Mangroves ecosystems are found in may parts of the world and consist of highly specialized plants occupying the intertidal zones.
The mangrove habitat is extreme. They grow in muddy tidal soil but the saline environment with its intense light, high temperatures, and wind is physiologically quite dry. To overcome this, mangrove leaves generally display a number of structural features including a thick walled waxy epidermis, salt glands and well developed water storage tissue giving a somewhat succulent character to the leaves.
In addition the plants require special mechanisms to enable them to survive and even thrive in conditions which make uptake of water, oxygen and nutrients very different from those which operate on the land. Throughout the area the vertical pneumatophores or ‘breathing roots’ arise from horizontal or lateral roots immediately below the soil surface. Some have conspicuous stilt roots. Not surprisingly, the dispersal of the seeds and fruits of the mangrove is by sea currents. The partly developed seedling floats for a time, settles and develops rapidly sending our secondary roots and leaves. The root systems protect the foreshores in storm surges.
Overseas studies have shown that in some coastal waters, 80% of fish caught commercially were linked to food chains dependent ultimately on mangroves.
Several birds e.g. the mangrove Honeyeater, the mangrove Kingfisher and the mangrove Warbler are confined to the mangrove habitats.
Mangrove communities are possibly the most remarkable in the world because they form in a special area where land meets sea. The trees and shrubs grow between the mean sea level and the highest tides. During high tides the roots are fully submerged in salt water which is devastating to most plants. The continual rise and fall of the tide means constant change. Salinity is not the only changing factor. The tides alter the temperature, the supply of nutrients and oxygen levels in the soil and water. In the mangroves organisms as diverse as crabs,oysters and barnacles co-existing with snakes, lizards, insects and birds will be found.
Within a mangrove there is a constant turnover of living and non-living matter making mangroves the most fertile ecosystems in the world. They are essential to the life cycles of many fish and other marine animals especially in their role as nurseries.
Mangrove soils are quite distinctive. These soils are poorly drained, saline, oxygen deficient, fine grained and rich in organic matter. The organic matter in the soil originated from decomposition of plant litter produced by the mangrove trees themselves. The debris is slowly broken down under slightly acidic conditions by microorganisms such as bacteria and fungi. Hence the smell.
The most outstanding feature of mangroves is their ability to grow in
salt water. Mangroves have a number of ways of coping with the high salt of their environment. In general they tolerate relatively higher internal salt concentrations in their sap than do most land plants. Mangroves also remove salt by storing it in older leaves before they fall.
However mangroves may be divided into two broadly different classes depending on whether they secrete salt or exclude salt. The ‘salt secretors’ absorb water plus small quantities of salt into their roots but this salt is later concentrated and actively removed by secretion through special leaf glands. The ‘salt excluders’ allow less salt to enter their root systems with the water. Salt excluders don’t allow salt into their system in the first place. Whilst salt secretors store the salt in older leaves before the leaves drop off. Some mangrove have very specialised leaves which may be waxy or even thicker than normal. Many of the mangroves studied allow for this type of leaf. A good example of such a mangrove is Rhzyophora stylosa.
Factors affecting mangroves and their adaptations
The main abiotic factors that mangroves are:
- WAVES AND CURRENTS
- SUBSTRATUM (soil)
Mangroves have adapted to these factors in many different ways. Some of ways mangroves have adapted include:
- The exclusion of salt by preventing it from entering cells by means of semipermeable membranes in their roots,
- The excretion of salt through salt glands
- The accumulation of salt only to shed the leaves at the end of the growing season.Plants that are salt resistant are called Halophytes.
Salinity Root systems that increase aeration.
- pneumatophores (peg like erect roots growing upwards)
- prop or stilt roots (curving roots which arch out from the trunk)
- buttress roots (blade like structures)
Reproduction Large seeds for dispersal-the seeds are large and buoyant which allow them to be dispersed by the water when they fall.
- Some mangroves produce young offspring, embryos that remain attached to the parent plant until they are mature enough to cope with the extreme conditions on their own. They then fall, take root and establish new plants.
Unstable soil – Cable like root systems-these root systems allow plants to anchor themselves in the mud and excessive water.
Gas exchange Lenticels – Breathing pores enabling the plant even when partially submerged to acquire oxygen. These are located on various structures eg pneumatophores, prop roots and main trunks of certain types of mangroves.
An estuary is a semi-enclosed coastal body of water where salt water from the ocean mixes with fresh water flowing from the land. Estuaries are dynamic environments that are changing throughout time within a set of natural extremes. Estuarine ecology has evolved to cope with this highly changeable environment, however when anthropogenic stresses cause a widening of environmental extremes (e.g. higher nutrient loadings, lower pH and hypoxia), the ecosystem can be changed (sometimes permanently) in various ways. The term “eutrophication” refers to the process of organic enrichment in an aquatic ecosystem, commonly caused by elevated nutrient loadings. Organic enrichment leads to higher rates of decay resulting in a decline in dissolved oxygen throughout the system and potentially leading to the local extinction of higher orders of the foodweb. Australia has more than 900 relatively large estuaries and many smaller estuaries, with most located along the northern, eastern and southwest coasts. Some of these estuaries are the site of major cities, towns and coastal communities.
Cudgera Creek meets the sea at Hastings Point where the natural entrance is kept open most of the time due to the geomorphology of the headland and longshore movements. There are three major branches of the creek:
- Christies Creek, opposite the main entrance
- Cudgera Creek main channel, which runs south approximately 3.5km to Pottsville, draining the Cudgera Creek locality in the west and surrounding agricultural lands
- An arm of Cudgera Creek, which continues south, draining SEPP14 Wetland southwest of Pottsville shopping centre.
- The Cudgera Creek catchment is approximately 50km2 in size, comprising large forested remnants, agricultural land, residential developments including Koala Beach and Seabreeze Estates and several new developments that discharge stormwater to the creek.
- The catchment also includes the Hastings Point STP, treating effluent to tertiary level and pumped to a dune injection system behind the beach north of Hastings Point. Historically, in times of flood or when sewage pump station fails, effluent can overflow into Christies Creek and subsequently into Cudgera Creek
There are numerous threatened flora and fauna recorded within or near the Cudgera Creek catchment including shorebirds, sedge frogs and bat species. A search of the Atlas of NSW Wildlife of a 10km2 region found 38 threatened terrestrial animal species and 28 flora species. During historic and current site investigations, many aquatic fauna species were seen, with the estuary floors in many places covered to varying degrees with worm and yabbie holes. Stingray feeding depressions were in the mid‐sections of the system indicating the presence of a developed trophic food chain in the estuaries.
There are patchy seagrass beds throughout the creek, however the extent of overall biomass loss/growth is not known due to the lack of regular monitoring.