Phenology of scyphozoan jellyfish species in a eutrophication and climate change context.

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.

Nutrient overload promotes the transition from top-down to bottom-up control and triggers dystrophic crises in a Mediterranean coastal lagoon.

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.