Halophytes of Salt Marshes

Many of the investigations of halophytes have been performed with plants from natural habitats that differ in terms of the ratios between important factors such as the degree of salinity and HM concentrations. Such differences are well manifested in river estuaries and coastal lagoons. Investigations in this direction were started some time ago and performed in several estuaries, such as the York River (USA) (Drifmeyer 1981), Ems Estuary salt marshes (Holland) (Otte et al. 1991), Suir Estuary salt marshes (Ireland) (Fitzgerald et al. 2003), and some others.

Relatively independent changes in the two parameters, HM concentration and salinity, are characteristic of such habitats, and this hampers any analysis of the action of each factor. Problems also arise due to the incomplete characterization of each of the locations where plant material was collected, and differences in plant species composition. Therefore, the most valuable reports are those where several plant species that inhabit the same region are compared.

Among the early studies, the investigation performed in salt marshes along the salinity gradient of the York River estuary (Drifmeyer 1981) seems characteristic. While a 3.5-4.5-fold salinity gradient was attained across the six locations tested, the levels of Mn, Cu, Zn, and Ni in Spartina alterniflora plants varied 2.5-3.5-fold, and the level of Fe by as much as ninefold. However, the authors concluded that, in plant tissues, "the levels of these metals ... did not correlate strictly with the salinity of either river or sediment pore water." In the absence of direct data on the metal content of the water, and due to the high probability that there were several sources of contamination across the rather large territory studied (more than 40 km along the coast), which had a dense human population, the author's conclusion that "... trace element uptake . is not greatly affected by salinity" can only be considered a preliminary one.

The work of Otte et al. (1991) presented information on the accumulation of Zn, Cu, and Cd in the roots and shoots of four plant species in salt marshes at two locations in the Ems Estuary, upstream (Dyksterhusen) and downstream (Petkum) of Ems (a difference of 2' N in the northerly direction). Rather representative are data for Triglochin maritima (Juncaginaceae) and two dicotyledonous species (Aster tripolium and Spergularia maritima, Cariophyllaceae), which grew under moderate salinity and strong HM pollution (Dyksterhusen) or heavy salinity and moderate HM pollution (Petkum). It turned out that, in the region of severe pollution (Dyksterhusen), all three HMs (Cd, Cu and Zn) accumulated to the greatest degree, whereas in the area with high salinity (Petkum), they reduced their accumulation in the roots but favored HM - especially Zn - translocation to shoots. As a result, the accumulation factor increased from 0.1 to 3.5 for Cu and from 0.4 to 6.3 for Zn (data for Cd were not significant). In contrast, the Zn and Cd contents in the roots of T. maritima and A. tripolium doubled in the region of high salinity (the effect of salinity on Cu was less pronounced or absent), but their translocation to shoots was suppressed.

More complete data were presented by Fitzgerald et al. (2003) for the Suir Estuary (Ireland). It turned out that, at four locations along the inner Suir Estuary that were analyzed in the study, Cu and Pb salt concentrations in sediment samples declined approximately twofold from the upper to lower point along the stream. However, there was no significant difference between the Cu concentrations in the roots of six tested plant species. Another pattern was observed for Pb. In two species, the highest Pb accumulation in the roots was detected mostly downstream (Checkpoint: the area with the least metal but with the highest salt concentration). In this area, the Pb content was 239.0 |imol kg-1 dry weight in Aster tripolium and 517.4 |imol kg-1 dry weight in Spartina spp., which was more than twice as high as in plants growing in other habitats. Correspondingly, the highest Pb concentrations in shoots were 360.5 and 290.5 |imol kg-1 dry weight, which also differed from those in plants growing in other habitats. Despite the fact that there were no significant differences between the Cu concentrations in the roots of all of the studied species at different locations in the estuary, the highest concentration in the shoots of Schoenoplectus tabernaemontani was observed immediately downstream of the site of pollution, significantly (threefold) exceeding the value for the unpolluted region; in Spartina spp, the highest Cu concentration was detected in the next region downstream

(fourfold higher than in the region located upstream). In general, the authors concluded that there was a common trend in the Cu and Pb contents of the two dicotyledonous species studied: the shoot/root ratio displayed a tendency to increase as the salinity increased. This trend was particularly evident for Pb in A. tripolium.

Only indirect data are available on the effects of salinity on HM accumulation by plant tissues in Tagus Estuary salt marshes (Portugal) (Reboreda and Cacador 2007; see references therein concerning the Tagus Estuary). Cu, Pb and Cd accumulation in the roots, stems and leaves of two plant species was investigated. Spartina maritima (Poales, Poaceae) plants inhabited the low marsh, and so were subjected to more severe salinity than Halimione portulacoides (Caryophyllales, Chenopodiaceae) plants, which inhabited the middle marsh. It was established that H. portulacoides accumulated twice as much Cd and Pb as S. maritima (taking into account the difference between the HM contents of the habitat sediments). Relative Cu absorption by the roots of S. maritima was significantly lower than that of the roots of H. portulacoides. However, under severe salinity, the Cu concentration in the leaves of this plant species was slightly higher than in H. portulacoides.

Among the studies of HM pollution in the Sheldt (Belgium) Estuary, Du Laing et al. (2008; see also references on the Sheldt Estuary therein) applied a special approach. Soil samples and sediments from four locations were flooded with waters containing different levels of salinity (0.5, 2.5, and 5 g l-1 NaCl for 250 days), and duckweed (Lemna minor) was grown (for 4 weeks) on the surface water. The salinity was found to primarily enhance the mobility of Cd, and its uptake by duckweed increased by as much as fivefold compared with the control. Moreover, the effect was also observed at a lower salinity. When the salinity of the flood water was increased from 0.5 to 5 g l-1 NaCl, the Cd concentrations in duckweed increased by a factor of 4. However, Zn concentrations in duckweed were only slightly enhanced by the salinity, while Ni uptake was not affected at all (excluding one treatment, where a large increase was observed). Although an effect of salinity on the total Cu concentration in surface water samples was not detected, the copper concentrations in the plants slightly (but significantly) increased.

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