Ecophysiology of Indus Delta Mangroves

Abstract

Mangroves are characteristically present in the delta of rivers, estuaries, and sea coast. Mangrove tidal swamp forest occurs on the coast of both Baluchistan and Sindh provinces, in Pakistan. Whilst their occurrence in the former is largely restricted to Porali river delta, to the north west of Karachi, the Indus delta mangroves represent the sixth largest mangrove block worldwide and, in 1983, occupied some 410,000 ha of virtually the entire Sindh coast. The present mangrove forest is almost entirely composed of Avicennia marina (Known locally as timer), with few small populations of Ceriops tagal. Seventy Five percent of the total Indus delta mangrove forest are under severe grazing pressure and widely degraded. Excessive browsing by camels, lopping by inhabitants of coastal villages for cattle fodder and substantial decrease in Indus discharge due to diversion of river water to irrigation system, have contributed to severe degradation. Growth and from the mangroves of the Indus delta is generally extremely poor. There are numerous gaps in the forest, trees are commonly stunted and bushy and stems are often crooked and hollow. The significance of mangrove lies in its economic importance. The product of mangroves include the mud itself, used fertilizer in India, potash, food fodder, fiber, medicine, pulp for paper, matches, thatching material and charcoal. More important than material products are the secondary benefits from mangrove such as protection against erosion, cyclones, tidal bores, and production of particulate and dissolved organic matter that enrich Down Stream fisheries in coastal waters. Considering the importance of mangrove it is imperative that serious attempts be made to protect and rehabilitate the threatened Indus delta mangrove ecosystem. Present investigation is the initial information gathering exercise to understand mechanism of osmoregulation (how they survive in salinity) of Indus delta mangroves. Propagules of Avicennia marina, Rhizophora mucronata and Ceriops tagal were grown in pots containing sandy soil and treated with nutrient solution. They were allowed to establish into young seedlings for 6 months. The young plants were then treated with nutrient solution containing 0, 25, 50, 75 and 100% sea water fortified with nitrogen. Growth and ecophysiological parameters were recorded, six months and one year after induction of salinity (i-e at both seedling and sapling level). Growth was greatly stimulated in 50% seawater which proved to be the optimum salinity for both species. Dry matter accumulation in shoot and root was also enhanced in 50% seawater. Increasing salinity decreased tissue water and osmotic potentials and stomatal conductance in all three species. Measurements showed that young leaves had more negative water and osmotic potentials than the old ones at all salinity levels. The accumulation of proline was more pronounced in shoots than roots. Proline content was minimum in 0% salinity in both species and maximum in 100% suggesting the intra-cellular osmotic adjustment in response to salt stress. It was not however sufficient enough to act as osmolyte like glycine betaine and sugar alcohols. Total oxalate, acid and water soluble oxalate decreased with increase in salinity. This indicate probably little role of Oxalates in osmoregulation. Soil electrical conductivity in the field (Sandspit and vicinity) increased during the dry months (October to March). Despite the increase in soil salinity water and osmotic potential in plants dropped to less negative from more negative with decrease in ambient temperature. Stomatal conductance was also low during the dry and relatively cold months. Our data indicate that: 1.The growth of mangroves was very slow especially in high salinity. 2. Aviceena marina is more salt tolerant of the 3 mangrove species studied. 3. Optimum growth in 50% indicates that mangroves thrive best where fresh water mixes with seawater. 4. Plant produce proline as an osmoregulator to tolerate high level of salinity along with other compatible solutes. 5. A. marina always maintains a more negative water and osmotic potential, whereas in C. tagal and Rhizophora mucronata water and osmotic potential become progressively more negative with the increase in salinity stress. 6. Low xylem tension indicate that both species are excluding salt but A.marina is also secreting salts. 7. Rhizophora mucronata and A. marina as compared to C. tagal can open stomata more even at very high salinity perhaps due to the maximized leaf area. These results inducate that A.marina not only tolerate high salinity but also show a steady growth. R. mucronata also showed fast growth at the start of the experiment. It seems that in arid environment A.marina is a better choice for rehabilitation. R. mucronata may also be introduced in the threatened mangrove ecosystem as it also shows good results but we will have to bring the seedling of this species all the way from Miani Hor (Sonmiani) near Porali River to the Indus Delta.