Seasonal Production and Biomass of the Seagrass, <Emphasis Type="Italic">Halodule wrightii</Emphasis> Aschers. (Shoal Grass), in a Subtropical Texas Lagoon

A study of Halodule wrightii in a shallow subtropical Texas lagoon was performed to obtain seasonal data on its physiological ecology. Leaf production and biomass dynamics of H. wrightii were intensively monitored along with the underwater light environment at a 1.2-m depth study site over a 21-month period from June 1995 to February 1997. The annual photosynthetically active radiation (PAR) flux of 6,764 mol m⁻² year⁻¹ was more than twice as high as 2,400 mol m⁻² year⁻¹, the minimum annual PAR required for maintenance of growth. As light intensity declined, blade chlorophyll a/b ratios increased suggesting that the plants were photo-adapting. Seasonal trends were evident in shoot growth and biomass. Compared to other Halodule populations in Texas, H. wrightii in LLM displayed slow growth and low biomass, high leaf tissue N content, and low C/N ratio but high N/P ratio of 38 suggesting that the plants were phosphorus-limited.     

Fe–Mg diffusion in olivine I: experimental determination between 700 and 1,200<Emphasis Type="Bold">°</Emphasis>C as a function of composition,crystal orientation and oxygen fugacity

We have determined Fe–Mg diffusion coefficients in olivines from different sources (Nanga Parbat, Pakistan and San Carlos, Arizona, USA) at atmospheric pressure as a function of composition, oxygen fugacity (10⁻⁵–10⁻¹² Pa) and temperature (700–1200°C) using thin films produced by pulsed laser deposition and RBS to analyze the concentration profiles. We have characterized the nano-scale structure and composition of the thin films annealed at various conditions and shown that the nature of the film (e.g. crystallinity, wetting behavior) depends strongly on the annealing conditions. If these variations are not taken into account in the form of boundary conditions for modeling the diffusion profiles, artifacts would result in the diffusion data. The diffusion coefficients obtained from 75 experiments reveal that (i) between fO₂ of 10⁻⁵ and 10⁻¹⁰ Pa, diffusion along all three principal crystallographic directions in olivine, [100], [010] and [001], are described by a constant activation energy of ∼200 kJ/mol, precluding any temperature dependence of diffusion anisotropy and change of mechanism of diffusion at temperatures between 950 and 1200°C, (ii) diffusion coefficients increase with oxygen fugacity at fO₂ > 10⁻¹⁰ Pa, with an fO₂ exponent that lies between 1/4 and 1/7, and (iii) at fO₂ below 10⁻¹⁰ Pa, and consequently at temperatures below ∼900°C, diffusion becomes weakly dependent/independent of fO₂, indicating a change of diffusion mechanism. Activation energy of diffusion at these conditions is slightly higher, ∼220 kJ/mol. The data, including the change of mechanism, are analyzed in terms of point defect chemistry in Part II of this work to derive an equation that allows calculation of diffusivities in olivine over its entire field of stability. Availability of directly measured data at temperatures down to 700°C imply that for the first time diffusion coefficients can be interpolated, rather than extrapolated, for modeling most natural systems.     

A field study of nitrogen storage and nitrate reductase activity in the estuarine macroalgaeEnteromorpha lingulata (Chlorophyceae) andGelidium pusillum (Rhodophyceae)

