Zooplankton long-term changes in the NW Mediterranean Sea: Decadal periodicity forced by winter hydrographic conditions related to large-scale atmospheric changes?

Copepod, chaetognath, decapod larva, siphonophore and jellyfish monthly abundances, from 1974 to 2003 at Point B (northwestern Mediterranean), were obtained with the ZooScan. Principal component analysis (PCA) was performed on zooplankton, and another PCA on local environment. Almost-decadal periods (1974–1982, 1983–1991, 1992–1999, and 2000–2003) were distinguished in the 1st PC of zooplankton, and that of local environment (1974–1980, 1981–1991, 1992–1998, and 1999–2003). The 1st PC of local environment was correlated with winter North Atlantic Oscillation (NAO) until early 1990s. In early 1980s, all groups increased and the majority of the decade abundances were above the long-term average for most groups. In the 1990s, all decreased, and in early 2000s they increased. This synchrony suggests bottom-up control as main mechanism structuring these groups. The 1980s were characterized by low winter temperature and high salinity. We hypothesize that phytoplankton production was favored during that decade due to increased nutrient uprise to surface by strong winter vertical mixing. In the 1990s salinity decreased probably to the detriment of vertical mixing and carrying capacity of the system. These results stress the role of salinity as physical forcing on water-column stability, in the NW Mediterranean, and the importance of winter conditions to determine the state of pelagic ecosystems. ► We analyzed 30 years of 5 zooplankton monthly abundance at Point B with the ZooScan. ► Main modes of zooplankton abundance and local environment had quasi-decadal changes. ► We did not find dominance of jellyfish from the late 80s related to water warming. ► Abundances dropped in the early 90s coinciding with winter-hydrography periodicity. ► The link of local hydro-climate with winter NAO is not temporary stationary.