Info

Tissue cyclitol concentration (mM)

in pea. Another interesting observation is that polyols, which are the dominating compatible solutes in mangroves (Box 7.1) may have an additional function as effective radical scavengers (Orthen et al. 1994). Li and Ong (1998) and Sun et al. (1999) studied this in the gametophytes of the mangrove fern Acrostichum aureum. Polyol concentration was strongly correlated with Na+ accumulation in the tissue as determined by substrate salinity (Fig. 7.21A). This involves hardening to salinity stress as it is seen when the gametophytes grown at up to 150 mM NaCl are transferred to 340 mM NaCl for two days and then, Fv/Fm is measured at an irra-diance of 400 |molm-2s-1 following 30min dark adaptation (Fig. 7.21B) where high Fv/Fm is correlated with cyclitol content (Fig. 7.21C).

Different strategies in niche occupation are also involved. Lovelock and Clough (1992) give an example for mangroves of the Daintree River in Australia (17° S, 147° E). In Rhizophora there is stress avoidance as leaves are oriented nearly vertically and thus reduce light absorption. Bruguiera parviflora has small horizontal leaves, which are rich in xanthophylls functioning in dissipation of excitation energy. Larger horizontally arranged leaves of Bruguiera gymnorhiza tend to heat up more strongly and are therefore more subject to photodamage. Thus, B. parviflora dominates the canopy, whereas B. gymnorhiza is less abundant at the top of the canopy.

7.5.4 Interacting Factors: Salinity, Irradiance, Elevated CO2

Environmental factors can interact. Salinity and irradiance stress may be additive, e.g. between two sympatric mangrove species at saline sites Aegiceras corniculatum was found to be favoured where excess radiation was less frequent and Avicennia marina under conditions of persistent excess irradiance (Christian 2005). Very interesting findings on the interactions between salinity and irradiance were obtained in studies of seedling establishment and growth (Ball 2002). Mangrove seedlings need full sunlight and the formation of gaps in the mangrove forests is essential for regeneration. At full sunlight for most species there were no substantial differences in seedling survival after 12 months at low and high salinity except for 2 species Bruguiera parviflora and Ceriops australis. At 30% sunlight both species showed similarly high survival rates of seedlings at high and low salinity; however, at high irradiance they required high salinity in addition to the full sunlight. Obviously low salinity conditions induced sensitivity to high irradiance in these two mangrove species (Fig. 7.22).

Elevated atmospheric CO2-concentration, pCo, modulates water use and carbon gain and one might expect that it affects salt tolerance of mangrove trees. However, a comparative study of two mangrove species differing in salt tolerance, i.e. Rhizophora apiculata and Rhizophora stylosa, showed that when relative growth rates were limited by salinity, i.e. 350 mM as compared to 125 mM NaCl at the root level, pCO2 elevated from 340 to 700 ppm had little effect on the growth rates. However, elevated pCa O stimulated growth when it was limited by air humidity,

Fig. 7.22 Survival of seedlings of Bruguiera parvi-flora (triangles) and Ceriops australis (circles) at low and high salinity, respectively, under full sunlight (A) and in the shade at 30% full sunlight (B), respectively. (After data of Ball 2002)

Fig. 7.22 Survival of seedlings of Bruguiera parvi-flora (triangles) and Ceriops australis (circles) at low and high salinity, respectively, under full sunlight (A) and in the shade at 30% full sunlight (B), respectively. (After data of Ball 2002)

i.e. at 43% as compared to 86% relative air humidity. Thus, elevated p£02 in the future could modify competitive potentials of different mangrove trees along salinity x aridity gradients, but it is unlikely that it will allow mangroves to expand into areas with salinities much more extreme than currently tolerated (Ball et al. 1997).

7.6 Nutrition

Mineral nutrition of mangroves is much determined by decomposition of litter (Mfilinge et al. 2002; Ochieng and Erftemeijer 2002) and the activity of microbial mats (Sect. 7.7.2). Mangroves may occasionally suffer phosphorus and nitrogen limitation especially in dwarf mangrove formations further inland of zonations (Cheeseman and Lovelock 2004; Parida and Das 2004; Lovelock et al. 2006a, b). Phosphorus deficiency inhibits water transport and hampers water relations of man groves (Lovelock et al. 2006b, c). High salinity inhibits uptake and reduction of nitrate (Pariada and Das 2004), and nitrogen fertilization may stimulate growth (Kao et al. 2001; Yates et al. 2002). However, other stress factors are dominating so that mineral supply is not very likely to become the limiting factor (Yates et al. 2002; Alongi et al. 2003). An interesting ion is K+ because under salinity stress when Na+ is accumulated K+ levels are normally reduced and this can lead to ion imbalances with adverse effects also in mangroves (Naidoo et al. 2002). However, since seawater has a K+-concentration of 10 mM this may not be a particular problem of stress in mangroves (Cram et al. 2002).

