Black Sea: Gelatinous Plankton Dominance
The Black Sea is a marine and coastal system previously dominated by top predators. Overfishing and increased nutrient input during the last 50 years, as well as climate change, triggered a shift of the system into a gelatinous plankton dominated regime in the late 80's, after which a population outburst of the invasive jellyfish Mnemiopsis leidyi occurred. The main feedback maintaining the regime is M. leidyi feeding on pelagic larvae and being better a competitor for zooplankton than the native jellyfish Aurelia aurita and pelagic fish. Ecosystem services related with food provision, biodiversity, aesthetic and recreational values, and nutrient cycling were affected by the regime shift. Management actions to restore the top predator regime include enforcement of fishing regulations, regional policies aimed to reduce excess nutrient input and the biological control of M. leidyi.
Type of regime shift
- Invasive Species Dominance
- Marine & coastal
- Small-scale subsistence crop cultivation
- Large-scale commercial crop cultivation
Spatial scale of the case study
- Sub-continental/regional (e.g. southern Africa, Amazon basin)
Continent or Ocean
- Eastern Europe and Asia Minor
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Key direct drivers
- Harvest and resource consumption
- External inputs (eg fertilizers)
- Species introduction or removal
- Global climate change
- Small-scale subsistence crop cultivation
- Large-scale commercial crop cultivation
Key Ecosystem Processes
- Primary production
- Nutrient cycling
- Aesthetic values
- Food and nutrition
- Livelihoods and economic activity
- Cultural, aesthetic and recreational values
Spatial scale of RS
Time scale of RS
- Contemporary observations
Confidence: Existence of RS
- Well established – Wide agreement in the literature that the RS exists
Confidence: Mechanism underlying RS
- Well established – Wide agreement on the underlying mechanism
Marine ecosystems, such as the Black Sea, can experience regime shifts between top predator and gelatinous plankton dominated regimes when pelagic fish overfishing and high nutrient concentration in the water column is present. The Black Sea experienced the following regimes:
Top predator dominated regime (until the 1950's)
This regime is described by low phytoplankton abundance, moderate to high abundance of zooplankton, low abundance of native gelatinous plankton (mainly the cnidarian A. aurita), low to moderate stocks of pelagic fish as sprat and anchovy, and high stocks of predator fish species as bonito, bluefish and mackerel. This top predator dominated regime is characterized by low fishing pressure, low levels of nutrients in the water column and well-oxygenated and clear waters.
Gelatinous plankton dominated regime (from early 1960's to present)
This regime is described by high phytoplankton abundance, low to moderate abundance of zooplankton, high abundance of gelatinous plankton (mainly the invasive ctenophore M. leidyi) leading to occasional blooms, low stocks of pelagic fish as sprat and anchovy, and low stocks (or even disappearance) of top predator fish species. This gelatinous plankton dominated regime is associated with eutrophication due to high concentration of phytoplankton and nutrients in the water column, therefore increasing the water turbidity and decreasing dissolved oxygen in the water.
Drivers and causes of the regime shift
The Black Sea shifted from being a top predator dominated system, to a gelatinous plankton one, due to the combined effects of a series of external driving forces and a strong shock, the introduction of an invasive jellyfish. The main driving force diminishing the response capacity of the system was the strong fishing activity due to increasing food demand, which had an effect on the population stock of predatory and pelagic fish and a consequent alteration of the dynamics in the food web (top-down control). The second key force was the increase of nutrient input due to the runoff from agricultural and urban activities, increasing the availability of nutrients in the water column of the sea and consequently raising the amount of phytoplankton and enhancing water turbidity (bottom-up dominance). Closely related to the nutrient input rise was the increase in the number of water mixing events as a response to more severe winters caused by global climate change, enhancing the availability of nutrients.
The fragility of the system due to the effect of the main drivers was demonstrated by a first outburst of the native gelatinous plankton species (A. aurita) by the end of the 1970's. By the early 1980's an invasive gelatinous plankton species (M. leidyi) was introduced in the system by means of evacuated ballast waters from ships arriving from outside areas. This species rapidly outnumbered the local gelatinous plankton (A. aurita) and the pelagic fish competing for zooplankton, setting itself as an important shock, rapidly allowing a shift to a gelatinous plankton regime dominated by a species with no natural predators.