The purpose of this field study was to determine the relationship between environmental conditions, particularly high nitrate (NO ₃ ⁻ ), low salinity events, and both nitrogen (N) storage (NO ₃ ⁻ , ammonium [NH ₄ ⁺ ], free amino acids [FAA], protein, and total N) and nitrate reductase (NR) activity in the macroalgaeEnteromorpha lingulata andGelidium pusillum in the lower Mobile Bay estuary (Alabama, USA). The environmental conditions at the collection site varied over the growing season with the most notable changes due to late winter and spring runoff entering the estuary (1–30 psu, 0.3–25.8 μM NO ₃ ⁻ , 0.9–12.5 μM NH ₄ ⁺ , 3–28°C, 61–2,375 μmol PAR m⁻² s⁻¹). Principal component analysis reduced the six environmental variables measured to three principal components. Stepwise, multiple regression analysis was then used to examine the relationship between the principal components and the internal NO ₃ ⁻ , NH ₄ ⁺ , and FAA pools and NR activity. The results indicate that changes in inorganic N availability and salinity rather than changes in irradiance determine patterns of N storage and NO ₃ ⁻ reduction. BothE. lingulata andG. pusillum are capable of taking up and storing NO ₃ ⁻ when it becomes available. Greater NO ₃ ⁻ availability produced larger NH ₄ ⁺ and FAA pools along with higher rates of NR activity inE. lingulata, but notG. pusillum, suggesting thatE. lingulata is able to metabolize NO ₃ ⁻ more rapidly during high NO ₃ ⁻ , low salinity events. Differences in the susceptibility ofE. lingulata andG. pusillum to NH ₄ ⁺ inhibition and salinity stress combined with their different growth strategies help to explain the seasonal trends in total N. Total N inE. lingulata ranged from 2.57% to 6.39% dw, while the slower growingG. pusillum showed no significant variation in total N content (3.8–4.1% dw). These results led to the conclusion thatE. lingulata responds more quickly thanG. pusillum to high NO ₃ ⁻ , low salinity events and that these events have a larger effect on the overall N content ofE. lingulata.     

Impacts of temperature and nutrients on coastal lagoon plant communities

We investigated the independent and interactive effects of nutrient loading and summer water temperature on phytoplankton, drift macroalgae, and eelgrass (Zostera marina) in a coastal lagoon mesocosm experiment conducted from May through August 1999. Temperature treatments consisted of controls that approximated the 9-yr mean daily temperatures for Ninigret and Point Judith Lagoons in Rhode Island (United States) and treatments approximately 4°C above and 4°C below the controls. Nutrient treatments consisted of the addition of 6 mmol N m⁻²d⁻¹ and 0.5 mmol P m⁻² d⁻¹ to mesocosms 4°C above and 4°C below the 9-yr daily mean. Nutrient enrichment produced marked phytoplankton blooms in both cool and warm treatments during early summer. These were replaced after midsummer by dramatic growths of macroalgal mats ofEnteromorpha flexuosa and, to a lesser degree,Cladophora sericea. No phytoplankton blooms were observed in the cool unenriched treatments, but blooms did develop in the mean temperature and warm mesocosms during the second half of the summer that were similar in intensity, though of shorter duration, than those observed earlier in the enriched systems. Macroalgal blooms did not occur in the unenriched mesocosms. Sustained warm water temperatures markedly decreased eelgrass density and belowground production and increased the time interval between the initiation of new leaves, particuarly when the biomass of macroalgae was high. The negative effect of elevated water temperature on eelgrass was significantly increased under conditions of elevated inorganic nutrient input. By the end of summer, virtually all of the measures of eelgrass health declined in rank order from cool, to mean, to cool enriched, to warm, to warm enriched treatments. It is likely that the marked declines in eelgrass abundance observed during recent decades in the Northeast have resulted from an interaction of increasing nutrient enrichment combined with increasing summer water temperatures.     

Stable isotope analyses (δ<Superscript>15</Superscript>N and δ<Superscript>13</Superscript>C) of the trophic relationships of<Emphasis Type="Italic">Callinectes sapidus</Emphasis> in two North Carolina estuaries

