• Kelp transitions
    • Summary
    • Categorical attributes
    • Detail information
    • Regime shift Analysis
      • References

Kelp transitions

Main contributors: Juan Carlos Rocha

Other contributors: Reinette (Oonsie) Biggs, Garry Peterson

Last update: 2011-02-28

Summary

Kelp forests are marine coastal ecosystems located in shallow areas where large macroalgae ecologically engineer the environment to produce a coastal marine environmental substantially different from the same are without kelp.  Kelp forests can undergo a regime shift to turf-forming algae or urchin barrens. These shift leads to loss of habitat and ecological complexity. Shifts to turf algae are related to nutrient input, while shifts to urchin barrens are related to trophic-level changes. The consequent loss of habitat complexity may affect commercially important fisheries. Managerial options include restoring biodiversity and installing wastewater treatment plants in coastal zones.

Categorical attributes

Impacts

Ecosystem type:’

  • Marine & coastal

Key ecosystem processes:

  • Primary production

Biodiversity:

  • Biodiversity

Provisioning services:

  • Fisheries
  • Wild animal and plant products
  • Other crops (eg cotton)

Regulating services:

  • Natural hazard regulation

Cultural services:

  • Recreation
  • Aesthetic values

Human well-being:

  • Food and nutrition
  • Livelihoods and economic activity
  • Cultural
  • Aesthetic and recreational values

Links to other regime shifts:

  • Freshwater eutrophication
  • Fisheries collapse
  • Hypoxia

Drivers

Key drivers:

  • Harvest and resource consumption
  • External inputs (e.g. fertilizers
  • pest control
  • irrigation)
  • Global climate change

Land use:

  • Large-scale commercial crop cultivation
  • Fisheries
  • Conservation
  • Land use impacts are primarily off-site (e.g. dead zones)

Key attributes

Spatial scale:

  • Local/landscape (e.g. lake
  • catchment
  • community)

Time scale:

  • Months
  • Years

Reversibility:

  • Hysteretic (difficult to reverse)
  • Readily reversible

Evidence:

  • Models
  • Paleo-observation
  • Contemporary observations
  • Experiments

Confidence: existence of the regime shift

  • Well established – Wide agreement in the literature that the RS exists

Confidence: mechanisms underlying the regime shift

  • Well established – Wide agreement on the underlying mechanism

Detail information

Alternative regimes

Kelps are marine coastal ecosystems dominated by macroalgae typically found in temperate areas. This group of species forms submarine forest with three or four layers, which provides different habitats to several species. Kelp ecosystems are important to maintaining important industries such as, lobster and rockfish fisheries, tourism based on fishing, recreational diving and kayaking, as well food and pharmaceutical products derived from kelp (e.g. tooth paste).

Three different self-reinforcing regimes can be identified:

Kelp forests are highly productive ecosystems dominated by canopy-formed algae in cold-water rocky marine coastlines. Among the biota associated with kelp forest are marine mammals, fishes, crabs, sea urchins, mollusks, other algae and biota that live on the kelp themselves (Steneck et al 2002). At least 4 trophic levels are found in kelp forests. Apex predators such as sea otters, cod, pollock, hake, and haddock are common. These predators regulate populations of species at lower trophic levels. In particular the regulation of sea urchins populations, who are an important consumer of kelp, is important to maintain kelp forests.

Urchin barrens are an alternative regime in which there is no kelp forest and substantial populations of urchins on the sea bed. This regime has low or no populations of apex predators and large populations of herbivores that keep macroalgae a very low population levels, preventing the regrowth of the kelp forest which is important habitat for apex predators (Steneck, R., et al. 2004). Ecosystem services related to apex predator fisheries and recreational activities are substantially reduced. However populations of bottom dwelling species can increase. In some places urchins have themselves become a valuable fishery. While in others, populations other harvested species such as lobster and crab have increased.

Turf-forming algae regime is similar to urchin barrens in that there is no kelp forest, however rather than dominance by urchins the seabed is dominated by turf forming algae. This regime also lacks apex predators (Gorman, D., et al. 2009). Ecosystem services related to fisheries and recreational activities are substantially reduced. However, populations of invertebrate species such as lobster and crab can be higher.This regime shift is associated with a change in habitat in shallow marine coastal ecosystems. Three different self-reinforcing regimes can be identified:

Drivers and causes of the regime shift

This regime shift is driven by two key direct drivers: reducing populations functional groups (Steneck et al 2002; Steneck, R., et al. 2004) and input of nutrients (Gorman, D., and Connell, S. 2009). Reductions in populations of key functional groups is due to the hunting of sea otter and the fishing to large apex predator fish species. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. Nutrient input is often due to the output of wastewater from urban settlement and agriculture in nearby coastal areas. However it can also be due to naturally occurring offshore upwelling events. Nutrients addition favors the growth of turf algae over kelp. Strong rain events and floods can produce shock events for kelp ecosystems by providing a pulse input of nutrients and turbidity, which can stimulate the growth of turf algae.

