Non-stationary covariance function modelling in 2D least-squares collocation

Standard least-squares collocation (LSC) assumes 2D stationarity and 3D isotropy, and relies on a covariance function to account for spatial dependence in the ob-served data. However, the assumption that the spatial dependence is constant through-out the region of interest may sometimes be violated. Assuming a stationary covariance structure can result in over-smoothing of, e.g., the gravity field in mountains and under-smoothing in great plains. We introduce the kernel convolution method from spatial statistics for non-stationary covariance structures, and demonstrate its advantage fordealing with non-stationarity in geodetic data. We then compared stationary and non-stationary covariance functions in 2D LSC to the empirical example of gravity anomaly interpolation near the Darling Fault, Western Australia, where the field is anisotropic and non-stationary. The results with non-stationary covariance functions are better than standard LSC in terms of formal errors and cross-validation against data not used in the interpolation, demonstrating that the use of non-stationary covariance functions can improve upon standard (stationary) LSC

Simultaneous Effects of Light Intensity and Phosphorus Supply on the Sterol Content of Phytoplankton

Sterol profiles of microalgae and their change with environmental conditions are of great interest in ecological food web research and taxonomic studies alike. Here, we investigated effects of light intensity and phosphorus supply on the sterol content of phytoplankton and assessed potential interactive effects of these important environmental factors on the sterol composition of algae. We identified sterol contents of four common phytoplankton genera, Scenedesmus, Chlamydomonas, Cryptomonas and Cyclotella, and analysed the change in sterol content with varying light intensities in both a high-phosphorus and a low-phosphorus approach. Sterol contents increased significantly with increasing light in three out of four species. Phosphorus-limitation reversed the change of sterol content with light intensity, i.e., sterol content decreased with increasing light at low phosphorus supply. Generally sterol contents were lower in low-phosphorus cultures. In conclusion, both light and phosphorus conditions strongly affect the sterol composition of algae and hence should be considered in ecological and taxonomic studies investigating the biochemical composition of algae. Data suggest a possible sterol limitation of growth and reproduction of herbivorous crustacean zooplankton during summer when high light intensities and low phosphorus supply decrease sterol contents of algae

Effect of lipid composition on the toxicity of trichlorobenzene isomers to diatoms. I. Short-term effects of 1,3,5-trichlorobenzene

Cultures of Cyclotella meneghiniana and Melosira varians were split into four flasks. Two flasks of each taxon were used for exposure experiments and two were used as controls. Exposure to 1,3,5-trichlorobenzene was initiated in one flask of each taxon in the 1 lth hour of the light period on a 16:8 h L/D regime. Another exposure was initiated four hours later in the same day (the 15th hour of the light period) in the second experimental flask of each taxon. These experiments were conducted at 15°C and 20°C for 2–4 days and were analyzed for lipid class composition. Cyclotella demonstrated some short-term changes in lipid class composition when exposures were initiated in the 11th hour of the light cycle at 20°C and in both exposures at 15°C. In contrast, short-term effects were only observed in Melosira when the exposure was initiated in the 11th hour of the light period at 20°C. Control cells demonstrated periodicities in lipid composition that appeared to be entrained with the light/dark cycle. The cultures that demonstrated toxicity effects were characterized by low triacylglycerol content, low neutral/polar lipid ratios, and a pattern of chlorophyll synthesis at the time of exposure. It is suggested that rapidly growing cells are more susceptible to short term effects of this trichlorobenzene isomer. Furthermore, timing of the initial exposure may alter toxicity results and this may be a consequence of lipid class composition at the time of exposure.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48079/1/244_2005_Article_BF01141353.pd