The present study focused on detecting variations in trophic relationships among blue crab (Callinectes sapidus) consumers according to water quality along two estuaries in North Carolina. Stable isotope (δ¹⁵N and δ¹³C) analyses of particulate organic matter and bivalve(Rangia cuneata andCorbicula fluminea) food sources were examined in combination with an Isosource mixing model. Results suggest that blue crab δ¹³C values increased significantly with increasing salinity from upper to lower sites along the Neuse River estuary (NRE; R2 = 0.87, p < 0.01) and Alligator River estuary (R² = 0.92, p < 0.01). There was a positive relationship between blue crab δ¹⁵N values and nitrate concentrations for the NRE (R² = 0.48, p = 0.12). This study found that blue crab δ¹³C values increased with salinity from upper to lower regions along both estuaries. Results suggest that blue crab production may have used alternative food sources that were isotopically (δ¹³C) depleted, especially in the upper NRE, and enriched sources in the mid to lower regions of both estuaries. Consumers sampled from the upper NRE may be influenced by higher nitrogen input from urban land use and municipal wastewater.     

Denitrification capacity in a subterranean estuary below a Rhode Island fringing salt marsh

Coastal waters are severely threatened by nitrogen (N) loading from direct groundwater discharge. The subterranean estuary, the mixing zone of fresh groundwater and sea water in a coastal aquifer, has a high potential to remove substantial N. A network of piezometers was used to characterize the denitrification capacity and groundwater flow paths in the subterranean estuary below a Rhode Island fringing salt marsh.¹⁵N-enriched nitrate was injected into the subterranean estuary (in situ push-pull method) to evaluate the denitrification capacity of the saturated zone at multiple depths (125–300 cm) below different zones (upland-marsh transition zone, high marsh, and low marsh). From the upland to low marsh, the water table became shallower, groundwater dissolved oxygen decreased, and groundwater pH, soil organic carbon, and total root biomass increased. As groundwater approached the high and low marsh, the hydraulic gradient increased and deep groundwater upwelled. In the warm season (groundwater temperature >12 °C), elevated groundwater denitrification capacity within each zone was observed. The warm season low marsh groundwater denitrification capacity was significantly higher than all other zones and depths. In the cool season (groundwater temperature <10.5 °C), elevated groundwater denitrification capacity was only found in the low marsh. Additions of dissolved organic carbon did not alter groundwater denitrification capacity suggesting that an alternative electron donor, possibly transported by tidal inundation from the root zone, may be limiting. Combining flow paths with denitrification capacity and saturated porewater residence time, we estimated that as much as 29–60 mg N could be removed from 11 of water flowing through the subterranean estuary below the low marsh, arguing for the significance of subterranean estuaries in annual watershed scale N budgets.     

Spatial distribution characteristics of stable carbon isotope compositions in desert plant Reaumuria soongorica (Pall.) Maxim

In order to investigate the distribution characteristics of stable carbon isotope ratios (δ ¹³C) in the desert plant Reaumuria soongorica, the δ ¹³C values of leaves were measured in 407 individuals of 21 populations. Soil physicochemical properties including soil water content, soil total dissolved solids, soil total nitrogen, soil total phosphorus and soil organic content were also analyzed in order to survey the major factors influencing δ ¹³C values on spatial variation. Leaves and soil samples were simultaneously collected from the ten major distribution areas in Northwest China at altitudes from 394 m to 1 987 m above sea level, at latitudes from 36°10′N to 44°33′N, and at longitudes from 81°43′E to 106°37′E. These ten areas include Shihezi, Baicheng, Yiwu areas in Xinjiang Uygur Autonomous Region; Anxi, Zhangye, Baiyin, Lanzhou areas in Gansu Province; Shapotou, Yinchuan areas in Ningxia Hui Autonomous Region; and Alashan County in Inner Mongolia Autonomous Region. The results show that the δ ¹³C value of R. soongorica ranges from −22.77‰ to −29.85‰ with an average of −26.52‰. Foliar d13C values in R. soongorica are not significantly correlated with altitude, latitude or longitude, and a spatial distribution trend of d13C values of R. soongorica is not obvious on a large scale. However, when d13C values of two R. soongorica populations under the same climate conditions are compared, δ ¹³C values increase obviously from east to west and from north to south. As none of the soil total dissolved solids, soil total nitrogen, soil total phosphorus, and soil organic content shows a uniform trend from east to west and from north to south, we suppose that the small-scaled spatial distribution pattern of δ ¹³C values of R. soongorica is mainly controlled by the soil water content. Translated from Quaternary Sciences, 2006, 26(6): 947–954 [译自: 第四纪研究]     

The relation of seasonal pattern in stable carbon compositions to meteorological variables in the leaves of <Emphasis Type="Italic">Sabina</Emphasis><Emphasis Type="Italic">przewalskii</Emphasis> Kom. and <Emphasis Type="Italic">Sabina chinensis</Emphasis> (Lin.) Ant.

