer 2000, submerged plants were sampled by harvesting all material enclosed by three 0.5 m2 weighted plastic frames, which were randomly tossed from the boat at each station. Plants were harvested underwater by breaking stems at the sediment surface, and collecting the aboveground material in nylon mesh bags. The bags were immersed into the water several times to dislodge loosely attached periphyton, and the plant material was held in an ice chest until processing in the laboratory. Sampling of Chara, which was <30 cm tall during this study, was done with three samples of a small Ponar dredge (total area sampled 0.08 m2 ). During winter, or when water depth was greater than 2 m, we ?rst surveyed the station by randomly swimming over the bottom (using diving lights when visibility was very low). If no plants were observed, sampling frames were not deployed, and dredge samples were not taken.

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Starting in August 2000, when a large-scale mapping of submerged plants was done (Havens et al., 2002), the sampling method was modi?ed to increase ef?ciency and ensure greater consistency in sampling at different times of the year and at stations of varying depth. The vascular plants and Chara were collected by triplicate samples of a tool constructed from two standard garden rakes, bolted together to create a tongs-like device. The degree of opening was constrained by a chain between the two handles, so that three replicate samples would harvest approximately 1 m2 of lake bottom. When the rakes are in a closed position, the distance between adjacent tines is approximately 10 mm, allowing for collection of very small plants. Collected material was placed into plastic bags and stored on ice as above. Periphyton was removed in the laboratory by agitation of the plants under water in plastic trays. A preliminary comparison of quadrat versus tong sampling indicates a good agreement between methods, over the range of plant taxa and sediment types sampled. In regions of the lake with sand sediments, the regression equation comparing the two methods is: y = 0.72x + 0.55 (r 2 = 0.72, P < 0.05, n = 5), where y is the total plant biomass sampled by quadrats and x is biomass sampled by the tongs. A similar relationship, with a slightly greater variability, was obtained for peat sediments: y = 0.85x + 0.34 (r 2 = 0.57, P < 0.05, n = 6). Sand and peat are the dominant sediment types in the near-shore zone. 2.3. Laboratory methods Concentrations of total and volatile solids were determined according to Standard Methods (APHA, 1985). Concentrations of phytoplankton chlorophyll a were determined with a spectrophotometer, after ?ltration of samples onto Whatman GF/F ?lters, grinding on a tissue grinder, and extraction in 90% acetone for at least 2 h in the dark at 0 ? C. Plant samples were sorted by species, stripped of epiphyton, and dried in ovens at 60 ? C until two successive measurements of biomass con?rmed that weight loss no longer was occurring. Dry weight biomass was expressed on a per area basis (g DW m?2 ). In addition to documenting trends in plant biomass over time, we also quanti?ed the percentage of sampling sites having plants, and the number of plant species observed, normalized to number of sites (because this attribute of sampling intensity varied over the study period). The spatial distribution of submerged plants was examined by visual examination of maps of the sampling sites, with station locations color-coded to correspond with presence versus absence of plants. 2.4. Statistical analyses Data collected on particular sampling dates were averaged across stations, and are presented as sequential bar graphs with standard errors, to illustrate temporal trends in water quality conditions and submerged plant biomass and species composition. Relationships between environmental attributes were evaluated with least-squares regression, and stepwise regression was used to predict plant biomass from one or more of the environmental variables. Statistical analyses were done using SYSTAT? Version 10.2. To address the problems of skewed data distributions and heterogeneous variability, all data were log 10 transformed prior to conducting the statistical analyses.

