Impacts of Mangrove Conditions on Zooplankton Communities
Chapter 1
Zooplankton Presence in Mangroves: A Comparison of the Zooplankton Communities in
Intact and Cleared Mangrove Areas in Bocas Del Toro, Panama
Kait Frasier
Introduction
Examinations of the impacts of deforestation generally focus on tropical rainforests, or
northern coniferous woodlands. However, an alarming amount of clear-cutting is currently
taking place in an extensive, yet far less recognized habitat: the world’s mangrove forests
(Ogden 2001; UNEP 1995).
Background
Mangroves, characteristic of tropical and subtropical coastlines, are currently gaining
increasing recognition as important, yet undervalued and poorly understood ecosystems.
These woody plants, which grow at the interface between land and sea, include an estimated
70 species of mangroves covering approximately 181 000 km2 on 5 continents world-wide
(Kathiresan and Bingham, 2001; Spalding et al., 1997). Mangrove trees and shrubs develop
structurally complex, dense and highly productive ecosystems in areas of low wave action
and high sediment availability (Alongi, 2002). Extensive aerial and sub-tidal prop root
networks (laterally-extending roots that stabilize the plants in loose, muddy coastal soils),
Frasier 2
dense canopies, and fluctuating water conditions allow these forests to support unique
collections of flora and fauna.
According to analyses performed by Alongi (2002), it has been estimated that
mangroves currently cover approximately two-thirds of the territory they covered 50 years
ago. This decline in coverage is expected to continue for at least the next two decades at
similar rates in the absence of large-scale intervention. A synthetic study done by Kathiresan
and Bingham in 2001 characterized the global plight of mangrove forests as follows:
“Measurements reveal alarming levels of mangrove destruction. Some estimates put
global rates at one million ha y-1, with mangroves in some regions in danger of
complete collapse. Heavy historical exploitation of mangroves has left many
remaining populations severely damaged.”
Causes of mangrove deforestation include coastal development, timber harvest,
exploitation of fisheries and mining. Physical consequences for marine environments include
increased sedimentation and erosion, excess nutrient flow, altered food chains and changes in
tidal flow (Alongi, 2002). The ecological impacts of these changes have only recently begun
to be explored and understood.
Mangroves and Zooplankton Communities
As mangrove deforestation continues to alter coastlines worldwide, these forests are
becoming recognized as important nursery habitats for many organisms, including a wide
Frasier 3
variety of zooplankton. “Zooplankton” includes both strictly microscopic organisms
(“holoplankton”) as well as meroplankton, the larval stages of benthic invertebrates, which
exhibit planktonic characteristics during early development. Meroplankton may account for
as much as 70% of zooplankton found in a given area (Kathiresan, 2001). In a 1990 study,
Dittel and Epifanio found decapod larvae in densities of up to 1000 individuals per m3 in
Costa Rican mangroves, and further suggested that these larvae were exported on outgoing
tides, while incoming tides brought older stages back to the mangrove habitat. Total
zooplankton abundances of up to 105 individuals per m3, and biomasses up to 623 mg per m3
have been recorded in some mangrove waters (Robertson and Blaber, 1992). This is
significantly higher than the abundances recorded in offshore waters. However, work done
by Goswami shows that this pattern is not consistent in all mangrove environments (1992).
Factors Impacting Zooplankton Density
Many fish and invertebrate species found in mangroves are highly mobile, especially
in late, pre-settlement stages, and may exhibit varying degrees of habitat selection. Some
fish larvae swim at speeds approaching 30 cm/min and can detect favorable habitats more
than 1 km away (Leis et al, 1996). In terms of invertebrate mobility, copepods, for example,
swim at maximum speeds of 2 cm/s, or 60 cm/min (Ferrari et al., 2003). This capacity for
efficient and directional travel could allow zooplankton to actively select mangrove habitats
if doing so favors survival.
Frasier 4
A second factor potentially influencing zooplankton presence in coastal environments
is the number of spawning individuals in each area. Some species exhibit pelagic or “open”
spawning behaviors, allowing colonization to occur from distant populations (Young, 1995).
