THE DYNAMIC FUNCTIONING OF MANGROVES

Mangrove ecosystems are essentially tropical to subtropical ecosystems, structurally dominated by trees and shrubs, some herbaceous plants and vines with associated biota. Mangroves predominantly occur along coastal areas and inhabit the fringes of estuaries (Nybakken 2005). Mangroves, seagrasses and coral reef tropical ecosystems are discrete ecosystems frequently occurring in close proximity to one another and interact with one another through the exchange of energy in the form of dissolved organic matter and faunal migration (Kitheka 1997).

Factors needed for the development of mangroves

Mangroves occur on soft, muddy, dark substrata that are frequently waterlogged, creating an anoxic environment due to reduced interstitial circulation and high bacterial activity (Nybakken 2005). These ecosystems occur in coastal areas or estuaries which are well protected from wave action. The latter explain why mangroves develop most extensively in regions behind coral reefs (Hogarth 1999). Reduced wave action allows for the settling out of fine silts and sediments which associated organic matter suspended in river inflows. In addition reduced wave action is required for the settling and establishment of new seedlings (Nybakken 2005). Mangroves are essentially facultative halophytes and have unique adaptations to cope with high salinities. Mangroves are terrestrial flowering plants that have reinvaded salt water and hence cannot survive in water of too high salinity. Being and estuarine ecosystem, mangroves experience continuous fluctuations in salinity with tidal action (Hogarth 1999). The distribution of mangrove forests is dictated by the relative sea surface temperature. They are mainly distributed within the winter position of the 20 °C isotherm (Nybakken 2005). However, these ecosystems may occur further south or north where currents bring warm water to the east coast of continents. Due to their sensitivity to freezing, mangroves do not extend into temperate habitats (Nybakken 2005). Mangroves require tidal action for their survival as the latter inundates roots with oxygenated salt water and replenishes nutrients. Tidal action prevents soil salinities from reaching lethal levels especially in areas with high rates of evapotranspiration (Nybakken 2005). The duration of tidal flooding dictates the degree of sedimentation in mangrove ecosystems. However, if root systems (aerial roots) are submerged for too long, mangroves can literally downed due to a lack of oxygen (Hogarth 1999).
Mangroves have adaptations that allows then to outcompete their terrestrial counterparts.

Mangroves are facultative halophytes, inhabiting stressful anoxic, saline environments because of their inability to compete with terrestrial freshwater angiosperms (Nybakken 2005). These ecosystems thrive in these seemingly stressful environments due to the acquisition of physiological, morphological and reproductive adaptations that allow them to cope with anoxia and osmotic problems.Many species of mangrove plants such as Bruguiera sp actively excrete salt via the roots, whereas other accumulate salt in older leaves which they later shed (for example Xylocarpus sp) (Hogarth 1999). Most mangroves have succulent sclerophylous leaves containing specialized water storing tissue. Alternatively as seen in Avicennia sp and Sonneratia sp, the leaves can contain salt exuding glands on the ventral and dorsal surfaces (Hogarth 1999). To reduce the osmotic gradients for the outward diffusion of water from the plant tissue, some mangrove species store amino acids in their internal fluid (Nybakken 2005). Rhizophora is less successful in preventing salt uptake, and as a consequence the internal salt concentrations may reach as high as 3 ‰, more than 100 times that of terrestrial plants (Branch and Branch 1981). Mangroves have a range of xeromorphic features in order to cope with the osmotic loss of water. These features include a thick leaf epidermal tissue layer, covered by a waxy cuticle. The leaves often contain fine hairs on the ventral surfaces and stomata are sunken (Hogarth 1999).
As adaptations to anoxic conditions most mangrove plants are shallow rooted with horizontal cable roots extending just below the mud surface. In Avicennea marina, vertical above ground pencil roots extend from the cable roots (Branch and Branch 1981). These pencil roots or pneumatophores contain lenticels which function in aeration of the roots (Nybakken 2005). Similarly Bruguera gymnorrhiza and Xylocarpus sp have knee roots and blade roots respectively, that branch from its cable roots and functions in gas exchange (Branch and Branch 1981). Rhizophora sp lacks cable roots but are shallowly anchored by a system of prop roots. In addition stem tendrils which extend from the braches or stem functions in gas exchange (Nybakken 2005). Sonneratia alba has above ground pneumatophores similar to that of Avicennia sp, however these are not pencil-like but can be more than 10 cm in diameter and are associated with fungal hyphae aiding in aeration and nutrient acquisition (Hogarth 1999). In most mangroves the section of the pneumatophores (or above ground root) penetrating the soil is often adapted with specialized aerenchyma tissue. Aerenchyma has a regular arrangement of air spaces in its interior called lacunae lending both floatation and aeration throughout the plant (Hogarth 1999).

