Relationship between body condition of American alligators and water depth in the Everglades, Florida

Feeding opportunities of American alligators (Alligator mississippiensis) in freshwater wetlands in south Florida are closely linked to hydrologic conditions. In the Everglades, seasonally and annually fluctuating surface water levels affect populations of aquatic organisms that alligators consume. Since prey becomes more concentrated when water depth decreases, we hypothesized an inverse relationship between body condition and water depth in the Everglades. On average, condition of adult alligators in the dry season was significantly higher than in the wet season, but this was not the case for juveniles/subadults. The correlation between body condition and measured water depth at capture locations was weak; however, there was a significant negative correlation between the condition and predicted water depth prior to capture for all animals except for spring juveniles/subadults which had a weak positive condition–water depth relationship. Overall, a relatively strong inverse correlation occurred at 10–49 days prior to the capture day, suggesting that current body condition of alligators may depend on feeding opportunities during that period. Fitted regression of body condition on water depth (mean depth of 10 days when condition-water depth correlation was greatest) resulted in a significantly negative slope, except for spring adult females and spring juveniles/subadults for which slopes were not significantly different from zero. Our results imply that water management practices may be critical for alligators in the Everglades since water depth can affect animal condition in a relatively short period of time.     

Spatial, geomorphological, and seasonal variability of CDOM in estuaries of the Florida Coastal Everglades

This paper demonstrates the usefulness of fluorescence techniques for long-term monitoring and assessment of the dynamics (sources, transport and fate) of chromophoric dissolved organic matter (CDOM) in highly compartmentalized estuarine regions with non-point water sources. Water samples were collected monthly from a total of 73 sampling stations in the Florida Coastal Everglades (FCE) estuaries during 2001 and 2002. Spatial and seasonal variability of CDOM characteristics were investigated for geomorphologically distinct sub-regions within Florida Bay (FB), the Ten Thousand Islands (TTI), and Whitewater Bay (WWB). These variations were observed in both quantity and quality of CDOM. TOC concentrations in the FCE estuaries were generally higher during the wet season (June–October), reflecting high freshwater loadings from the Everglades in TTI, and a high primary productivity of marine biomass in FB. Fluorescence parameters suggested that the CDOM in FB is mainly of marine/microbial origin, while for TTI and WWB a terrestrial origin from Everglades marsh plants and mangroves was evident. Variations in CDOM quality seemed mainly controlled by tidal exchange/mixing of Everglades freshwater with Florida Shelf waters, tidally controlled releases of CDOM from fringe mangroves, primary productivity of marine vegetation in FB and diagenetic processes such as photodegradation (particularly for WWB). The source and dynamics of CDOM in these subtropical estuaries is complex and found to be influenced by many factors including hydrology, geomorphology, vegetation cover, landuse and biogeochemical processes. Simple, easy to measure, high sample throughput fluorescence parameters for surface waters can add valuable information on CDOM dynamics to long-term water quality studies which can not be obtained from quantitative determinations alone.     

Sawgrass (Cladium jamaicense) responses as early indicators of low-level phosphorus enrichment in the Florida Everglades

Anthropogenic phosphorus (P) inputs to the Florida Everglades have produced dramatic changes in the wetland vegetation of this otherwise oligotrophic system. While the proliferation of undesirable plant species in response to enrichment has been well documented, nutrient-related changes in the physiological and morphological attributes of existing vegetation, prior to any shifts in species composition or changes in the spatial extent of certain taxa, have yet to be adequately characterized. In this experiment, three sawgrass-dominated areas were enriched with P for 3 years at rates of 0.4 g P/m²/year (HP), 0.1 g P/m²/year (LP), or 0 g P/m²/year (controls) to assess potential impacts of P-enriched discharges from stormwater treatment areas into the Everglades. Elevated concentrations of TP in rhizomes and leaves and reduced ratios of leaf N:P were detected in HP plants within ~1 year at most sites. Live leaf densities, plant heights, and plant densities of the HP groups were generally higher than LP and control groups after 2 years, a pattern that was evident even after major fire events. Total aboveground biomass was significantly elevated in both HP and LP treatments at two of the three sites after 3 years. No change in species composition was detected during the study. Planned hydrologic restoration measures will increase P loads into parts of the Everglades that have not previously experienced anthropogenic P enrichment. Monitoring native vegetation such as sawgrass can be a sensitive and relatively robust means of detecting unintended P enrichment in these areas prior to shifts in vegetation community composition or changes in area cover of key species.     