Others have spawning patterns that retain larvae in the same area, as opposed to sending
them into outgoing currents. These behavioral differences could cause the presence of
certain types of invertebrates in coastal environments to be highly reliant on the existence of
established, spawning adults in those areas. Meanwhile, the occurrence of pelagicallyspawned species in a given area may be independent of the size and presence of local adult
populations.
Finally, zooplankton survival may be higher in environments that have more favorable
substrate, food availability, and limited active predation (McFarland et al., 1985; Cocheret de
la Morinière et al., 2004). The extensive, dense root networks of mangrove forests are
thought to retain nutrients and sediment carried in runoff from adjacent land. Both of these
functions may benefit organisms seeking food and substrate. Mangrove roots also provide
complex environments for vulnerable organisms seeking shelter from the harsher conditions
characteristic of open waters, as well as for organisms seeking concealment from predators
(Kathiresan and Bingham, 2001).
In consideration of all of these factors: the potential ability of zooplankton to select
more favorable environments, coupled with varying amounts of input through spawning, and
the effects of favorable versus less favorable environments on zooplankton survival, it is
Frasier 5
possible that the presence or lack of mangrove forest habitats along coastlines may result in
variations in the composition of the zooplankton communities found in coastal waters.
Purpose
This study compares the zooplankton populations in intact mangrove environments to
those found in areas where mangroves have been cleared by human interference. The
differences between the populations sampled were analyzed by focusing on zooplankton
abundance, species diversity, and developmental stages represented in the two types of
environments. This information is useful for determining whether there are differences in the
abundance and/or diversity of the zooplankton communities in waters within intact mangrove
systems versus in cleared mangrove areas.
In the cleared mangrove areas involved in this study, almost all traces of the mangrove
structure have been removed, therefore the characteristics of the cleared environments are
significantly different from those of the intact mangrove forests. Cleared areas are
characterized by similar depths and distance from the shoreline, but lack the complex
underwater root structure, (limited to a few remaining snags), do not provide the overhead
cover available under the mangrove canopies, and do not carry the same nutrient density,
since they lack the organic production of mangrove communities (Krishnamurthy, 1982).
Mangrove destruction is an increasing problem along many coastlines worldwide
(UNEP, 1995). The information gathered by this study provides an initial look at the effects
of this destruction on the structure of zooplankton communities in Panama.
Frasier 6
Materials and Methods
Zooplankton Sampling
Plankton sampling methods were used in this study to measure zooplankton recruitment in
each type of environment studied: “intact” versus “cleared” mangrove systems. The goal
was to assess and compare the composition of the zooplankton communities at selected sites
where both types of systems were found in
close proximity.
Sampling was conducted in 6 pairs of
neighboring areas of cleared and intact red
mangrove (Rhizophora mangle) forest along
the coast of Isla Colon, off of the Caribbean
coast of Panama, shown in Figure 1.1. These
Figure 1.1. Site Location: Sampling was
conducted along Colon Island in the Bocas del
Toro province of Panama.
forested and deforested shorelines were wave-protected, inland-facing areas, characterized by
relatively low tidal exchange (approximately 30 to 60 cm) and very low wave action. Nearby
development was limited, consisting of non-industrialized farming and clear-cutting (often
slash and burn). Major commercial and industrial development was found only at a
considerable distance from the sampling sites. Collections took place during the second half
of June 2004.
Frasier 7
In the design of this study, pairs of areas were coupled into three sites. Each site
consisted of two pairs of associated cleared and intact areas, and was sampled twice over the
course of the six-day sampling period.
Sampling at an area involved the use of two different components - light traps and
plankton tows. These methods were used simultaneously in order to conduct a more
complete sampling of the zooplankton communities. Theoretically, phototactic zooplankton
are attracted into the light traps, while non-phototactic zooplankton are better sampled using
the tow (Doherty, 1987). The data from these two methods of sampling cannot be combined
for the purpose of statistical analysis. However, the two approaches are useful for
comparison, and aid in developing a more balanced picture of the zooplankton communities
and developmental stages present.
