Contrasting patterns of genetic diversity between the northern and southern populations of yellow bullhead catfish, Ameiurus natalis in North America

Environmental conditions have profound influence on the life history characteristics of a species, for instance, the yellow bullhead (Ameiurus natalis) population in its southernmost native range, South Florida was previously reported to be distinguished from other A. natalis populations in the United States by growth rates and feeding habits. Utilizing mitochondrial DNA control region sequence data, the present study sought to evaluate the patterns of intraspecific diversification of this North American freshwater catfish, A. natalis. The analyses have revealed the existence of four distinct matrilineal lineages (Mississippi, Gulf coast, Southeast Atlantic, and South Florida) with strong geographic specificity. Although all the lineages showed rapid demographic expansion, the South Florida lineage is shown to have greater genetic diversity than the remaining lineages. The persistence of more favorable environmental conditions and suitable habitats during the late-Pleistocene period in southern Florida could be the possible explanation for such genetic disparities among the lineages. However, taken together with the conclusion of previous studies, the present study predicted that the recurrent density-dependent process resulting from the altercation of hydrology in South Florida may cause the reduction in genetic diversity and put this species at risk in this region.     

Substrate heterogeneity and number of plant species in Everglades savannas (Florida,USA)

Environmental heterogeneity, especially that related to topography, has been proposed to influence numbers of plant species in different sized areas. Despite little variation in elevation, large numbers of vascular plant species occur in some habitats. This study explored possible relationships between number of plant species and substrateheterogeneity in two species-rich habitats, subtropical pine savannas and short-hydroperiod prairies, in the Long Pine Key region of Everglades National Park (Florida, U.S.A.). We examined relationships between numbers of vascular plant species and topographic heterogeneity by measuring numbers of species and elevations in differ-ent sizes of nested plots that spanned five orders of magnitude (0.1 m² to 1000 m² ) and that were located along two transects extending from pine savannas into short-hydroperiod prairies in different areas of Long Pine Key. We also classified substrates and soil depths in 1 m 2 sized submodules within the nested plots. Pine savannas occurred at higher elevations than adjacent short-hydroperiod prairies. Although differences occurred in substrate types and distribution within 1 m 2 plots, numbers of species were not associated with these differences. Vari-ances in elevations were similar in the smallest plots, but increased with area more rapidly in pine savannas than in short-hydroperiod prairies. Plot size explained about 85% of the variation in species numbers, which increased from 2040 per 1 m 2 to 80120 per 1000 m² . An interaction between habitat and scale explained 5% of the variation; more species occurred in shorthydroperiod prairies than pine savannas at scales <10 m² , but the re-verse occurred at scales >10 m². The number of species in pine savannas at scales of 1 m²and 10 m²was positively associated with variation in elevations; no significant relationships were obtained in short-hydroperiod prairies, which lack the fine-scale topographic variation of pine savannas. Our data indicate that substrate het-erogeneity, measured as variation in elevations, is not likely to be involved in the co-occurrence of many species within small areas of these savannas, but may influence numbers of species at larger scales of observation, es-pecially in pine savannas. Why many plant species occur within very small areas in these savannas remains un-answered.     

Experimentally derived salinity tolerance of hatchling Burmese pythons (Python molurus bivittatus) from the Everglades,Florida (USA)

In a laboratory setting, we tested the ability of 24 non-native, wild-caught hatchling Burmese pythons (Python molurus bivittatus) collected in the Florida Everglades to survive when given water containing salt to drink. After a one-month acclimation period in the laboratory, we grouped snakes into three treatments, giving them access to water that was fresh (salinity of 0, control), brackish (salinity of 10), or full-strength sea water (salinity of 35). Hatchlings survived about one month at the highest marine salinity and about five months at the brackish-water salinity; no control animals perished during the experiment. These results are indicative of a “worst-case scenario”, as in the laboratory we denied access to alternate fresh-water sources that may be accessible in the wild (e.g., through rainfall). Therefore, our results may underestimate the potential of hatchling pythons to persist in saline habitats in the wild. Because of the effect of different salinity regimes on survival, predictions of ultimate geographic expansion by non-native Burmese pythons that consider salt water as barriers to dispersal for pythons may warrant re-evaluation, especially under global climate change and associated sea-level-rise scenarios.     

