From coast to slope: Zooplankton communities shift in the Northern Alboran Sea.

The zooplankton composition of the Sound Scattering Layer (SSL) detected at multiples frequencies, 18, 38, 120 and 200 kHz, along the continental shelf of the Northern Alboran Sea (Mediterranean Sea) has been identified in July 2013 and 2014 by means of different plankton nets and the SSL dominant scatterers have been identified using zooplankton scattering models. Three acoustic patterns, based on their distinctive relative frequency response (r(f)), have been detected along the continental shelf, matching with three zooplankton communities; in coastal areas (25–40 m bottom depth), a homogeneous community composed by Fluid-like zooplankton, mainly crustaceans, exhibiting a r(f) peak at 120 kHz, (ii) in intermediate areas (40–190 m) a heterogeneous community composed by Fluid-like (crustaceans, chaetognaths, larvaceans, doliolids, Calycophora siphonophores), Elastic-shelled (heteropods) and Gas-bearing (fish larvae) zooplankton, exhibiting a r(f) peak at 38 kHz (iii) in the continental shelf edge (180–250 m), a zooplankton community dominated by Maurolicus muelleri fish larvae, exhibiting a r(f) peak at 18 kHz. Scattering models results showed that stronger scatterers as fish larvae (Gas-bearing), although scarce in samples, dominated the backscattering energy at almost all frequencies and locations. Fluid-like zooplankton contribution was only significant in coastal areas at high frequencies (120 and 200 kHz). The application of these findings to all echograms collected over the study area made it possible to obtain, the macro scale spatial distribution of the zooplankton communities as well as the SSL backscatter energy all over the continental shelf, highlighting the suitability of the use of acoustic data as an indicator of secondary production.