Observation of two-photon speckle patterns.

We report the observation of speckle patterns in quantum correlations within light that is scattered by a disordered medium. The random medium is illuminated with spatially entangled photon pairs, and fourth-order speckle patterns are spatially resolved by two independently scanning detectors. Spatial entanglement gives two-photon speckle a much richer structure than ordinary one-photon speckle. Our experiments demonstrate that two-photon speckle from a surface scatterer and a volume scatterer look entirely different.

Optical characterization of periodically-poled KTiOPO4.

We demonstrate how the Maker fringes that are observable in spontaneous parametric down-conversion (SPDC) give a direct visualization of the poling quality of a periodically-poled crystal. Identical Maker fringes are observed in the optical spectrum of collinear SPDC and the temperature dependence of second harmonic generation. We analyze these Maker fringes via a unified treatment of the tuning curve in crystals with small and slowly-varying deformations of the poling structure. Our theoretical model, based on a Fourier analysis of the poling deformations, distinguishes between duty-cycle variations and variations of the poling phase. The analysis indicates that the poling phase is approximately fixed, while the duty-cycle typically varies between 36% and 64%.

Chronic ecotoxicity of mixtures of Cu, Zn, and Cd to the zebra mussel Dreissena polymorpha.

Organisms in contaminated freshwater ecosystems are often exposed to a variety of toxicants for their entire lifetime. To evaluate the ecological consequences of these long-term contaminations, the effects of mixtures of heavy metals on the filtration rate and survival of the freshwater mussel Dreissena polymorpha were studied during chronic exposure. In laboratory experiments, mussels were exposed to equitoxic mixtures of Cu + Zn, Cu + Cd, Zn + Cd, and Cu + Zn + Cd in concentrations causing a 50% decrease in filtration rate in short-term (48 hr) experiments. The filtration rate was measured once a week, during a 9- to 10-week exposure period. For all metal combinations effects on mortality increased when exposure time was prolonged from 48 hr to 9-10 weeks. In contrast, the effects on filtration rate did not increase, indicating that the filtration rate was related to the metal mixture concentration in the water, but not related to the metal concentrations in the mussels. Consequently, the effects on mortality and filtration rate were not related. In short-term experiments Cu + Cd were more than concentrations additive, whereas in chronic experiments Cu + Cd were strongly less than additive, indicating a loss of potential for additivity during prolonged exposure. In general, Cu, Zn, and Cd did not affect each others uptake. It was concluded that the chronic effects of mixtures could not be predicted from their short-term effects nor from the chronic effects of the metals tested individually.

Chronic ecotoxicity of copper and cadmium to the zebra mussel Dreissena polymorpha.

In order to evaluate ecological consequences of the long-term presence of metals in aquatic ecosystems, we investigated the filtration rate and survival of zebra mussels (Dreissena polymorpha) during chronic exposure to Cu and Cd. The filtration rate was measured once a week in laboratory experiments lasting 9-11 weeks. The lowest Cu concentration tested (13 micrograms/L) did not affect the filtration rate and survival of D. polymorpha, but the lowest Cd concentration (9 micrograms/L) did affect the filtration rate, but had no effect on survival. The EC50 for Cd decreased markedly from 388 micrograms/L to 27 micrograms/L when the exposure time was lengthened from 48 hours to 10 weeks. The largest decrease in EC50 for Cd was observed during the first week of exposure. In contrast, the EC50 for Cu did not decrease with increasing exposure time (chronic EC50: 43 micrograms/L). Since the chronic LC50 for Cd was 130 micrograms/L, the filtration rate appeared to be a far more sensitive endpoint for ecotoxicological laboratory experiments than mortality. D. polymorpha was capable of regulating the body concentration of the essential metal Cu at low concentrations in the water (13 micrograms/L). Cd was accumulated at every Cd concentration in the water, suggesting that Cd could not be regulated by D. polymorpha. It is concluded that the relation between short-term and long-term ecotoxicity was different for each metal and could not be predicted from the results of the short-term experiments.

Biomonitoring of heavy metals in the Western European Rivers Rhine and Meuse using the freshwater mussel Dreissena polymorpha.

Since 1976, active and passive biological monitoring programs using the freshwater mussel Dreissena polymorpha have been carried out to study trends in the bioavailability of heavy metals in the rivers Rhine and Meuse. The Cd concentration in mussels exposed in the river Rhine has decreased from 74 mg kg(-1) in 1976 to 1.5 mg kg(-1) in 1988. In the river Meuse, however, the Cd concentration in Dreissena polymorpha has increased from 6 mg/kg (-1) to 22 mg kg(-1). Cu concentrations in mussels have not changed. Comparison of heavy metal concentrations in mussels from Lake Heerhugowaard, Lake Markermeer, Lake Maarsseveen and the river Dieze demonstrated that background concentrations for Cu, Zn, Cd and Pb in Dreissena polymorpha are about 12, 110, 1 and 0.5 mg kg(-1), respectively. Mussels from these four locations showed seasonal variation in heavy metal concentrations. This may be caused by both changes in the bioavailability of the metals and by the annual growth and reproductive cycle of the mussels. Therefore, one should take care that animals are collected at the same location and in the same section in long-term active biological monitoring programs.