Introduction

Condition

Pressure

Response

Acknowledgment

Introduction

Giant kelp (genus Macrocystis) or 'string kelp' is a large, canopy forming plant, which grows in dense beds along the inshore subtidal reefs of south-east South Australia, Victoria and Tasmania. Kelp forests occur in cold, nutrient-rich waters and are among the most beautiful and biologically productive habitats in the marine environment. Individual plants can grow up to 30 metres tall, forming tall spectacular forests with fronds providing a dense canopy which shade and modify understorey reef communities.

Giant kelp forests create habitats of outstanding ecological and economic significance, representing areas of high biodiversity and productivity; and providing key habitats for the recruitment of economic species such as abalone and rock lobster. In addition, the large kelp plants themselves represent a major keystone species, influencing the hydrological and light environment, and the recruitment of rocky inshore fish and invertebrates. As plant parts break off in rough sea conditions, the rafts of drifting seaweed are important both as food and as a means of shelter and transport for invertebrates, assisting their dispersal. When beached, the rafts become an important nesting and foraging habitat for shorebirds, particularly migratory species. Giant kelp forests are also of very high recreational and tourism value, providing one of the greatest diving experiences in temperate waters.

Concerns have been voiced over the decline in extent of giant kelp beds. A recent report (Edyvane 2003) reviewed all of the available information - ranging from Admiralty charts from the late 1800s to Landsat satellite imagery from 1999 - and showed large fluctuations in the extent of the beds.

Condition

• Giant kelp is a short-lived, perennial temperate macroalgae, which shows pronounced inter-annual population fluctuations.
   
• Unlike many perennial temperate macroalgae, reproduction in giant kelp is not seasonal and is more closely linked to food supply levels-particularly high nitrogen levels in the water-and environmental conditions such as cool water temperatures. This reproduction strategy means the kelp can respond readily to unpredictable fluctuations in resource availability and environmental conditions.
   
• Major kelp loss episodes coincided with the 1972 El Nino, the 1982-83 El Nino and the 1987 El Nino (all of which were strong events) for all regions. The extent of recovery after these events varied. In all regions, kelp forests have been unable to fully recover to their original historical levels.
   
• Historical trends have been evaluated for seven major kelp distributions - Binalong, Freycinet, Maria Island, Eaglehawk Neck/Fortescue Bay, Port Arthur, Bruny Island, and Southport/Recherché Bay. However there is no agreement about interpretation of the data. Edyvane (2003) interprets the data as follows.
   
• All regions showed a consistent decline in extent over the 1944-99 period except for Port Arthur, which showed an increase of 33%.
   
• Over the last 20 years, all regions showed loss apart from Fortescue Bay, which had a 61% increase, and north Bruny Island, which had a 2% increase.
   
• Over the last 10 years, all regions showed a decline apart from Binalong with a 9% increase.
   

According to Edyvane (2003), the greatest losses have occurred in the north-eastern region of the distribution range-that is, Freycinet and Maria Island-while the smallest losses have occurred in areas under the influence of colder waters, such as, Bruny Island and Port Arthur.

Barrett (pers. comm.), a marine biologist at the Tasmanian Aquaculture and Fisheries Institute (TAFI), suggests that the preceding historical analysis is overly pessimistic and has been influenced by the choice of the final year of the period (1999), which shows particularly low areas of kelp. An alternative interpretation is that the plant is fluctuating dramatically in abundance, rather than declining, in response to the current extraordinary environmental conditions. He sees that the main loss has been in the Mercury Passage over the last 40 years. He also argues that the mapping methods are not reliable and that larger areas of the kelp are present but simply not developing surface canopies due to warmer nutrient-poor surface waters.

Pressure

• Reproduction and growth in Macrocystis pyrifera is strongly coupled to environmental fluctuations - particularly sea temperatures and nutrients. Cold, nutrient-rich springtime waters are optimal for growth and reproduction. In Tasmania a number of factors-all of which have a strong correlation with El Nino episodes-have probably combined to dampen reproductive success including the:
   
• rise in minimum water temperatures;
   
• declining influence of Sub-Antarctic waters;
   
• increasing influence of subtropical Eastern Australian Current waters; and
   
• Reduction in seasonal and inter-annual variability in sea temperatures on the east coast.
   
• Storm events are also an important source of kelp forest mortality on the east and south coast of Tasmania. For example, the major storm event of 25 March 1972 (under strong El Nino conditions) resulted in the loss of many kelp beds on the east and south coasts. El Nino episodes typically result in an increase in the frequency and severity of storms.
   
• Other possible future pressures on the giant kelp forests are the spread of urchin barrens-particularly the black sea urchin (Centrostephanus rodgersii)-and the spread of Japanese wakame sea kelp (Undaria pinnatifida) into habitat previously occupied by giant kelp.
   

Response

• A recent report argues that the status of giant kelp appears to have deteriorated to the point where it may qualify as a threatened species, based on three criteria (as defined by the Threatened Species Protection Act 1995) (Edyvane 2003):
   
• an observed, estimated, inferred or suspected reduction of at least 50% over the last 10 years in area of occupancy (Criteria A1);
   
• continuing decline, inferred, observed, or projected in area of occupancy, number of locations or populations, number of mature individuals (Criteria B2); and
   
• extreme fluctuations in number of populations, number of individuals (Criteria B3).
   
• However, there are a number of counter views about whether the plant warrants threatened species listing and protection, including the following.
   
• Craig Sanderson, a marine biologist advising SeaCare, argues that the clearly apparent large fluctuations in area indicates that the giant kelp has the capacity to make very large scale, rapid recoveries from low numbers. He has drafted a report called: 'Restoration of String Kelp (Macrocystis pyrifera) habitat on Tasmania's east and south coasts' for Seacare Inc as part of a Natural Heritage Trust program. It covers many of the issues surrounding giant kelp in Tasmania and is due to be released shortly.
   
• There is some doubt whether the plant is actually in decline or, rather, simply expressing its naturally dynamic and opportunistic growth habit in response to a relatively strong variation in environmental conditions caused by a combination of El Nino, the straddling of an oceanographic convergence zone and, possibly, global warming. In a similar situation along the west coast of North America, the plant can shift 100's of kilometres north or south in response to changing water temperature conditions (Dayton et al. 1992).
   
• Neville Barrett raises further strong doubts about both the relevance of IUCN listing criteria-such as minimum areas, which were largely developed for terrestrial plants-for assessing marine plants with their strong linear zonation (Barrett pers. comm.).
   
• Neville Barrett also suggests a mechanism for the low apparent areas in the aerial mapping and surveys compared to direct observations. He suggests that the difference in water temperature and nutrient levels between the top and the bottom of the water column in some years is enough to stop the giant kelp from achieving a floating surface canopy, thus making it almost impossible to detect with aerial methods. This could account for some of the large fluctuations in area in the report by Edyvane (2003).
   
• Research into the genetics of Macrocystis shows clearly that Macrocystis pyrifera and Macrocystis angustifolia share the same genes and can successfully interbreed (Coyer et al. 2001). It may be that M. angustifolia, which lives in the warmer waters of northern Tasmania, is an environmental variant to M. pyrifera, and when grown in the same conditions, will develop in the same way.
   
• There is uncertainty surrounding the management of this species in spite of the efforts to collate the available information. It appears further research into the biology of the species may be necessary to clarify the situation.
   

Acknowledgment

Richard Mount, TAFI, University of Tasmania