 

Other drivers can also contribute to this regime shift. Pollution discharges and sedimentation may play a synergetic role as stressors. In Tasmania for example, global warming has favored the reproduction of urchins which acting in synergy with lobster fishing has reduced kelp resilience (Ling et al. 2009). El Niño events and global warming can generate water stratification. As consequence, nitrogen concentration declines and kelp forest growth becomes limited by the availability of nitrogen (Steneck et al 2002).

Impacts on ecosystem services and human well-being

Shift from kelps forest to urchin barrens or turfs

The main ecosystem impact of the loss of kelp forests is the loss of habitat complexity. Kelp is a three-dimensional structure that offers shelter and food for many species; urchin barrens and turfs do not have such characteristic. This loss is associated with the reduction of the food web complexity and loss of functional groups (Steneck et al 2004), with varying effects on fisheries. Some valuable fish species may diminish since kelp forests provide nursery areas. Invertebrate species such as lobster and crab can increases in population (Steneck et al 2002). Along with hosting high marine biodiversity, kelp forest provide ecosystem services related to recreation for divers. In addition, kelps support a multi-million dollar industry of canopy-cropping for alginates (Steneck et al 2002). This product is commercially important in pharmaceutical and chemical industry. These services can reduced or loss by this regime shift.

The ecosystem service impacts of algae turfs are likely to be similar to those related to coastal eutrophication. Such effects include abundance of rich-nutrient environment species as shellfish, bad odors and the associated consequences for recreational and aesthetic values.

Shift from urchin barrens or turfs back to kelp forest.

The recovery of kelp forest increases habitat complexity and is expected to increase biodiversity. On the other hand, the benefits gained by urchin and lobster fisheries may be diminished, since the recovery of functional groups and increasing biodiversity will control their populations. However, other species of commercial fisheries and tourism may recover, although it is not always the case.

Management options

Options for enhancing resilience

Restoring biodiversity in functional groups. For kelp forests it is particularly important to maintain apex predators (Estes et al 2011). A high profile version of this strategy has been efforts to recover sea otter populations on the west coast of North America, by protecting sea otter populations (Steneck et al. 2002).

Reducing nutrients can be done by reducing runoff from agricultural areas, as well as improving water treatment and managing runoff in urban areas (Gorman, D., et al. 2009). Reducing nutrients income will attack the positive feedback that maintain turfed landscapes and inhibit kelp recruitment (Gorman, D., and Connell, S. 2009).

Targeted removal of sea urchins and measures to maintain the abundance of predators like cod, sea otters and sheep-head through fishery controls may provide stability to kelp forest ecosystems (Steneck et al 2002; Steneck, R., et al. 2004).

Options for reducing resilience to encourage restoration or transformation

Transplantation of kelp-forming species has been proposed as a restoration strategy in highly fragmented seascapes. This strategy is likely to increase the supply of kelp propagules to colonize new areas and maintain free substratum for canopy recruitment (Gorman, D., and Connell, S. 2009). However this strategy has not been tested.

Regime shift Analysis

Feedback mechanisms

Kelps regime is maintained by a healthy food web, usually with 4 trophic levels that keep urchin population under control. Urchin regime is maintained by reduction of predators due to lack of habitat or fishing pressure. Turf-forming algae regime is maintained by an environment over enriched by nutrients either from sediments from land or upwelling nutrients from the deep ocean. Note that canopy-forming algae and turf-forming algae are functional groups competing for resources and space in the ecosystem. All feedbacks are local and well established.

Kelp Forest

Competition feedback (Local, well established): Canopy forming algae and turf forming algae are two set of species that compete for space, light and nutrients. The consumption of such resources reduces the availability of the resource for its own progeny and for other species. For this reason, such relationship is represented as a balancing feedback loop. There are two competition feedbacks in figure 1. The resulting ecosystem is a kelp forest when canopy-forming algae is dominating such competition, reducing in turn the possibility of turf forming algae to develop.

Predation feedback (Local, well established): The relationship between predator and prey in the food web can be viewed as a feedback. In a nutshell, the more abundant the prey is, the more resource the predator has and vice-versa, producing a balancing feedback loop. Such feedback is represented here aggregated across functional groups: between apex predators and lobsters - meso-predators; between the later and urchins; and between urchin sand algae groups. Lobsters and other meso-predators help maintain kelp forests by regulating urchin populations at low densities.

Structure feedback (Local, well established): Canopy forming macroalgae maintain more complex habitat structure than in turns favor the presence of high biodiversity. When diverse predatory species are present in the ecosystem, these predators regulate urchin populations, which maintains kelp forests.

Urchin barren

Predation feedback (Local, well established): The ecosystem is dominated by urchin barrens when the predation feedback amongst urchins and kelps is strong, while the predation feedback amongst lobster or meso-predators and urchins is weak. It may be related with changes in water temperature that favors urchin barren establishment, or fishing pressure that reduces meso-predator abundance.

Structure feedback (Local, well established): As urchin barrens dominate, less structural habitat complexity is provided by kelp forest. Thus, meso-predators habitat requirements may be affected, reducing their abundance and the predation pressure on urchins.