Limnological characteristics of a subtropical constructed wetland in south Florida (USA)

The Everglades Nutrient Removal Project (ENRP) was a 1544 ha constructed wetland built by the South Florida Water Management District as part of Everglades restoration efforts. The limnology of this wetland is characterized over its 60-month operational history. The ENRP received agricultural runoff containing high levels of C, N, P and other dissolved constituents; had moderately high alkalinity with a circumneutral pH; and had low to moderate DO. The ENRP provided substantial treatment (concentration reduction from inflow to outflow) for Al, Fe, NH₄, NO_x, SRP, TP, TSS and turbidity (high-treatment variables), while Secchi depth increased markedly. These changes were judged biologically significant. Dissolved oxygen, and water temperature had well defined annual cycles, while some level of seasonality was noted for Al, alkalinity, Ca, conductivity, DOC, Fe, hardness, K, Mg, Mn, Na, NH₄, pH, Secchi depth, SiO₂, TOC, TN, turbidity, and TSS. The ENRP was P limited based on TN:TP molar ratios. Dissolved ions were dominated by Ca, Cl, Na, and HCO₃; the stoichiometric balance of both major and minor ions was similar throughout the wetland. The downstream settling of TSS was associated with increased light penetration, but did not appear important in sediment accretion. The adsorption of P to Ca, and perhaps Al and Fe, precipitates is thought to have been an important nutrient removal mechanism. Although there was little net reduction in DOC, we speculate that some incoming material was degraded and replaced by new DOC produced within the wetland.     

Impact of black bullhead (Ameiurus melas) on turbidity in a diked wetland

Benthivorous fish in shallow, aquatic systems have been correlated with increased turbidity and declines in macrophyte production and wildlife use. Bullheads have been credited with increasing turbidity, but this has been seldom tested and has not been studied in a diked marsh. To assess the relationships of black bullhead (Ameiurus melas) and turbidity, we assembled mesocosmsin the Show Pool Management Unit of The Ottawa National Wildlife Refuge, OH, U.S.A. We stocked treatment enclosures with different biomasses of black bullhead at weekly intervals. Mean turbidity within treatment enclosures was significantly higher than within controls but remained lower than that of the open marsh. Both surface and bottom turbidity increased with adult and juvenile black bullhead biomass. Turbidity increased with fine sand concentration only in the presence of juvenile fish. Wind speed and direction were significant influences on the turbidity of the open marsh, but not within control enclosures. That treatment turbidity – even at extreme biomasses – remained significantly lower than the turbidity of the open marsh implicates fetch in having a greater influence on a marsh's turbidity than the presence of black bullhead. The greater impact of benthivorous fish on turbidity within shallow systems may be an indirect one through the destruction of macrophytes and subsequent destabilization of unconsolidated substrates.     

A hydrologic assessment of the Everglades Nutrient Removal Project, a subtropical constructed wetland in South Florida (USA)

The Everglades Nutrient Removal Project (ENRP), a 1544-ha constructed wetland in south Florida, was intensively monitored throughout its five-year operational history. Water budgets for the ENRP and each of its interior treatment cells were dominated by surface flows (≥85% of inflows; ≥68% of outflows) with smaller contributions from precipitation, evapotranspiration, groundwater flux, and change in storage. The mean water depth, hydraulic loading rate for surface water, and nominal hydraulic retention time for the entire wetland were 0.6 m, 3.1 cm d⁻¹ and 17.7 d, respectively, and were comparable to values anticipated in design. The east flow-way was slightly shallower (0.2 m) and received proportionately more flow (61%) than the west flow-way. The hydrology of other treatment wetlands is often driven by surface flows. All treatment cells in the ENRP were to some extent hydraulically short-circuited. There was net groundwater inflow to the ENRP from Water Conservation Area 1 (WCA-1) resulting from significant head differences between these wetlands. Groundwater outflow to the adjacent farmlands was greatest in Cell 2 and substantially exceeded groundwater inflow. All hydrologic parameters exhibited seasonality to some extent; fluctuation in water depth and groundwater inflows corresponded with the seasonal change in stage in WCA-1. Errors in the ENRP and individual cell water budgets were generally less than 10% and within the range of errors for water budgets from other wetlands.     