The seasonal variation of foliar δ¹³C values in Sabina przewalskii Kom. and Sabina chinensis (Lin.) Ant. was measured. The relationships between foliar δ¹³C values and branch δ¹³C values as well as environmental factors (monthly total precipitation, monthly average air temperature, monthly average soil temperature, monthly total solar duration, relative humidity, atmospheric pressure, vapor pressure, wind speed and potential evaporation) were investigated. The results showed that the foliar δ¹³C values were negatively correlated with air pressure, and positively correlated with air temperature, precipitation, vapor pressure, potential evaporation, solar duration, wind speed and soil temperature. No significant relationship between δ¹³C values and relative humidity was detected. This demonstrates that the foliar δ¹³C of Sabina is a successful empirical indictor of these meteorological factors within the usual range of C₃ whole-leaf δ¹³C values. Furthermore, the δ¹³C signature of leaf tissue is similar to that of wood tissue and the responses of δ¹³C values in S. przewalskii Kom. to environmental factors are also relatively stronger than that of S. chinensis (Lin.) Ant. These results provided strong evidence that it is feasible to extract climatic information from tree-ring δ¹³C series and S. przewalskii Kom. is a dendroclimatologically promising tree species.     

Bubble nucleation in rhyolite and dacite melts: temperature dependence of surface tension

Surface tension (σ) profoundly influences the ability of gas bubbles to nucleate in silicate melts. To determine how temperature impacts σ, experiments were carried out in which high-silica rhyolite melts with 5 wt% dissolved water were decompressed at temperatures that ranged from 775 to 1,085°C. Decompressions were also carried out using dacite melts with 4.3 wt% dissolved water at 1,150°C. Water bubbles nucleated in rhyolite only when decompressions exceeded 95 MPa at all temperatures. Bubbles nucleated in number densities that increased as decompression increased and at hotter temperatures at a given amount of decompression. After correcting decompression amounts for temperature differences, values for σ were estimated from nucleation rates and found to vary between 0.081 and 0.093 N m⁻¹. Surface tension decreases as temperature increases from 775 to 875°C, but then increases as temperature increases to 1,085°C. Those values overlap previous results, but only when melt viscosity is less than 10⁴ Pa s. For low-viscosity rhyolite, there is a strong correlation of σ with temperature, in which σ increases by 6.9 × 10⁻⁵ N m⁻¹ C⁻¹. That variation is robust for 5–9 wt% dissolved water, as long as melt viscosity is ≤10⁴ Pa s. More viscous rhyolite deviates from that correlation probably because nucleation is retarded in stiffer melts. Bubbles nucleated in dacite when decompressions exceeded 87 MPa, and occured in one or more events as decompression increased. Surface tension is estimated to be 0.083 (±0.001) N m⁻¹ and when adjusted for temperature agrees well with previous results for colder and wetter dacite melts. At a given water content, dacite melts have lower surface tensions than rhyolite melts, when corrected to a fixed temperature.     

Salt Spray Induces Osmotic Adjustment and Tissue Rigidity in Smooth Cordgrass, <Emphasis Type="Italic">Spartina alterniflora</Emphasis> (Loisel.)

