Interdiffusion of K and Na in Alkali Feldspars: Homogenization Experiment

The rates of homogenization of coherent exsolution lamellaein cryptoperthites have been used to evaluate K-Na interdiffusion in alkali feldspars.An average interdiffusion coefficient (Ď) is between 2 and 4 x l0-17 cm2/sec for a synthetic cryptoperthite at 600°C in the composition range Or23-Or53, and between 6 and 12 x 10-16 cm2/sec for a natural cryptopertlite at 650°C in the composition range Or0-Or85. These values are for diffusion (801) at 1 atmosphere pressure. K-Na interdiffusion coefficients calculated from published K and Na self-diffusion coefficients and thermochemical solution data for alkali feldspars depend strongly on composition.The minimum Ď\u27s calculated are similar to the average Ď\u27s determined from the homogenization experiments. Finite-difference models of the homogenization experiments using the calculated,composition-dependent interdiffusion coefficients are in good agreement with our experimental results. Homogenization experiments appear to be successful in determining average interdiffusion coefficients which are consistent with diffusion data obtained using other techniques.The method is especially useful for determining Ď\u27s in the range of l0-16 to l0-19 cm2/sec

Active and passive processes associated with initial settlement and post-settlement dispersal of suspended meiobenthic copepods

Settlement by suspended meiofaunal copepods into shallow depressions was investigated in a large, recirculating laboratory flume. Initial settlement was examined under various near-bottom flow regimes, and the interaction of flow with post-settlement behavior was investigated. Cylindrical depressions of constant depth were constructed with a range of aspect ratios (diameter and depth combinations) to mimic natural microtopographic features. Copepods were released into the flume (at nominal flows of 2, 5 and 7 cm sec−1), and settlement/ distribution in the mimic pits was observed. Four experiments were conducted using a total of five species. The first tested for copepod substrate preferences in still water. The second tested for passive settlement into azoic sediment using predictions, based on hydrodynamics, of the ability of depressions of different aspect ratios to act as passive collectors. The patterns of initial settlement of living, freeze-killed and bead-mimic meiofauna were compared. Algal-enriched sediment (to test for active habitat selection) was used in two experiments; one to test for active choice upon initial settlement and the other to test for active habitat selection via post-settlement dispersal into closely-paired pits. Although copepods were capable of active habitat selection in still water, no species tested was able to initially select the preferred habitat in moving water. Moreover, the same copepod species tested in moving water were generally deposited among depressions in the same manner as passive particles. Post-settlement behavior influenced copepod distribution at a low nominal current velocity (2 cm sec−1) as more individuals of both species located (and remained in) algal-enriched depressions after four hrs. Under high flow (8 cm sec−1), copepods were unable to select enriched over non-enriched depressions, either because they did not re-emerge from non-enriched pits or because shear velocity was too high to permit active habitat selection

Temperature Tolerance and Stress Proteins as Mechanisms of Invasive Species Success

Invasive species are predicted to be more successful than natives as temperatures increase with climate change. However, few studies have examined the physiological mechanisms that theoretically underlie this differential success. Because correlative evidence suggests that invasiveness is related to the width of a species' latitudinal range, it has been assumed – but largely untested – that range width predicts breadth of habitat temperatures and physiological thermotolerances. In this study, we use empirical data from a marine community as a case study to address the hypotheses that (1) geographic temperature range attributes are related to temperature tolerance, leading to greater eurythermality in invasive species, and (2) stress protein expression is a subcellular mechanism that could contribute to differences in thermotolerance. We examined three native and six invasive species common in the subtidal epibenthic communities of California, USA. We assessed thermotolerance by exposing individuals to temperatures between 14°C and 31°C and determining the temperature lethal to 50% of individuals (LT50) after a 24 hour exposure. We found a strong positive relationship between the LT50 and both maximum habitat temperatures and the breadth of temperatures experience across the species' ranges. In addition, of the species in our study, invasives tended to inhabit broader habitat temperature ranges and higher maximum temperatures. Stress protein expression may contribute to these differences: the more thermotolerant, invasive species Diplosoma listerianum expressed higher levels of a 70-kDa heat-shock protein than the less thermotolerant, native Distaplia occidentalis for which levels declined sharply above the LT50. Our data highlight differences between native and invasive species with respect to organismal and cellular temperature tolerances. Future studies should address, across a broader phylogenetic and ecosystem scope, whether this physiological mechanism has facilitated the current success of invasive species and could lead to greater success of invasives than native species as global warming continues