Biophysical Interactions

The role of the BIOSPHERE 

All plant and animal species living in kelp forests make up the biosphere of this ecosystem. These elements are a significant part of the biotic environment and are essential to the existence of kelp forests.

 In the kelp forests of the Great Southern Reef (GSR), scientists recognise the high levels of biodiversity and endemism, for example, 

• “What makes the Great Southern Reef such a distinctive ecosystem is that not only is it incredibly biodiverse, but a high proportion of the species it hosts are found nowhere else on earth” - Stefan Andrews, Ocean Imaging Director and Science Communicator and 

• The GSR is also a global biodiversity hotspot for bryozoans, chordates, crustaceans, echinoderms, molluscs and sponges. Scientists estimate that between 20–60% of all species are endemic to the GSR.

• Read and watch the  'storymap' on the GSR  and read the article.

• "Kelp forests contain a diversity of marine species on the seafloor, in the understorey and throughout the water column including fish, molluscs (sea snails), bryozoans (lace corals), polychaetes (worms), crustaceans (crabs, isopods, amphipods), echinoderms (sea urchins, sea stars) and sponge." Read more in DPI Macroalgae fact sheet 

Figure 11 outlines the important  producer and consumer groups and the trophic levels in a kelp forest ecosystem. 

Kelp is key to the operation of this ecosystem as kelp produces  large amounts of biomass, and supplies food and energy to the entire system. 

Kelp forests are also important in recycling nutrients as all parts of the kelp plant absorb nutrients directly from the seawater. When kelp plants lose their grip on the hard subsurface they begin to die and decompose. Many of the microscopic decomposers produce nutrients for kelp and other species like  sponges and bryozoans  to absorb and consume. 

Kelp plants can be considered a keystone species because the whole ecosystem is dependent on kelp being present, and the ecosystem collapses without this species. Scientists have also described kelp as an 'ecosystem engineer' that is, kelp actually builds the ecosystem and creates habitat for species such as marine mammals, fish and invertebrates by providing a source of food and shelter. 

Kelp creates habitat by influencing  the abiotic and biotic aspects of the environment. Some examples of what kelp does for the ecosystem are - 

• it oxygenates the water

• it absorbs carbon dioxide

• it recycles nutrients

• it traps and stabilises sediments

• it provides a nursery for young plants and fish by reducing water flow and reducing the amount of sunlight which reaches the sea floor.

Figure 10 shows how a kelp forest ecosystem in California functions when sea otters are present and when they are not. The symbiotic relationship between kelp plants and sea otters, is known as mutualism. 

In this type of ecological relationship both species depend on each other and benefit each other. Kelp plants need sea otters to eat black urchins. If there are too many black urchins grazing on kelp plants, the kelp forest can completely disappear or be replaced by a much less diverse ecosystem called a turf bed.  The sea otter needs the kelp forest as a place for protection during strong currents and a place to find food.

In south east Australia, rock lobsters are the natural predator of urchins. However, the reduction of the population of southern rock lobsters through overfishing, has meant that spiny urchins have been able to invade, modify and  overtake the kelp forests of some parts of Victoria and north east Tasmania.

• This process is also happening in Victoria Read and watch the video clip. Included is  a short news clip uploaded onto Facebook, showing a tv reporter visiting a scientist in Port Phillip Bay.  

Black urchins are native to the kelp forests of NSW. Black urchins have not caused the same overgrazing in NSW kelp forests as they have in Tasmania and Victoria. 

 The kelp forests in NSW seem to be resilient to most changes in the size and density of black urchins as natural predators still exist to keep the population of urchins in check. 

Biogeographical processes

The processes of invasion, modification and succession are important biogeographical processes that can be illustrated by examining the effects of invasive spiny urchins on kelp forest ecosystems. 

Invasion

In all three locations, California, north-east Tasmania and Victoria, the impacts of  the black urchin could be considered an example of a biogeographical process known as invasion. 

Invasion refers to when a species is not native to an area but establishes itself and dominates another area or ecosystem. Biological invasions usually have multiple negative effects on ecosystems, usually disrupt the interactions between species  and can often impact the whole ecosystem. 

(REFER to the section 

Human impacts - positive and negative 

to know more about this process.)

 Modification and Succession

Modification and succession are also biogeographical processes which are evident in kelp forests. 

Modification refers to to the changes which take place in an ecosystem after an increase or decrease in a species occurs, or after a new species is introduced.  

Some kelp forests in south east Australia have undergone the process of modification due to the impacts of 'tropicalisation.' These kelp forests have also experienced the process of invasion due to the arrival of the spiny urchins which often follow 'tropicalisation'. 

