chapter 10: mangrove forests and seagrass meadows

10.2 mangrove forests

10.2.1 what is a mangrove?

  • mangroves are woody trees or shrubs that flourish at sea/ land interface in sheltered tropical coastal and estuarine regions where fine sediment collects

  • can be divided into two categories:

    • true mangroves → they only occur in estuarine/tidal environments

    • mangrove associates → they only occur in e.g. rainforests, or more inland, but still waterlogged, and little to no salinity

  • three basic forms of mangroves based on shore morphology, sediment, and salinity:

    • riverine mangroves → they form in low tidal ranges and a dominance of freshwater flows, such as deltas of major rivers. This is applicable to most of the mangroves in asia

    • tide dominated mangroves → fully intertidal, often in full strength seawater and are subject to high wave action, also termed fringing mangroves

    • basin mangroves → occur to the landward side of fringing mangroves, where there are lower tidal currents and wave action, but where salinity can be high and variable due to evaporation and rainfall

10.2.2 where are mangroves found?

  • strictly tropical and their distribution across the globe coincides very closely with coral reefs, but also found in tropical regions where local conditions do not favour coral reef growth

  • diversity is highest int eh indo-west pacific

  • mangroves develop where shallow sloping, gnerally muddy shores are available within this region, especially lagoon, estuaries, and river deltas

  • fringing mangrove species are found at the sea edge, and across most of the world these pioneer tree tend to be species of Rhizophora

  • other true mangrove families inhabit conditions behind these fringing species

  • two main factors limit plant distribution:

    • increased salt levels

    • water logged sediment

10.2.3 how mangroves deal with living in a marine environment

  • on soft shore sediments, high salinity levels and water being maintained within the sediment are crucial to the survival of a marine organism, but both cause extreme problems for terrestrial palnts used to living on dry soil with minimum salt content

waterlogged sediment

  • the sediment is exceptionally low in oxygen

  • mangroves have three main morphological adaptations for this and they all involve the root structure that exit the sediment, enabling air to be taken in through pores called lenticles and keep the section of the root under the sediment oxygenated

    • aerial roots → leave the tree from up to 2 m and then penetrate the soil, giving the tree support

    • knee root → periodically break teh surface during growth; the aerial section is rich in lenticles

    • pneumatophores → vertical tubes emerging every 15 to 30 cm from horizontal roots (this section is under the water)

dealing with increased salt levels

  • exclusion of salt by the roots

  • tolerance of sat in tissue (much higher in mangroves than other plants)

  • secretion of excess salt through the bark or by shedding leaves

  • only taking water from the upper 50 cm of the soil (freshwater is less dense than salt water, and hence occurs in the upper layers of the soil in these systems)

  • mangroves cool their leaves by having it at an angle to the sun

10.2.4 how do mangroves reproduce

  • mangroves produce flowers which are pollinated by wind or by animal-led pollination

  • most mangroves demonstrate vivipary → following pollination, the embryo remains on the parent plant for up to months and only leaves as a fully developed seedling known as a propagule

  • the propagule drop into the water and float on top, developing roots. wherever the current/ water movement takes them, the roots eventually help teh seed either to lodge in the soil somewhere and grown, or they sink to the bottom and start growing

10.2.6 marine organisms associated with mangroves

  • sessile fauna are attached to submerged roots

  • stochastic pertubations (random chance or variables) such as physical disturbance can cause fluctuations in the species

  • fewer marine species are associated with mangrove trees above the water line

  • marine snails are able to feed on the mangrove leaves

    • they demonstrate an ontogenic ( through out their life cycle, they change) change in their diet

  • classic marine organisms associated with mangroves tend to inhabit burrows within the surrounding mud, like the mudskipper and crabs

  • crabs have a major influence on teh whole functioning of the mangrove system and are an example of an ecosystem engineer

    • they are able to digest the mangrove detritus completely unlike other organisms, allowing the carbon from the mangrove trees to enter the marine food web

  • crab activity is key to the healthy functioning of mangrove forests through recycling of organic material and bioturbation

10.3 seagrass meadows

10.3.1 what are seagrass meadows?

  • true marine angiosperms

  • grows in soft sediments in shallow coastal waters

  • network of rhizomes and roots

    • rhizomes spread horizontally connecting other seagrasses together

    • roots grow vertically downward into the sediment

  • two families:

    • Potamogetonaceae

    • Hydrocharitaceae

    • none of these are related to the terrestrial grasses

  • they posses three key attributes that enable them to colonize the marine environment:

    • leaves with sheaves adapted to high energy environments

    • hydrophilous (marine flowering plant), and thus submarine, pollination

    • extensive lancular systems that enable transport of oxygen to the below ground structures in anoxic sediments

  • diversity of seagrasses are low

10.3.2 where are seagrasses found?

  • they are found in all teh world’s oceans except for antarctica

  • some species have extensive distribution like Zostera marinea (eelgrass)

    • occurs in europe, on both coasts of north america

    • the northwest pacific

  • other species are more resticed

  • peak of diversity occurs in malaysia (the indopacific region), and seagrass species richness declines with distance along major currents form this point

  • seagrasses require a soft substratum that will enable the penetration of the roots

    • some species can grow on rocks, but most stick to sand and mud

  • some features of soft sediments make plant growth impossible:

    • highly mobile or exposed sediments

    • sediments with high inputs of organic matter resulting in reduced, anoxic conditions

  • seagrass beds can also change the particle size distribution of the sediment, enhancing the deposition of fine sediment particles

  • seagrasses are limited by the light levels

    • the maximum depth at which seagrass meadows are found is related to their compensation point, which i controlled by teh clarity of the water

    • in europe, eelgrass is found at depths of up to 6 m

    • of the coast of the island of malta, they have been found down up to 40 m

  • many seagrasses show a tolerance to a wide salinity range and seagrasses are often a major feature of estuaries and hypersaline lagoons

  • high level of nutrient run off can directly affect the grwoth of seagrass, but it also influences the competitive balance between the seagrass productivity and that of the algae associated with teh seagrass meadow, represented by epiphytic assmeblage growing on the leaves or by macroalgae that grow alongside

  • changes to the global cover of seagrass have raised much recent concern

    • 15 percent of seagrass worldwide have been lost over the 10 year period up to 2003, due to minaly a combination of human activities

10.3.3 reproduction and growth of seagrasses

  • they reproduce through both sexual (flowers and seeds) and asexual (vegetative through rhizome extensions) means

  • they are diocious, meaning they have separate male and female parts

  • clonal vegetative reporduciton through new modules

  • hydrophilous pollination

  • pollination by invertebrates

seagrass root structures:

  • lacunar systems to transport oxygen

  • seagrasses are known to be able to release oxygen in the sediments through their roots to create a small oxic zone

salt adaptations in seagrasses

  • cell wall remodeling

  • accumulation of organic osmolytes

  • active ion transport (Na+ comes in but is packed away)

  • increase in antioxidants to counter damage

  • ion transport processes in plants to achieve osmotic homeostasis

ecological role of seagrasses

  • 50 to 60% of the biomass is underground, in roots or horizontal rhizomes

socioeconomic importance

  • fisheries: seagrass ecosystems support commercial and recreational fishing industries by providing nursery and foraging grounds for many fish and shellfish species

  • carbon sequestration: teh high productivity reduces carbon in hte atmosphere

  • coastal protection: seagrass stabilize the seabed, reducing erosion and protecting against storm surges

  • tourism: teh clear waters and rich marine life of mangroves and seagrass meadows can attract tourists who enjoy snorkelling, diving, and boating