Salt spray is one of many abiotic factors that can influence plant productivity and species composition in coastal ecosystems. However, little is known about how marsh plants respond physiologically to the accumulation of sea aerosols on foliar tissues. In this study, experimental microcosms maintained in controlled greenhouse conditions were used to evaluate how low- (1.7 mg dm⁻² day⁻¹, weekly averages) and high- (8.6 mg dm⁻² day⁻¹) salt-spray loads would influence plant–water relations in Spartina alterniflora (Loisel.). While no differences in plant performance (e.g., changes in biomass and leaf area) were observed between the treatments and control plants, a number of physiological modifications attributed to salt spray were observed. In general, salt-treated plants underwent significant decreases in water potential (Ψ) and osmotic potential (Ψ _π) and increases in leaf conductance (g) and bulk modulus of elasticity (ε). It is likely that these physiological responses were used to generate lower Ψ while maintaining osmotic and water homeostasis. That is, by decreasing Ψ _π and increasing g and ε, more efficient water flow through the soil–plant–atmosphere continuum can be achieved, thus generating lower Ψ without promoting loss of turgor.     

Chromium-induced depression of <Superscript>15</Superscript>N content and nitrate reductase activity in rice seedlings

Hydroponic experiments were conducted to investigate the influence of chromium (Cr) exposure on assimilation of ¹⁵N-labeled nitrate in rice (Oryza sativa L. cv. XZX 45) seedlings. Results showed that exposure of young seedlings to both Cr compounds led to a linear decrease in relative growth rate, nitrate reductase (NR) activity and tissue ¹⁵N accumulation though responses differed between the two Cr variants, in which potassium chromate [Cr(VI)] caused more severe effects than chromium chloride [Cr(III)]. Analysis of subcellular distribution of Cr showed that Cr deposition in cell wall was the highest in tissues of rice seedlings exposed to Cr(III), whereas the largest deposition of Cr in the roots was associated with cytosol in the Cr(VI) treatments. Analysis of a partial correlation between tissue fractions of Cr, NR activity and ¹⁵N content revealed that depression of NR activity in plant materials of rice seedlings exposed to Cr(III) was mainly associated with a decrease in ¹⁵N accumulation, whereas inhibition of NR activity from exposure of plants to Cr(VI) was most likely due to the presence of Cr in cytosol. Information collected here indicated that mechanisms of transport, subcellular distribution and phytotoxicity of the two Cr compounds were different in rice seedlings.     

Thermal diffusivity of quartz to 1,000°C: effects of impurities and the α-β phase transition

Lattice thermal diffusivities of eleven orientated natural and synthetic quartz samples were measured using a laser-flash apparatus (LFA). For α-quartz, thermal diffusivity (D) decreases with temperature, so that directionally averaged values of D(T) between 20° and 500°C can be fit with D(T) = 1/(0.0017T + A), where A varies from 0.12 to 0.18 among the samples and D is in mm²/s. For β-quartz, D decreases very slightly or is constant with temperature. Values of D measured along [001] exceed those measured along [100] at all temperatures studied. A sharp decrease of D(T) marks the α-β phase transition, consistent with correlation of 1/D with C _P in the damped harmonic oscillator model. Due to the rapidity of the laser-pulse and the dynamic nature of the measurements, the raw data show evidence of latent heat being released at the transition temperature over ∼50 ms. D(T) values within 10 K of the transition are affected both by latent heat release and by the phase change. For our suite of samples, D at all temperatures varies by ±7% from the overall average. This range is outside the 2% experimental uncertainty. To ascertain causes of D variations, we characterized out samples using infrared absorption spectroscopy, electron microprobe analyses and secondary ion mass spectrometry. Differences between the samples seem to result from the interplay of different impurity types: cations that replace Si in the silicate tetrahedra, cations squeezing into the lattice, and OH. Although limitations in quantifying amounts of trace impurities and poor understanding of where impurities reside in the lattice hamper interpretation, it seems that samples containing more substitutional impurities (e.g., Al³⁺) have higher diffusivities along [001], those with more interstitial defects (e.g., Li⁺) have higher diffusivities along [100], and that OH reduces D.     

