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.