Light Traps
Light traps, the first sampling method used, trap phototactic, mobile zooplankton
capable of both detecting the presence of a light source and swimming towards it effectively.
Light traps were deployed for one hour after sunset, between the hours of 7:00 and 8:30 pm.
In intact mangrove areas, the light traps were placed within the root structure, where depths
varied between 3 and 4 feet. In cleared areas, the traps were deployed at a distance from the
shoreline within the area that had likely been occupied by mangroves prior to deforestation.
This distance was approximated based on the distance over which neighboring intact
mangroves extended before giving way to open water. Traps were arranged along an
Frasier 8
imaginary transect line, parallel to shore, which
crossed areas of both of the two types of
environments of interest.
Light traps were constructed using 7.6 Liter (2
gallon) clear plastic water jugs, inverted, with
attached, mesh-lined cod-ends made of
perforated PVC tubing (Figure 1.2). A yellow
dive glow stick, suspended inside the bottle
from the top of each trap, was used as the light
Figure 1.2. Light trap design
source. Three entry points for zooplankton
were possible through funnel-shaped openings in the bottles’ sides which led inward to holes
measuring approximately 1 cm in diameter. The funnel shapes and the small size of the entry
points were designed to limit the ability of the zooplankton to leave the traps after having
entered. The water in the trap was filtered through 220 m mesh in the cod-end at the end of
the sampling period. The remaining sample was a concentration of the trapped organisms
contained in a small amount of water.
Plankton Tows
Detachable cod-end
The second type of sampling involved a
diver pulling a plankton tow (Figure
Mesh-lined holes
Figure 1.3. Plankton tow design
1.3) through the waters in the vicinity
Frasier 9
of the light traps for 1 minute, (approximately 20 meters), during the period of time that the
light trap was deployed. A second diver followed, holding the cod-end of the tow at the level
of the front end. The purpose of the plankton tows was to sample the less mobile
zooplankton present in the water column. They primarily trap individuals that do not swim
into light traps because they are either ineffective swimmers or do not exhibit phototacticity.
Tows also capture those individuals unable to actively avoid the oncoming net. Unlike light
traps, tows do not select preferentially for phototactic versus non-phototactic zooplankton.
The plankton tow used had an opening diameter of 30 cm, and a cod-end mesh size of
200 m. Organisms were inactively drawn into the net as it passed through the water column
and collected in the end of the tow. (Note: the mesh size used for filtration in the tow was
20m smaller than that used in the light traps). In intact mangrove areas, the tow was pulled
through partially open waters found behind the most waterfront trees and through small
channels within the mangrove forest. These waters were somewhat more accessible than
those in which the light traps were set, but drag paths remained within the root structure. In
the cleared mangrove areas, the tows were pulled along a straight line, parallel to the shore.
Sampling Period
Sampling was conducted for 6 nights around the new moon, when spawning activity is
highest and the zooplankton community is usually at its peak density (McFarland, W.N. et al.
1985). Collections were begun two days prior, and continued three days after the new moon.
Frasier 10
One site, composed of two intact areas and two associated cleared areas, was sampled each
night. Each site was sampled twice over the course of the study.
Complete sampling at an area consisted of:
1) Deploying a light trap for one hour after sunset, between 7:00 and 8:30 pm,
2) Pulling a plankton tow through the neighboring area for one minute, (approximately
20 meters) during the period of deployment of the light trap.
Sample Analysis
After trapping, samples were preserved within three hours of capture, in order to limit
the effect of predation within the samples on sample composition. The contents of the codends of the traps and tows were rinsed with filtered seawater into formalin-safe containers.
Formalin was then added to each sample at a final concentration of 2-4% formalin. The
preserved samples were condensed by removing excess liquid, and kept for analysis at a later
date.
Sample analysis was done by light microscope, using Instant Ocean® to dilute the
samples to facilitate the separation and counting processes. All of the individuals in each
sample were counted and identified to phylogenetic order, whenever possible. Decapods
were further categorized to suborders (crab, shrimp, or lobster) and by general life stage:
zoea, megalope, or postlarval. Reproductive individuals were noted when identified.