Influence of water depth on nest success of the endangered Cape Sable seaside sparrow in the Florida Everglades

The restoration of the Florida Everglades rests largely on the ability of managers to re-create a more natural hydrologic regime throughout the remaining natural areas. The Cape Sable seaside sparrow, an endangered subspecies endemic to the freshwater marl prairies of the Everglades, has suffered from changes in the depth and the timing of water flows through its habitat. However, it remains unclear what temporal and spatial aspects of water inputs (both managed and natural) affect nesting success. We monitored 429 nests in two of the six extant sparrow subpopulations over 10 breeding seasons and a variety of water levels. Using an information-theoretic approach, we find that nests initiated early in the breeding season experience substantially higher success rates than those initiated later. We suggest that this seasonal effect is due to a change in predator abundance or activity levels as the season progresses, which are tied to the increase in water levels that accompany the onset of the wet season. In addition, nest success is influenced to a lesser degree by where sparrows choose to nest across the landscape, the height of base water levels within the sparrow's breeding season and the height of water levels when nests are active. Our observation of extreme variability in nest success over the span of a single season suggests that successful late-season breeding, although shown to be important for population recovery, is a rare event. Management actions that maximize the success of late-season broods or increase the number of early broods are warranted, but the ecosystem implications of such actions are poorly understood.     

Seasonal changes in the abundance and distribution of submerged aquatic vegetation in a highly managed coastal lagoon

The inflow of fresh water into coastal lagoons is a key factor influencing the structure and function of these ecosystems. Biscayne Bay, a coastal lagoon adjacent to the city of Miami, is located downstream of the Everglades ecosystem where the extensive water management system now in place has modified the historical hydrology, replacing groundwater and overland flows with pulsed releases from canals. In areas where canals discharge directly into littoral habitats, an environment with low-mean salinity and high-salinity variability is created. In this study, we characterize the salinity patterns of nearshore habitats (<500 m from shore) and document patterns of seasonal abundance and distribution of submerged aquatic vegetation (SAV) to evaluate the impacts of water management practices. Seagrasses were the principal component of the SAV community during the 2005 dry season (mean cover = 25.5%), while macroalgae dominated during the wet season (mean cover = 33.4%). The distribution and abundance of SAV were directly related to the tolerance of each taxon to salinity patterns. Seagrass species with high tolerance to low and variable salinity such as Halodule wrightii and Ruppia maritima were found only in canal-influenced areas and increased in abundance and spatial distribution in the wet season when freshwater inflow is highest. The dominance of rhizophytic macroalgae during the wet season was determined by the appearance and high abundance of Chara, a taxon commonly associated with freshwater environments. Thalassia testudinum, the most abundant seagrass species, was found throughout the study region, but decreased in abundance in the canal-influenced areas during the wet season when lower, more variable salinity resulted in lowered productivity. The data presented here showed a significant relationship between salinity patterns and the seasonal abundance and distribution of SAV. These findings support the use of SAV as appropriate indicators of changes in water quality resulting from future restoration projects associated with the Everglades Restoration Plan, which will once again modify the delivery of fresh water into littoral habitats with unknown ecological consequences. Handling editor: S. M. Thomaz     

Tree island pattern formation in the Florida Everglades

The Florida Everglades freshwater landscape exhibits a distribution of islands covered by woody vegetation and bordered by marshes and wet prairies. Known as “tree islands”, these ecogeomorphic features can be found in few other low gradient, nutrient limited freshwater wetlands. In the last few decades, however, a large percentage of tree islands have either shrank or disappeared in apparent response to altered water depths and other stressors associated with human impacts on the Everglades. Because the processes determining the formation and spatial organization of tree islands remain poorly understood, it is still unclear what controls the sensitivity of these landscapes to altered conditions. We hypothesize that positive feedbacks between woody plants and soil accretion are crucial to emergence and decline of tree islands. Likewise, positive feedbacks between phosphorus (P) accumulation and trees explain the P enrichment commonly observed in tree island soils. Here, we develop a spatially-explicit model of tree island formation and evolution, which accounts for these positive feedbacks (facilitation) as well as for long range competition and fire dynamics. It is found that tree island patterns form within a range of parameter values consistent with field data. Simulated impacts of reduced water levels, increased intensity of drought, and increased frequency of dry season/soil consuming fires on these feedback mechanisms result in the decline and disappearance of tree islands on the landscape.     