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3. Results 3.1. Physical and chemical trends During the 3-year study, mean daily water depths at the sampling stations varied from approximately 0.5 to 2.5 m (Fig. 2A). From May to October 1999, depths increased, coincident with the rapid rise in surface elevation of the lake after a period of heavy rainfall in the watershed. Depths at the stations then declined, to an average of 0.8 m after the lake drawdown in spring 2000. During spring and summer 2000, water depths at the sampling stations were relatively constant. Depths were low during spring and early summer 2001, coincident with the drought, which dropped surface elevation of the lake to a record low. Depths at the sampling stations then increased to near 2 m and gradually declined toward the end of the study (May 2002). Submerged plants were likely exposed to a wide range of irradiances, due both to changes in water depth (Fig. 2A) and amounts of light-attenuating materials in the water (Fig. 2B–C). Havens (2003) documented that non-volatile solids are the main factor attenuating light in the near-shore region of this lake. Phytoplankton can contribute substantially to light attenuation at certain times and locations, but this is not a common situation. Concentrations of chlorophyll a varied directly with water depth (log chlorophyll a = 1.04log depth + 1.15, r 2 = 0.59, P < 0.001, n = 17), but non-volatile solids did not correlate signi?cantly with depth (r 2 = 0.15, P = 0.10). 3.2. Trends in submerged plant biomass and species composition The biomass of submerged plants was low in 1999 (Fig. 3A), when depths were greatest, and high in autumn 2000 and 2001 when depths were low, though not so low that sites with submerged plants became dry. Biomass was also reduced in winter. The plant species composition displayed a pronounced succession (Fig. 3B). Prior to the time of drawdown and drought, when plant biomass was low (May 1999 to April 2000), the community was dominated by sparse stands of Vallisneria and/or Potamogeton. Hydrilla comprised a substantial portion of total biomass on one occasion (October 1999). These stands of vascular plants appeared to be remnants of larger beds that were located in sheltered areas, where they were able to survive during the years with high water. As biomass of submerged plants increased after the drawdown, Chara became dominant, accounting for between 50 and 100% of submerged plant biomass from June 2000 to October 2001. The exception was January 2001, when biomass was low during winter, and dominated by Vallisneria occurring in one dense stand at the southern end of the lake. In autumn 2001, the community structure changed: vascular plants increased and Chara dramatically declined in relative biomass. From November 2001 onward, Chara accounted for no more than 5% of total plant biomass. The dominant taxa now were Hydrilla, followed by Potamogeton, a mixed community of Najas and Utricularia, and Vallisneria. A temporal trend was also observed in overall frequency of plants (Fig. 4). In 1999, when water level was high, plants were found at 5–30% of the stations. In summer 2000, following the drawdown, the percent occurrence of plants increased to 65–80%, and averaged 45% thereafter. The total number of species collected during each sampling trip increased by nearly three fold from 1999 to 2002. This was due to the occurrence of Najas, Utricularia,

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Fig. 2. Variation in water depth (A), phytoplankton chlorophyll a concentrations (B), and non-volatile solids concentrations (C) at the submerged plant sampling stations in Lake Okeechobee. Data are means from all of the stations in a particular sampling event, with standard error bars.

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Fig. 3. Variation in total biomass of submerged plants (A), and relative biomass of different plant species (B) during the 3-year study at Lake Okeechobee. Data in panel A are means from all of the stations on a particular sampling event, with standard error bars. Data in panel B are for Chara spp. (CHA), Hydrilla verticillata (HYD), Vallisneria americana (VAL), Potamogeton illinoiensis (POT), and other less frequently encountered taxa (OTH), including Najas quadalupensis and Utricularia spp.

and Ceratophyllum in samples from 2002. These taxa were not found in the lake during the earlier years (1999–2000). In regard to spatial distribution (Fig. 5), only eight stations had plants in October 1999, and ?ve of these occurred at the southern end of the lake, sheltered by islands. In July 2000, 2 months after the drawdown, the submerged vegetation had increased in spatial extent to 16 stations. However, this vegetation was still concentrated in the southern region. By October 2000, submerged plants had spread around the full extent of the near-shore zone. A total of 22 stations had plants, and these were uniformly distributed from north to south. At that time, water clarity was high in the entire near-shore region, whereas in July the lake had remained turbid in the north.

