What happened in the mid-1990s? The coupled ocean-atmosphere processes behind climate-induced ecosystem changes in the Northeast Atlantic and the Mediterranean

Northeast Atlantic marine ecosystems such as the Bay of Biscay, Celtic Sea, English Channel, Subpolar Gyre region, Icelandic waters and North Sea as well as the Mediterranean Sea show concomitant ‘regime shift’-like changes around the mid-1990s, which involved all biota of the pelagial: phytoplankton, zooplankton, pelagic fish assemblages, demersal fish assemblages and top predators. These shifts were caused by complex ocean-atmosphere interactions initiating large-scale changes in the strength and direction of the current systems, that move water masses around the North Atlantic, and involved the North Atlantic Oscillation (NAO), the Atlantic Meridional Overturning Circulation (AMOC), and the subpolar gyre (SPG). The contractions and expansions of the SPG and fluctuations of the Atlantic Multidecadal Oscillation (AMO) play a key role in these complex processes. Small pelagic fish population trends were the sentinels of these changes in the mid-1990s in the ecosystems under investigation

Gulf of Cadiz zooplankton: community structure, zonation and temporal variation

This study provides the first comprehensive description of the Gulf of Cadiz (SW Spain) marine zooplankton community. To do this we compiled information from a number of surveys and monitoring programs carried out in the area over the last 14 years. Using this time-series of quarterly (3 times per year) resolution, we describe their general composition, spatio-temporal variability and relationship with environmental factors. Zooplankton is dominated by copepods (mainly Clausocalanus, Paracalanus, Oncaea and Oithona), except in warm periods, when outbreaks of cladocerans (mainly Penilia avirostris but also Podon intermedius and Pleopis polyphemoides) outnumber copepods, accounting for more than 80% of total abundance on occasion. Zooplankton diversity was found to be higher in deeper waters while the density of most taxa (but not their abundance) decreased towards the ocean. On top of this general coast-ocean pattern a north-south gradient could also be distinguished resulting in three distinct zones. In relation to environmental structuring, mesoscale processes, sea surface temperature, winds, river runoff and primary productivity were identified as main factors shaping this zonation. Overall, zooplankton abundance showed a decreasing trend during the study period (2001-2015), caused by a drop of summer abundances, that was mainly driven by cladocerans.Versión del editor3,26