Invertebrate Biodiversity in the Ala Wai Estuary
By Jamie Takayesu and Dustin Shigaki
Shimamoto P1 and Kay P3
January 5, 2011
Abstract:
The purpose of this lab was to investigate the type of the invertebrate composition of the
Ala Wai estuary. We analyzed zones 2 and 4 of the estuary, looking at the invertebrates we
found in the canal bed. Using a deployed plate, we were able to calculate the approximate the
relative amounts of invertebrates found in the estuary’s soil. We found that the majority, 98%,
of the organisms in the mud were amphipods. The other two percent were composed of
Chironomids, Annelids, Nematodes and Platyhelminthes. We monitored pH and salinity, but
there was little difference between zones 2 and 4, thus we were unable to see the effects of these
two variables.
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Intro:
The Ala Wai Canal is a man-made estuary that extends 3.2 km from the ocean (Glenn,
1995). An estuary is a semi-enclosed coastal body of water with freshwater flowing into it and a
connection to the open sea. Because of the freshwater input, the salinity of an estuary is lower
than that of sea water, and is called brackish (Weis, 2008). The Mānoa, Pālolo and Makiki
Valley streams empty into the canal. The canal displays the characteristics of a salt wedge
estuary (Glenn, 1995), which has minimal mixing of salt and fresh water, forming a wedge of
salt water that is thickest near the sea and thinnest further inland. The extent the salt wedge
enters inland depends on the flow of the river. The salt water remains at the bottom of the
estuary, whereas the surface layer carries fresh water toward the sea. This circulation helps to
pump nutrients, derived from decomposing plant and animal remains, into the estuary
(Britannica, 2010). The salinity of estuaries changes in response to the tidal cycle, as the tide
assists with the circulation of brackish water. Therefore, plants and animals must be able to
respond quickly to changes in salinity and must tolerate a wide range of salinities. Few
organisms have evolved to adapt to these conditions, so estuaries tend to have lower biodiversity
than other coastal habitats in the same region (Weis, 2008).
In the Ala Wai, the most abundant organism found in the sediment was amphipods.
Amphipods are any invertebrates of the order Amphipoda (class Crustacea) and are normally
found in sea, lakes, rivers, sand beaches, caves and warm, moist habitats on many tropical
islands. Body length of amphipods in midlatitude regions, including Hawaii, normally vary
between 4 to 10 mm long (Britannica, 2010). Because they are scavengers of carrion, amphipods
typically burrow into the soft mud at the bottom of the water source. Other organisms are not as
common. Isopods, relatives of the “potato bug,” are crustaceans. Aquatic forms of isopods feed
on algae and thus are usually located near aquatic plants. Larvae of midges, insects that
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resemble mosquitoes, also inhabit the seabed. Midge larvae are food sources for many types of
fish, which are abundant in the Ala Wai (Britannica, 2010). The last group of organisms
expected in the Ala Wai is annelids. Annelids are invertebrates that can either exist on land or in
water. Most of these organisms hide at the bottom of the riverbed to hide from predators
(Britannica, 2010).
The majority of the invertebrates we found in the estuary were amphipods. However,
according to a study done by the University of Hawaii at Manoa along the Ala Wai Canal and its
tributaries, amphipods represented less than 2.5 percent of the total organisms collected in the
Canal and Manoa-Palolo streams. The organisms were collected from the water using a mesh
net. In their experiment, copepods were found throughout the Canal and the two streams. They
were found in most abundance in fresh water, and toward the ocean, the number of copepods
collected greatly decreased. Even so, copepods were one of the most frequently collected
organisms that were captured by the zooplankton tows (Miller, 1975).
Determining the population density of any region is done by proportions. If we know
how many organisms are in one set of plates, we can approximate the number of species per
square meter in a given zone. Of course this number may not be accurate, especially if we do not
find any organisms of a particular phylum. Other environmental factors may come into play,
such as the location of the plate in the zone.
There are some conditions that could have affected the amount of biodiversity or biomass found
in each zone. We monitored salinity and pH to account for a few discrepancies. Average
b
 f (t )dt
salinity is found by evaluating
a
ba
where a is the first date when data was gathered, b is the
last date when data was gathered and f (t) is the salinity during time t. We can approximate the
4
average salinity of zones 2 and 4 using the trapezoidal rule by dividing the graph into n-1
subdivisions where n is the number of data values collected. The same method was used to find
the average pH. Data values that were collected on the same day were averaged and considered
one data value. Also, since we deployed plates, we are concerned with the salinity gathered that
the bottom of the water. The salinity and pH were gathered from the Iolani Portal, which did
chemical sampling using a YSI 6920 V2 sonde and a YSI MDS 650. To measure salinity, we
used the units ppt (parts per thousand), which is a fraction of the amount of salt per 1,000 grams
(1 kilogram) of salt water.
