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The bloom dynamics and trophic ecology of salps and doliolids in Storm Bay, Tasmania

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Ahmad Ishak, NHB (2014) The bloom dynamics and trophic ecology of salps and doliolids in Storm Bay, Tasmania. PhD thesis, University of Tasmania.

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Abstract

Zooplankton are important grazers of primary production and play a
central role in the transfer of energy from primary producers to higher order
consumers. Zooplankton are sensitive to environmental variability, making them
useful indicators of climate change; importantly, their physiology is strongly
coupled to temperature, they exhibit generally short life cycles and they are
excluded from most pressures associated with commercial fishing. However,
given the diversity of organisms found in the pelagic environment the responses
of different groups of zooplankton to environmental variability are most likely
different. In this study I have investigated the bloom dynamics and trophic
ecology of dominant thaliaceans in Storm Bay: two species of salp (Thalia
democratica and Salpa fusiformis) and two species of doliolid (Dolioletta sp. and
Doliolum sp.). Storm Bay is a region of dynamic oceanography that is influenced
by (i) warm, low nutrient waters from the East Australian Current in the summer,
(ii) cooler, nutrient-rich subantarctic waters in the winter, (iii) the Leeuwin
(Zeehan) Current flowing along the west coast and (iv) flows from the Derwent
Estuary. Key challenges in this study included the fragility of the gelatinous
zooplankton, their unpredictable presence in Storm Bay and the absence of
doliolids during certain years.
Monthly field trips were undertaken to Storm Bay for three consecutive
years (November 2009 to March 2012) to investigate the blooms of salps and doliolids and the causes of their patchy distribution. Collections of zooplankton
were made at five sites and seven environmental parameters were recorded
(temperature, salinity, rainfall, diatom stocks, chlorophyll a concentration,
presence of the heterotrophic dinoflagellate Noctiluca scintillans and total
phytoplankton abundance).
Relationships between the distribution of thaliaceans and environmental
parameters in Storm Bay were examined using the BIOENV (Biology-
Environment) procedure of PRIMER, which highlighted that the bloom patterns
of salps and doliolids in Storm Bay were not uniform in time or space due to the
variability in environmental parameters. The top three drivers of thaliacean
distribution and abundance, according to BIOENV, were salinity, temperature
and diatom stocks, with a correlation of 0.433. It was clear that each species
showed different environmental preferences. Of the doliolids, Dolioletta sp.
preferred lower temperatures (mean SST 13.42-14.93 °C) and higher salinity
(mean SSS 33.91-34.62) than Doliolum sp. (mean SST 16.35-16.76 °C; mean SSS
32.95-33.83). The salp T. democratica showed a preference for higher
temperatures (mean SST 15.85-17.4 °C) and slightly lower salinity (mean SSS
34.34-34.40) than S. fusiformis (mean SST 14.64-15.38 °C; mean SSS 34.57-
35.11).
The dietary preferences of salps were investigated using two methods of
gut content analysis: Scanning Electron Microscopy (SEM) and High Performance
Liquid Chromatography (HPLC). Using SEM, I obtained micrographs of 31
different species of plankton, including copepods, in the guts of the four species of thaliaceans. HPLC confirmed that diatoms, cryptophytes and green algae were
the main dietary preferences for salps.
To investigate further where each species of salp fitted within the
planktonic food web in Storm Bay, carbon and nitrogen concentrations and
stable isotopic profiles were measured on T. democratica and S. fusiformis.
Because of the fragility of salps, an extension of this research project involved
comparing three different methods of preparation of salps for elemental
analyses (freshly collected and incised salps rinsed with small volume of Milli-Q
filtered water, thawed salps incised and rinsed in small volume of Milli-Q filtered
water and freshly collected salps incised and rinsed with ammonium formate).
The best method was then used for isotopic analysis of salps and seawater.
Carbon and nitrogen elemental analyses were found to show the most consistent
results if fresh specimens were incised and rinsed with Milli-Q prior to analysis.
T.democratica had higher carbon and nitrogen values than S. fusiformis, and
solitary forms of both species had higher carbon and nitrogen contents than the
aggregate forms. Comparison with the literature confirmed the relatively low
carbon and nitrogen concentrations of these gelatinous organisms when
compared to crustacean plankton. The present study does point to the need to
consider the life stages separately for any research, e.g. ecosystem modelling,
that is attempting to produce realistic carbon budgets for a system.
This study provided further insight into the current understanding of the
impacts of environmental variability on important marine zooplankton;
specifically on salps and doliolids. Further, this study increased our knowledge of the dietary preferences of salps and added significant information to the littleknown
diets of doliolids. It also identified some issues with methods used for
preparing gelatinous species for biochemical analyses and provided
recommendations for optimal preparation of specimens. These findings will
significantly increase our ability to determine how climate-driven oceanographic
changes will affect the distribution of these important zooplankton species in
Australian waters and in other areas globally.

Item Type: Thesis (PhD)
Keywords: Salps, Doliolids, Thaliaceans, Storm Bay, Thalia democratica, Salpa fusiformis, Dolioletta sp., Doliolum sp.
Copyright Holders: The Author
Copyright Information:

Copyright 2014 the author

Date Deposited: 02 Jun 2016 00:17
Last Modified: 02 Jun 2016 00:17
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