Circadian variations in photosynthetic assimilation and estimation of daily phytoplankton production

In August 1984, hourly measurements of photosynthetic characteristics were carried out during 96 h, at 5 and 10 m, on a natural population of phytoplankton in the St Lawrence Estuary. Synchronous circadian variations of similar amplitude (max./min.: 2 to 3) were observed at the two depths in both the photosynthetic capacity (P _m ^B ) and the photosynthetic efficiency (^B). Maximum values occurred at around noontime and minima during the night. Estimates of daily specific productivity were computed with and without the observed circadian variability. Large differences (15 to 70%) were evidenced between estimates.Contribution to the programs of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec) and of the Maurice Lamontagne Institute (Department of Fisheries and Oceans)     

Biosorption of thorium on the external shell surface of bivalve mollusks: the role of shell surface microtopography

External shell surface (ESS) of bivalve mollusks is known to adsorb various metals dissolved in ambient water in high concentration. It is hypothesized here that the surface microtopography of the thin organic coating layer, periostracum, or calcareous shell (if periostracum was destroyed) plays a major role in the adsorption of actinides on ESS. Thorium (natural alpha-emitter) was used in short-term biosorption experiment with shell fragments of five bivalve mollusks. After a 72 h exposure to Th (∼6 kBq L⁻¹), thorium concentration was measured on ESS using laser ablation inductively coupled plasma mass spectrometry; the distribution and density of alpha tracks were subsequently visualized by α-track autoradiography. A trend in reduced Th concentrations on the ESS was observed depending upon the species tested: (group 1 ∼4000 μg g⁻¹) Chlamys islandica (M.), Mercenaria mercenaria (L.), Dreissena polymorpha (P.) > (group 2 ∼1200 μg g⁻¹) Crassostrea virginica (G.) ? (group 3 ∼150 μg g⁻¹) Mytilus edulis L. The microtopography of ESS was characterized by scanning electron microscopy revealing the high porosity of the calcareous surface of C. islandica and M. mercenaria, lamellate surface of periostracum in D. polymorpha, uneven but a weakly porous surface of periostracum of C. virginica, and a nearly smooth surface of the periostracum of M. edulis. This work has demonstrated, for the first time, the presence of a strong correlation between concentration of adsorbed Th and ESS microtopography, and the role of the periostracum in this process is discussed.     

Acclimatization to intertidal conditions modifies the physiological response to prolonged air exposure inMytilus edulis

Both the tolerance of air exposure and the metabolic means by which mussles,Mytilus edulis, face air exposure can be modified by intertidal acclimatization. Short-term intertidal acclimatization enhanced the tolerance of air exposure most in early summer. Prevention of air breathing increased the mortality of wild and non-acclimatized cultured (i.e., control) mussels, but only slightly enhanced that of intertidally acclimatized mussels, suggesting their increased dependence upon anaerobic metabolism. In June, anaerobic end products accumulated in the adductor muscle of cultured but not of wild mussels. Intertidal acclimatization shifted the pattern and rates of metabolite accumulation. Control mussels accumulated succinate and alanine in the adductor muscle while intertidally acclimatized mussels accumulated strombine + alanopine, albeit at far lower levels. Pyruvate kinase (PK) from adductor muscle of intertidally acclimatized mussels was gradually activated during prolonged air exposure, while that of control and wild mussels was more inhibited. In July and August, during prolonged air exposure, the PK from the most viable mussles generally was the most inhibited. Prevention of air breathing did not markedly change the time course of PK inhibition during air exposure. Cultured mussels used in the present study were obtained from growers in Québec, and wild mussels were collected from the Baie des Chaleurs (Québec, PQ) in 1988.     

Mercury cycling in litter and soil in different forest types in the Adirondack region, New York, USA.

The fate of mercury in decomposing leaf litter and soil is key to understanding the biogeochemistry of mercury in forested ecosystems. We quantified mercury dynamics in decomposing leaf litter and measured fluxes and pools of mercury in litterfall, throughfall, and soil in two forest types of the Adirondack region, New York, USA. The mean content of total mercury in leaf litter increased to 134% of its original mass during two years of decomposition. The accumulation pattern was seasonal, with significant increases in mercury mass during the growing season (+4.9% per month). Litterfall dominated mercury fluxes into the soil in the deciduous forest, whereas throughfall dominated fluxes into the coniferous forest. The increase in mercury mass in decomposing deciduous litter during the growing season was greater than could be accounted for by throughfall inputs during the growing season (P < 0.05), suggesting translocation of mercury from the soil to the decomposing deciduous litter. This internal recycling mechanism concentrates mercury in the organic horizons and retards transport through the soil, thereby increasing the residence time of mercury in the forest floor. A mass balance assessment suggests that the ultimate fate of mercury in the landscape depends upon forest type and associated differences in the delivery and incorporation of mercury into the soil. Our results show that incorporation of mercury into decaying leaf litter increases its residence time in the landscape and may further delay the recovery of surface waters, fish, and associated biota following control of mercury emissions to the atmosphere.     

Mélange vertical et capacité photosynthétique du phytoplancton estuarien (estuaire du Saint-Laurent)

Time series of chlorophyll a, photosynthetic capacity and many physical parameters were sampled hourly for 167 h in August, 1975, at an anchor station located in the Middle Estuary of the St. Lawrence River, Canada. Sampling was carried out during the transition from neap tides to spring tides. The long-and short-term variations in chlorophyll a are coupled with the advection of water masses which depends on tidal currents. Vertical mixing also influences the chlorophyll a concentration of the cells, since it modifies the physiological state of the phytoplankton. Furthermore, circadian periodicities were observed in the photosynthetic capacity, suggesting that the phytoplankton of this area have a homogeneous light history due to strong vertical mixing. Under these conditions, the photosynthetic capacity is adapted to the mean light intensity in the mixed layer. The semimonthly (M _f) variations of the mean light intensity in the mixed layer depend on the M _f variations in the vertical mixing, whereas in the short-term, the variations in mean light intensity in the mixed layer are circadian.