Materials & Methods:
Materials:
 Boat
 Hester-Dendy Multi-plate invertebrate sampler
 Hand nets
 Bucket
 Plankton net
 Light microscope
 Bowls and petri dishes
 Pipet
 Forceps
Method:
1. Drop assembled deployed plate in zones two and four.
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2. Return in two weeks to collect plates. While collecting plate, be sure to use hand net to
capture any loose sediment.
3. Disassemble deployed plates and clean plates in a bucket. The bucket should now
contain water and all sediment and organisms.
4. Strain contents of bucket using the plankton net. Repeat until bucket is completely
empty.
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5. Using the pipet, place some of the condensed sample onto a petri dish. Add water until
organisms can clearly be seen under microscope.
6. Use a light microscope under 10x40 magnification to find organisms.
7. Remove, catalogue and count organisms.
Data:
Zone 2 Retrieved: 10/30/10 Note: Did not count all of the organisms, but just the organisms in a
fourth of the mud collected. (Salinity of 35.56 ppt and pH of 7.51)
Organism
Amphipod
Ave
Size
(mm)
4
Chironomid
Annelid
7
Quantity
(% of total
species)
158
(96.93%)
Color
2
(1.23%)
Tan/Brown
3
(1.84%)
Tan/ Brown
Picture
Brownish gray
N/A
7
Zone 4 Retrieved 12/11/10. Note: Plentiful rain on 12/10/10. Plate moved from one end of the
zone to the other throughout the trial. Pulled up much debris when pulling up plates. Counted
all of the organisms. (Salinity of 31.67 ppt and pH of 7.46)
Organism
Quantity
(% of total
species)
320
(98.77%)
Color
Amphipod
Ave
Size
(mm)
5
Platyhelminthes
2
Annelid
8
1
(0.31%)
1
(0.31%)
Tan/ clear with
black center
Red
Nematode
2
2
Tan/clear
Brownish gray
Picture
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4
Average Salinity:
Zone 2: 30.374 ppt
Zone 4: 35.270 ppt
Average Salinity in Zone 2
40
Salinity (PPT)
30
35.57
34.03
35
31.67
26.42
27.24
25.87
7-Jul
25
13-Jul
11-Aug
20
2-Oct
15
30-Oct
11-Dec
10
5
0
Average Salinity Zone 4
38
37.05
37
36.34
Salinity (PPT)
36
35
7-Jul
35.13
34.77
13-Jul
33.98
11-Aug
34
33
32
31
30
2-Oct
32.52
30-Oct
11-Dec
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Average pH:
Zone 2: 7.355 pH
Zone 4: 7.510 pH
Average pH in Zone 2
9
7.79
7.63
8
7.31
7.61
7.66
7.46
7-Jul
6.48
7
13-Jul
pH
6
11-Aug
5
2-Oct
4
30-Oct
3
13-Nov
11-Dec
2
1
0
Average pH of Zone 4
8
7.86
7.8
7.76
7.8
7.57
7.6
7-Jul
7.44
13-Jul
pH
7.4
11-Aug
7.15
7.2
2-Oct
30-Oct
7
6.89
6.8
13-Nov
11-Dec
6.6
6.4
Discussion
Because we could only collect a small number of data, it is difficult to make conclusions
on the ideal living conditions of the organisms. We only have one data set per zone meaning that
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we cannot determine change over time. Zone 2 has far less organisms than Zone 4 has. We need
to find a significant factor that is likely to bring about such a difference in number. It is certain
that pH does not have much of an effect on the population density of the organisms since. The
difference of 7.510 and 7.355 in pH equals a 1.325e-8 change in the concentration of hydrogen
ions. This should not affect the organisms in a significant way. We can also assume that salinity
does not affect the organisms. The difference of the salinities is just 5 ppt, which again should
not be sufficient to cause a huge change in the number of organisms. The one thing that most
likely brought about the huge difference in number was the debris that we pulled up. The debris
functions like the plates; it provides a hiding spot for the smaller organisms. Copious amounts of
debris were extracted from Zones 4 and 5. More hiding space means that more organisms will
survive, and more organisms will gather in that particular zone.
