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International Journal of Marine Science 2013, Vol.3, No.41, 333-343
http://ijms.sophiapublisher.com
337
Table 1 Mean squares and their significance values derived from one-way ANOVA of turbidity, pH, NO
x
, PO
4
and NH
4
between
treatments for each time sampled. NH
4
data were Log
10
(n+1) transformed. N.B. * =
p
<0.05; ** =
p
<0.01; *** =
p
<0.001. Bolded
LSD results are shown where treatments are significantly different
Time interval
df
Sources of variation Turbidity
pH
NOx
NH
4
PO
4
0
2
6
Treatment
Residual
0.01ns
0.03
0.00ns
0.00
0.3ns
0.19
0.01ns
0.01
0.05ns
0.04
30 mins
2
6
Treatment
Residual
299.29***
IM>C,CM
4.19
2.52***
C>CM>IM
0.04
23.09ns
14.39
4.54***
IM>CM>C
0.05
14.51ns
7.85
3 hrs
2
6
Treatment
Residual
117.87***
IM>C,CM
4.85
3.62***
C>CM>IM
0.06
0.55ns
0.14
4.93***
IM>CM>C
0.05
0.96***
IM>CM>C
0.01
1 day
2
6
Treatment
Residual
25.77*
IM>C,CM
0.75
1.86*
CM,C>IM
0.06
0.30ns
0.15
3.63***
C,IM>CM
0.14
0.68***
IM>CM>C
0.00
2 days
2
6
Treatment
Residual
22.47***
IM>CM>C
0.31
2.34***
CM,C>IM
0.03
1.11ns
0.17
5.51***
IM>C,CM
0.01
0.72***
IM>CM>C
0.01
3 days
2
6
Treatment
Residual
12.20***
IM>CM>C
0.37
1.75***
CM>C>IM
0.02
0.42ns
0.22
4.99***
IM>C,CM
0.01
0.59***
IM>CM>C
0.01
4 days
2
6
Treatment
Residual
12.20**
IM>C,CM
0.52
107**
CM>C>IM
0.03
0.42ns
0.43
4.83***
IM>C,CM
0.01
1.21***
IM>CM>C
0.01
5 days
2
6
Treatment
Residual
15.57**
IM,CM>C
0.81
0.70**
CM>C>IM
0.05
0.03ns
0.12
4.12***
IM>C,CM
0.01
1.11***
IM>CM>C
0.01
treatments (IM>CM>C), at all times observed, except
at 30 min post disturbance (Table 1).
Available data for silicate (SiO
2
) is limited to initial
conditions and 2 and 5 days after sediment
disturbance (Figure 3d). Silicate levels at control sites
were ~ 550
μ
g/L prior to and following the gyttja
mixing event. Silicate levels rose markedly over the 5
day period in both the control mesocosms and the
impact mesocosms, with the IMs and CMs exhibiting ~
2000 µg/L and 1300 µg/L, respectively, on Day 5
(Figure 3f).
3.2 Cell abundance and diversity
Total cell abundance in surface waters (0-0.5 m) at the
start of the experiment was 20,000-30,000 cells/mL
and remained in this range until Day 2, when the IMs
showed a significant increase in cell number (Figure
4a, Table 2). By Day 4, cell abundance in the IMs
peaked and exceeded 55,000 cells/mL while, other
treatments exhibited mean maxima of <30,000
cells/mL and no net growth in cell abundance.
One-way ANOVA, followed by post hoc LSD tests,
indicated significantly higher cell densities in IMs
(IM>CM=C) on Days 2, 3, 4 and 5 (Table 2).
Temporal trends in abundance of Cyanophyceae were
similar to that shown for total cell abundance, as
cyanophytes were the dominant cell types present
(numerically >65%) and were the taxa that exhibited
the strongest growth response following gyttja
disturbance (Figure 4b). As with total cells, one-way
ANOVA of Cyanophyceae abundance demonstrated
significant differences among treatments at Days 2-5
(IM>CM=C, Table 2).
Although abundance of Bacillariophyceae was below
150 cells/mL at the commencement of the experiment
and in all CMs and Control samples throughout the
experiment, numbers in the IMs just 3 hours after the
disturbance of gyttja were >1000 cell/mL (Figure 4c).
The number of microalgal taxa increased dramatically
shortly after disturbance, with most new taxa being of
benthic origin, e.g.
Achnanthes
,
Achnanthidium
,
Amphora
,
Cocconeis
,
Craticula
,
Cymbella
,
Fragilaria
and
Melosira
. After 24 hours, the abundance of