Spatial and temporal variability of zooplankton off New Caledonia (Southwestern Pacific) from acoustics and net measurements

Abstract

Spatial and temporal distribution of zooplankton off New Caledonia in the eastern Coral Sea was studied during two multidisciplinary cruises in 2011, during the cool and the hot seasons. Acoustic measurements of zooplankton were made using a shipborne acoustic Doppler current profiler (S-ADCP), a scientific echosounder and a Tracor acoustic profiling system (TAPS). Relative backscatter from ADCP was converted to biomass estimates using zooplankton weights from net-samples collected during the cruises. Zooplankton biomass was estimated using four methods: weighing, digital imaging (ZooScan), ADCP and TAPS. Significant correlations were found between the different biomass estimators and between the backscatters of the ADCP and the echosounder. There was a consistent diel pattern in ADCP derived biomass and echosounder backscatter resulting from the diel vertical migration (DVM) of zooplankton. Higher DVM amplitudes were associated with higher abundance of small zooplankton and cold waters to the south of the study area, while lower DVM amplitudes in the north were associated with warmer waters and higher abundance of large organisms. Zooplankton was largely dominated by copepods (71-73%) among which calanoids prevailed (40-42%), with Paracalanus spp. as the dominant species (16-17%). Overall, zooplankton exhibited low abundance and biomass (mean night dry biomass of 4.7 6 2.2 mg m 3 during the cool season and 2.4 6 0.4 mg m 3 during the hot season) but high richness and diversity (Shannon index $4). Substantially enhanced biomass and abundance appeared to be episodically associated with mesoscale features contributing to shape a rather patchy zooplankton distribution.

Mesozooplankton structure and functioning in the western tropical South Pacific along the 20° parallel south during the OUTPACE survey (February–April 2015)

“…30 At a regional scale, similar results were described by Smeti et al (2015) for the NECTALIS campaigns in July (NECTALIS 1 campaign) and December (NECTALIS 2) in the Coral Sea. They identified 10 common taxa explaining 63% of the similarity in all zones identified in both studies: Clausocalanus/Paracalanus, Appendicularians, Oncaea, Oithona, Corycaeus, Ostracods, Chaetognaths, Lucicutia, Mecynocera and Acartia.…”

“…Generally, frontal aggregation of zooplankton biomass can occur at anti-cyclonic eddies peripheries (Riandey et al, 2005;Smeti et al, 2015). In oligotrophic regions, eddies potentially represent highly productive areas for zooplankton, positively impacting the biomass of all subsequent levels of the food web (Rissik & Suthers, 2000;Smeti et al, 2015). Other mesoscale activities were observed in the CE-MA region during OUTPACE between 170-180°W (Rousselet et al this issue) which could explain the relative increase in zooplankton biomass at SD-9.…”

“…One joint program, Zonal Flux (April l5-May 14, 1996), was an equatorial transect cruise made during a -La Nina‖ event (April-May 1996) in the equatorial Pacific upwelling. In the WTSP, zooplankton 10 studies are rare and largely confined to the Coral Sea (Le Borgne et al, 2010;Smeti et al 2015). In the eastern tropical South The OUTPACE survey (Oligotrophy to UlTra-oligotrophy PACific Experiment, 18 February and 3 April 2015), aboard the RV L'Atalante, was designed specifically to sample a variety of trophic conditions along a west-east transect 15 covering 4000 km in the SE Pacific Ocean from the western part of the Melanesian archipelago (New Caledonia) to the western were boundary of the South Pacific gyre (French Polynesia).…”

“…The survey path between stations SD1 to SD5 passed through a succession of cyclonic and anticyclonic eddies, but the distance between sampling stations was unfortunately not sufficient to map them. A few studies have related the impact of mesoscale structure on zooplankton distribution in the region (Le Borgne et al 1985;Smeti et al, 2015). Around Mare (the southernmost Loyauté Island), Le Borgne et al (1985) found similar zooplankton enrichment not correlated with chlorophyll increase, associated 30 with diverse mesoscale processes and in particular the island mass effect leeward (west) of Mare.…”

“…The cyclonic or anticyclonic nature of the eddy influences not only the distributions of zooplankton abundance and biomass from the center to the edge of the eddy, but the taxonomic distribution as well ( Riandey et al 2005). Generally, frontal aggregation of zooplankton biomass can occur at anti-cyclonic eddies peripheries (Riandey et al, 2005;Smeti et al, 2015). In oligotrophic regions, eddies potentially represent highly productive areas for zooplankton, positively impacting the biomass of all subsequent levels of the food web (Rissik & Suthers, 2000;Smeti et al, 2015).…”