Amphipods are normally found burrowing in the mud, and since we took our
sample from the canal bed, it makes sense that we found a lot of amphipods. Also, amphipods
are nocturnal and remain at the bottom of the canal during the day, which explains why we
captured so many amphipods. Midges were not as common because they are mainly found near
aquatic plants. Since we did not uproot any plants, the small amounts of midges are justified. In
the UH experiment, the scientists predominantly found copepods, however many copepods are
planktonic, drift in seawaters. There are supposedly many more copepods that are benthic, live
on the ocean floor, but we did not find any in the Ala Wai Estuary. This discrepancy, as
mentioned, may be due to the time of our experiment. Copepods are only present on incoming
tides, and are more abundant during certain times of the year. Moreover, the experiment by UH
was done thirty years ago throughout the whole canal. The toxicity and chemical composition
might have slightly changed since then, and they might have found copepods in areas other than
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those that we studied. We did not find many annelids, which corresponds with the data collected
by UH. The main difference between our experiment and the one done by UH is that their
experiment was focused on finding the planktonic zooplankton, whereas we looked at benthic
invertebrates.
The percent biological composition in zones two and four were fairly similar. As far as
pH and salinity, neither should have significantly influenced the percentages of each organism
that we found. Because our methods were slightly different for zones two and four (for zone 2,
we quartered our sample because of the mass amounts of organisms that we were finding), the
total amount of organisms found should not be comparable. However, despite this difference,
the percentages still were consistent with each other, thus we can be sure that our estimates justly
reflect the biodiversity in the sediment of this section of the Ala Wai Canal.
Errors in the experiment include not having more than one set of data per zone. Because
our plates were stolen or lost on more than one occasion, we did not have sufficient time to do
multiple trials. Also, there definitely are errors calculating average salinity and pH because we
used an approximation method to evaluate an integral. However, because of the uneven lengths
of the subintervals it would be far more accurate to use the trapezoidal rule that to find the
arithmetic mean of the values of salinity and pH. Error can also be attributed to human error; we
may not have counted all of the organisms. Also, because we are finding the amount of
organisms per square meter, and each zone is much larger than one square meter, any error in our
sample will be greatly magnified. Our data might not correctly correspond to the actual density
of organisms.
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Conclusion

We found that amphipods made up about 98% of the total biomass in the sediment of
zones 2 and 4 of the Ala Wai Canal. Chironomidae, Annelids, Platyhelminthes and
Nematodes totaled less than 2% of the organims found. Salinity and pH has no apparent
effect on the differences between the zones.

The Ala Wai is an estuary that supports many different organisms. If we know what kind
of organisms live there, we can make more informed decisions about regulating
construction along the Ala Wai to preserve the biodiversity.

If we were to further investigate the microbiology of the sediment in the Ala Wai, we
would do more trials to validate our existing data. We would also try to find areas of the
Ala Wai with more drastic differences in pH and salinity to further analyze the impact of
these two factors.
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Works Cited
"Amphipod." Encyclopædia Britannica. 2010. Encyclopædia Britannica Online. 24 Nov. 2010
<http://www.britannica.com/EBchecked/topic/21587/amphipod>.
"Annelid." Encyclopædia Britannica. 2010. Encyclopædia Britannica Online. 24 Nov. 2010
<http://www.britannica.com/EBchecked/topic/26308/annelid>.
"Boundary Ecosystem." Encyclopædia Britannica. 2010. Encyclopædia Britannica Online. 24
Nov. 2010 <http://www.britannica.com/EBchecked/topic/75627/boundary-ecosystem>.
Campbell, N. (2008). AP Edition Biology 8th ed. California: Pearson Benjamin Cummings.
Glenn, C. (1995). Scientific studies and History of the Ala Wai Canal, an artificial tropical
estuary. Hawaii: University of Hawai’i Press <http://scholarspace.manoa.hawaii.edu/
handle/10125/2327>
"Isopod." Encyclopædia Britannica. 2010. Encyclopædia Britannica Online. 24 Nov. 2010
<http://www.britannica.com/EBchecked/topic/296466/isopod >.
Judith S. Weis, J. Emmett Duffy "Estuary". In: Encyclopedia of Earth. Eds. Cutler J. Cleveland
(Washington, D.C.: Environmental Information Coalition, National Council for Science
and the Environment). [First published in the Encyclopedia of Earth May 20, 2008; Last
revised Date May 20, 2008; Retrieved November 23, 2010
<http://www.eoearth.org/article/Estuary>
Miller, J. (1975). Ecological studies of the biota of the Ala Wai Canal. Hawaii: University of
Hawai’i Press < http://scholarspace.manoa.hawaii.edu/bitstream/10125/17329/1/HIMBTR32.pdf>