Three-dimensional structure of a swarm of the salpThalia democraticawithin a cold-core eddy off southeast Australia

Abstract

[1] Swarms of the salp Thalia democratica periodically occur off southeast Australia following the austral spring bloom of phytoplankton. In October 2008 a filament of upwelled water was advected south by the adjacent East Australian Current and formed a 30 km diameter cold-core eddy (CCE). The three-dimensional structure of a subsurface swarm of T. democratica within the eddy was examined using both oblique and vertical hauls and an optical plankton counter (OPC) deployed on a towed body. The CCE displayed distinct uplift of the nutricline and elevated fluorescence. Net samples show the zooplankton community was dominated by T. democratica, comprising 73%-88% of zooplankton abundance. The size distribution of T. democratica measured from net samples was 0.5-5 mm and was used to interpret the OPC transects, which showed the swarm formed a 15 km diameter disc located 20-40 m deep in the center of the eddy. The maximum salp abundance was in the pycnocline and coincided with the subsurface fluorescence maximum. The mean abundance of T. democratica size particles within the disc was 5003 individuals m À3 (ind. m À3 ), contrasted with only 604 ind. m À3 at the outer edge of the eddy. The vertically concentrated and horizontally constrained disc-shaped salp swarm occurred at the interface of salp-bearing inner shelf water and nutrient-rich upwelled water in a CCE. The physical processes that formed the CCE on the inshore edge of the western boundary current led to the largest density of salps recorded.Citation: Everett, J. D., M. E. Baird, and I. M. Suthers (2011), Three-dimensional structure of a swarm of the salp Thalia democratica within a cold-core eddy off southeast Australia,

Entrainment and retention of the coastal larval fish assemblage by a short‐lived, submesoscale, frontal eddy of the East Australian Current

“…The importance of eddies for fish production was proposed in Bakun's (2006) three stage model of favorable reproductive habitat-enrichment, larval concentration, and retentionproviding an explanation for successful fisheries recruitment in boundary current systems (Logerwell and Smith 2001;Kasai et al 2002;Okazaki et al 2003). Furthermore, the advection of eddies can be up to 5-to 10-fold less than the boundary current itself (Mooers and Fiechter 2005;Everett et al 2011), giving the biological community time to grow and become nektonic.…”

“…Cyclonic (cold-core) eddies can further increase primary and secondary productivity through the uplifting and doming of isotherms and nutriclines (Lee et al 1991;Kimura et al 1997;Everett et al 2011).…”

Decadal Distribution and Abundance Trends for the Late Stage Copepodites of Pseudocalanus spp. (Copepoda: Calanoida) in the US Northeast Continental Shelf Ecosystem.

“…Though there is no record of salps directly feeding on Pseudocalanus spp., these filter feeders ingest a wide size range of particulate matter and can substantially reduce the quantities of phytoplankton, bacteria, and microzooplankton that other mesozooplankton use for food (Paffenhofer, 1994;Vargas andMadin, 2004, Bernard et al, 2012). Several studies have proposed that high predation rates by salps indirectly limit the growth of copepod populations (Dubischar and Bathmann, 1997;Halsband-Lenk et al, 2001;Everett et al, 2011). This study has found that salps and Pseudocalanus spp.…”

Massive salp outbreaks in the inner sea of Chiloe Island (Southern Chile): possible causes and ecological consequences

“…Aggregations have generally been described by the presence of geological barriers (i.e., shoreline and seafloor) and hydrographic (i.e., horizontal fronts and vertical discontinuities) (reviewed by Graham et al, 2001) or mesoscale eddies (Everett et al, 2011). However, the effect of man-made barriers is less well documented (Purcell et al, 2007;Doyle et al, 2008;Baxter et al, 2011;Mianzan et al, 2014).…”