Trophic substances

Also among recent discoveries in brain chemistry are the so-called trophic substances, which are believed to be secreted front nerve terminals. One of these is nerve gnnvrfi factor (NGF). It has been established that... 

Biomagnincation- Increase in the concentration of a chemical substance as it is progresses to higher trophic levels of a food chain. 

A. I. Popov and O. G. Chertov, On humic substances as a direct nutritive component of plant-soil trophic system. The Role of Humic Substances in the Ecosystems and in Environmental Protection ( J. Drozd, S. S. Gonet, N. Senesi, and J. Weber, eds.), Polish Society of Humic Substances, Wroclaw, 1997, p. 993. 

In a first attempt to derive characterization factors with QSARs, the entire dataset of plastics additives was included, and aquatic ecotoxicity was predicted for two different trophic levels. This generated characterization factors that did not correspond well with the ones derived from experimental data [30]. Hardly surprising, but a clear indication that two trophic levels are unsufficient. A second attempt to derive characterization factors with QSARs are currently being performed [31]. In this second attempt, substances that are difficult to model in QSAR models have been removed from the dataset and the ecotoxicity has been predicted for three different trophic levels instead of two. However, results have not yet been obtained from this second attempt. If the results show that it is possible to derive reliable characterization factors by the use of QSARs, the current data gap regarding characterization factors for human toxicity and ecotoxicity could be... 

It has been shown that these substances, and especially PCBs, can occur in elevated concentration in Arctic fauna. As the diet of many Arctic populations relies to a vast extent on marine mammals that represent high trophic levels, human exposure has been shown to be considerably high compared to industrialized areas. 

Food Chain Bioaccumulation. Because of the rapid hydrolysis of HDI in water and the ease with which this substance is metabolized in higher trophic animals (see Section 2.3), it is not expected that this substance will bioconcentrate in aquatic organisms, or bioaccumulate in the food chain. Neither TDI and MDl, nor their diamine hydrolysis products, TDA and MDA, have been found to bioaccumulate in fish (Cyprinus carpio) in river model studies (International Isocyanate Institute 1990). No information on BCFs and food chain bioaccmnulation could be foimd for HDI in the available literature however, a BCF of approximately 100 was calculated using the method of Veith et al. (1979), which indicates a very low bioaccumulation potential for HDI. Further studies on the bioaccumulation of HDI do not appear to be warranted. 

To increase the reliability of assessing the role of the World Ocean in the global carbon cycle, a more detailed description is needed of the production processes in ocean ecosystems. Along with the physical and chemical processes of transformation and motion of carbon in the ocean medium, the biological processes play an important role. In particular, phytoplankton, just like the nutrient elements, assimilates dissolved C02 from saltwater. As a result, an organic substance is formed that partially goes to the nutrient chains of the trophic pyramid of a given basin of the World Ocean and partially descends to bottom sediments. The totality of all the... 

Thus far, quality objectives for chemical substances are derived from the most sensitive organisms in acute and chronic toxicity test batteries that determine NOEC values for different trophic levels. The pT-method similarly determines specific sample dilution levels that are devoid of adverse effects toward (micro)organisms of a standardized test battery. Common to both approaches is the more frequent use of water-column test organisms as opposed to benthic-dwelling organism that reflect more intimate contact with sediment. This practice is primarily based on the fact that standardized bioassays capable of appraising sediment porewaters and elutriates are presently more numerous than solid-phase tests for whole-sediment assessment. As more of these latter tests become developed and standardized (see Chapters 12 and 13, volume 1 of this book on amphipod and chironomid tests), their more frequent use will contribute to a better understand of the toxic effects of sediment-bound contaminants. 

Such a procedure carries the risk of underestimating the toxicity of investigated substances with regard to an entire ecosystem. It is important that toxicity tests be conducted simultaneously with several bioindicators, that is, with a battery of bioassays characterized by different sensitivities and representing different trophic levels.19 Such an approach is often applied in research on environmental samples, which are usually complex mixtures of compounds with unknown physicochemical properties. 

---