Pattern of nutrient availability and plant community assemblage in Everglades Tree Islands, Florida, USA

We address the relative importance of nutrient availability in relation to other physical and biological factors in determining plant community assemblages around Everglades Tree Islands (Everglades National Park, Florida, USA). We carried out a one-time survey of elevation, soil, water level and vegetation structure and composition at 138 plots located along transects in three tree islands in the Park’s major drainage basin. We used an RDA variance partitioning technique to assess the relative importance of nutrient availability (soil N and P) and other factors in explaining herb and tree assemblages of tree island tail and surrounded marshes. The upland areas of the tree islands accumulate P and show low N concentration, producing a strong island-wide gradient in soil N:P ratio. While soil N:P ratio plays a significant role in determining herb layer and tree layer community assemblage in tree island tails, nevertheless part of its variance is shared with hydrology. The total species variance explained by the predictors is very low. We define a strong gradient in nutrient availability (soil N:P ratio) closely related to hydrology. Hydrology and nutrient availability are both factors influencing community assemblages around tree islands, nevertheless both seem to be acting together and in a complex mechanism. Future research should be focused on segregating these two factors in order to determine whether nutrient leaching from tree islands is a factor determining community assemblages and local landscape pattern in the Everglades, and how this process might be affected by water management.     

Decomposition of macrophyte litter in a subtropical constructed wetland in south Florida (USA)

The South Florida Water Management District has constructed large treatment wetlands (stormwater treatment areas (STAs)) to reduce total phosphorus concentrations in agricultural runoff before this water enters the Everglades. An important component of nutrient removal and storage in these systems is incorporation of nutrients into aquatic macrophytes and burial of this biomass in the sediments. However, decomposition of plant biomass before burial returns nutrients to the water column and may reduce STA treatment efficiency. As part of research on biogeochemical control of STA performance, we conducted a summer (July–September) and a long-term (12-month) experiment (February–February) that measured decomposition rates and release of chemical constituents from dominant aquatic macrophytes in a constructed wetland located in south Florida. The rank order of mean decomposition rates was Najas/Ceratophyllum (0.0568 d⁻¹) > Pistia (0.0508 d⁻¹) > Eichhornia (0.0191 d⁻¹) > submerged Typha (0.0059 d⁻¹) > aerial Typha (0.0008 d⁻¹). Summer decomposition rates were generally higher than rates from the long-term experiment, which suggested a temperature effect. Decomposition rates were negatively correlated with litter C:N and C:P molar ratios and cellulose and lignin content and positively correlated with N and P content. There was no significant difference in decomposition rates among sampling stations despite the fact that there was a decreasing gradient in water column inorganic phosphorus and nitrogen concentrations at these sites. Relatively little of the initial P mass remained in the litter of all species, except Typha, by the end of both experiments. First-order decomposition models derived using nonlinear regression generally had explanatory power, i.e. accounted for variance, comparable to more complex decreasing-coefficient models. Decomposition rates for the species examined in this study were within the range of published values when comparisons were made either by species or by plant group.     

Patterns of vertical stratification in a subtropical constructed wetland in south Florida (USA)

The South Florida Water Management District (District) has built large treatment wetlands, known as Stormwater Treatment Areas (STAs), to reduce excess phosphorus loading to the Everglades. The District conducted research in a prototype treatment wetland, the Everglades Nutrient Removal Project (ENRP), to study biogeochemical processes that are important to treatment performance. Vertical profile measurements of water temperature, dissolved oxygen, pH, conductivity and transmission of photosynthetically active radiation (PAR) were made in open-water areas and sites dominated by emergent, floating or submersed vegetation over an annual cycle. Relative thermal resistance to mixing was used to infer the strength of thermal stratification. Long-term diel variation in temperature at the surface and bottom of an open-water and a vegetated site also was measured. Open-water sites were nearly isothermal and had minimal thermal stratification, while vegetated sites were all thermally stratified to some degree. The highest surface water temperatures (>35 °C) occurred in submersed vegetation where much of the light absorbed by leaves and stems was reflected as heat. Oxygen was uniformly low (<4 mg L⁻¹) in emergent and floating vegetation and attributed to shading, high biological oxygen demand and limited reaeration at the surface. Depressed oxygen levels at open-water areas were attributed to high sediment oxygen demand. The highest oxygen concentrations occurred in submersed vegetation beds. Water column pH was unstratified and near circumneutral in the open water and at emergent and floating vegetation sites, while pH was markedly stratified in submersed vegetation, where surface values at times exceeded 9.0. High surface oxygen and pH levels in submersed vegetation were consistent with intense photosynthesis. Conductivity increased at the bottom of emergent and submersed vegetation but not at open-water or floating vegetation sites. PAR transmission was strongly reduced at all sites due to shading and/or absorption by dissolved organic carbon. Light extinction coefficients were markedly higher at vegetated sites compared to the open water. Peak irradiance shifted to longer wavelengths (538–643 nm) and both short (<400 nm) and long (>700 nm) wavelengths were largely attenuated at 60 cm relative to the surface. Long-term monitoring at a vegetated site revealed periods of inverse thermal stratification and dampened diel variation in temperatures at the bottom compared to open water.     

