The study site is located in a drain along Upper
Changi Road East, Singapore at latitude 1° 20' 2.48" S and longitude 103°
57' 23.63" E. The drain transports
ground water from Upper Changi Road and spans a total of about 400 metres
before depositing the water into a concretised natural canal, Sungei Ketapang. Near the junction where the drain meets the
canal, it is flanked by two narrow strips of sediment with little grass with a
total area of about 4.32 square metres.
Sungei Ketapang then transports to water to the
sea at East Coast Park some three kilometres away. Water level in Sungei Ketapang is consequently
affected by the diurnal tides experienced by the Singapore coastlines. During low tides, the canal carry water from
the land to the sea and the converse happens during high tides. Along with the incoming seawater from the high
tide, aquatic organisms are transported inland. During rising tides beyond 1.8 metres in
height, water from the concretised canal starts to overflow into the drain
(Plate 1) and at a height of two metres, the water floods the sediment strips
behind the concrete slabs of the drain and completely quickly submerges the
sediment strips.
Fig. 1. Location of
study site and other sites (Pasir Ris Park and East Coast Park) with similar Uca annulipes communities and layout of
the study site showing connection between drain and concretised Sungei Ketapang
(inset).
On these sediment strips, Uca annulipes and U. rosea can
be seen emerging and feeding when the sediment strips are exposed. In the cracks of the drain other species of
crustaceans, Episesarma spp., Scylla olivacea, Varuna yui, Perisesarma eumolpe,
can be seen as well. Along the perimeter
of the drain, mud mounds belonging to mud lobsters Thalassina anomala were also observed. As the most visible and common crustacean
seen in the drain is the Uca annulipes,
this species has been selected for the purpose of this study.
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Plate 1. (a)
concretised Sungei Ketapang overflowing into the drain (circled) during a
rising tide, (b) water from concretised Sungei Ketapang (background)
overflowing into the drain during a rising tide, (c) sediment strips exposed
during receding tide and
(d) dominant crab species Uca annulipes seen foraging on sediment strips.
(d) dominant crab species Uca annulipes seen foraging on sediment strips.
The water flowing in the drain was collected at the
junction of the drain and concretised Sungei Ketapang, mid and beginning of the
drain (Fig.1 inset), at one hour intervals over a six hours period twice a
month for a period of 12 months. Salinity
of the water collected was measured using a refractometer. The corresponding tidal height was also
recorded.
Sediment
samples were collected from (i) behind the concrete slabs of the drain at the
junction where the drain meets the concretised canal, (ii) mid-point and
(iii) beginning of the drain and weighed. Samples were then dried in an oven at 80 °C for 24 hours and then weighed again to determine the amount of water in the samples. Fixed amounts of water were then added to the samples and re-suspended in a shaker. After 24 hours, salinity of the suspensions was determined using a refractometer. These replicates were processed for each sediment sample. Sediment samples were also collected from East Coast Park and Pasir Ris Park (Fig. 1) where
Uca annulipes were also found for comparisons.
(iii) beginning of the drain and weighed. Samples were then dried in an oven at 80 °C for 24 hours and then weighed again to determine the amount of water in the samples. Fixed amounts of water were then added to the samples and re-suspended in a shaker. After 24 hours, salinity of the suspensions was determined using a refractometer. These replicates were processed for each sediment sample. Sediment samples were also collected from East Coast Park and Pasir Ris Park (Fig. 1) where
Uca annulipes were also found for comparisons.
Sediment from the junction between the drain and
the canal were collected and placed in airtight Ziploc bags and transported
back to the laboratory for analysis of particle size using the method outlined
by Buchanan (1984). The analysis repeated
for East Coast Park and Pasir Ris and the results compared to determine if
there were any differences in the samples using GRADISTAT v.4.0.
The presence of transient fauna were
observed and documented with a digital camera and identified. The transient fauna was compared with those typically
found in Singapore’s waterways and mangroves. Data collection for drain water salinity,
population density and fauna comparison were carried out twice a month, over a
period of 12 months.
Average population density of the
most visible species, Uca annulipes,
was estimated by enumeration of occupied fiddler crab burrows twice a month
over a period of 12 months. Burrows with
chimneys were typically ignored because they were not typically made by U. annulipes. The resulting density was then compared
densities of similar populations found naturally in Singapore.
Uca
annulipes were collected individually from the study
site, East Coast Park and Pasir Ris Park. They were then placed in buckets along with
about 1.5cm depth of sediment from the same location. In order to ensure the sustainability of the
population, only male crabs were captured. They were then acclimatised at room
temperature in their sediment and buckets in which they were collected for at
least 24 hour period before subjected to 72-hour static acute toxicity tests. The tests were carried out according to the
guidelines on acute toxicity testing as described by Parrish (1985) and Rand &
Petrocelli (1985). The aim of the acute
toxicity tests was to determine the mean lethal salinity limits (LC50).
Ten male crabs (7mm ≤ carapace width ≤ 15mm)
were placed in each of the five 15-litre tanks. Solutions in each tanks were made up to the
required salinities of 0 ‰, 10 ‰,
20 ‰, 30‰ and 40‰
using Instant Ocean® salt and aerated, de-chlorinated tap water. Evaporation was minimised by covering the lids
to reduce water loss and variations in salinity. The crabs in the tanks were checked for
mortality after 6, 12, 24, 36, 48, 60 and 72 hours. Crabs which have an inactive appearance were
repeatedly prodded with a glass rod and then subsequently turned upside down.
After five minutes, if no movement was detected, the crabs were declared dead. The crabs’ behaviour were also recorded as
‘slightly torpid’ if the crabs became active after gentle prodding and ‘very
torpid’ if only little or slight sluggish movement was detected after repeated
prodding as recommended by Owen (2002). Replicates tests were carried out and
experiment repeated on crabs from East Coast Park and Pasir Ris Park.
The percentage mortality from the duplicates of
each site were analysed and determined if they were significantly different
from each other. If there is no significant
difference (p > 0.05), they were combined to increase the confidence of the
estimate for LC50 for each salinity (Owen, 2002). Time-toxicity graphs were plotted using the 24,
48, 72 hour median mortality data. These
will serve as an indication whether and when acute lethality stopped in each
tests. The point of the graph where the
curves are asymptotic with the time axis will be the LC50. This is also an indication for the salinity at
which 50% of the test population can live for an indefinite period (Rand & Petrocelli, 1985). The
analysis were carried out using MINITAB (MINITAB, Inc., Release 16.1.0, 2010)
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