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Fig. 4. Variation in the % occurrence of submerged plants at the sampling sites in Lake Okeechobee.

The submerged plant community was affected by low water in spring 2001. In June of that year, 13 stations had plants. Those stations were located rather far offshore, where there still was suf?cient water for their growth. When the water level rose again in fall 2001, submerged plants recovered at re-inundated areas: in October 2001, 20 stations had plants. There was a subsequent decline, but by May 2002, 11 stations again had plants. Areas of high biomass occurred both in the northern and southern regions of the lake. 3.3. Relationships between submerged plants and environmental conditions The biomass of submerged plants displayed an inverse relationship with water depth (log biomass = ?1.69 log depth + 0.62, r 2 = 0.22, P = 0.03, n = 21) and non-volatile solids (log biomass = ?1.36 log non-volatile solids + 1.74, r 2 = 0.32, P = 0.02, n = 18). Cutoff points, beyond which plants did not occur, could also be identi?ed (depth >2.2 m, non-volatile solids >30 mg l?1 ), but below these cutoffs, it was not possible to accurately predict whether or not plants would occur. A stepwise multiple regression model, considering depth, chlorophyll a, non-volatile solids, and color, retained depth and non-volatile solids as signi?cant predictor variables. The model explains 43% of the variation in average plant biomass (log biomass = ?1.17 log depth ? 1.01 log non-volatile solids + 1.48, r 2 = 0.43, P = 0.02, n = 18), and provides a moderately good ?t to observed data during the 3-year study period (Fig. 6).

4. Discussion This study examined the recovery of a submerged plant community when stress from several years of high water was alleviated by a lake draw down and a subsequent natural drought. Because we had an extensive monitoring program in place prior to the dramatic

76 K.E. Havens et al. / Aquatic Botany 78 (2004) 67–82 Fig. 5. Maps showing the presence (black circles) vs. absence (open circles) of submerged plants on six selected occasions during the 3-year study of Lake Okeechobee, to illustrate temporal changes in the spatial distribution of plants. The grey areas correspond to a wetland region with emergent vascular plants.

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Fig. 6. Biomass of submerged plants in Lake Okeechobee, as measured in the sampling program (black symbols), vs. predictions from a stepwise regression model (solid line) based on water depth and concentration of non-volatile suspended solids.

changes in water level, it was possible to treat the events as a natural experiment, and evaluate spatial and temporal responses of the plant community as the environmental conditions changed. The correlational statistics that we used cannot establish cause–effect relationships, but they do provide insight into possible controlling factors for the submerged plant community. Recovery of the submerged plants in Lake Okeechobee began when 0.3 m of water was intentionally discharged from the lake, and was immediately followed by a greater decline in water level (?2 additional meter of depth) during the drought. The recovery of plants was prolonged, taking approximately 2 years until a moderate biomass and diverse assemblage of vascular plants was re-established in the lake. Recovery included an initial period (spring 2000) with little observed response, perhaps because the water remained turbid immediately after the draw down, or simply re?ecting the time required for plants to colonize the landscape where they had nearly been eliminated. For example C. zeylanica, the dominant Chara species in Lake Okeechobee, requires 8–10 days just for oospore germination under favorable conditions (Proctor, 1960). Dense blooms of cyanobacteria occurred in the near-shore zone during the period without plants (Havens et al., 2001). Submerged plants began to recover at the southern end of the lake in summer 2000, and they expanded around the lake shore by October of that year. However, for over a year, vascular plants were rare and the community was dominated by Chara. During the 3-year period, major transitions in species dominance occurred quickly—in particular, strong dominance by Chara spp. “switched” on and off within 1-month time frames (June to July 2000, October to November 2001). A sudden onset of Chara in June 2000 (Fig. 3B) was consistent with observations in other shallow lakes (see Scheffer, 1998). It may re?ect that a dense sedimentary bank of charophyte oospores can survive multiple years of ?ooding (e.g. de Winton et al., 2000; Bonis and Grillas, 2002), and then respond quickly to increased irradiance (Takatori and Imahori, 1971). Controlled experiments using plants from Lake Okeechobee (Grimshaw et al., 2002; Grimshaw et al., unpublished) recently documented that the threshold irradiance