How the regime shift worked
Under conditions of low fishing pressure and nutrient input, the Black Sea was dominated by top predators until the 1950's. High population of pelagic fish, which feed on gelatinous plankton A. aurita, kept the populations of this species in check. In addition, low nutrients limited the growth of phytoplankton.
Overfishing over a period of approximately 40 years reduced the population stock of pelagic fish, reducing the competition for zooplankton with gelatinous plankton, therefore weakening the top-down dynamics in the food web. At the same time, the bottom-up dynamics of the system were altered by the increased nutrient input, which enhanced turbidity and the growth of phytoplankton and facilitated a connected increase in zooplankton. Under these circumstances the system lost resilience and the accidental introduction of an invasive and highly competitive species of gelatinous plankton (M. leidyi), became the disturbance which pushed the system over a tipping point towards a gelatinous plankton dominated regime. M. leidyi took advantage of low pelagic fish population and high abundance of zooplankton to increase its population and dominance over the system.
The gelatinous plankton dominated regime is characterized by eutrophic and turbid waters, high concentration of nutrients and high fishing pressure. The most important feature of this regime is the high concentration of the invasive ctenophore M. leidyi, and the low abundance of pelagic fish. This regime is maintain by 1) the reinforcement feedback between M. leidyi and pelagic fish, as the M. leidyi feeds on the pelagic fish's larvae; 2) M. leidyi competes better than pelagic fish when the concentrations of zooplankton increase and 3) M. leidyi is a better competitor than A. aurita for zooplankton. These three facts, associated with the impacts of the external main drivers on the system, keep high concentrations of M. leidyi and maintain the system within this regime.
Impacts on ecosystem services and human well-being
The intensified fishing pressure in the gelatinous plankton dominated regime provides a high value on food provision given that more fish is harvested and actually used while decreasing the other ecosystem services found before the shift. The collapse of top predators and an increased pressure on pelagic fish causes a direct impact on biodiversity, in terms of species diversity (changes in species richness and evenness) while the booms of gelatinous plankton, especially in coastal areas, may impact on aesthetic and recreational values. The increase of nutrient input from agriculture and sewage has a direct impact on the nutrient cycling function, enhanced by a higher frequency of mixing events in cooler winters.
Human well-being is impacted positively in terms of food and nutrition for those potential consumers of the Black Sea fisheries, while livelihoods and economic activity is enhanced at a global scale through fisheries enterprises harvesting in the sea, regardless of their origin. Cultural, aesthetical and recreational values are reduced for those inhabitants of the coastline and potential tourists, while livelihoods and tourist dependant economic activities in the region are also affected.
No measures were taken to prevent the regime shift. However the main actions that could have been taken to enhance resilience should have aimed to avoid overfishing of predatory and especially pelagic fish. Being a top-down dominated system, it is important to identify keystone species in top trophic levels of the Black Sea and manage for them. Properly enforced fishing regulations leads to healthy fish populations, hence balanced primary production. On the other hand, nutrient load in the Black Sea could have been controlled to avoid eutrophication.
The Black Sea is a complex dynamic system with multiple linkages and mechanisms. Therefore, more than one action is required to encourage restoration. Currently, the system presents low phytoplankton production and increasing stocks of small pelagic fish. This was achieved by reducing fishing pressure, decreasing nutrient load originated from farming and sewage in the Black Sea Basin, and reducing populations of the invasive species M. leidyi through the introduction of the ctenophore B. ovata which feeds on M. leidyi. Warming hydroclimate is also leading to a restoration of the desirable regime.
Since the ecosystem is showing signs of hysteresis, more active intervention that considers well-implemented cross-scale and adaptive ecosystem-based management is required to avoid switching back to an undesirable regime. Therefore management actions should include enforcement of fishing regulations that consider location and time of year where/when fish spawning and reproduction take place, restrictions regarding fishing gear, and establishment of fishing quotas and economic alternatives for fishermen. It is also required to enforce regional policies that aim for a shift to the use of organic fertilizers, agroforestry and mechanisms to treat sewage.
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