Tidal currents, winds and the morphology of phytoplankton spatial structures

Chlorophyll a, nutrients and salinity distributions were studied at two spatial scales (10 cm and 0.25 to 2.5 km) in the St. Lawrence Estuary (Quebec, Canada), in order to investigate the role of tidal currents and winds in the formation and maintenance of spatial structures. Data were collected according to a synoptic sampling pattern using three sampling platforms simultaneously, and they were analyzed using analysis of variance. The sampling pattern was repeated on four occasions during July 1980.Analyses of variance indicated significant spatial heterogeneities of about the same magnitude at the two scales studied for chlorophyll and nutrients, whereas salinity showed only large–scale variability. At the kilometer scale, the frequency distribution spectra of patch length for chlorophyll showed the existence of patches of various dimensions between 0.2 and 6.0 km with a dominance of small patches (≤0.5 km). Frequency maxima were usually observed at the smaller (≤0.5 km) and larger (≥2.0 km) scales for the nutrients and only at larger (≥2.0 km) scale for salinity. The distribution spectra of patch dimensions were characteristic for each sampling experiment, depending on tidal currents and prevailing wind conditions. Estimated patch dimensions were larger parallel to the current direction than perpendicular to current direction, implying that spatial structures are elongated in the sense of the current direction. Higher winds have, first, a tendency to increase the small–scale structure of the environment by breaking up larger patches into smaller patches, before structures are completely eliminated. The implication of these findings is that different results could be obtained depending on the sampling strategy used (sampling either at anchor stations or at random, independent of current direction), which could lead to different conclusions

Subsurface chlorophyll maximum and hydrodynamics of the water column

The vertical distributions of chlorophyll a (in vivo fluorescence) and hydrodynamic properties were monitored in the Gulf of St. Lawrence (Canada) from 6 to 10 August 1983, using an automatic yo-yo profiling system and a chain of 4 current meters. Spectral analyses of temperature and in vivo fluorescence series showed that dominant frequencies were associated with internal waves (∼16 h inertial frequency). A subsurface chlorophyll maximum was continuously observed in the lower part of the 20 m thick photic layer, at a depth corresponding with maximum vertical stability of the water column, just above the nutricline.The depth of maximum phytoplankton production, at least on sunny days, corresponded to that of the subsurface chlorophyll maximum and of the maximum in vertical stability. This close association persisted despite strong horizontal advection and vertical movements caused by internal waves. Photosynthetic adjustment did occur in the water column: higher vertical stability at depth favored shade adaptation of the phytoplankton in the layer of maximum stability, as compared to the more light-adapted cells of the upper well-mixed layer. At our sampling station, vertical turbulent diffusion seemed to be high enough to replenish nutrients in the photic layer, so that they never became completely exhausted, even in surface waters. Therefore, the observed subsurface chlorophyll maximum not only resulted from environmental conditions more favorable for phytoplankton accumulation and growth, but it also involved active photosynthetic responses of phytoplankton

Patterns of community variability depend on habitat variability and habitat generalists in natural aquatic microcosms

Habitat variability is largely an external mechanism influencing community variability by affecting abundances and precipitating other community changes but the nature of this influence is poorly understood. The absence of systematic quantitative studies appears to be a major reason for this deficiency. To address the problem, we have evaluated community and population variability in invertebrate communities collected from 49 coastal Jamaican rock pools with contrasting levels of habitat variability. We calculated a multivariate index of habitat variability based on temporal changes in physicochemical variables. Variability in diversity indices (Simpson.s and Shannon-Wiener), evenness (2 measures), and species richness represented community variability while species rank correlations and community constancy represented changes in community structure. Additionally, we analyzed the impact of three habitat generalists (harpacticoid copepod (Nitocra spinipes Boeck), cyclopoid copepod (Orthocyclops modestus Herrick), and the ostracod (Potamocypris sp.)) on overall community variability. As habitat variability increased, both community and population variability increased. Community structure (ranked abundances) was more variable in variable habitats compared to non-variable habitats but communities in these variable habitats retained greater constancy of composition suggesting that highly variable habitats are dominated by a few species with good dispersal abilities. Rare species may come and go, but the dominant species persist in these habitats. Habitat generalists influenced temporal community variability differently, especially evenness (based on the Shannon-Wiener index). Positive relationships were found between the variability in evenness and population variability of the ostracod and cyclopoid copepod. A negative relationship was found between the variability in evenness and the variability of harpacticoid copepods. Our study suggests that individual communities or assemblages respond independently and asynchronously to environmental factors, a view originally proposed by Gleason (1917).We conclude that the form of community structure in variable habitats remains constant. The species composition and relative abundances can change over time but the relative abundance of the dominant species stays high and the remaining species, regardless of their numbers,make relatively small contributions to the overall community variability pattern

