Water column phytoplankton, effect

Comparison with other Studies. How do the results of our investigation compare with similar studies Our results corroborate the data provided in a similar study of the effect of UV-B on primary productivity in the southeastern Pacific Ocean (35). In the latter study, it was noted that enhanced UV-B radiation caused significant decreases in the productivity of surface and deep samples. Compared to ambient, primary productivity decreased with increasing doses of UV-B. In another study in which in situ experiments using natural Antarctic phytoplankton populations, it was noted that incident solar radiation significantly depressed photosynthetic rates in the upper 10-15 meters of the water column (36). It was also found that the spectral region between 305 and 350 nm was responsible for approximately 75 percent of the overall inhibitory effect. 

This scenario should be considered as a maximum effect of the hypolim-netic contribution because it does not include regeneration of 15N in the hypolimnion from sinking phytoplankton and the subsequent transport of this 15N back into the upper water column. Clearly, this kind of dilution is insufficient to account for the slow labeling of POM at the start of the experiment. 

The accumulation of hydrophobic contaminants in phytoplankton plays a significant role in the transport and fate of these potentially toxic compounds. However, the limited amount of available field data indicate that partitioning models fail to adequately predict the distribution of these compounds in the water column. Several hypotheses have been proposed to explain these differences. In this chapter we propose additional explanations for these differences. We hypothesize that assumptions in the partitioning model about the rate of uptake, mechanism of uptake, and effect of phytoplankton growth also contribute to these deviations. 

PAHs enter the environment from both natural and man-made sources, and the anthropogenic point and nonpoint sources are the major sources. The nonpoint sources are diffuse sources disseminated through the air and waterways. In aquatic systems, PAH-enriched particles or floes may settle to the lake s bottom under calm conditions and accumulate in the sediments. Once the PAH-enriched particles have accumulated in the lake s floor, they may undergo a number of changes that are mediated by chemical or microbial activities. As a result, the bound PAHs can be released from the sediment into the water phase. Once they enter the water column, they may also enter phytoplankton. The PAHs in phytoplankton may then bioaccumulate in the food web. This can cause both acute and chronic effects in fish, birds and other mammals that feed on aquatic organisms (Zhang, 1998). 

In order to establish the sources of DMS we have made measurements of dimethylsulphoniopropionate (DMSP the precursor of DMS) and attempted to relate the two compounds to phytoplankton species and abundance. We have also investigated variations in DMS and DMSP with depth through the water column, with respect to diel cycling and monitored the effect of high and low nitrate concentrations on DMSP levels in laboratory cultures and in the N. Sea. 

The ongoing research into such specific N fractions is critical to address the fundamental issue of isotopic alteration of exported organic N in the water column and sediments. This work also raises a host of new questions. Consider the case of diatom frustule-bound N (Fig. 34.7). The organic N of diatoms exported from the surface layer could differ from the integrated sinking N, for instance, if the isotope effect of nitrate assimilation by diatoms differs from that of the entire phytoplankton population. Moreover, the of N trapped and preserved within the diatom... 

The effect of phytoplankton bloom on Fe distribution during the austral summer in coastal areas was studied by Frache et al. (130). Measurements of dissolved and particulate Fe along vertical profiles in the Wood Bay (Ross Sea) were carried out on samples collected during the summer of 1993-1994. The authors did not present the result of each single profile, but reported the mean concentration of Fe through the water column before and after the ice pack melted (Figure 5.11). The metal concentration in samples collected in the first 10 m was 16 nM when pack ice was present the profile presented a minimum concentration of 6 nM at a depth of 50 m. After the ice melted the dissolved concentration in the first 10 m was reduced to a mean value of 8.4 nM. At the same time as the depletion of Fe in the dissolved phase an increase in Fe was detected in the particulate phase. The mean Fe concentration in the particulate in the first 10 m before the ice melt was 1.6 pg g after pack melt the mean value increased to 20 pg g. ... 

Table 1 is the compilation of pigment data obtained from analyses of the sediment trap samples. The data reveal, from the increase in the relative amounts of pheophytin-a and pheophor-bide-a, a large degree of Mg and Mg plus phytol loss, respectively. The deeper sample, upon microscopic examination, was found to contain a majority of broken, relative to intact, phytoplankton cells (diatom dominated) occurring mainly in fecal pellets. Thus, the change in the distribution of tetrapyrrole pigments, occurring with depth in the water column, reveals the combined effects of senescence/death and predation. The latter is evidenced by the increase in pheophorbide-a relative to pheophytin-a (37,40). 

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