Systematic study of hydrogen incorporation into Fe-free wadsleyite

The H₂O content of wadsleyite were measured in a wide pressure (13–20 GPa) and temperature range (1,200–1,900°C) using FTIR method. We confirmed significant decrease of the H₂O content of wadsleyite with increasing temperature and reported first systematic data for temperature interval of 1,400–1,900°C. Wadsleyite contains 0.37–0.55 wt% H₂O at 1,600°C, which may be close to its water storage capacity along average mantle geotherm in the transition zone. Accordingly, water storage capacity of the average mantle in the transition zone may be estimated as 0.2–0.3 wt% H₂O. The H₂O contents of wadsleyite at 1,800–1,900°C are 0.22–0.39 wt%, indicating that it can store significant amount of water even under the hot mantle environments. Temperature dependence of the H₂O content of wadsleyite can be described by exponential equation C_{\textH2 \textO} = 6 3 7.0 7 \texte ^{- 0.00 4 8T} , C_{{{\text{H}}_{2} {\text{O}}}} = 6 3 7.0 7 {\text{e}}^{ - 0.00 4 8T} , where T is in °C. This equation is valid for temperature range 1,200–2,100°C with the coefficient of determination R ² = 0.954. Temperature dependence of H₂O partition coefficient between wadsleyite and forsterite (D _wd/fo) is complex. According to our data apparent D_wd/fo decreases with increasing temperature from D _wd/fo = 4–5 at 1,200°C, reaches a minimum of D _wd/fo = 2.0 at 1,400–1,500°C, and then again increases to D _wd/fo = 4–6 at 1,700–1,900°C.     

Link between convection and meridional gradient of sea surface temperature in the Bay of Bengal

We use daily satellite estimates of sea surface temperature (SST) and rainfall during 1998–2005 to show that onset of convection over the central Bay of Bengal (88–92°E, 14–18°N) during the core summer monsoon (mid-May to September) is linked to the meridional gradient of SST in the bay. The SST gradient was computed between two boxes in the northern (88–92°E, 18–22°N) and southern (82–88°E, 4–8°N) bay; the latter is the area of the cold tongue in the bay linked to the Summer Monsoon Current. Convection over central bay followed the SST difference between the northern and southern bay (ΔT) exceeding 0.75°C in 28 cases. There was no instance of ΔT exceeding this threshold without a burst in convection. There were, however, five instances of convection occurring without this SST gradient. Long rainfall events (events lasting more than a week) were associated with an SST event (ΔT ≥ 0.75°C); rainfall events tended to be short when not associated with an SST event. The SST gradient was important for the onset of convection, but not for its persistence: convection often persisted for several days even after the SST gradient weakened. The lag between ΔT exceeding 0.75°C and the onset of convection was 0–18 days, but the lag histogram peaked at one week. In 75% of the 28 cases, convection occurred within a week of ΔT exceeding the threshold of 0.75°C. The northern bay SST, T _N , contributed more to ΔT, but it was a weaker criterion for convection than the SST gradient. A sensitivity analysis showed that the corresponding threshold for T _N was 29°C. We hypothesise that the excess heating (∼1°C above the threshold for deep convection) required in the northern bay to trigger convection is because this excess in SST is what is required to establish the critical SST gradient.     

The stability of sapphirine-quartz and hypersthene-sillimanite-quartz assemblages: an experimental investigation in the system FeO−MgO−Al2O3−SiO2 under H2O and CO2 conditions

