Abstract
The size structure of zooplankton communities in the Gulf of Lion, NW Mediterranean Sea, was studied in May 2010 and January 2011. The integrated physical and biological measurements provided a 3D view with high spatial resolution of the physical and biological variables and their correlations over the whole gulf. The effects of physical processes such as freshwater input, coastal upwelling, and water column mixing by winds on phytoplankton and zooplankton distributions were analyzed using these data. During the winter, strong northerly winds mixed the water column, and the vertical distributions of biological variables were uniform over most of the gulf while there were local hot spots with high chlorophyll a (chl a) concentrations in front of the Rhone mouths and in coastal areas. During the spring, light winds and water column stratification resulted in less vertical mixing, and the Rhone River freshwater plume spread over a large part of the gulf. The nutrients delivered by the freshwater input encouraged high primary production in the surface layer. In the pycnocline, a thin layer of high particle concentration was associated with these high phytoplankton biomasses. Three habitats were distinguished based on statistical analysis performed on biological and physical variables: (1) the coastal area characterized by shallow waters, high chl a concentrations, and a steep slope of the normalized biomass size spectrum (NBSS); (2) the area affected by the Rhone with high stratification and flat NBSS slope; and (3) the continental shelf with a deep mixed layer, relatively low particle concentrations, and moderate NBSS slope. Defining habitat is a relevant approach to designing new zooplankton sampling strategies, validating distribution models and including the zooplankton compartment in trophodynamic studies.
Highlights
Continental margins are generally highly productive areas compared to the open sea and provide the main spawning habitats for small pelagic fishes (Palomera et al 2007, Planque et al 2007, Bellido et al 2008, Nicolle et al 2009)
The wind conditions and river flows, which are the main factors of non-stationarity over short time scales in the Gulf of Lion, were constant during both surveys, resulting in the model predicting nearly stable physical structures
This was in agreement with the low values of currents inferred from ADCP data
Summary
Continental margins are generally highly productive areas compared to the open sea and provide the main spawning habitats for small pelagic fishes (Palomera et al 2007, Planque et al 2007, Bellido et al 2008, Nicolle et al 2009). Physical processes have direct effects on the production of the first trophic levels by mixing different water masses and bringing nutrients from terrestrial runoff and coastal upwelling. To understand how these environmental conditions affect zooplankton distributions at the mesoscale, physical and biological variables must to be measured simultaneously at a sufficiently high resolution to detect fine. Recent advances in in situ (Bellido et al 2008, Giannoulaki et al 2011, Tugores optical and acoustic sensors have made such high- et al 2011, Salas-Berrios et al 2013), only a few studresolution measurements of physical and biological ies have used this concept for zooplankton.
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