The impact of jellyfish on the estuarine ecosystems: the Guadiana study case

Muha, Teja Petra

http://hdl.handle.net/10400.1/7722

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Authors

Muha, Teja Petra

Advisor(s)

Chícharo, Alexandra

Ben-Hamadou, Radhouan

Chícharo, Luís

Abstract(s)

The presence of newly introduced species of jellyfish in the estuarine systems can result in diverse disruptions at different levels of the food web affecting native competitor species, predators and prey. Newly introduced species of jellyfish can poses a threat to this system by affecting the well- established food web dynamics. There was a noticeable increase in jellyfish presence in years after the Alqueva dam in relation to the species Aurelia aurita, Blackfordia virginica, Maeotias marginata and Catostylus tagi. B. virginica has become the most widely spread species within the Guadiana Estuary. Its presence might be an important factor influencing the pelagic ecosystem in Guadiana by predation effect and competition with other predators. As zooplankton presents a main food source for zooplanktivorous fish and fish larvae the jellyfish species may be able to over compete the native fish population or decrease the food supply to insufficient level. This could directly result in decrease of zooplankton abundance correlated to an increase of phytoplankton and consequently in eutrophication. Presumably there is a potential for jellyfish species annual growth and establishment. This work presents a food web model located in the Guadiana Estuary situated between South of Spain and Portugal where the impact of jellyfish B. virginica was evaluated in a model based on the variation of biomass of each state variable (mgC m-3). B. virginica has been present in the estuary since 2002 with the highest abundance of 31.2 ind. m-3 measured in 2008. The Guadiana Estuary had experience significant changes related to fresh water availability as Alqueva dam was built in 2002. What impact does water discharge and pattern of precipitation rate related to NAO has on present ecosystem has been evaluated through the nutrient and seston concentration in the model. Extential sampling data from year 1997 up to 2012 was conducted in Guadiana Estuary which data were used through the process of statistical analysis for the model functioning. Guadiana Estuary is divided into three sub-areas: upper with stations Alcotium and Gueirros do Rio; middle with sampling at Almada de Ouro and Foz de Odeleite; and lower estuary where samples were taken at Esteiro Carrasqueira, Barra, River Plume (Pluma) and Praia de St. Antonio station. Presence of B. virginica normally takes place in brackish zone where ETM zone is present which is characterized by mixed salinity. We have developed a seasonal food-web model based on the annual presence of jellyfish B. virginica, which is from beginning of June to the end of August. We have used 6 groups of marine organisms represented as state variables in the model. Groups of phytoplankton, zooplankton, ichthyoplankton (fish eggs and fish larvae separated), B. virginica, juvenile European anchovy (Engraulis encrasicolus) and their biomass, distribution, and diet were used from survey data. Food web model was created within the most important groups in the ecosystem and presented in the conceptual diagram. For our theoretical network model we have worked on a predator- prey relation expressed with the Michaelis- Menten exponential response. Through different sensitivity tests we have shown the potential impact of jellyfish species on the present food web through different scenarios. Statistical analysis based on average abundance rate of B. virginica and zooplankton compared to abiotic conditions was done for an easier clarification and comparison with the results from the model. The results obtained by the model developed in this thesis are in conformity with field measurements to what it concerns biomass values of each individual group in the model. The model shows a significant impact of B. virginica over all groups of species presented in the model. Nutrient and seston concentration appears to be the most influential trigger for the majority of the food web dynamics. Sensitivity analysis has proven that the phytoplankton, zooplankton and juvenile anchovy are the most sensitive organisms in the whole food web influenced by nutrient availability and water discharge. B. virginica, fish eggs and fish larvae biomass has proven to vary upon these conditions tested in different runs though they are not affected directly. There is significantly high jellyfish biomass increase in case of high nutrient concentrations. In similar situation if B. virginica is not present there is a high rise in ichthyoplankton biomass, juvenile anchovy and zooplankton. Situation with low water discharge and high nutrient amount appears to be the most devastating for the estuarine ecosystem. In situation with low nutrient conditions the phytoplankton, fish eggs, fish larvae and B. virginica biomass increases through summer but zooplankton and juvenile anchovy biomass decreases. Trend of this groups tend to change in different nutrient conditions. Significant changes in biomass content are noticed between all groups. Compared to low nutrient concentration simulations in high nutrient concentration present that all state variables have an initial increase and only due to the predatory effect the decrease of certain groups occur. Detritus production trend follows up the movement of biomass increase and decrease. There is a strong correlation of B. virginica increase due to predational effect over zooplankton which levels seem to be detrimental for the fish eggs. Fish larvae group appears to be the most resistant group for the B. virginica pressures on the ecosystem. By the results in the model we can see that the NAO index can reflect a pattern of each individual biomass group in the food web as it is partially responsible for the nutrient concentration in the estuary. The most impacted group by B. virginica is zooplankton which controls phytoplankton growth from top down. These relation causes bottom up control as zooplankton biomass influences the ichthyoplankton and juvenile anchovy biomass. In case of high winter water discharge the detritus production is higher and the turbidity at the mouth of the estuary attracts higher amount of adult’s fish to spawn. Higher fish eggs biomass causes higher and faster growth of B. virginica which is able to consume higher amount of zooplankton and with that controls the decrease in juvenile anchovy population and fish larvae survival rate. Possible presented scenario is over- predation of zooplankton which can lead to phytoplankton bloom.