The excess input of nutrients that triggers eutrophication processes is one of the main destabilizing factors of coastal ecosystems, being coastal lagoons prone to suffer these effects and present dystrophic crises. This process is aggravated by the current trend of rising temperatures and more frequent torrential rains due to climate change. We observed that the Mar Menor lagoon had a great capacity for self-regulation of its trophic web and resistance to the eutrophication process, but after 30 years of nutrient input due to the change in the agricultural regime in its drainage basin in the 1990s, the lagoon ecosystem has suffered several of these events. In this work, we characterize the seasonal dynamic of the pelagic system during the last dystrophic crises. Phosphorus and nitrogen alternate as the limiting nutrient for phytoplankton proliferation. The entrance of phosphorus is mainly related to vacation periods, while nitrogen inputs, both superficial and sub-superficial, are more related to chronic high nitrates concentration in the water table after the agricultural activities carried out in the area changed. Our analysis reveals that the summer season is prone to suffer periodical hypoxia events when the N/P ratio decreases, and the temperature rises. In the Mar Menor, the ecological balance has been maintained in recent decades thanks to, among other mechanisms, the spatial and temporal segregation of top-down control over phytoplankton exerted by three species of jellyfish. However, the deep reduction in the abundance of the summer jellyfish species and the excessive proliferation of phytoplankton has meant the loss of this control. Moreover, we have registered a decline in the abundance of all the other zooplanktonic groups during the dystrophic crises. We suggest that management actions should address the input sources of water and nutrients, and an integrated management of the activities carried out throughout the watershed.

The study of jellyfish blooms has gained attention in the recent decades because of the importance of forecasting and anticipating them and avoiding their interference with human activities. However, after thirty years of scientific effort (monitoring systems, empirical laboratory and field studies, modeling, etc.), the occurrence of blooms remains unpredictable, and their consequences unavoidable. Climate change, eutrophication, overfishing, coastal construction, and species translocation have been suggested as stressors that increase them, but robust evidence to support these claims is limited. The widespread belief that jellyfish blooms are “increasing in number” has been challenged in recent years. Among the gelatinous zooplankton, the bloom forming species are concentrated in the class Scyphozoa, and the number of species with at least one recorded bloom has increased during the last decade. The analyses of long-term time series show seasonality in the dynamic of each blooming jellyfish species population, but the blooms vary in intensity and there are years of an unexplained absence of jellyfish. In this review, we focus on the current state of knowledge, uncertainties and gaps in the critical points that can strongly influence the intensity of the bloom or even lead to the absence of the medusa population. These points include ephyrae, planulae and scyphistoma natural, predatory or fishing mortality, the molecular pathway of strobilation, benthic population dynamics, planula settlement and ephyra to medusa transition success. Some of these points account for certain empirical laboratory evidence under controlled conditions, and are difficult to be studied on the field, but the different sources of non-typically recorded variability need to be addressed to improve our understanding of jellyfish population dynamics.

142 scientific publications have been reviewed on the characteristics of the scyphozoans with respect to their ability to develop blooms and the most significant environmental characteristics that determine them. Special attention was paid to depth, temperature, salinity, chlorophyll concentration, and the habitat of the 39 registered blooming genera. After the review, we find that over the past decades, the number of scyphozoan blooming-species is higher than previously recorded, increasing from circa 14% to 25% of the class. Species that inhabit depths less than 27.1 m are prone to produce blooms, particularly in semienclosed areas with low rates of water renewal and high thermal amplitudes. Temperature appears as the main environmental factor controlling blooms, but food availability is essential to sustain the proliferations. Interspecies variability in the response to environmental factors observed in this work suggest that bloom predictive models should be constructed species-habitat-specific.

The uprising interest in gelatinous zooplankton populations must cope with a lack of robust time series of direct abundance observations in most of the ecosystems because of the difficulties in sampling small, fragile organisms, and of the dismissal of jellyfish as a nuisance. Most of the hypotheses about their dynamics are built on a few species and ecosystems and extended to the whole group, but the blooms are registered mainly for the members of the Class Scyphozoa that dwell in temperate, shallow waters. Within the scyphozoans, our knowledge about their phenology relies mainly on laboratory experiences. Here we present a long-term analysis of the phenology and life cycle of three scyphozoan species in an ecosystem affected by eutrophication in a climate change context. We have found that the phenology is directed by temperature, but not modified by different thermal and ecological regimes.