Distribution and abundance of larvaceans in the Southern Ocean

Larvaceans (also known as appendicularians) are zooplankton that inhabit most oceans, coastal waters and estuaries, and are often found in abundances that are second only to copepods among the meso-zooplankton. They form a gelatinous “house” through which they circulate seawater to filter very small particles onto mucopolysaccharide mesh. This concentrates sub-micron and micron-sized protists to 100 -1000 times the ambient concentrations. The house is discarded when the mesh clogs which can occur several times per day. These contribute significantly to the large particles of marine detritus known as “marine snow”. In addition, larvaceans form tightly compacted, rapidly sinking faecal pellets that contribute to carbon “export” from surface waters. Thus larvaceans play a rather unusual role in marine microbial food-webs, by transferring matter across many orders of magnitude in organism size, and moving it into the ocean depths. Previously there was little known about larvaceans in the Southern Ocean. The aims of this study were to increase the knowledge of the ecological role of the larvaceans species: by first determining their distribution and abundance in the Sub-Antarctic Zone (SAZ), Southern Oceans Permanently Open Ocean Zone (POOZ) and the Sea Ice Zone (SIZ) and secondly, by determining the diet of Southern Ocean larvaceans. Larvaceans were collected during four Southern Ocean marine science voyages between 2006 and 2008 which surveyed the different zones through different seasons in the East Antarctic sector of the Southern Ocean. Larvaceans were collected using a number of sampling devices; a purpose-built ring net, Rectangular Mid-Water Trawl (RMT1), Working Party 2 (WP2) net, HYDRO-BIOS MultiNet, Visual Plankton Recorder (VPR) and the Continuous Plankton Recorder (CPR). Larvacean distributions were complex. A significant fraction of the net hauls (55%) contained no specimens, while others obtained high abundances (maximum 57.8 ind. `m^-3`). The average abundances of larvaceans from the variety of nets used were 1.4 ± 5.4 ind. `m^-3`, and for the CPR were 6.4 ± 29.7 ind. `m^-3`. The surveys revealed that for the period of this study larvacean abundances varied between the Southern Ocean zones with the lowest abundances in the SAZ (0.6 ± 2.6 ind. `m^-3`, maximum = 15.9 ind. `m^-3`), highest in the POOZ (2.8 ± 10.6 ind. `m^-3`, maximum = 57.8 ind. `m^-3`) and 1.4 ± 4.8 ind. `m^-3` (maximum = 49.7 ind. `m^-3`) in the SIZ. Possible controls on larvacean distributions were evaluated by comparing them to physical (latitude, longitude, water temperature, salinity, light and sea-ice) and biological (chlorophyll and total zooplankton) distributions. Significant correlations occurred with physical parameters of latitude, longitude, fluorescence, irradiance, water temperature and salinity and the biological distributions of other Southern Ocean zooplankton. The diet of the Southern Ocean larvacean was determined using scanning electron microscopy (SEM) and stomach dissections from the samples collected from the ring net during January to March 2006. Protists ranging from 3.5 μm – 240 μm were the main food items. By considering the alignment and sizes of the largest ingested protist, Corethron pennatum, the inferred feeding house mesh size was 5 - 82 μm. From this study Southern Ocean larvaceans were estimated to contribute ~10.5 million tonnes of (wet) biomass to the Southern Ocean. Comparing this to the ~250 million tonnes of copepods, which are considered to be a keystone taxon, emphasizes the need to consider the importance of larvaceans to Antarctic food webs