7.7 Aquatic Communities 7.7.1 Macroalgae in Mangroves

Macroalgae in mangroves grow between the roots but mainly epiphytically on the pneumatophores and trunks of trees (Post 1963). Species diversity is mainly given by red algae of the genera Bostrychia, Caloglossa and Stictosiphonia, although brown algae may also occur, e.g. mats of Hormosira banksii in SE-Australia (Karsten 1995). About 15-20% of the total biomass of mangrove communities is represented by these macroalgae (Karsten 1995).

The macroalgae are subject to the same stress conditions and even more so than the woody mangrove plants, e.g. changing salinity and desiccation at low tide. They show a broad salinity tolerance between 1/5- and 2/1-strength sea water (Karsten and West 1993). Some mangrove algae are also desiccation tolerant (Biebl 1962; see Sect. 11.4), e.g. Stictosiphonia arbuscula can lose up to 95% of its tissue water and recover within several hours when rewetted (Karsten 1995). Accumulation of compatible solutes (Sect. 7.4) plays a large role in the red algae in response to salinity and shows a rather high chemical diversity including floridoside, dige-neaside, D-sorbitol, D-dulcitol, D-mannitol and isethionic acid (Box 7.1) (Karsten 1995, 1996; Karsten et al. 1995a,b, 1997a,b). Different compatible solute spectra have been found in Bostrychia tenuisissima from different geographic provenance in Australia, i.e. sorbitol plus dulcitol and sorbitol plus digeneaside, respectively, and this was controlled genetically (Karsten et al. 1995b). In the mangrove fern Acrostichum aureum the gametophyte uses D-pinitol and the sporophyte D-1- O -methyl-muco-inositol (Sun et al. 1999).

Low irradiance is an important factor limiting the growth of macroalgae in mangrove forests. While the canopy of the trees may receive photosynthetic photon flux densities (PPFD) up to 2,500|imolm-2s-1, the algae may not obtain more than 60-100 |molm-2s-1. This is not only due to shading by the trees but also to turbid water with organic materials and debris (Karsten 1995). Bostrychia sim-pliciuscula and species of Caloglossa may still show positive relative growth rates at the very low PPFD of 2.5 |molm-2s-1 (Karsten and West 1993; Karsten et al. 1994).

7.7.2 Microbial Mats

Microbial mats are an essential aspect of mangrove ecosystems. The surface muds are zones of net heterotrophy. Light limitation beneath mangrove forests might mean that photosynthesis by benthic microalgae only makes a minor contribution to primary productivity (Alongi 1994), but Karsten (1995) arrives at the estimate that microbial mats account for 5-20% of the total mangrove productivity. They are largely composed of diatoms, cyanobacteria, sulphur bacteria, purple sulphur bacteria and sulphate reducing bacteria (Table 7.4). Their thickness as given in Table 7.4 is 10-12 mm but can also be as much as 80-120 mm (Hussain and Khoja 1993). Root associations of mangroves with halotolerant N2-fixing bacteria have been shown to improve N-supply and to contribute to the high productivity of mangrove ecosystems (Zuberer and Silver 1978, 1979; Sengupta and Chaudhuri 1991).

The main stress factors, as for the other mangrove communities are large varying amplitudes of salinity, irradiance and desiccation. Cyanobacteria use glycosylglyc-erol as compatible solute (Karsten 1996) and form scytonemin, pterins and myco-sporine-like amino acid compounds as ultraviolet light protectives (Karsten et al. 1998; see Sect. 11.2.1.2).

Table 7.4 Layers in microbial mats of mangroves (after Karsten 1995)

Thickness from top (mm) Organisms

0-2 Fine sand plus diatoms

2-4 Cyanobacteria

4-6 Sulphur bacteria

6-8 Purple sulphur bacteria

8-11 Sulphate reducing bacteria

7.8 Mangroves as Endangered Ecosystems with Numerous Benefits for Man and the Need for their Conservation

Mangroves are among the most endangered ecosystems on earth (Springer 2002). They have been frequently considered to be useless and are disappearing rapidly. However, they are very unique, and with their characteristic physiognomic beauty they are among the outstanding natural heritages we have. Moreover, they have numerous direct benefits for us (Oo 2002). They are pioneer communities at the interface between sea and land and stabilize coastlines. Some mangrove trees provide useful wood for fuel and the production of charcoal and particularly resistant timber for construction purposes. They provide fodder and medicine. They serve as nursery grounds for breeding of marine life, for fish and crabs of coral reefs, and sustain the economical basis of coastal fisheries (Ellison 2002). They are also used for establishing ponds for the culture of fish and prawns.

Mangroves are among the most productive ecosystems of the world. If we take the productivity of macroalgae as 15-20% and that of microbial mats as 5-20%, the productivity of trees would be 80-60%. This underlines the diversity of the mangrove communities.

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