Phase relations and mineral chemistry involving the phases garnet (Gt), spinel (Sp), hypersthene (Hy), sapphirine (Sa), cordierite (Cd), sillimanite (Sil) and quartz (Qz) have been experimentally determined in the system FMAS (FeO−MgO−Al₂O₂−SiO₂) under low fO₂ and for various H₂O/CO₂ conditions. Several compositions were studied with 100 (Mg/Mg+Fe) ratio ranging from 64 to 87 with excess quartz and sillimanite. Our data do not show any differences in Gt−Cd stability and composition as a function of H₂O, CO₂ and H₂O−CO₂ (±CH₄) content, in good agreement with a previous experimental study at lower temperature (Aranovich and Podlesskii 1983). At 1,000° C and 11 kbar, under CO₂-saturated conditions, cordierite grew from a crystalline mix unseeded with cordierite. Thus, under water-absent conditions, cordierite will have a high-P stability field in the presence of CO₂. If water has a pressure stabilizing effect on cordierite, then our results would indicate that the effects of H₂O and CO₂ are of the same magnitude at high temperature. Our data support the theoretical P-T grid proposed by Hensen (1986) for high-T metapelites and are largely consistent with the high-temperature experimental data of Hensen and Green (1973). The univariant boundary Gt+Cd=Hy+Sil+Qz, which marks the disappearance of Hy−Sil−Qz assemblages, has a negative dP/dT slope above 1,000° C and a positive one below this temperature. Extrapolation of our data to iron-free systems shows that the high-P breakdown limit of Mg-cordierite has a negative slope in the range 1,025–1,300° C and probably positive below 1,000° C. This indicates a maximum of stability for Mg-cordierite at around 1,000° C and 13 kbar. Because of the curvature of the univariant reactions En+Sil=Py+Qz, Mg−Cd=En+Sil+Qz and Gt+Cd=Hy+Sil+Qz, the iron-free invariant point involving the phases Py, En, Cd, Sil and Qz probably does not exist. Sapphirine—Qz-bearing assemblages are stable only at temperatures above 1,050° C. At 1,075° C, the joint Gt−Sa is stable up to 11 kbar. At higher pressure, garnet, sapphirine and quartz react according to the reaction Gt+Sa+Qz=Hy+Sil. Reequilibrated sapphirines are more aluminous than the theoretical endmember Mg₂Al₄SiO₁₀ due to AlAl=MgSi substitutions [100(Al₂O₃/Al₂O₃+FeO+MgO) in experimental sapphirines ranges from 50.5 to 52.2]. Sapphirine in the assemblage Sa−Cd−Sil−Qz shows a decrease in Al content with decreasing temperature and pressure, such that the alumina isopleths for sapphirine have a slight negative dP/dT slope. A similar decrease in Al content of sapphirine with temperature is also observed in Sa−Sil−Qz assemblages.     

Temperature Tolerance of Early Life History Stages of Black Mangrove <Emphasis Type="Italic">Avicennia germinans</Emphasis>: Implications for Range Expansion

Avicennia germinans (L.) L. (black mangrove) grows at its North American latitudinal limit in coastal salt marshes of Louisiana, USA. To assess low-temperature tolerance of A. germinans, we evaluated the survival and developmental progress of three early life history stages (dispersal, stranded, and seedling stages) to three temperatures (5.7°C, 2.5°C, and −6.5°C) of four durations (2, 6, 12, and 24 h). Duration and temperature of exposure decreased mangrove survivorship, particularly 24-h exposure at −6.5°C. Although there was not a significant effect of stage on survivorship overall, both survival analysis and final mortality analysis indicate that dispersal stage had the greatest survivorship. Treatments had little impact on developmental progress of surviving propagules, but propagules exposed to −6.5°C were most susceptible to fungal infection. Greater cold temperature tolerance of the dispersal stage may be a mechanism for northward range expansion in Louisiana.     

Effects of an Omnivorous Katydid,Salinity, and Nutrients on a Planthopper-<Emphasis Type="Italic">Spartina</Emphasis> Food Web