Effects of Differences in Prescribed Fire Regimes on Patchiness and Intensity of Fires in Subtropical Savannas of Everglades National Park, Florida

Abstract We investigated effects of fire frequency, seasonal timing, and plant community on patchiness and intensity of prescribed fires in subtropical savannas in the Long Pine Key region of Everglades National Park, Florida (U.S.A.). We measured patchiness and intensity in different plant communities along elevation gradients in “fire blocks.” These blocks were prescribed burned at varying times during the lightning season and at different frequencies between 1995 and 2000. Fire frequency, seasonal timing, and plant community all influenced the patchiness and intensity of prescribed fires. Fires were less patchy and more intense, probably because of drier conditions and pyrogenic fuels, in higher elevation plant communities (e.g., high pine savannas) than in lower elevation communities (e.g., long‐hydroperiod prairies). In all plant communities fires became increasingly patchy and less intense as the wet season progressed and moisture accumulated in fuels. Frequent prescribed fire resulted in increased patchiness but a wider range of intensities; higher intensities appeared to result from regrowth of more flammable vegetation. Our study suggests that frequent early lightning season prescribed fires produce a wider range of post‐fire conditions than less frequent late lightning season prescribed fires. Our study also suggests that natural early lightning season fires readily carried through pine savannas and short‐hydroperiod prairies, but lower elevation long‐hydroperiod prairies functioned as firebreaks. Natural fires probably crossed these firebreaks only during drier years, potentially producing large landscape‐level fires. Knowledge of how patchily and intensely fires burn across a savanna landscape should be useful for developing landscape‐level fire management.     

A method of determining rooting depth from a terrestrial biosphere model and its impacts on the global water and carbon cycle

We outline a method of inferring rooting depth from a Terrestrial Biosphere Model by maximizing the benefit of the vegetation within the model. This corresponds to the evolutionary principle that vegetation has adapted to make best use of its local environment. We demonstrate this method with a simple coupled biosphere/soil hydrology model and find that deep rooted vegetation is predicted in most parts of the tropics. Even with a simple model like the one we use, it is possible to reproduce biome averages of observations fairly well. By using the optimized rooting depths global Annual Net Primary Production (and transpiration) increases substantially compared to a standard rooting depth of one meter, especially in tropical regions that have a dry season. The decreased river discharge due to the enhanced evaporation complies better with observations. We also found that the optimization process is primarily driven by the water deficit/surplus during the dry/wet season for humid and arid regions, respectively. Climate variability further enhances rooting depth estimates. In a sensitivity analysis where we simulate changes in the water use efficiency of the vegetation we find that vegetation with an optimized rooting depth is less vulnerable to variations in the forcing. We see the main application of this method in the modelling communities of land surface schemes of General Circulation Models and of global Terrestrial Biosphere Models. We conclude that in these models, the increased soil water storage is likely to have a significant impact on the simulated climate and the carbon budget, respectively. Also, effects of land use change like tropical deforestation are likely to be larger than previously thought.     

Evaluation of and insights from ALFISH: a spatially explicit, landscape-level simulation of fish populations in the Everglades

We present an evaluation of a spatially explicit, age-structured model created to assess fish density dynamics in the Florida Everglades area. This model, ALFISH, has been used to compare alternative management scenarios for the Florida Everglades region. This area is characterized by periodic dry downs and refloodings. ALFISH uses spatially explicit water depth data to predict patterns of fish density. Here we present a method for calibration of ALFISH, based on information concerning fish movement, pond locations and other field data. With the current information, the greatest coefficient of determination achieved from regressions of ALFISH output to field data is 0.35 for fish density and 0.88 for water depth. The poor predictability of fish density mirrors the empirical findings that hydrology, which is the main driver of the model, only accounts for 20–40% of the variance of fish densities across the Everglades landscape. Sensitivity analyses indicate that fish in this system are very sensitive to frequency, size and location of permanent ponds as well as availability of prey.     