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for net growth of C. zeylanica is substantially lower than that of a vascular plant (V. americana) from the lake. The combination of a dense oospore bank and the ability to start growth at a lower irradiance may have given Chara a head start in colonizing the near-shore landscape when water level declined in spring 2000. As indicated, oospores of C. zeylanica can germinate in the dark (Proctor, 1960). In contrast, the sudden transition from Chara to vascular plants in November 2001 was not expected, and may have been driven by physical and meteorological conditions (see below). The recovery sequence of submerged plants in Lake Okeechobee is of general interest for two reasons: (1) it is the ?rst to be documented in a subtropical lake; and (2) it differs from what has been observed in temperate lakes (e.g. Blindow, 1992; van den Berg et al., 1999). In a long-term (30-year) study of Veluwemeer (The Netherlands), van den Berg et al. (1999) noted that at sites with dense Chara, the taxon “always return in the subsequent year, independent of water or Secchi depth.” They concluded that Chara is “self-stabilizing” because of its dense propagule bank, and because it is a superior competitor with vascular plants (Potamogeton) under clear water conditions. As concentrations of total phosphorus in Veluwemeer declined and Secchi transparency increased in the late 1970s, Chara recolonized the lake in a “step by step” manner, replacing Potamogeton. Similarly, Blindow (1992) documented a sequence in which Chara replaced Potamogeton in a Swedish lake. Scheffer (1998) and van Nes et al. (2002), as well as many others, have described the dominance of Chara in shallow lakes as a stable state. In contrast to these conclusions, our data suggest that in Lake Okeechobee, Chara is an ephemeral member of the community, dominating for approximately 1 year after high water stress was removed. Our ?eld observations indicate that its dense growth may have facilitated re-establishment of vascular plants, perhaps by improving underwater irradiance in areas where the vascular plants later appeared. Kimber et al. (1995) documented that while germination of Vallisneria seeds was not sensitive to light, the survival of seedlings was quite light-sensitive, with low survival 2–5% of surface irradiance, compared to high survival at surface irradiance of 9–25%. When Chara expanded in Lake Okeechobee in June to October 2000, we observed dramatic increases in Secchi transparency and reduced concentrations of solids that coincided in time and space with the moving front of Chara towards deeper water. It is uncertain whether this moving front was one of germinating oospores or establishing vegetative fragments. Within the dense Chara lawn, water clarity was particularly high, and scattered Potamogeton and Vallisneria plants emerged and grew towards the water surface (see Havens et al., 2001, Fig. 7b for a representative photograph). However, the vascular plants did not expand to replace the Chara until November 2001, despite favorable conditions for their growth in the interceding period. This raises two questions: (1) why did vascular plants not gradually replace Chara after they began to occur in the lake in summer 2000; and (2) what triggered the rapid switch from Chara to vascular plants in November 2001? In regard to the ?rst question, we suspect that it simply is a matter of competition for resources and space. With low water level, high transparency, and little wave energy in the shoreline areas, dense lawns of Chara were able to persist. Under these conditions, Chara is expected to out-compete vascular plants such as Potamogeton, as noted previously (Blindow, 1992, van den Berg et al., 1999). In regard to the second question, the rapid decline of Chara in November 2001 may have been caused by physical stress, in particular,

