Zooplankton in the Ligurian Sea: Part II. Exploration of their physical and biological forcing functions during summer 2000

A survey of the biological and physical oceanography of the Ligurian Sea was conducted in the late summer of 2000. Forty-one stations were sampled for nutrients, oxygen, fluorescence and hydrographic information. Acoustic backscatter measurements were used to estimate abundance of small (<5 mm) zooplankton biovolume versus depth and the distribution of northern krill, Meganyctiphanes norvegica. Net-tow and underwater video data were collected to identify the zooplankton present. These data were used to analyze the Ligurian Sea ecosystem for physical and biological linkages that control zooplankton abundance and distribution. Results are compared with those from a similar study conducted in 1999. Hydrographic sampling showed a dome of dense water in the southwestern middle of the basin. The highest chlorophyll a (Chl a) concentrations were measured in this area, while small zooplankton biovolume was evenly distributed throughout the survey. Integrated values of Chl a and small zooplankton biovolume in 2000 were greater than in 1999. Meganyctiphanes norvegica, siphonophores and salps were the dominant components of the macrozooplankton population in the upper 200 m. In the sampled depth strata, siphonophore abundance did not change during the day, while M. norvegica were only caught at night. Acoustic backscatter data show that higher densities of M. norvegica occurred in deeper water and in the western and southwestern areas of the Ligurian Sea.