Vegetation differentiation in the patterned landscape of the central Everglades: importance of local and landscape drivers

Aim We present a model to account for self‐assembly of the slough–ridge–tree island patterned landscape of the central Everglades in southern Florida via feedbacks among landforms, hydrology, vegetation and biogeochemistry. We test aspects of this model by analysing vegetation composition in relation to local and landscape‐level drivers. Location We quantified vegetation composition and environmental characteristics in central Water Conservation Area (WCA) 3A, southern WCA‐3A and southern WCA‐3B in southern Florida, based on their divergence in water management and flow regimes over the past 50 years. Methods In 562 quadrats, we estimated species coverages and quantified maximum, minimum and average water depth, soil depth to bedrock, normalized difference vegetation index (NDVI) and proximity to the nearest tree island. We used non‐metric multi‐dimensional scaling (NMS) to relate compositional variation to local and landscape‐level factors, and evaluated environmental differences among eight a priori vegetation types via anova . Results Water depth and hydroperiod decreased from sloughs to ridges to tree islands, but regions also differed significantly in the abundance of several community types and the hydroregimes characterizing them. NMS revealed two significant axes of compositional variation, tied to local gradients of water depth and correlated factors, and to a landscape‐scale gradient of proximity to tall tree islands. Sawgrass height and soil thickness increased toward higher ridges, and NDVI was greatest on tree islands. Main conclusions This study supports four components of our model: positive feedback of local substrate height on itself, woody plant invasion and subsequent P transport and concentration by top predators nesting on taller tree islands, compositional shifts in sites close to tree islands due to nutrient leakage, and flow‐induced feedback against total raised area. Regional divergence in the relationship of community types to current hydroregimes appears to reflect a lag of a few years after shifts in water management; a longer lag would be expected for shifts in landscape patterning. Both local and landscape‐level drivers appear to shape vegetation composition and soil thickness in the central Everglades.     

End-of-season soil water depletion in relation to growth of herbaceous vegetation in a sub-humid Mediterranean dwarf-shrub community on two contrasting soils

Dwarf-shrub communities of Sarcopoterium spinosum dominate large areas of the landscape on hilly, eastern Mediterranean rangelands. Colonisation of new areas depends on the establishment of seedlings that must compete for water with the ubiquitous annual herbaceous species during the spring-winter growing season and also survive the first hot, dry summer. The present study investigated the role of the herbaceous vegetation patches growing between S. spinosum shrubs on the depletion of soil water during the critical transition period between the cool, rainy season and the dry summer. Dense and sparse herbaceous vegetation stands were established in S. spinosum dwarf-shrub communities by differential use of fertiliser on two contrasting soil types – a terra rossa overlying hard limestone where seedling establishment is low and a pale rendzina overlying a soft chalk substrate where seedling establishment is high. Soil water in the main root zone of the herbaceous vegetation between the shrubs was monitored with protected gypsum block sensors permanently placed at two depths (10 and 33 cm). Soil water depletion during the transition from the wet to the dry season was significantly more rapid under dense vegetation only on the terra rossa soil where the herbaceous vegetation also matured more rapidly than on the rendzina soil. However, in both habitats and under both dense and sparse vegetation, soil water depletion during the transition period left very little available water in the rooting zone of the herbaceous vegetation to maintain shrub seedlings throughout the summer. It was concluded that the difference in shrub seedling establishment success in the two habitats mainly reflects the differences in accessibility of water below the rooting zone of the herbaceous vegetation growing on the two contrasting soil types.     

