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

Abstract

Pelagic tunicates, including salps, are an important category of gelatinous zooplankton and yet are relatively understudied. Salps regularly occur in intermittent swarms and can quickly become the most dominant zooplankton within an area. My thesis explores the trophic interactions of salps within the zooplankton community using stable isotope analysis, the environmental drivers of salp population dynamics, and the occurrence of salp deposition on the sea floor. The trophic niche of salps within the oceanic zooplankton community was investigated using stable isotopes of carbon and nitrogen. Zooplankton and suspended particulate organic matter (POM) were sampled in three different water types: inner shelf (IS), a cold core eddy (CCE; cyclonic) and a warm core eddy (WCE; anti-cyclonic). Recent upwelling in the IS water type resulted in lower than expected trophic enrichment for all zooplankton species (0.53 compared to 3.4 ), and the salp Thalia democratica was depleted 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 on chlorophyll a and nitrate concentrations, the WCE was characterised as an oligotrophic environment and was associated with an increased trophic level for omnivorous zooplankton (copepods and euphausiids) to a similar level as carnivorous zooplankton (chaetognaths). This study shows that trophic relationships among the zooplankton are dynamic and can vary across water types. The demographic characteristics of three salp swarms were studied to examine factors influencing variations in salp swarm magnitude. The interannual abundance of Thalia democratica during spring was related to the rates of asexual reproduction (buds per chain). T. democratica abundance was significantly higher in October 2008 (1312 individuals m-3) than 2009 and 2010 (210 and 92 individuals m−3, respectively). Salp abundance was negatively related to buds per chain and relative growth rates, implying a faster release rate. As T. democratica abundance was significantly positively related to food >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 to investigate the temporal resolution of a salp swarm. The model used size-dependent reproduction and mortality, where growth was dependent on food consumption (chlorophyll a biomass) and temperature. Average generation time (12 days) and mean abundances of each stage correspond to previously reported values. Salp ingestion rate and the doubling time of chlorophyll a were the most influential parameters, negatively influencing salp biomass and abundances for each individual stage. Forcing the model with a 10-year temperature and chlorophyll a time-series identified that salp abundances off the coast of Sydney (34ºS) were significantly greater during spring and summer compared to autumn and winter. This is consistent with observations of salp swarms which occur after the spring phytoplankton bloom. Salp swarm abundance appears to be related to the availability of food (chlorophyll a per salp) rather than absolute abundance (chlorophyll a biomass). Mass depositions of the large salp Thetys vagina were observed on the Tasman Sea floor in 2008 and 2009, prompting examination into the potential of salp carcasses to act as food fall events to the benthos. Over 1700 carcasses were identified on the sea floor and benthic fish and crustaceans were observed feeding on the carcasses. Analysis of a 30-year trawl survey database determined that salp biomass (wet weight, WW) in the eastern Tasman Sea regularly exceeds 100 t km−3. T. vagina has a carbon (31% dry weight, DW) and energy (11 kJ g−1 DW) content more similar to that of phytoplankton blooms, copepods and fish than to that of jellyfish (cnidarians), with which they are often grouped. Given their abundance, rapid export to the seabed and high nutritional value, salps are likely to be a significant input of carbon to pelagic and benthic food webs, which until now, has been largely overlooked.