Vertical distribution and movements of brown bullhead (Ameiurus nebulosusLesueur 1819) in Motuoapa Bay,southern Lake Taupo,New Zealand

Ultrasonic telemetry was used to determine the swimming depths of brown bullhead (Ameiurus nebulosus) in Motuoapa Bay at the southern end of Lake Taupo, New Zealand. Sonic transmitters equipped with a pressure sensor were surgically implanted in 12 adult fish (295–345 mm FL). Three automatic receivers recorded a total of 61335 data points on swimming depth between 26 November 1998 and 7 November 1999. Brown bullheads swam in water from 0 to 17 m deep but showed some seasonal variations. They were more active at night when they used shallow water extensively, especially during the winter months. They were most mobile in spring when they left Motuoapa Bay, suggesting migratory behaviour. During winter the fish remained more stationary, only moving around inside the bay. The results of this study provide guidance for establishing a brown bullhead control program should it become necessary.     

Patterns of Root Dynamics in Mangrove Forests Along Environmental Gradients in the Florida Coastal Everglades,USA

Patterns of mangrove vegetation in two distinct basins of Florida Coastal Everglades (FCE), Shark River estuary and Taylor River Slough, represent unique opportunities to test hypotheses that root dynamics respond to gradients of resources, regulators, and hydroperiod. We propose that soil total phosphorus (P) gradients in these two coastal basins of FCE cause specific patterns in belowground biomass allocation and net primary productivity that facilitate nutrient acquisition, but also minimize stress from regulators and hydroperiod in flooded soil conditions. Shark River basin has higher P and tidal hydrology with riverine mangroves, in contrast to scrub mangroves of Taylor basin with more permanent flooding and lower P across the coastal landscape. Belowground biomass (0–90 cm) of mangrove sites in Shark River and Taylor River basins ranged from 2317 to 4673 g m⁻², with the highest contribution (62–85%) of roots in the shallow root zone (0–45 cm) compared to the deeper root zone (45–90 cm). Total root productivity did not vary significantly among sites and ranged from 407 to 643 g m⁻² y⁻¹. Root production in the shallow root zone accounted for 57–78% of total production. Root turnover rates ranged from 0.04 to 0.60 y⁻¹ and consistently decreased as the root size class distribution increased from fine to coarse roots, indicating differences in root longevity. Fine root biomass was negatively correlated with soil P density and frequency of inundation, whereas fine root turnover decreased with increasing soil N:P ratios. Lower P availability in Taylor River basin relative to Shark River basin, along with higher regulator and hydroperiod stress, confirms our hypothesis that interactions of stress from resource limitation and long duration of hydroperiod account for higher fine root biomass along with lower fine root production and turnover. Because fine root production and organic matter accumulation are the primary processes controlling soil formation and accretion in scrub mangrove forests, root dynamics in the P-limited carbonate ecosystem of south Florida have a major controlling role as to how mangroves respond to future impacts of sea-level rise.     