Effects of hydrologic and water quality drivers on periphyton dynamics in the southern Everglades

Everglades periphyton mats are tightly-coupled autotrophic (algae and cyanobacteria) and heterotrophic (eubacteria, fungi and microinvertebrates) microbial assemblages. We investigated the effect of water column total phosphorus and nitrogen concentrations, water depth and hydroperiod on periphyton of net production, respiration, nutrient content, and biomass. Our study sites were located along four transects that extended southward with freshwater sheetflow through sawgrass-dominated marsh. The water source for two of the transects were canal-driven and anchored at canal inputs. The two other transects were rain-driven (ombrotrophic) and began in sawgrass-dominated marsh. Periphyton dynamics were examined for upstream and downstream effects within and across the four transects. Although all study sites were characterized as short hydroperiod and phosphorus-limited oligotrophic, they represent gradients of hydrologic regime, water source and water quality of the southern Everglades. Average periphyton net production of 1.08 mg C AFDW⁻¹ h⁻¹ and periphyton whole system respiration of 0.38 mg C AFDW⁻¹ h⁻¹ rates were net autotrophic. Biomass was generally highest at ombrotrophic sites and sites downstream of canal inputs. Mean biomass over all our study sites was high, 1517.30 g AFDW m⁻². Periphyton was phosphorus-limited. Average periphyton total phosphorus content was 137.15 μg P g⁻¹ and average periphyton total N:P ratio was 192:1. Periphyton N:P was a sensitive indicator of water source. Even at extremely low mean water total phosphorus concentrations ( ≤ 0.21 μmol l⁻¹), we found canal source effects on periphyton dynamics at sites adjacent to canal inputs, but not downstream of inflows. These canal source effects were most pronounced at the onset of wet season with initial rewetting. Spatial and temporal variability in periphyton dynamics could not solely be ascribed to water quality, but was often associated with both hydrology and water source.     

Testing the importance of nest concealment: does timing matter?

ABSTRACT.   Nest concealment by vegetation is considered an important factor affecting predation rates for many passerines and, therefore, is frequently measured in studies examining nest predation. However, the time when concealment measurements are made may affect the results of such studies, particularly in highly seasonal ecosystems where characteristics of the vegetation later in the breeding period may differ considerably from those at the time of nest-site selection. We used artificial nests baited with quail ( Coturnix sp.) eggs in a highly seasonal tropical dry forest in Jalisco, western Mexico, to test the effects of seasonal change in concealment on nest predation. We placed 40 open-cup, artificial nests in shrubs at the end of the dry season and again at the beginning of the rainy season in 2007, and monitored the fate of the nests and the degree of concealment by vegetation during both periods. Nest concealment was significantly greater during the wet season than during the dry season. The percentage of nests predated was marginally higher during the dry (100%) than the wet (72.5%) season, and daily nest survival was lower during the dry than the wet season. Our results suggest that, in highly seasonal environments such as tropical dry forests, delayed measurement of nest concealment after nest completion rather than during nesting may constitute a significant source of error.     

Water relations and gas exchange in Coespeletia moritziana (Sch. Bip) Cuatrec., a giant rosette species of the high tropical Andes

Giant rosettes are ones of the most striking features of the vegetation in the high tropical Andes, with Coespeletia moritziana reaching the highest altitudes up to 4,600 m a.s.l. Different from other giant rosettes, this species grows on rock outcrops with poorly developed soils and where water availability may be limited. Two questions are addressed in this study: How does this species respond in terms of water relations to maintain favorable gas-exchange conditions? Considering that adult plants rely on a water-reserving central pith, how do early stages respond to this environment??s extreme conditions? Water relations and gas-exchange studies were carried out on juveniles, intermediate and adult C. moritziana plants during wet and dry seasons in Páramo de Piedras Blancas at 4,200 m a.s.l. Adult plants maintained higher leaf water potentials (??_L) during the wet season, however, no differences between stages were found for the dry season. Minimum dry season ??_L were never near the turgor loss point in any of the stages. Juveniles show a more strict stomatal control during the dry season to maintain a favorable water status. Net photosynthesis significantly decreased in intermediate and juvenile stages from wet to dry seasons. Our results suggest that C. moritziana resists more extreme conditions compared to other Andean giant rosettes     

