Salp swarms in the Tasman Sea: Insights into their population ecology

Pelagic tunicates, including salps, are an important category of gelatinous zooplanktonand yet are relatively understudied. Salps regularly occur in intermittent swarms and canquickly become the most dominant zooplankton within an area. My thesis explores thetrophic interactions of salps within the zooplankton community using stable isotopeanalysis, the environmental drivers of salp population dynamics, and the occurrence ofsalp deposition on the sea floor.The trophic niche of salps within the oceanic zooplankton community was investigatedusing stable isotopes of carbon and nitrogen. Zooplankton and suspended particulateorganic matter (POM) were sampled in three different water types: inner shelf (IS), acold core eddy (CCE; cyclonic) and a warm core eddy (WCE; anti-cyclonic). Recentupwelling in the IS water type resulted in lower than expected trophic enrichment for allzooplankton species (0.53 compared to 3.4), and the salp Thalia democratica wasdepleted in 15N compared to POM. Trophic enrichment of zooplankton within the CCE(2.74) was higher than the IS, and more similar to expected results (3.4). Based onchlorophyll a and nitrate concentrations, the WCE was characterised as an oligotrophicenvironment and was associated with an increased trophic level for omnivorouszooplankton (copepods and euphausiids) to a similar level as carnivorous zooplankton(chaetognaths). This study shows that trophic relationships among the zooplankton aredynamic and can vary across water types.The demographic characteristics of three salp swarms were studied to examine factorsinfluencing variations in salp swarm magnitude. The interannual abundance of Thaliademocratica during spring was related to the rates of asexual reproduction (buds perchain). T. democratica abundance was significantly higher in October 2008 (1312individuals m-3) than 2009 and 2010 (210 and 92 individuals m−3, respectively). Salpabundance was negatively related to buds per chain and relative growth rates, implyinga faster release rate. As T. democratica abundance was significantly positively related tofood >2 μm in size and negatively related to the proportion of non-salp zooplankton,salp swarm abundance may depend on the abundance of larger phytoplankton(prymnesiophytes and diatoms) and competition with other zooplankton. A discrete-time, size-structured Thalia democratica population model was developed toinvestigate the temporal resolution of a salp swarm. The model used size-dependentreproduction and mortality, where growth was dependent on food consumption(chlorophyll a biomass) and temperature. Average generation time (12 days) and meanabundances of each stage correspond to previously reported values. Salp ingestion rateand the doubling time of chlorophyll a were the most influential parameters, negativelyinfluencing salp biomass and abundances for each individual stage. Forcing the modelwith a 10-year temperature and chlorophyll a time-series identified that salp abundancesoff the coast of Sydney (34ºS) were significantly greater during spring and summercompared to autumn and winter. This is consistent with observations of salp swarmswhich occur after the spring phytoplankton bloom. Salp swarm abundance appears to berelated to the availability of food (chlorophyll a per salp) rather than absoluteabundance (chlorophyll a biomass).Mass depositions of the large salp Thetys vagina were observed on the Tasman Seafloor in 2008 and 2009, prompting examination into the potential of salp carcasses toact as food fall events to the benthos. Over 1700 carcasses were identified on the seafloor and benthic fish and crustaceans were observed feeding on the carcasses. Analysisof a 30-year trawl survey database determined that salp biomass (wet weight, WW) inthe eastern Tasman Sea regularly exceeds 100 t km−3. T. vagina has a carbon (31% dryweight, DW) and energy (11 kJ g−1 DW) content more similar to that of phytoplanktonblooms, copepods and fish than to that of jellyfish (cnidarians), with which they areoften grouped. Given their abundance, rapid export to the seabed and high nutritionalvalue, salps are likely to be a significant input of carbon to pelagic and benthic foodwebs, which until now, has been largely overlooked