This research investigated the factors influencing the structure of the phytoplanktori
community in Chaffey Dam, which is located in sub-tropical Australia. In particular, the
research aimed to determine the influence of light at time scales ranging from seconds to
seasons, on the growth and photophysiology of the cyanobacterium Anabaena circinalis.
On a large scale, field monitoring programs between 1987 and 1997 indicated that the
phytoplankton community of Chaffey Dam was dominated by colonial or relatively large
phytoplankton that move either with the aid of flagella or can be positively buoyant. Diatoms
contributed only a minor component, which may be the result of the reservoir being stratified
for much of the year. Several of the dominant taxa bloomed in each of the seasons during the
eleven year period, with some blooms lasting >9 months, indicating that environmental
variability between seasons can be low. In contrast to other studies, A. circinalis was more
likely to grow and bloom during the cooler months (March-October). A two-year intensive
monitoring program (1995-1997) identified a seasonal progression that was similar in both
years. Chlorophytes occurred in spring, Ceratium in mid summer, a relatively clear period in
February, A. circinalis in March and cryptomonads in winter.
On a smaller scale, short-term (2-3 day) in-situ and laboratory enclosure experiments found
that the light and nutrient requirements of the dominant taxa varied. In comparison to most
other phytoplankton, A. circinalis cells disappeared at very rapid rates when supplied
irradiances <10 (umol photons m-2 s-1. Over several days of darkness, the filaments broke
apart and the cell numbers declined. The experiments also showed that at certain times, field
populations of A. circinalis were subject to high losses at all irradiances.
Laboratory studies investigating the influence of inter- and intra-daily changes in light
availability showed that the growth rate of A. circinalis was not affected by the frequency of
daytime light:dark cycles, indicating that the rate of water mixing will not have major
influence on its growth if the total daily light dose is maintained. It was also found that
A. circinalis cultures did not accumulate large reserves of energy in the form of carbohydrate,
other than that required for one night. This strategy may enable the colonies to have a high
level of buoyancy each morning so that they float quickly to the surface waters and obtain
sufficient light each day to minimise losses. However, this strategy limits the ability of
A. circinalis to grow and maintain vital cell processes during extended periods of low
irradiances and may be a factor causing them to be susceptible to cell breakdown.
Weekly measurements of algal growth rates in Chaffey Dam identified two factors that may
have acted singly or simultaneously to influence the development of A. circinalis blooms
during 1996 and 1997. The blooms developed during a 4-6 week period when the mean
irradiance in the surface mixed layer (SML) was sufficient to prevent high losses. Secondly,
the blooms developed when soluble phosphorus in the epilimnion was relatively high but
soluble nitrogen was low. This may have favoured A. circinalis, which has the potential to fix
atmospheric nitrogen. The decline of A. circinalis blooms was correlated with a deepening of
the SML and a reduction of the mean daytime irradiance within the SML. Their decline did
not appear to be related to nutrient limitation or to changes in zooplankton concentrations.
This research also developed a physiological technique for tracking daily changes in the mean
daytime irradiance of A. circinalis and for estimating cell growth rate. This method is based
on chlorophyll-a fluorescence quenching analysis of the state transition mechanism, which
regulates light availability between the photosystems. The mean daytime irradiance of
A. circinalis showed a strong relationship with the degree of non-photochemical quenching
(qn), whereas the relative change to the maximum fluorescence showed a strong relationship
with cell growth. It is anticipated that this method will provide a useful research tool for
determining the relative importance of light and other factors on the net growth of
A. circinalis and other cyanobacteria.
| Identifer | oai:union.ndltd.org:ADTP/218969 |
| Date | January 2001 |
| Creators | Green, Damian William, n/a |
| Publisher | University of Canberra. Science &Design |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Damian William Green |
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