Turf-forming algae

Competition feedback (Local, well established): In turf-forming algae dominance regime, the competition feedback is favoring turfs, reducing space, nutrients and light availability for kelps to develop.

Drivers

Shift from kelp forest to urchin barrens or turfs

Important shocks (e.g. droughts, floods) that contribute to the regime shift include:

Rain and floods (regional, well established): Strong rain events and floods represent therefore shock events for kelp ecosystems given the pulse input of nutrients. Nutrients in turn unbalance the competition between kelps and turfs favoring the development of the later; which can use the excess of nutrients faster than kelps and also take advantage from the turbidity conditions generated by nutrients.

ENSO (global, well established): El Niño events or global warming events may generate water stratification. As consequence, nitrogen concentration declines and kelps become nitrogen limited (Steneck et al 2002). In addition pollution discharges and sedimentation may play a synergetic role as stressors.

The main external direct drivers that contribute to the shift include:

Overfishing (regional, well established): overfishing functional groups is one of the most important drivers of kelp transitions (Steneck et al 2002; Steneck, R., et al. 2004; Estes et al. 2011). Fishing pressure reduce control of mid predators on urchins favoring the formation of turfs. When fishing is strong enough on urchins, it may favor the formation of turfs as well.

Nutrients inputs (regional, well established): Input of nutrients is another key driver of the regime shift, both natural from deep ocean upwelling or anthropogenic runoff (Gorman and Connell 2009). Nutrients inputs increase sedimentation and turbidity, favoring conditions for turf to outcompete kelps.

The main external indirect drivers that contribute to the shift are:

Population growth (global, speculative): Population growth leads to higher demand of food.

Food demand (local-regional, speculative): Higher food demands usually stimulate intense agriculture, both as expansion of agricultural frontier or increase of fertilizers use to increase yield. It also increase fishing pressure on the food web.

Agriculture (regional, well established): Agriculture often requires the use of fertilizers. When soils are eroded or washed, fertilizers run downstream increasing nutrients input to lakes and rivers.

Urban growth (global, well established): Urban growth in coastal zones increase the production of sewage that is rich in nutrients. It also increase the water runoff on the urban landscape, which also transports nutrients into coastal water.

Deforestation (regional, well established): Deforestation and poor agricultural management can accelerate, in magnitude and frequency, the nutrients runoff from agricultural lands. Deforestation increase landscape fragmentation and facilitates landscape conversion to agriculture. Both reduce the capacity of the landscape to retain water in the soil, accelerating erosive processes and runoff of nutrients (Smith and Schindler 2009).

Global warming (regional, speculative): Global warming is expected to increase average water surface temperature. It is also expected to increase the gradient between land and ocean temperatures, strengthening winds parallel to the coast and as result increasing upwelling of deep ocean water (Bakun et al. 2010). This could increase nutrients inputs on coastal ecosystems at the regional scale. On the local scale, global warming has favored the reproduction of urchins which acting in synergy with lobster fishing has reduced kelp resilience (Ling et al. 2009).

Slow internal system changes that contribute to the regime shift include:

Kelp fragmentation (regional, well established): Gorman and Connell (2009) also report that the loss of kelp dominated areas undermine kelp’s ability to reestablish in disturbed areas. While deforested areas surrounded by kelp patches are more likely to return to the kelp regime, isolated kelp disturbed patches are more likely to stick in the turfed regime.

Summary of Drivers # Driver (Name) Type (Direct, Indirect, Internal, Shock) Scale (local, regional, global) Uncertainty (speculative, proposed, well-established) 1 Kelp fragmentation Internal Regional Well-established 2 ENSO Shock Regional Well-established 3 Floods Shock Regional Well-established 4 Precipitation Shock Regional Well-established 5 Nutrients input Direct driver Local-Regional Well-established 6 Fishing pressure Direct driver Local-Regional Well-established 7 Demand for food and fiber Indirect driver Local-Regional Well-established 8 Agriculture Indirect driver Local-Regional Well-established 9 Fertilizers use Indirect driver Local-Regional Well-established 10 Urbanization Indirect driver Local-Regional Well-established 11 Sewage Indirect driver Local-Regional Well-established 12 Deforestation Indirect driver Regional Well-established 13 Global warming Indirect driver Global Well-established 14 Human population growth Indirect driver Global Well-established 15 Ocean upwelling Indirect driver Regional Well-established Summary of Ecosystem Service impacts on different User Groups References (if available) Provisioning Services Freshwater Food Crops Feed, Fuel and Fibre Crops Livestock Fisheries Wild Food & Products Timber Woodfuel Hydropower Regulating Services Air Quality Regulation Climate Regulation Water Purification Soil Erosion Regulation Pest & Disease Regulation Pollination Protection against Natural Hazards Cultural Services Recreation Aesthetic Values Cognitive & Educational Spiritual & Inspirational

Citation

Acknowledge this review as:

Juan Carlos Rocha, Reinette (Oonsie) Biggs, Garry Peterson. Kelp transitions. In: Regime Shift Database, www.regimeshifts.org. Last revised: 2011-02-28

References

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