The process of 'tropicalisation' refers to the introduction of a number of tropical species to temperate kelp forests. Over the last ten years, the East Australian current has carried species such as spiny urchins further south, as far as north east Tasmania. The spiny urchins  consume kelp and algae on the rocky reefs. This creates an 'urchin barren'.

Succession refers to the process of ecological succession, whereby ecosystems regenerate in a structured way after a disturbance.  There are two types of ecological succession - primary and secondary. Primary  succession occurs over a long period of time for example after a volcanic eruption. 

Secondary succession occurs if an ecosystem recovers and re-establishes after a natural disturbance or event. For example, kelp forests can usually regrow after storm damage. 

Some kelp forests along the NSW coast have undergone the process of secondary succession after human induced disturbance. Succession has been 'kick-started' and managed  by scientists so that kelp forests can regenerate by themselves. 

• Watch the video and interviews with  scientists Dr Verges and Dr Marzinelli - here they explain the complexities of kelp ecosystems, human interactions, tropicalisation and a restoration project called 'Operation Crayweed' . 

• Refer to the section 

Human impacts - positive and negative  to learn more about modification, invasion, 'tropicalisation' and restoration. 

The role of the HYDROSPHERE 

Elements of the hydrosphere including ocean currents, ocean temperature, water quality, tides and waves provide the conditions for kelp forests to establish and thrive. These elements are part of the abiotic environment and are extremely important to the survival of kelp forests.

Some requirements and preferences of kelp forests include -

•  cooler ocean water of less than 20 degrees celsius, on average

• cool ocean waters often contain nutrient upwellings which assists growth

• normal ocean salinity levels

• normal seawater chemistry, including normal ocean water pH of approximately 8.2

• ocean water quality, including turbidity, should be at least at  moderate levels. Kelp are negatively affected when there is polluted water eg, with sewage or sediments.

• Shallow water is preferred - Most kelp grow to a depth of up to 30 metres and most kelp grow below the low tide level. 

Hydrological Processes

Normal hydrological processes such as ocean currents, flooding or storm waves with high energy impact kelp forests.

Storms create waves with high energy which can damage the holdfasts of kelp plants. A healthy kelp forest can regrow and show resilience to this type of natural stress. 

Often storms also produce high levels of precipitation and flooding. Rain, stormwater and floodwaters can erode soil and carry sediment into coastal rivers, bays  and seas. Sediment can be deposited and  smother kelp plants reducing its ability to photosynthesise.

Ocean currents are a significant element of the hydrosphere and are important because they distribute heat around the globe. Ocean currents generally travel horizontally and are not dependent on tidal flows. 

See figure 13  which shows the major ocean currents which flow around Australia. 

A more detailed map of ocean currents can be seen in the State of the Environment map. 

Biogeographical processes 

Modification 

The process of modification can be seen in the way both water quality and rising ocean temperatures negatively impact kelp forests. Both of these elements are important aspects of the hydrosphere.

• Water quality 

Polluted sewage water which flowed from Sydney's sewerage system  directly onto the coast up until the 1990's shows how flows in the hydrosphere can affect kelp forests. 

Two large sewage treatment plants in Sydney's east and at north - east discharged primary treated sewage through a short pipeline close to Sydney's coastline. There was substantial  community concern and protests about coastal water quality during this time.

Ultimately a long term solution was developed which transported the sewage through a much longer pipeline, called the ocean outfall pipeline system. Unfortunately, kelp forests were affected by the poor water quality and much of the kelp died. The impact on Sydney's kelp forest went unnoticed until researchers such Coleman, Verges and Marzinelli  measured the impact around 2003. 

Much of the kelp forest was modified ( it disappeared) due to the impact of this threat. That is, the  kelp forest did not grow back after water quality problems were fixed in the 1990's. So geographers can conclude that the kelp forest had low resilience and was highly vulnerable to threats. 

Refer to the section Human impacts - positive and negative  to learn more about the disappearance of Sydney's kelp forests. 

• • Rising ocean temperatures

Ocean currents have an important role in distributing heat around the globe. However, the changing climate has changed some ocean currents like the the Eastern Australian Current (EAC), see figure 13 for its location.

How has the EAC affected kelp forests?

Scientists recognise that as ocean temperatures have risen, the EAC has become stronger and now flows a lot further south than before. The warmer waters of the EAC have impacted kelp forests along NSW coasts eg, near the Solitary islands and even the kelp forests of Tasmania.

The warmer waters bring with them other species eg, tropical fish and urchins which feed off the kelp until there is no kelp left. This process is known as 'tropicalisation' and it leads to a significantly modified ecosystem.

Read more in the previous section on the Biosphere and in the section on 

Human impacts - positive and negative .