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Mélange vertical et capacité photosynthétique du phytoplancton estuarien (estuaire du Saint-Laurent)

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Contribution au programme du Groupe interuniversitaire de recherches océanographiques du Québec (GIROQ)     

Who’s where? Ecological uncertainty shapes neophobic predator avoidance in Trinidadian guppies

Behavioral Ecology and Sociobiology - Behavioural diversity is a basic component of biodiversity, with implications in ecological interactions at the intra- and interspecific levels. The...     

Small-scale gradients of phytoplankton productivity in the littoral fringe

Inshore-offshore transects and time-series sampling programs were carried out on the South shore of the St. Lawrence Estuary on several occasions during the summer season of 1978, 1979 and 1986. In 1981, a time-series sampling program was also conducted on three occasions during the summer. The sampling program was carried out to test the hypothesis that higher phytoplankton biomass and productivity occurs in the littoral zone than in the offshore zone. Our results showed consistently higher nutrient concentrations and lower seasonal variability in the littoral zone than offshore. However, biomass indicators (chlorophyll concentrations and phytoplankton cell counts) showed lower values nearshore than offshore, in contrast to primary production and photosynthetic capacity which were higher nearshore than offshore. These differences are interpreted with reference to the grazing activity of benthic filter-feeders, which probably helped to reduce the phytoplankton biomass in the littoral zone, while at the same time, they contributed to the rapid recycling of nutrients, thereby allowing much higher phytoplankton productivity nearshore than offshore. These results are in keeping with the ecological concept suggesting that exploitation can have a rejuvenating effect on an ecosystem which often is translated into enhanced productivity.     

Freedom Space for Rivers: A Sustainable Management Approach to Enhance River Resilience

River systems are increasingly under stress and pressure from agriculture and urbanization in riparian zones, resulting in frequent engineering interventions such as bank stabilization or flood protection. This study provides guidelines for a more sustainable approach to river management based on hydrogeomorphology concepts applied to three contrasted rivers in Quebec (Canada). Mobility and flooding spaces are determined for the three rivers, and three levels of “freedom space” are subsequently defined based on the combination of the two spaces. The first level of freedom space includes very frequently flooded and highly mobile zones over the next 50 years, as well as riparian wetlands. It provides the minimum space for both fluvial and ecological functionality of the river system. On average for the three studied sites, this minimum space was approximately 1.7 times the channel width, but this minimum space corresponds to a highly variable width which must be determined from a thorough hydrogeomorphic assessment and cannot be predicted using a representative average. The second level includes space for floods of larger magnitude and provides for meanders to migrate freely over a longer time period. The last level of freedom space represents exceptional flood zones. We propose the freedom space concept to be implemented in current river management legislation because it promotes a sustainable way to manage river systems, and it increases their resilience to climate and land use changes in comparison with traditional river management approaches which are based on frequent and spatially restricted interventions.     

Does radioactive contamination affect the shell morphology of the pond snail <Emphasis Type="Italic">Lymnaea stagnalis</Emphasis> in the exclusion zone of the Chernobyl NPP (Ukraine)?

A distinct external shell abnormality, coarse seam, was found in a high percentage of the pond snail Lymnaea stagnalis (L.) collected near Chernobyl Nuclear Power Plant (NPP), Ukraine. The negative cytogenetical and hematological effects of long-term irradiation on pond snails in the Chernobyl Exclusion Zone have been repeatedly reported; consequently, it has been assumed that radioactivity may also affect shell morphology. However, due to the absence of this shell abnormality in pond snails collected from two radioactive sites in Russia, it can be concluded that the appearance of abnormal shells in snail population near Chernobyl NPP cannot directly be explained by the radioactive contamination.     

Alteration of shell nacre micromorphology in blue mussel Mytilus edulis after exposure to free-ionic silver and silver nanoparticles

This study describes the morphology of inner shell surface (ISS) of the blue mussel Mytilus edulis Linnaeus after short-term exposures to radiolabeled silver in free-ionic (^110mAg⁺) and engineered nanoparticulate (^110mAgNPs, <40 nm) phases. Radiolabeled silver in starting solutions was used in a similar low concentration (∼15 Bq mL⁻¹) for both treatments. After exposure experiments radiolabeled silver was leached from the ISS using HCl. It concentration for shells from both treatments was ∼0.5 Bq mL⁻¹. Whole ISS of young individuals and prismatic layer of adults showed no evidence of any major alteration process after silver uptake. However, the nacre portion of adult mussels exposed to both treatments revealed distinct doughnut shape structures (DSS) formed by calcium carbonate micrograins that covered the surface of aragonite tablets. Scanning electron microscope (SEM) imaging revealed the existence of only minor differences in DSS morphology between mussels exposed to Ag⁺ and AgNPs. From literature survey, DSS were also found in bivalves exposed to Cd²⁺. The DSS occurring in a specimen of a field-collected bivalve is also shown. Formation of distinctive DSS can be explained by a disturbance of the shell calcification mechanism. Although the occurrence of DSS is not exclusively associated with metal bioavailability to the mussels, the morphology of DSS seems to be linked to the speciation of the metal used in the uptake experiments.