High plasma buffering and the absence of a red blood cell beta-NHE response in brown bullhead (Ameiurus nebulosus)

The high non-bicarbonate buffer capacity of brown bullhead (Ameiurus nebulosus) plasma was postulated to function as an alternative mechanism for the protection of red blood cell (RBC) intracellular pH (pHi) in the absence or attenuation of a RBC adrenergic response. The requirement for protecting RBC pHi arises from the presence of a Root effect haemoglobin in bullhead. In support of this hypothesis, bullhead RBCs incubated in vitro with isoproterenol (10(-8)-10(-5) mol l(-1)) or forskolin (10(-4) mol l(-1)) exhibited significant cyclic AMP accumulation, but failed to exhibit cell swelling or significant Na(+) or Cl(-) accumulation; plasma pH (pHe) was also unaffected. Similarly, no significant effect on RBC water content, Na(+) or Cl(-) concentration, or pHe was detected in bullhead blood incubated with 8-bromo cyclic AMP (10(-4)-10(-2) mol l(-1)) in vitro. These results suggest that while bullhead RBCs possess a beta-adrenoreceptor linked to cyclic AMP formation, stimulation of this adrenergic receptor does not result in measurable activation of a Na(+)/H(+) exchanger.     

Rapid responses of vegetation to hydrological changes in Taylor Slough, Everglades National Park, Florida, USA

We analyzed the dynamics of freshwater marsh vegetation of Taylor Slough in eastern Everglades National Park for the 1979 to 2003 period, focusing on cover of individual plant species and on cover and composition of marsh communities in areas potentially influenced by a canal pump station (“S332”) and its successor station (“S332D”). Vegetation change analysis incorporated the hydrologic record at these sites for three intervals: pre-S332 (1961–1980), S332 (1980–1999), post-S332 (1999–2002). During S332 and post-S332 intervals, water level in Taylor Slough was affected by operations of S332 and S332D. To relate vegetation change to plot-level hydrological conditions in Taylor Slough, we developed a weighted averaging regression and calibration model (WA) using data from the marl prairies of Everglades National Park and Big Cypress National Preserve. We examined vegetation pattern along five transects. Transects 1–3 were established in 1979 south of the water delivery structures, and were influenced by their operations. Transects 4 and 5 were established in 1997, the latter west of these structures and possibly under their influence. Transect 4 was established in the northern drainage basin of Taylor Slough, beyond the likely zones of influence of S332 and S332D. The composition of all three southern transects changed similarly after 1979. Where muhly grass (Muhlenbergia capillaris var. filipes) was once dominant, sawgrass (Cladium jamaicense), replaced it, while where sawgrass initially predominated, hydric species such as spikerush (Eleocharis cellulosa Torr.) overtook it. Most of the changes in species dominance in Transects 1–3 occurred after 1992, were mostly in place by 1995–1996, and continued through 1999, indicating how rapidly vegetation in seasonal Everglades marshes can respond to hydrological modifications. During the post-S332 period, these long-term trends began reversing. In the two northern transects, total cover and dominance of both muhly grass and sawgrass increased from 1997 to 2003. Thus, during the 1990’s, vegetation composition south of S332 became more like that of long hydroperiod marshes, but afterward it partially returned to its 1979 condition, i.e., a community characteristic of less prolonged flooding. In contrast, the vegetation change along the two northern transects since 1997 showed little relationship to hydrologic status.     

Phosphorus removal by the Ceratophyllum/periphyton complex in a south Florida (USA) freshwater marsh

Removal of phosphorus (P) by Ceratophyllum demersum L. and associated epiphytic periphyton was quantified by measuring the disappearance of soluble reactive P (SRP) from microcosms during 1-h in situ incubations conducted over a 1-year period. Initial P concentrations in these incubations ranged from 30 to >10,000 μg P L⁻¹. Phosphorus removal was proportional to initial P concentrations and was weakly correlated with solar irradiance and water temperature. Removal rates (0.6–32.8 mg P m⁻² d⁻¹) and k_v coefficients (0.68–1.93 h⁻¹) from experiments run at low initial P concentrations (up to 200 μg P L⁻¹) were comparable to results reported for other macrophytes. Removal rates from experiments run at the highest (>10,000 μg P L⁻¹) initial P concentrations (5300 and 11,100 mg P m⁻² d⁻¹) most likely represented luxury nutrient consumption and were not thought to be sustainable long term. We were unable to determine a V_max for P removal, suggesting that the nutrient-storage capability of the C. demersum/periphyton complex was not saturated during our short-term incubations. Based on N:P molar ratios, the marsh was P limited, while the C. demersum/periphyton complex was either N limited or in balance for N and P throughout this study. However, despite its tissue stoichiometry, the C. demersum/periphyton complex always exhibited an affinity for P. It appeared that the biochemical mechanisms, which mediate P removal, at least on a short-term basis, were more influenced by increases in ambient P levels than by tissue nutrient stoichiometry.     