Academic Problem-Solving and Students’ identities as engineers

Socially constructed identities and language practices influence the ways students perceive themselves as learners, problem solvers, and future professionals. While research has been conducted on individuals’ identity as engineers, less has been written about how the language used during engineering problem solving influences students’ perceptions and their construction of identities as learners and future engineers. This study investigated engineering students’ identities as reflected in their use of language and discourses while engaged in an engineering problem solving activity. We conducted interviews with eight engineering students at a large southeastern university about their approaches to open and closed-ended materials engineering problems. A modification of Gee’s analysis of language-in-use was used to analyze the interviews. We found that pedagogical and engineering problem solving uses of language were the most common. Participants were more likely to perceive themselves as students highlighting the practices, expectations, and language associated with being a student rather than as emerging engineers whose practices are affected by conditions of professional practice. We suggest that problem solving in an academic setting may not encourage students to consider alternative discourses related to industry, professionalism, or creativity; and, consequently, fail to promote connections to social worlds beyond the classroom. By learning about the ways in which language in particular settings produces identities and shapes problem solving practices, educators and engineering professionals can gain deeper understanding of how language shapes the ways students describe themselves as problem-solvers and make decisions about procedures and techniques to solve engineering problems

Investigating the role of time in affective forecasting: temporal influences on forecasting accuracy.

Using extensive diary data from people taking their driver's license exam, the authors investigated the role of time in affective forecasting accuracy. Replicating existing findings, participants grossly overestimated the intensity and duration of their negative affect after failure and only slightly overestimated the intensity and duration of their positive affect after success. Extending existing findings, participants accurately predicted a decrease of their affective reactions over time but underestimated the speed with which this decrease would occur. In addition, they showed greater forecasting accuracy for positive affect than negative affect when the exam was distant and greater forecasting accuracy for negative affect than positive affect when the exam was close. The motivational processes underlying these findings are being discussed. © 2007 by the Society for Personality and Social Psychology, Inc

Quiescent and shear-induced crystallization of linear and branched polylactides

The quiescent and shear-induced isothermal crystallization behavior of linear and long chain branched (LCB) polylactides (PLAs) was investigated at a temperature of 130oC. LCB-PLAs were produced by the reaction with a multi-functional chain extender, Joncryl©. In quiescent crystallization the presence of the LCB structure accelerated the nucleation process and reduced the induction time, depending on the level of branching. The impact of shear strain, and shear rate on crystallization was also examined. The shear-induced crystallization of the linear and LCB-PLAs was affected by both the total shear strain and shear rate. The crystallization kinetics of the LCB-PLAs was more affected by shear than that of the linear PLA. The crystalline morphology of the linear and LCB- PLAs under quiescent and step shear conditions was examined using a Linkam optical shearing system. An increase in the spherulite density was observed in the strained melt of both linear (33 %) and LCB-PLAs (15 %), in comparison with those of unstrained counterparts. Optical micrographs confirmed that the crystal nucleation was affected by the shear flow. . Long chain branching significantly promoted the nucleation density (6.7 times), although it diminished the crystal growth rate from 4.4 to 2.0 µm/min

The role of color diagnosticity in object recognition and representation

Abstract The role of color diagnosticity in object recognition and representation was assessed in three Experiments. In Experiment 1a, participants named pictured objects that were strongly associated with a particular color (e.g., pumpkin and orange). Stimuli were presented in a congruent color, incongruent color, or grayscale. Results indicated that congruent color facilitated naming time, incongruent color impeded naming time, and naming times for grayscale items were situated between the congruent and incongruent conditions. Experiment 1b replicated Experiment 1a using a verification task. Experiment 2 employed a picture rebus paradigm in which participants read sentences one word at a time that included pictures of color diagnostic objects (i.e., pictures were substituted for critical nouns). Results indicated that the ''reading'' times of these pictures mirrored the pattern found in Experiment 1. In Experiment 3, an attempt was made to override color diagnosticity using linguistic context (e.g., a pumpkin was described as painted green). Linguistic context did not override color diagnosticity. Collectively, the results demonstrate that color information is regularly utilized in object recognition and representation for highly color diagnostic items

Rheological and foaming behavior of linear and branched polylactides

In this work, a chain extender (CE), Joncryl © ADR, was added to a polylactide (PLA) at two concentrations, 0.4 and 0.7 wt%, using two different strategies. The rheological properties and foamability of the neat PLA and PLA containing the CE were studied. The steady and transient rheological properties of the neat PLA and CE-treated PLAs revealed that the introduction of the CE profoundly affected the melt viscosity and elasticity. The linear viscoelastic properties of CE-enriched PLAs suggested that a long chain branching (LCB) structure was formed from the reaction with the CE. LCB-PLAs exhibited an increased viscosity, more shear sensitivity, and longer relaxation time in comparison with the linear PLA. The LCB structure was also found to affect the transient shear stress growth and elongational flow behavior. LCB-PLAs exhibited a pronounced strain hardening, whereas no strain hardening was observed for the linear PLA. Batch foaming of the linear and LCB-PLAs was also examined at foaming temperatures of 130, 140, and 155 ᵒC. The LCB structure significantly increased the integrity of the cells, cell density and void fraction