Effects of past, present and possible future seawater environments on sea cucumbers and the sediments they process

Vidal Ramirez, Francisco (2017). Effects of past, present and possible future seawater environments on sea cucumbers and the sediments they process PhD Thesis, School of Biological Sciences, The University of Queensland. doi:10.14264/uql.2017.124

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Author Vidal Ramirez, Francisco
Thesis Title Effects of past, present and possible future seawater environments on sea cucumbers and the sediments they process
School, Centre or Institute School of Biological Sciences
Institution The University of Queensland
DOI 10.14264/uql.2017.124
Publication date 2017-01-13
Thesis type PhD Thesis
Supervisor Sophia Dove
Gene Tyson
Maria Byrne
Total pages 194
Total colour pages 13
Total black and white pages 181
Language eng
Subjects 0602 Ecology
0699 Other Biological Sciences
0605 Microbiology
Formatted abstract
Future climate change is predicted to have deleterious impacts on coral reefs, leading to a decline in
the ability of these systems to provide ecological and human services that are both economically
and socially valuable. Much of the projected decline is focused on the negative effects that
increased sea surface temperature and ocean acidification (OA) are expected to have on key
calcifiers (Scleractinian corals), present in the reef ecosystem. There has been significantly less
research effort directed towards the impacts of warming and acidification on other components of
the ecosystem, especially holothurians and the sediment communities with which these
echinoderms interact. Sediments are essential to reefs as the microbes that inhabit them, recycle
nutrients in these otherwise poor nutrient environments; and key to the release of essential nutrients
from the sediments is bioturbation by organisms such as holothurians.

My PhD Thesis aims to investigate: 1) The short-term impact of Holothuria atra (one of the most
abundant Indo-Pacific holothurians) over the seawater carbonate chemistry, nutrient recycling and
OA buffering capacity (AT/DIC) within a sedimentary environment (Chapter 2); 2) The long-term
effects (2 months) of co-varying winter temperature/pCO2 Scenarios over H. atra and sediment
associated biota. Consequently, the aim is to test the impacts of such Scenarios over calcium
carbonate dissolution, AT/DIC, nutrient recycling and O2 flux produced by the animals and
sediment-associated organisms on reef ecosystems (Chapter 3); 3) The long-term effects of summer
temperature/pCO2 Scenarios over the performance of H. atra and other organisms in regards to the
same responses tested in Chapter 3, but tested when more extreme conditions than in winter occur
(e.g., temperatures above MMM and a greater lack of DOM than in winter) (Chapter 4). Seasons
have not been replicated, but each long-term experiment encompasses 67% of the season in terms
of length. The response variables were tested in Chapter 3 and 4 under present day conditions (PD:
+0oC, +0 ppm pCO2), pre-industrial conditions (PI: -1oC, -100 ppm pCO2 below PD, to estimate
potential impacts produced by PD), and two IPCC Scenarios (RCP4.5: +1.8oC, +180 ppm pCO2;
and RCP8.5: +3.6oC, +570 ppm pCO2). All Scenarios included diurnal and seasonal variability.

The results showed that in a short-term period (Chapter 2), in presence of H. atra, there was a
greater CaCO3 dissolution (~290 mg CaCO3 m-2h-1) and TAN (NH3 + NH4+) production (~45%)
than in sediments without H. atra. However, H. atra was not able to modify most of the carbonate
parameters and AT/DIC, leading to the conclusion that H. atra most likely will not assist reef
calcifiers by the modification of OA buffering capacity.

In winter (Chapter 3), the only direct effect of H. atra on the system was a 34% increase in net
daytime O2 production and modification of sediment infauna. However, such changes did not
mitigate the observed decreases in O2 production under future climate Scenarios. PI generated a
~62% greater TAN uptake than PD, suggesting that under PD processes associated with nitrogen
such as ANAMMOX may have already been affected in reefs. Calcium carbonate dissolution was
observed always under RCP8.5, regardless the presence of H. atra. Likewise, all other seawater
parameters were influenced either by day/night fluctuations or Scenario, likely assisted by an
observed overall decrease in microbial abundance and change in microbial composition under
RCP8.5 (analysed by qPCR and 16S amplicon sequencing, respectively). Therefore, processes that
may impact calcification rates and AT/DIC may have been affected, such as sulfur-oxidation and
sulfate-reduction, regardless of H. atra.

In summer (Chapter 4), H. atra significantly increased AT/DIC; however, such increase did not
significantly modify the downward trend observed for AT/DIC under RCP8.5. Net CaCO3
calcification rates were highly variable and were not modified by any of the factors tested
(presence/absence of animals, time of day and Scenarios). AT appeared to be the only parameter
that significantly correlated to calcification rates, explaining 10% of the variation observed.
Microbial abundance increased significantly under RCP8.5 compared to PD; however, this increase
was proportional across taxa, yielding no apparent change in microbial composition. Therefore, the
lack of change in microbial composition may help explain the insignificant changes observed for
summer calcification rates and AT/DIC over any potential of the animals at this period of the year.

This study demonstrates that H. atra had a low impact on most of the variables tested compared to
sediment-associated biota (principally prokaryotes). The animals were never able to counter the
downward trends observed for different parameters (e.g., AT/DIC) under future climate Scenarios
linked to current rates of fossil fuel burning. Future business-as-usual Scenarios produced
significant effects on sediment microbes. Therefore, most changes were driven principally by
abiotic factors (PI conditions were generally similar to PD conditions with RCPs producing the
most negative impacts over the response variables), potentially aided by changes to sediment
microbes rather than the action of H. atra.
Keyword Holothuria atra
sea cucumbers
bacteria
microalgae
climate change
IPCC scenarios
Heron Island

Document type: Thesis
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Created: Fri, 13 Jan 2017, 18:36:13 EST by Francisco Vidal Ramirez on behalf of Learning and Research Services (UQ Library)