Aquatic systems trophic levels

Significant concentrations of cyanotoxins have been found to accumulate in the tissues of macroinvertebrates such as mollusks and crustaceans, presenting an indirect route of exposure for invertebrates, fish, and aquatic mammals at higher trophic levels (Negri and Jones 1995). In natural systems, mortality among benthic invertebrate herbivores is probably low because most bloom-forming bacteria are planktonic and only periodically come into contact with the benthos. Nevertheless, Kotak et al. (1996) determined that enhanced mortality of snails at the end of a bloom cycle in Canadian lakes was due to consumption of Microcystis cells that had formed a scum on the surface of macrophytes. Oberemm et al. (1999) found that aqueous microcystins, saxitoxins, and anatoxin-a all resulted in developmental delays in fish and salamander embryos. Interestingly, more severe malformations and enhanced mortality were observed when larvae were exposed to crude cyanobacterial extracts than to pure toxins applied at natural concentrations (Oberemm et al. 1999). 

Previous studies have found that cyanotoxic compounds may accumulate in sym-patric plants as well as in the tissues of herbivorous fish and invertebrates (reviewed in Zurawell et al. 2005). The accumulation of cyanotoxins at these trophic levels provides a direct path to both aquatic and, potentially, terrestrial consumers (Negri and Jones 1995 Kotak et al. 1996 Giovannardi et al. 1999). However, these compounds are rarely encountered in higher trophic levels in freshwater systems (Kotak et al. 1996 Zurawell et al. 2005). Nevertheless, attempts to minimize cyanotoxins in water bodies for recreational use should remain a major focus of environmental and public health managers, especially in light of the evidence that low doses may still have sublethal effects on the larval development of aquatic vertebrates (Oberemm et al. 1999). 

Terrestrial Detritus. Variability in ecosystem response to fertilization may be attributed in part to the interaction of aquatic and terrestrial ecosystems. In contrast to the many aquatic ecosystems in which higher trophic levels are fueled almost entirely by organic matter originating in the water column, other systems are driven by inputs of particulate and dissolved organic matter from land. The importance of this land-water interaction in regulating system metabolism has been obvious to stream ecologists for some... 

Cottus cognatus), deepwater sculpin (Myoxocephalus thompsoni)], and lake trout. Lower food web components were collected from 11 locations in the lake on seven different cruises, including all seasons over a 2 year period. The fish were collected from three sites over the same 2 year period. The concentrations of PCBs in each of these food web components are shown in Fig. 14. The data are lipid-normalized to allow for better comparison across trophic levels. Note that concentrations increase with increasing trophic level, even when normalized to lipids. These data demonstrate that PCBs biomagnify in Great Lakes food webs, and are the most comprehensive and self-consistent data in the world for PCBs in a food web of a large aquatic system. 

Once produced PAHs can be transported through the atmosphere or the water column if directly discharged via uncombusted petroleum. In the air, PAHs partition between the gas and particle phases, can undergo photochemical and oxidation reactions, be washed out by precipitation and deposit to aquatic surfaces by both wet and dry deposition. Once in the aquatic system, PAHs partition between the dissolved and particulate phases, can undergo photochemical reactions and bioaccumulate in the lower trophic levels. 

The levels of uranium in aquatic organisms decline with each successive trophic level because of very low assimilation efficiencies in higher trophic animals. Bioconcentration factors measured in fish were low (Mahon 1982 Poston 1982 Waite et al. 1988) and were thought to arise from the extraction of uranium from the water or simply from the accumulation of uranium on gill surfaces (Ahsanullah and Williams 1989). In plants, uptake of uranium may be restricted to the root system and may actually... 

Overall, these results suggest a general pattern of carbon flow in lakes on the central Amazon floodplain. Nutritious plant materials, primarily derived from C-3 plants (phytoplankton, periphyton, C-3 macrophyte leaves, tree fruits, and seeds) are selectively consumed by aquatic herbivores and detritivores and dominate the organic carbon flow to higher trophic levels. Some C-3 plant materials and the bulk of C-4 plants decompose and release their organic carbon, predominantly in the dissolved form, to the water column. The labile component of this DOC, dominated by C-4 plant carbon, is rapidly consumed by heterotrophic bacterial communities and released as either CO2 or CH4 to the atmosphere. The more refractory DOC component has a much slower turnover rate and tends to persist in the system where it is eventually exported to the main river channel. 

These are primary producers in aquatic systems and therefore play a key role in the food chain and in the transport of xenobiotics into higher trophic levels... 

Finally, data indicate that di- -butyl phthalate can partition from food and water into a variety of organisms. Studies using radioactively labeled di- -butyl phthalate have shown accumulation of radioactivity in aquatic invertebrates (Sanders et al. 1973) and fish (Wofford et al. 1981). Most of the accumulated radioactivity is apparently in the form of the primary metabolite, mono- -butyl phthalate (Howard 1989). Numerous experiments have shown that the accumulation of di- -butyl phthalate in the aquatic and terrestrial food chain is limited by biotransformation (i.e., transformation of chemical compounds within a living system), which progressively increases with trophic level (Staples et al. 1997). 

Jackson, T.A. 1986. Methyl mercury levels in a polluted prairie river-lake system seasonal and site-specific variations, and the dominant influence of trophic conditions. Canad. Jour. Fish. Aquat. Sci. 43 1873-1887. 

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