Decomposition responses to phosphorus enrichment in an Everglades (USA) slough

The effects of phosphorus (P) enrichment ondecomposition rates were measured in a Ploading experiment conducted in an oligotrophicmarsh in the northern Everglades, USA. In thisstudy, eighteen 2.5 m² enclosures(mesocosms) were placed in a pristineopen-water (slough) wetland and subjectedweekly to 6 inorganic P loads; 0, 0.2, 0.4,0.8, 1.6 and 3.2 g·m^–2g·yr^–1. Phosphorus accumulated rapidly in the benthicperiphyton and unconsolidated detrital (benthicfloc) layer and significantly higher Pconcentrations were recorded after 1 yr of Paddition. In contrast, a significant increasein surface soil (0–3 cm) TP concentrations wasmeasured in the surface soil layer only after 3yr of loading at the highest dose. Plantlitter and benthic floc/soil decompositionrates were measured using litter bags,containing sawgrass (Cladium jamaicenseCrantz) leaves, and cotton (cellulose) strips,respectively. Litter bag weight losses weresimilar among treatments and averaged 30% atthe end of the 3 yr study period. Litter Nconcentrations increased over time by anaverage of 80% at P loads < 1.6g·m^–2·yr^–1, and by > 120% at Ploads 1.6 g·m^–2·yr^–1.In contrast,litter P concentrations declined up to 50% inthe first 6 months in all P loads and onlysubsequently increased in the two highestP-loaded mesocosms. Cotton strip decaydemonstrated that benthic floc and soilmicrobial activity increased within 5 mo of Paddition with more significant treatmenteffects in the benthic than the soil layer. The influence of soil microbial transformationswas shown in porewater chemistry changes. While porewater P levels remained close tobackground concentrations throughout the study,porewater NH₄ ⁺ and Ca²⁺increased in response to P enrichment,suggesting that one significant effect of Penrichment in this oligotrophic peat system isenhanced nutrient regeneration.     

Heterogeneity of phosphorus distribution in a patterned landscape,the Florida Everglades

The biologically mediated transfer of nutrients from one part of a landscape to another may create nutrient gradients or subsidize the productivity at specific locations. If limited, this focused redistribution of the nutrient may create non-random landscape patterns that are unrelated to underlying environmental gradients. The Florida Everglades, USA, is a large freshwater wetland that is patterned with tree islands, elevated areas that support woody vegetation. A survey of 12 tree islands found total soil phosphorus levels 3–114 times greater on the island head than the surrounding marsh, indicating that the Florida Everglades is not a homogeneous oligotrophic system. It was estimated that historically 67% of the phosphorus entering the central Everglades was sequestered on tree islands, which are ~3.8% of the total land area. This internal redistribution of phosphorus onto tree islands due to the establishment of trees may be one reason that marshes have remained oligotrophic and may explain the spatial differentiation of the patterned Everglades landscape.     

Denitrification in Marl and Peat Sediments in the Florida Everglades

The potential for denitrification in marl and peat sediments in the Shark River Slough in the Everglades National Park was determined by the acetylene blockage assay. The influence of nitrate concentration on denitrification rate and N₂O yield from added nitrate was examined. The effects of added glucose and phosphate and of temperature on the denitrification potential were determined. The sediments readily denitrified added nitrate. N₂O was released from the sediments both with and without added acetylene. The marl sediments had higher rates than the peat on every date sampled. Denitrification was nitrate limited; however, the yields of N₂O amounted to only 10 to 34% of the added nitrate when 100 μM nitrate was added. On the basis of measured increases in ammonium concentration, it appears that the balance of added nitrate may be converted to ammonium in the marl sediment. The sediment temperature at the time of sampling greatly influenced the denitrification potential (15-fold rate change) at the marl site, indicating that either the number or the specific activity of the denitrifiers changed in response to temperature fluctuations (9 to 25°C) in the sediment. It is apparent from this study that denitrification in Everglades sediments is not an effective means of removing excess nitrogen which may be introduced as nitrate into the ecosystem with supply water from the South Florida watershed and that sporadic addition of nitrate-rich water may lead to nitrous oxide release from these wetlands.     