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the uprooting of plants by wind-driven waves. In subtropical south Florida, October to March is a period of increased wind velocity, whereas summer is a period of relative calm. Earlier studies have implicated wind as a factor that can disrupt Chara lawns in shallow lakes. Blindow et al. (2002), for example suggested that a period of “instability” in the typical dominance of Chara in Lake Krankesjon, Sweden, was associated with “windy weather.” In Lake Okeechobee, winter declines of Chara are well documented, and generally have been linked with low underwater irradiance (Steinman et al., 1997, 2002a), since autumn to winter also is the time when water levels are highest in the lake. However, it is possible that the scouring effects of wind-driven waves are an equally important factor. The Lake Okeechobee Hydrodynamic Model (Jin et al., 2000) indicates that there can be high current velocities in the bottom layer of the water column in the shoreline region, and substantial sediment shearing stress during times of high wind. In November 2001, when Chara declined, lake surface elevation had increased to a more typical range (near 4.5 m mean sea level). Two weeks prior to sample collection, a frontal system moved across the lake, with wind velocities in excess of 32 km h?1 for three successive days (compared to velocities of <10 km h?1 prior to this event). Coincident with the loss of Chara, the concentration of non-volatile solids increased by three-fold and the total biomass of submerged plants was very low. Observations in the ?eld during November 2001 indicated substantial amounts of Chara ?oating in the turbid water, further supporting the concept that it was uprooted by waves. The subsequent increase in vascular plants following the loss of Chara in Lake Okeechobee is consistent with both our ?eld observations and research on other shallow lakes. The scattered Potamogeton and Hydrilla plants that had occurred within the Chara lawn probably survived because of their more extensive root systems (Chara has no true roots). These plants extended upwards into the water column, where photosynthetic tissues had adequate irradiance for net growth. The plants then may have expanded laterally by vegetative growth to produce the dense beds observed in winter to spring 2002. It is of interest that there was not a phytoplankton bloom when the Chara was taken out in November 2001; this may simply have been due to the time of year (short photoperiod, lower water temperature) and the windy conditions, which do not favor development of surface blooms. Further insight into the potential importance of wind on Chara dynamics in shallow lakes is obtained by comparing the characteristics of Lake Veluwemeer, where Chara persists as the dominant submerged plant (van den Berg et al., 1999), with Lake Okeechobee, where Chara is ephemeral. H?kanson (1982) developed a simple indicator of wind impacts on the bottom of a lake—the dynamic ratio, calculated as the square root of lake surface area, divided by mean depth. Lakes with larger dynamic ratios are more susceptible to the effects of wind. The dynamic ratio of Lake Okeechobee (13.9) is more than three times that of Veluwemeer (4.0), and therefore, even if exposed to the same wind ?elds, Lake Okeechobee would not be as amenable to supporting poorly rooted plants such as Chara. In addition, Lake Okeechobee is impacted by strong frontal systems during the winter months, and tropical storms at an average rate of one every year (during the last century). Hence, the extent of wind impacts on submerged plants in Lake Okeechobee may be extreme relative to what occurs in smaller shallow lakes. However, because more than 35% of the large lakes (>500 km2 ) in the world have mean depths <5 m (Herdendorf, 1990), wind effects on shallow-rooted macro-algae such as Chara might be relatively common.

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In regard to the long-term prospects for the submerged plant community in Lake Okeechobee, one may expect the cycle of high and low biomass to repeat itself in the future, since the lake remains the main reservoir for collection of water from a large drainage basin. Large water storage facilities are envisioned in a 30-year plan of hydrologic, biological, and water quality restoration for south Florida (USACE, 1999), and these may allow for a less variable water level regime in Lake Okeechobee, and a less dynamic community. Given that wind action still will play an important role, even under more moderate lake levels (Jin et al., 2000), it seems likely that vascular plants, rather than Chara, will dominate that community. Acknowledgements The authors are grateful to D. Anson, H.J. Carrick, C. Hanlon, H.J. Grimshaw, and A.D. Steinman for providing assistance in the ?eld and laboratory. Discussions with H.J. Grimshaw were helpful in formulating parts of the discussion section of this paper. The authors are grateful to S. Gray, B. Gu, J. Vermaat, and two anonymous reviewers for providing constructive comments on earlier versions of the paper. References
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