Marine ecology

The Importance of Seagrass Beds in Supporting Biodiversity

The seagrass bed has a richer associated community than surrounding bare sediments. They are foundation species that facilitate the colonization of other species.

Concetto importante: Since seagrass beds are important for nursery, they can attract predators. Predation pressure is the major force structuring the assemblages found within seagrass beds. This creates the Edge Effect: Distance of a prey from the edge of a bed is an important factor that regulates the predation pressure.

The density of the blades and rhizome systems, which increase moving in the middle of the meadow, tends to deter large predators from entering the bed. The central denser part provides more shelter for smaller animals. Predators are much more successful at the edge. At the edge, there is more predation that stimulates more and faster growth to reduce the possibility of being preyed upon.

Another important factor that influences the diversity and the structure of the communities is the habitat fragmentation: fragmentation of the

Seagrass beds are often found in many smaller patches. They can be naturally generated by specific environmental conditions or due to human activities. The consequences of having seagrass beds in smaller patches include:

• The overall area is reduced, which may impact big species that require a large area.
• Small patches can form islands separated by bare dangerous sediment, potentially isolating organisms from the main population.
• There is an increase in the edge effect, which leads to an overall increase in predation pressure.
• Water flow and sediment deposition can be altered.

However, there are different points of view on this matter. Bowden et al (2001) demonstrated that there is higher diversity of sediment-dwelling invertebrates in large Zostera marina patches compared to several small patches. McNeill and Fairweather (1993) found that several small patches had a higher overall diversity than continuous patches of the same area.

Seagrass beds have exceptionally high biomass and productivity. They provide large amounts of carbon input into coastal systems. The high leaf biomass produced by seagrass is also harvested by humans for a range of uses, such as packing material.

The construction of the coastline can damage the seagrass. Indirect effects are related to runoff, such as discharges and wastewater from industries, aquaculture, and river inputs. Another indirect effect is the overfishing of herbivores that consume seaweeds, causing a trophic cascade. Invasive species are also introduced accidentally by humans, such as Caulerpa taxifolia from the museum of Monaco, which has started to colonize the Mediterranean. Natural disturbances, such as climate change, storms, hurricanes, and wasting disease, also affect seagrass. For example, Zostera marina was affected by wasting disease in 1930, leading to a decline in seagrass cover and abundance of all related benthic species. However, we can restore damaged areas by using different methods, such as using the whole plant, sprigs, or seeds.

Mangrove forests consist of woody trees or shrubs that live at the sea/land interface in sheltered tropical and subtropical coastal and estuarine regions. They cannot tolerate continual and complete immersion in seawater.

Exchange of gas and oxygen retention. The root system:

• Thickening of the root epidermis which reduces the loss of O2 to the outside anoxic environment
• Development of special spongy aerenchyma for O2 transport along all the root
• In the epidermis of the roots that exit the sediment there are special pores called lenticels, which favor the gas exchanges keeping the section of root within the sediment supplied with O2
• Some species have roots that only partially fit into the sediment avoiding exposure to deeper-laying anoxic sediments, to increase the area for exchange the oxygen
• Aerial roots/branch (with lenticels) to aid oxygen uptake
• Root extensions that project into the air outside sediment to collect O2 and oxygenate the underground root parts (Pneumatophores)

Three main adaptations:

1. Rhizopohora: roots which leave the tree up to 2 m from the ground to absorb O2 + aerial roots
2. Avicenna: pneumatophores emerging every 15-30 cm from horizontal roots
3. Roots periodically break the soil

surface during growth producing 'knees' above

Bruguiera: sediment surface through which air is taken through lenticels

High salt level

In the roots there is a system reducing of salt uptake. There are salt glands that secrete salt from the leaves. Translocation of the salt in the older leaves that soon will fall down... The sediment has a high concentration of salt, due to the osmosis water tends to move from water to the sediments, so great salinity differences between root and sediment, it is difficult in taking up water. They developed a great root biomass compared with the rest of the plant to enable water uptake when salinity is high. This ratio between below and above part of mangrove increases the amount of salinity in the sediment: high is salinity higher will be the above/below ratio. This additional growth of the roots causes less vertical growth using this energy.

Water conservation

To reduce the loss of water they have thick and succulent leaves to accumulate water.

1. epidermis covered by a thick cuticle
2. stoma is sunken and usually confined to underface
3. leaves are held at almost vertical orientations when exposed to full sunlight

Mangrove forests can be divided into a series of zones with different species dominating with increasing distance from shoreline:

• Subtropical forests: may have only 1 mangrove species
• Tropical forests: many species zoned in abundance

Red mangroves (fringing mangroves): pioneer species across most of the world areas roots tolerate full-strength seawater and tidal inundation, because of their roots system.

Black mangroves: roots tolerate only occasional inundation (high tide), pneumatophore would be submerged in low tide.

White mangroves: prefer sediment less waterlogged (rarely inundated).

All the species produce flowers and reproduce by sexual reproduction. Pollination mainly by animals. Rhizophoraceae pollinated by wind. The animal vectors for pollination are bats, birds, butterflies, and bees.

Most of the mangroves

Mangroves are viviparous: fruits start to germinate when they are still attached to the mother plant. Following pollination, the growing embryo remains on the parent plant for several months. The young plant does not leave the adult as seed or fruit but as a fully developed seedling known as propagule. Once ready, the mangroves propagules drop from the parent plant directly into the water where they are carried by wind to another muddy shore.

There are three phases in the life cycle of a mangrove propagule: floating horizontally, turning vertically after about 1 month when roots start to develop, negative buoyancy, and finally, the plant erects vertically after rooting.

A lot of organisms can live with mangroves, both terrestrial and aquatic. So, mangroves host an important biodiversity. There are many terrestrial plants that can live in association with mangroves. For example, there are epiphytic species like orchids. There are also many insects that graze the leaves of mangroves, reducing photosynthesis, growth, and reproduction. Additionally, there are vertebrates, reptiles, and amphibians.

That show high tolerance to salt. But the most important are birds, using them for food. There are marine organisms divide in sessile or vagile (mobile). Sessile is attached to the submerged roots and create a community called fouling community, this is very rich constituted by seaweeds and invertebrates. The composition can change every month, not stable. The vagile are above the water line, detritivores or grazers, they move from leaf to roots to sediment. Snails is an abundant group, in particular 2 genera: Littoraia (different species show distinct niche separation) and Terebralla palustris (show ontogenic change in diet). Another category is a vagile fauna inhabiting burrows within the mud. E.g. fishes and crabs. Mudskippers have excellent eyesight, have an amphibious behavior, able to walk across the mud surface thanks to his fins, they are able to retain into the mouth huge quantity of water to breathe. The crabs can be defined as ecosystem engineers, they are able to create and maintain the habitat.