不同时空尺度下土地利用对洱海入湖河流水质的影响

土地利用与入湖河流水质的关系存在时空差异。以洱海西部入湖河流及其小流域为研究对象,综合空间分析和数理统计手段,探讨两者随空间尺度和时间变化的关系,结果表明:选取的小流域、河岸带30m缓冲区、河岸带60m缓冲区和河岸带90m缓冲区4种尺度下,对入湖河流水质影响显著的土地利用类型为建设用地和植被(包括林地和牧草地),影响最大的空间尺度为小流域尺度,河岸带30m缓冲区次之;小流域尺度下,建设用地面积百分比与入湖河流COD和TP浓度呈正相关,植被面积百分比与NH_4~+-N浓度呈负相关,响应土地利用的主要水质指标为TN和TP,回归调整系数分别为0.624和0.579;季节性关联分析表明建设用地与COD、NH_4~+-N、TP的回归关系在雨季强于旱季,植被与COD、TP的回归关系在雨季强于旱季,雨季建设用地和植被面积变化引起COD浓度变化更快。在流域管理中,针对植被覆盖率低、建设用地占比高的白鹤溪和中和溪应重点加强雨季土地利用管控,增加植被覆盖率,合理开发建设用地。     

Seasonal habitat use by Elephants (Loxodonta africana) in the Mole National Park of Ghana

To avoid unnecessary waste of limited resources and to help prioritize areas for conservation efforts, this study aimed to provide information on habitat use by elephants between the wet and dry seasons in the Mole National Park (MNP) of Ghana. We compiled coordinates of 516 locations of elephants’ encounters, 256 for dry season and 260 for wet season. Using nine predictor variables, we modeled the probability of elephant's distribution in MNP. We threshold the models to “suitable” and “nonsuitable” regions of habitat use using the equal training sensitivity and specificity values of 0.177 and 0.181 for the dry and wet seasons, respectively. Accuracy assessment of our models revealed a sensitivity score of 0.909 and 0.974, and a specificity of 0.579 and 0.753 for the dry and wet seasons, respectively. A TSS of 0.488 was also recorded for the dry season and 0.727 for the wet season indicating a good model agreement. Our model predicts habitat use to be confined to the southern portion of MNP due to elevation difference and a relatively steep slope that separates the northern regions of the park from the south. Regions of habitat use for the wet season were 856 km² and reduced significantly to 547.68 km² in the dry season. We observed significant overlap (327.24 km²) in habitat use regions between the wet and dry seasons (Schoener's D = 0.922 and Hellinger's‐based I = 0.991). DEM, proximity to waterholes, and saltlicks were identified as the key variables that contributed to the prediction. We recommend construction of temporal camps in regions of habitat use that are far from the headquarters area for effective management of elephants. Also, an increase in water point's density around the headquarters areas and selected dry areas of the park will further decrease elephant's range and hence a relatively less resource use in monitoring and patrols.     

Ecology and distribution of diatoms in Biscayne Bay,Florida (USA): Implications for bioassessment and paleoenvironmental studies

The spatial and temporal distribution of planktonic, sediment-associated and epiphytic diatoms among 58 sites in Biscayne Bay, Florida was examined in order to identify diatom taxa indicative of different salinity and water quality conditions, geographic locations and habitat types. Assessments were made in contrasting wet and dry seasons in order to develop robust assessment models for salinity and water quality for this region. We found that diatom assemblages differed between nearshore and offshore locations, especially during the wet season when salinity and nutrient gradients were steepest. In the dry season, habitat structure was primary determinant of diatom assemblage composition. Among a suite of physicochemical variables, water depth and sediment total phosphorus (STP) were most strongly associated with diatom assemblage composition in the dry season, while salinity and water total phosphorus (TP) were more important in the wet season. We used indicator species analysis (ISA) to identify taxa that were most abundant and frequent at nearshore and offshore locations, in planktonic, epiphytic and benthic habitats and in contrasting salinity and water quality regimes. Because surface water concentrations of salts, total phosphorus, nitrogen (TN) and organic carbon (TOC) are partly controlled by water management in this region, diatom-based models were produced to infer these variables in modern and retrospective assessments of management-driven changes. Weighted averaging (WA) and weighted averaging partial least squares (WA-PLS) regressions produced reliable estimates of salinity, TP, TN and TOC from diatoms (r² = 0.92, 0.77, 0.77 and 0.71, respectively). Because of their sensitivity to salinity, nutrient and TOC concentrations diatom assemblages should be useful in developing protective nutrient criteria for estuaries and coastal waters of Florida.     

