Processes involved in the breakdown of wrack in an urbanised estuary
Giordana Cocco (Supervisors: Professor Gee Chapman, Associate Professor Ross Coleman and Dr Danny Roberts)
Seagrasses are important in many estuaries. They sustain high levels of primary and secondary production and provide a complex habitats. They contribute to cycling of nutrients (i.e. moving nitrogen and phosphorous into and out of the water), assist in oxygenating the sediment-water interface and facilitate the stabilisation of substrata by slowing water-movement and trapping sediments. Numbers of types and of each type of animals and plants are often larger in seagrass beds than in adjacent unvegetated habitats. Seagrasses are also economically important and of value to fisheries.
Many seagrasses are particularly sensitive to disturbance by people and to changes in environmental variables, particularly in shallow coastal waters where disturbance frequently leads to loss and fragmentation of habitat.
When seagrass dies or detaches from the bottom, it becomes detrital material and forms what is known as wrack. Wrack forms thick mats that either sink or float on the water surface or are deposited on the shore. Its density and distribution are often unpredictable and vary spatially and temporally.
When large amounts of macro-algae and seagrass wrack accumulate against seawalls, they sink onto the sediment and decompose. Due to reduced circulation of water and small amounts of oxygen, the process of decay of seagrass can be substantially slowed down. Seawalls may therefore have a deleterious effect on decomposition of wrack.
In areas where wrack constitutes a problem, removal is a short-term treatment of the symptom. Beds of wrack are a precious resource and an integral component of the food chain. It is difficult to establish the consequences of changing inputs of wrack into the ecosystem, because of the numerous effects that litter has on various physical, chemical and ecological attributes of an ecosystem.
The aim of my project is to add further to our understanding of the ecological processes involved in decomposition of wrack in estuarine habitats. Knowledge of these processes is a necessary prerequisite for responding to environmental, social and economical issues associated with accumulations of wrack on estuarine shores.
I am currently testing various models that might explain varying rates of breakdown of wrack. These include potential effects of urbanization, artificial structures such as seawalls, distance from shore, presence of macro-algae or macro-invertebrates among the decomposing wrack, location and season. The project also involves examining the distribution, density and composition of wrack at different spatial and temporal scales and identifying potential effects of wrack on seagrass beds, growth of macro-algae and oxygen and changes of nutrients in the sediments.
Several pilot studies have been done to test different methodologies for measuring decomposition of wrack. Different structures, ranging from meshed litter-bags to plastic containers with meshed windows to allow water to flow in and out have been trialled to minimize external material from entering the structures and thus confounding results.
The fieldwork is mainly done in Tuggerah Bay and Chittaway Bay, two embayments of the Tuggerah Lakes (Central Coast, NSW, Australia). The former is mostly undeveloped, while the latter is developed with residential housing along most of its foreshore. Build-up of wrack nearshore is a regular occurrence in this estuary and large amounts of nutrients and large accumulations of wrack are a serious ecological, social and economical issue.
Fieldwork is also being done in other estuaries along the coast of NSW, such as in Narrabeen Lagoon, north of Sydney and in Lake Illawarra, south of Sydney to increase the generality of the study. This broad range of investigations will contribute to a thorough understanding of the ecology of wrack and of its influences on associated habitats, so that results can be applied in the management of urbanized estuaries.
Coastal development is reported to increase wrack and in many urbanized estuaries along the NSW coast, mats of rotting wrack cause a serious ecological, social and economical problem. It is therefore important to understand the mechanisms that underlie the breakdown of dead plant and algal material and identify how and to what extent wrack affects the ecology of the estuary.
Wrack is a source of carbon and nutrients and an important component of estuaries. When floating, it can disperse plant propagules and be a source of food, shelter and passive transport for a number of invertebrates and fish, and indirectly for shorebirds which feed on invertebrates in the wrack. Often a substantial amount of wrack ends up washed onto the shoreline where it may assist in the establishment, growth and survival of saltmarsh by providing a structure against erosion and/or nutrients to the saltmarsh plants. Accumulations of seagrass wrack are also known to support formation of dunes through trapping and binding drifting sand.
On the other hand, wrack can cause degradation and smother the underlying fauna and flora due to reduced oxygen exchange and the toxic effects of hydrogen sulphide. Large clumps of decomposing wrack can degrade habitat and alter the physical and chemical attributes of surrounding sediments. Removal of floating seagrass and macroalgae wrack is a common activity of councils, especially in urbanized estuaries, where it is commonly seen as a degraded habitat of rotten plant material.
Despite the numerous studies and collection of beach-cast wrack for various purposes since Australian settlement, we are still lacking a comprehensive understanding of the ecology of wrack
Results of this study will be posted as they become available.