Mangroves are able to optimize the dispersal and survival of their seedlings by being viviparous. In this way the seed germinates and develops into a seedling while still attached to the parent plant. The seedling is only released once sufficient roots have developed. Once released into the water column, the seedling floats with prevailing currents to a new location where it is able to settle and set roots in shallow waters (Nybakken 2005).

Mangroves and Coral Reefs are never found in the immediate vicinity of one another

Mangroves and coral reefs are both tropical ecosystems that are frequently found in close proximity to one another and are interconnected via the exchange of energy in the form of organic matter and animal migration during spawning and feeding (Kitheka 1997). However, these two discrete ecosystems are never found in the immediate vicinity of one another due to the fact that they thrive in contrasting physical environments.
The interaction between mangroves and coral reefs are not clear cut. Coral reefs stabilize the seascape by dissipating wave action and over geological time create areas protected from wave action, favouring the development of mangroves, while mangroves (and seagrasses ) act as chemical and physical buffers to the influence of land runoff on coral reef ecosystems (Birkeland 1997). Mangroves have the capacity to filter land runoff, removing terrestrial particulate and dissolved organic matter, trap and bind sediment, essentially promoting downstream coral reef growth. However, sporadic events such as Hurricane Andrew that hit the Florida Peninsula in August 1992, is a reminder of why these two ecosystems occur some distance from each other (Hogarth 1999). Heavy precipitation and wave action flushed large quantities of accumulated material from mangrove and seagrass sinks into downstream coral reef ecosystems (Birkeland 1997). Coral reefs are extremely vulnerable to sedimentation, eutrophication in the form of dissolved organic matter and fluctuations in salinity. Fine sediments and silts cause clogging of the mouth parts of coral polyps and hence prevent respiration subsequently leading to smothering of these polyps. The inflow of fine silts increases the turbidity of reef water, essentially decreasing the amount of light reaching corals and so doing decreasing the photosynthetic ability of obligate mutualistic zooxanthallae (Birkeland 1997).

Associated with mangrove sediment and mud is large amounts of dissolved organic matter. Coral reef ecosystems thrive in oligotrophic waters (Koop et al. 2001). Eutrophication increases the growth of opportunistic fleshy macro algae which outcompete corals for space and prevent coral larval settlement (McCook 1999). Increased levels of dissolved organic matter also lead to sporadic algal bloom events which further increases the turbidity of the reef water. Increased levels of dissolved nitrogen and phosphorous tends to disrupt the mutualistic relationship between zooxanthallae and coral polyps and as a result reduce coral calcification (Ferrier-Pages et al. 2000). Coral reefs are stenohaline and are not able to tolerate the fluctuation in salinity which is brought about when brackish mangrove derived water enter highly saline coral reef water.

The success of mangrove ecosystems is attributable to their ability to exclude stronger terrestrial competitors from the seemingly stressful habitats in which they occur. Mangroves have unique physiological, morphological and reproductive adaptations which allow them to thrive in conditions in which their terrestrial counterparts would be unable to. Mangroves interact with other tropical coastal ecosystems (seagrasses and coral reefs) via the exchange of dissolved organic matter and provide a spawning ground for many reef fish and invertebrates. Due to the catastrophic effect that disturbed mangroves can have on coral reefs, these two ecosystems are never found in the immediate vicinity of one another.

References

Birkeland C (1997) Life and death of Coral Reefs. Chapman & Hall, New York, USA. ISBN 0412035413, pp 536

Branch G, Branch M (1981) The living shores of Southern Africa. Struik Publishers, Cape Town. ISBN 0869771159, pp 272

Ferrier-Pages C, Gattuso J, Dallot S, Jaubert J (2000) Effect of nutrient enrichment on growth and photosynthesis of the zooxanthallate coral Stylophora pistillata. Coral Reefs 19: 103-113

Hogarth P (1999) The Biology of Mangroves. Oxford University Press, New York. ISBN 0198502230, pp 228