Seasonal variation in seed bank composition and its interaction with nutrient enrichment in the Everglades wetlands

In the Florida Everglades, nutrient enrichment from agricultural outflow and the change in hydrology have collectively contributed to the expansion of cattails (Typha spp.). To assess the effectiveness of prescribed fire in controlling cattails and to predict vegetation dynamics after the fire, it is important to understand the seasonal variation of the soil seed bank and how the seed bank is affected by nutrient enrichment and fire. This paper investigates the effects of season, nutrient enrichment, and fire on soil seed bank species composition, richness, and density along a nutrient gradient in Water Conservation Area 2A (WCA 2A) of the Florida Everglades. Species richness was significantly affected by nutrient enrichment and season but not their interaction. Total seed density, however, was significantly affected by the interaction between nutrient enrichment and season. Yet, at species level, the relationship between seed density, nutrient enrichment and season varied. The highest seed density of cattail occurred in summer at highly enriched sites, but that of sawgrass occurred in fall regardless of enrichment; the seed density of water lily was very low regardless of season and nutrient enrichment, and the highest Amarathus seed density occurred at highly enriched sites year round. Moreover, germination timing differed greatly among species. While cattail seeds had a short incubation period and started to germinate 2–3 days after initiation of the germination assay, sawgrass seeds generally started to germinate 4 weeks later. Further, both the prescribed summer fire at the highly enriched site and the natural winter fire at the moderately enriched site reduced the seed density of cattail but not of sawgrass. Our results suggest that fire application for vegetation recovery in WCA 2A would benefit from explicitly considering seasonal dynamics of the seed bank.     

Temporal population genetic structure of eastern mosquitofish in a dynamic aquatic landscape

We analyzed the effect of periodic drying in the Florida Everglades on spatiotemporal population genetic structure of eastern mosquitofish (Gambusia holbrooki). Severe periodic drying events force individuals from disparate sources to mix in dry season relatively deep-water refuges. In 1996 (a wet year) and 1999 (a dry year), we sampled mosquitofish at 20 dry-season refuges distributed in 3 water management regions and characterized genetic variation for 10 allozyme and 3 microsatellite loci. In 1996, most of the ecosystem did not dry, whereas in 1999, many of our sampling locations were isolated by expanses of dried marsh surface. In 1996, most spatial genetic variation was attributed to heterogeneity within regions. In 1999, spatial genetic variation within regions was not significant. In both years, a small but significant amount of variation (less than 1% of the total variation) was partitioned among regions. Variance was consistently greater than zero among long-hydroperiod sites within a region, but not among short-hydroperiod sites within a region, where hydroperiod was measured as time since last marsh surface dry-down forcing fishes into local refuges. In 1996, all sites were in Hardy-Weinberg equilibrium. In 1999, we observed fewer heterozygotes than expected for most loci and sites suggesting a Wahlund effect arising from fish leaving areas that dried and mixing in deep-water refuges.     

Deep-rooted vegetation, Amazonian deforestation, and climate: results from a modelling study

The depth of the root system controls the maximum amount of soil water that can be transpired by the vegetation into the atmosphere during dry periods. Water uptake from deep soil layers has been found to contribute significantly to the dry season transpiration at some sites in Amazonia and it has been estimated that large parts of the evergreen forests in Amazonia depend on deep roots to survive the dry season. Thus, the presence of deep roots might provide a significant source of atmospheric moisture during the dry season, and one which would be affected by deforestation. We investigate the role of deep-rooted vegetation and its removal in the context of Amazonian deforestation using an atmospheric General Circulation Model (GCM). A distribution of deep roots is obtained by a numerical optimization approach. The simulated climate with the use of the calculated deep roots substantially improves the seasonal characteristics of the GCM. Three additional simulations are then conducted in order to isolate the effect of rooting depth reduction from other parameter changes associated with large-scale deforestation. Most of the climatic effects occur during the dry season and are attributed to the reduction of rooting depth. Dry periods are found to last longer, being more intense with drier and warmer air, while the wet season remains fairly unchanged. The implications of these climatic effects for the re-establishment of the natural evergreen forest are discussed.