Top–down and bottom–up effects interact to structure communities, especially in salt marshes, which contain strong gradients in bottom–up drivers such as salinity and nutrients. How omnivorous consumers respond to variation in prey availability and plant quality is poorly understood. We used a mesocosm experiment to examine how salinity, nutrients, an omnivore (the katydid Orchelimum fidicinium) and an herbivore (the planthopper Prokelisia spp.) interacted to structure a simplified salt marsh food web based on the marsh grass Spartina alterniflora. Bottom–up effects were strong, with both salinity and nutrients decreasing leaf C/N and increasing Prokelisia abundance. Top–down effects on plants were also strong, with both the herbivore and the omnivore affecting S. alterniflora traits and growth, especially when nutrients or salt were added. In contrast, top–down control by Orchelimum of Prokelisia was independent of bottom–up conditions. Orchelimum grew best on a diet containing both Spartina and Prokelisia, and in contrast to a sympatric omnivorous crab, did not shift to an animal-based diet when prey were present, suggesting that it is constrained to consume a mixed diet. These results suggest that the trophic effects of omnivores depend on omnivore behavior, dietary constraints, and ability to suppress lower trophic levels, and that omnivorous katydids may play a previously unrecognized role in salt marsh food webs.     

H<Subscript>2</Subscript>O storage capacity of MgSiO<Subscript>3</Subscript> clinoenstatite at 8–13 GPa, 1,100–1,400°C

We present H₂O analyses of MgSiO₃ pyroxene crystals quenched from hydrous conditions in the presence of olivine or wadsleyite at 8–13.4 GPa and 1,100–1,400°C. Raman spectroscopy shows that all pyroxenes have low clinoenstatite structure, which we infer to indicate that the crystals were high clinoenstatite (C2/c) during conditions of synthesis. H₂O analyses were performed by secondary ion mass spectrometry and confirmed by unpolarized Fourier transform infrared spectroscopy on randomly oriented crystals. Measured H₂O concentrations increase with pressure and range from 0.08 wt.% H₂O at 8 GPa and 1,300°C up to 0.67 wt.% at 13.4 GPa and 1,300°C. At fixed pressure, H₂O storage capacity diminishes with increasing temperature and the magnitude of this effect increases with pressure. This trend, which we attribute to diminishing activity of H₂O in coexisting fluids as the proportion of dissolved silicate increases, is opposite to that observed previously at low pressure. We observe clinoenstatite 1.4 GPa below the pressure stability of clinoenstatite under nominally dry conditions. This stabilization of clinoenstatite relative to orthoenstatite under hydrous conditions is likely owing to preferential substitution of H₂O into the high clinoenstatite polymorph. At 8–11 GPa and 1,200–1,400°C, observed H₂O partitioning between olivine and clinoenstatite gives values of D ^ol/CEn between 0.65 and 0.87. At 13 GPa and 1,300°C, partitioning between wadsleyite and clinoenstatite, D ^wd/CEn, gives a value of 2.8 ± 0.4.     

Point defects and cation tracer diffusion in (Ti<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Fe<Subscript><Emphasis Type="Italic">1−x</Emphasis></Subscript>)<Subscript>3−δ</Subscript>O<Subscript>4</Subscript>. II. Cation tracer diffusion

 Cation tracer diffusion coefficients, D_Me ^*, for Me=Fe, Mn, Co and Ti, were measured using radioactive isotopes in the spinel solid solution (Ti _x Fe _1−x )_3−δO₄ as a function of the oxygen activity. Experiments were performed at different cationic compositions (x=0, 0.1, 0.2 and 0.3) at 1100, 1200, 1300 and 1400 °C. The oxygen activity dependence of all data for D_Me ^* at constant temperature and cationic composition can be described by equations of the type D_Me ^*=D^∘ _Me[V]. C_V·a _O2 ^2/3+D_Me[I] ^∘·a _O2 ^−2/3·D_Me[V] ^∘ and D_Me[I] ^∘ are constants and C_V is a factor of the order of unity which decreases with increasing δ. All log D_Me ^* vs. loga _O2 curves obtained for different values of x and for different temperatures go through a minimum due to a change in the type of point defects dominating the cation diffusion with oxygen activity. Cation vacancies prevail for the cation diffusion at high oxygen activities while cation interstitials become dominant at low oxygen activities. At constant values of x, D_Me[V] ^∘ decreases with increasing temperature while D_Me[I] ^∘ increases.