Top predator dominated regime
The most important feedbacks that maintain the top predator dominated regime are the following:
- Pelagic fish and A. aurita feedback (reinforcing, regional scale, well-established): Pelagic fish and A. aurita compete for zooplankton. As pelagic fish feed on the native gelatinous plankton (A. aurita), an increase of the pelagic fish stocks decreases A. aurita populations, immediately followed by an increase of zooplankton concentrations which later leads to an increase of pelagic fish stocks.
- Pelagic fish, A. aurita and zooplankton feedback (balancing, regional scale, well-established): Pelagic fish and A. aurita compete for the same resource, zooplankton. In a top-down trophic web controlled by top predators, with moderate stocks of pelagic fish and low levels of A. aurita, pelagic fish populations indirectly control jellyfish populations.
- Nutrient cycling feedback (balancing, regional scale, well-established): An increase in the nutrient concentrations in the water column leads to an increase in the growth of phytoplankton, while the growth of phytoplankton removes nutrients from the water column. This balancing feedback keeps the nutrient and phytoplankton concentrations at low to moderate levels.
Gelatinous plankton dominated regime
The most important feedbacks that maintain the gelatinous plankton dominated regime are the following:
- Pelagic fish and A. aurita feedback (reinforcing, regional scale, well-established): In a regime with very low stocks of pelagic fish, the concentrations of native gelatinous plankton (A. aurita) trend to increase. This fact leads to a decrease on zooplankton populations followed by a decrease on the pelagic fish stocks.
- Pelagic fish, gelatinous plankton and zooplankton feedbacks (balancing, regional scale, well-established): On the other hand, the new invasive ctenophore M. leidyi, the native cnidarian A. aurita and pelagic fish compete also for the same resource, the zooplankton. The efficiency of M. leidyi consuming zooplankton is higher than the others. This better efficiency, combined to an increase in zooplankton due to a rapidly increase of phytoplankton, leads M. leidyi to dominate over A. aurita and pelagic fish, allowing an increase of M. leidyi populations.
- M. leidyi and pelagic fish feedback (reinforcing, regional scale, well-established): M. leidyi feeds on pelagic fish larvae. An increase in the abundance of M. leidyi therefore leads to a decrease of pelagic fish, directly followed by a high increase in zooplankton, and therefore, a rapidly increase in M. leidyi abundances.
Important shocks that contribute to the regime shift
- Introduction of the invasive species M. leidyi (regional, well-established): The increase of maritime trade in the Black Sea and poor regulation regarding the management of evacuated ballast waters are considered the main contributors for the introduction and spread of M. leidyi.
Main external direct drivers that contribute to the regime shift
- Overfishing of predatory fish (regional, well-established): Fishing activity in the area has primarily focused on big individual species, exerting strong fishing pressure over the predatory fish trophic level, dangerously decreasing its variability and population stock, and consequently disturbing the food web dynamics.
- Overfishing of pelagic fish (regional, well-established): Pelagic fish population has suffered strong fishing pressure due to the overfishing of predatory fish population in combination with the constantly increasing seafood demand. The decrease of pelagic fish populations (consumer or top-down control) had an effect on the variance of zoo and phytoplankton populations as a response to physical and environmental fluctuations explained by the following two external direct drivers.
- Nutrient input (regional, well-established): During the last fifty years, the Black Sea has suffered a constant increase of nutrient input related to urbanization processes and agricultural activities. The excessive availability of nutrients in the system implies an increase of phytoplankton and a consequently increased turbidity in the water, decreasing the capacity of top predators to locate pelagic fish prey and enhancing the growth of zooplankton and its availability for gelatinous plankton consumption.
- Frequency of mixing events (regional, well-established): The mixing events in the Black Sea increased as a response to the enhanced severity of the winter periods in the region, implying a rise in nutrient availability and phytoplankton.
Main external indirect drivers that contribute to the regime shift
- Population Growth (global, well-established): The accelerated growth of population has shown its effects in the analysed system by means of food demand and urbanization.
- Food Demand (global, well-established): The growth of food demand exerts an increasing pressure for intensifying the fishing and agriculture activities in the region. At the same time, agricultural activities contribute to an increasing input of nutrients to the Black Sea.