Apparent diffusion limitations on branchial CO2 transfer are revealed by severe experimental anaemia in brown bullhead (Ameiurus nebulosus)

In this study, the rapid (within 2 h) effects of acute anaemia on blood gas and acid-base status, as well as cardiorespiratory variables, were examined in brown bullhead (Ameiurus nebulosus). Anaemia was induced by blood withdrawal coupled to volume replacement with saline. Lowering haematocrit from the control value of 23.5+/-1.0% (mean+/-S.E.M.; N=37) to 5.9+/-0.3% (N=37) resulted in a significant increase (by 2.63+/-0.51 torr; N=7) in arterial CO(2) tension (PaCO(2)) over the subsequent 2-h period in the absence of a change in arterial O(2) tension (PaO(2)). Treatment with bovine carbonic anhydrase (CA) reduced the extent of the PaCO(2) increase to the point where it was not statistically significant. In both control and CA-treated fish, arterial pH decreased during acute anaemia; the acidosis was of mixed respiratory and metabolic origin in control fish and primarily metabolic in CA-treated fish. Inducing anaemia caused increases in both cardiac output (V*b) and heart rate that were similar in control and CA-treated fish. Experimental elevation of V*b equivalent to that observed during anaemia, but in the absence of lowered haematocrit, increased PaCO(2) significantly by 1.49+/-0.74 to 1.64+/-0.78 torr (N=5) without affecting PaO(2). These findings suggest that CO(2) excretion in bullhead, as in rainbow trout, is effectively diffusion-limited, and that approximately half of the increase in PaCO(2) measured during the initial 2 h of anaemia results from the impact of increased blood flow (hence decreased gill transit time) in a diffusion-limited system.     

Effects of Hurricane Andrew on stands of slash pine (Pinus elliottii var. densa) in the everglades region of south Florida (USA)

Few hurricanes affect intact stands of subtropical pines. We examined effects of winds in the eyewalls of Hurricane Andrew, where wind speeds were >200 km h^–1, on all remaining large mainland stands of Pinus elliottii var. densa (south Florida slash pine) on limestone outcroppings (rocklands) in the everglades region of southern Florida. We measured densities and sizes of trees and assessed damage and mortality in plots in old-growth stands in the Lostman's Pines (LOP) region of Big Cypress National Preserve and in second-growth stands in the Pines West (PIW) and Long Pine Key (LPK) regions of Everglades National Park. We also examined age-size relationships using sections from trees killed by the hurricane in LOP and LPK. We used the data to predict effects of recurrent hurricanes on the structure and dynamics of the old-growth stand and to compare effects of hurricanes on old- and second-growth stands.Slash pine was resistant to hurricane winds. Most trees in stands (68–76%) were not severely damaged; mortality in the three regions averaged 17–25% shortly after the hurricane and 3–7% during the following year. Mortality was positively associated with tree size; mean tree sizes decreased and size-selective thinning occurred in all stands. Nonetheless, local mortality ranged from 3–4% to 50–60% among plots in all stands. Such local variation in mortality resulted from clustering of large trees, especially in old-growth stands, and from microbursts during the hurricane, which affected all stands. Recurrent, intense hurricanes are predicted to kill larger trees, slowly opening new patches and increasing sizes of extant patches, thus resulting in almost continual presence of openings suitable for recruitment in old-growth stands. Age-size relationships also indicated that large trees in old-growth stands may survive 2–3 centuries. The combination of frequent openings and wind resistance of large trees is predicted to result in old-growth stands that are highly uneven aged, with trees locally distributed in similar-aged patches. The extent to which such stands deviate from demographic equilibrium, as well as turnover rates within stands, are likely to increase as the frequency of recurrent, intense hurricanes increases.Damage and mortality differed in old- and second-growth stands. Large trees were more, but small trees less likely to be damaged in old- than second-growth stands. In contrast, mortality was significantly lower in old- (LOP: 16.9% ± 3.1 [mean ± s.e.]) than second-growth stands (PIW: 22.5% ± 2.0; LPK: 25.2% ± 2.7). Total hurricane-related mortality was 30–60% higher in second- than old-growth stands. Size class structure, more uneven in old- than second growth stands prior to the hurricane, diverged even more afterwards. Hurricane Andrew removed