Page 16 - IJMS 878-Vol.3 No.41

International Journal of Marine Science 2013, Vol.3, No.41, 333-343
http://ijms.sophiapublisher.com
342
values of 0.5-0.8 to represent a moderate difference.
4 Discussion
4.1 Effects of sediment disturbance on water
quality and nutrients
Disturbance of the gyttja layer resulted in the
introduction of hydrous organic sediments and
associated dead and living microalgae into overlying
water within mesocosms. Physically disturbing the
gyttja not only increased turbidity but also affected
other water quality variables (e.g. minor decreases in
pH and dissolved oxygen). pH decreased to about 6.5
but recovered gradually to normal levels within 5
days. Under severely reduced pH conditions, the
availability of nitrogen and phosphorus can be
affected (Wetzel, 1975; Fisher et al., 1992; Bartram
and Balance, 1996). pH plays a role in mobilizing
phosphorus from the sediment into the water column
(Fisher and Wood, 2004).
The disturbance of gyttja introduced significant levels
of NH4
+ (~2000 µg/L) into overlying water, while
NOx levels, although slightly elevated, remained low
(<6 µg/L). This was due to the highly organic nature
of gyttja sediments. Bacterial utilization of oxygen
creates anoxic, reducing conditions which hampers
coupled nitrification-denitrification processes and
results in NH4
+ as the primary breakdown product of
organic matter decomposition (Bronmark and Hansson,
1998; Scheffer, 2001).
Elevated SiO3
－ concentrations also followed sediment
disturbance and continued to rise during the course of
the 5 day experiment. The latter effect may be due to
suspension of diatoms and subsequent degradation to
silicate. Release of dissolved silicate from undisturbed
sediments is largely dependent on remineralization of
bio-silicates from dead diatom cells (Srithongouthai et
al., 2003).
Sediments play an important role in phosphorus (P)
cycling in aquatic systems (Bostrom et al., 1988;
Berdalet et al., 1996; Caetano et al ., 1997; Scheffer,
2001). Sediments can act as both sinks and sources of
P (Bostrom et al., 1988; Clavero et al., 2000). In Myall
Lake, gyttja and overlying charophytes (Chara and
Nitella) serve primarily as sinks for P (Shilla et al.,
2006; Siong et al., 2006). Phosphates within the gyttja
of Myall Lake are highly bound and not readily
available to phytoplankton in overlying waters (Siong
et al., 2006). Gyttja disturbance introduced only very
small amounts of orthophosphate (PO4
-3) into the
water column, with concentrations of ~6 µg/L
(oligotrophic level) appearing within 30 minutes of
the mixing event. These levels dropped to ~2 µg/L
within 24 hr. Data from two years of monitoring in
Myall Lake show that TP levels are naturally low,
and that P is the most limiting macronutrient for
phytoplankton growth in the Myall Lakes (DIPNR,
2004).
4.2 Effects of gyttja disturbance on phytoplankton
Light availability, a major factor driving phytoplankton
growth (Yentsch, 1980; Eilers and Peters, 1988;
Gaevskii et al., 2000), decreased shortly after
sediment disturbance due to an immediate increase in
turbidity (>20 NTU), which remained at levels of 5-10
NTU on Days 1-5. Although significantly higher than
other treatments, it is unlikely that phytoplankton
growth was inhibited by these turbidity levels as they
are low relative to turbidity levels in most coastal
waters (Kendrick et al., 1998).
Typically, algal growth is either controlled or limited
by ambient light conditions or by the supply of critical
nutrients (e.g. nitrogen, silicate, phosphate) (Huisman
et al., 1999; Wild-Allen et al., 2002). Growth in
phytoplankton cell numbers, in response to increases
in NH 4, was apparent 2 days following the mixing
event, with peak cell abundance observed on Day 4.
This is in line with other studies, which show that
phytoplankton growth response to elevated dissolved
nutrient concentration occurs within a few days
(Chang and Rossmann, 1988; Schelske et al., 1995).
Although both Cyanophyceae and Chlorophyceae
were present at cell numbers >8000/mL at the start of
the experiment, cyanophytes showed the greatest
growth response following sediment disturbance.
Cyanophytes appear to be more competitive for available
nutrients when orthophosphate levels are low.
Cyanophytes associated with the gyttja in Myall Lake
are mostly Chroococcales and Nostocales, with a high
abundance of Aphanothece and Aphanocapsa (Dasey
et al. 2005). Potentially toxic benthic cyanobacteria
( Microcystis flos-aquae) were also found in the
surface gyttja layer but these large colonies quickly
sank out of the water column when suspended.
It was expected that sediment disturbance would
introduce benthic algae (diatoms and benthic