- Urbanization (regional, well-established): The different activities related to the growing urban areas within the Black Sea´s basin have an increased contribution of waste water, consequently raising the input of nutrients.
- Climate Change (global, well-established): More severe winters in the Black Sea region due to global climate alterations, have a direct effect on the frequency of mixing events, considerably increasing the amounts of these events during the year, hence increasing nutrient availability.
A threshold could be crossed when jellyfish becomes dominant and suppresses the pelagic fish recruitment, in combination with high fishing pressure over the pelagic fish. Oguz and Gilbert (2006), for example, suggest a precautionary limit for fishing pressure over pelagic fish of 0.5 FP (fishing pressure defined as the catch per stock per year).
Another threshold could be identified when nutrient inputs lead to algal blooms that cannot be controlled by zooplankton.
- Fishing pressure (regional scale, well-established): Regulated fishing activities avoid overexploitation of top predators, consequently avoiding cascades down the trophic web. Reducing the fishing pressure over pelagic fish enhances recovery of their populations. Healthy pelagic fish populations will therefore feed on zooplankton and avoid excess zooplankton to be consumed by M. leidyi, which is an efficient consumer when food density is high. Regulated fishing should empower fishermen and be enforced by institutions.
- Nutrient input (regional, well-established): Decrease of nutrient input to the Black Sea avoids eutrophication and phytoplankton blooms leading to low density of zooplankton and consequently decreases food availability for M. leidyi. Less nutrient input also reduces turbidity and enables predatory fish to find their food with fewer problems. Actors involved in this action are inhabitants and users of the Black Sea basin, as well as institutions at a regional level.
- Introduction of the bio-control species B. ovata (regional, contested): B. ovata is a comb jelly but unlike other similar species, it does not feed on crustacean zooplankton, but prefers other ctenophores like M. leidyi. Introduction of B. ovata to control populations of M. leidyi might be a potential solution to the problem of itsinvasion and an explanation why their populations have already started to decline in the Black Sea.
Ecosystem service impacts
The fisheries in the Black Sea in both regimes provide food production for the global market, benefiting consumers and fishery industry. This ecosystem service is enhanced in the gelatinous plankton dominated regime due to an increased harvest of the fish stock. However, continued overfishing activity could produce a hypothetical loss of this ecosystem service in the future if the fishery collapses. On the other hand, the pristine top predator dominated regime has higher stocks of predator and pelagic fish, hence a higher potential of food production. Nonetheless, food harvest in the top predator dominated regime was lower than in gelatinous plankton dominated regime, and this service is therefore not utilized.
Biodiversity is strongly altered. Some targeted fish species could become extinct in the gelatinous plankton dominated regime causing a loss of species richness. Most importantly, species evenness is strongly perturbed, leading to an ecosystem with a high abundance of gelatinous plankton and phytoplankton and a low number of individuals of the targeted fish species. As biodiversity is considered a supporting service necessary for the existence of the other ecosystem services and key processes, all users groups would be affected by its loss.
The nutrient cycling and hence primary production of this system is altered because of a higher input of nutrients from the use of fertilizers in agriculture and from sewage originated in the Black Sea basin. Changes in these key processes can impact on biodiversity and, indirectly, on the other ecosystem services provided.
In conclusion, consumers and fishing industry are the two main user groups benefited from the gelatinous plankton dominated regime due to the high levels of fish harvest. On the other hand, local population, potential tourists and tourist industry are clearly affected because of the loss of recreation and aesthetic values. Loss of biodiversity and changes in primary production and nutrient cycling affect the system's capacity to provide ecosystem services.
Uncertainties and unresolved issues
The Black Sea is showing some signs of hysteresis as populations of M. leidyi have already started to decrease due to recovering populations of small pelagic fish and current low phytoplankton densities. This could have also been influenced by the introduction of B. ovata. Further research is required to obtain more knowledge on the role of B. ovata. Taking into account that this is a social-ecological system, the main aim should not be returning to a pristine top predator dominated regime, but to one that sustainably balances human well-being by means of ecosystem services provision and a healthy marine ecosystem.