Frasier 11
Statistical Analysis
Sample composition data collected from counts were compared between associated
neighboring intact and cleared sampling locations. I tested for differences in sample
composition between areas using StatGraphics to run both paired-sample t-tests and
ANOVA comparisons of the means.
The data were tested in two ways using each method. First, the two sub-samples from
each site were used as independent observations, resulting in 11 data points each for cleared
and intact sites for the trap data set, and 10 each for the tow data set. Secondly, sub-samples
from each area were averaged for analysis, resulting in 6 observations for the trap data set
(one sample was lost – therefore one sub-sample was not averaged), and 5 observations for
the tow data set.
Trap and tow data sets were tested individually for differences between the means of
intact and cleared samples in the number of: invertebrates, decapods, crabs, shrimp, lobsters,
taxonomic orders, shrimp and stages of crabs.
Due to the large quantities of copepods present in the samples, (two to four orders of
magnitude more than other groups), these were excluded from the analyses. Individuals were
grouped as specifically as possible given the scarcity of identification keys and information
for this region. Special attention was given to trends in the presence of economically
significant decapods, including juvenile stages of crab, shrimp and lobster.
Frasier 12
Results
5000
Trap Data Set
Three major trends appear
among the samples:
Total Number of Individuals
4500
4000
3500
3000
2500
2000
1500
1000
500
0
Cleared
1) Total sample sizes were
larger in cleared areas than in
Intact
Figure 1.4. Mean number of individuals in cleared vs.
intact trap samples. Bars are means  Std deviation,
n=12.
intact areas, as shown in Figure 1.4 (single-sided p-value= 0.007). A paired sample t-test
using the un-averaged data strongly rejects the null hypothesis that the sample sizes are the
same, with 95% confidence.
2) The total numbers of decapods in the samples were larger in the cleared areas. (Singlesided p-values for uncombined and combined subtotals are 0.006 and 0.009, respectively).
The differences in average sample composition between the two types of sampling
environment are summarized in Figure 1.5. Decapods and cumaceans were markedly more
numerous in cleared areas, while ostracods and mysids were more common at intact sites.
The large error bars demonstrate the high variability found in sample composition in both
types of environment.
Frasier 13
1600
900
1400
800
700
600
1000
Number of Individuals
Number of Individuals
1200
800
600
500
400
300
400
200
200
100
Cleared
oe
a
Sh
ri m
pm
eg
alo
pe
Po
stl
arv
al
Sh
ri m
p
To
tal
Sh
rim
p
Lo
bs
ter
lar
va
e
Po
r ce
lla
nid
cra
bs
bs
cra
Sh
ri m
pz
Category
Taxon
Figure 1.5. Total number of individuals by taxon in
cleared vs. intact trap samples. (Bars are means 
Std deviation, n=12).
To
tal
eg
alo
pe
Po
stl
arv
al
Cr
ab
s
Cr
ab
m
oe
a
0
Cr
ab
z
De
c
Sto apod
a
ma
to p
od
a
Cu
ma
ce
a
Iso
po
Am
da
ph
ipo
da
My
si d
ac
Ta
e
a
na
ida
ce
Cla
a
Cn
do
ida
ce
ra
ria
Os
ns
t
& C raco
da
ten
op
ho
Po
ra
lyc
ha
Eu
ete
ph
a
au
sia
Ch
ce
ae
a
tog
na
tha
Cir
r
Co ipedi
a
ma
tul
ida
e
0
Figure 1.6. Breakdown of subtaxa and
developmental
stages
represented
within
Decapoda in cleared vs. intact trap samples. (Bars
are means  Std deviation, n=12).
Intact
3) Among the decapods, the shrimp populations were significantly larger, on average, in
cleared samples, according to the ANOVA test (p-value = .0314, F-value= 6.25). The
breakdown of sub-taxa and life stages represented within the order Decapoda, in Figure 1.6,
reveals that crab populations in both areas tended to be dominated by zoeal stages, while
shrimp populations were primarily found in megalopal stages.
Frasier 14
Tow Data Set
None of the trends in the trap data set were found in the tow data set. In fact, none of the
differences tested for were found to be significantly important. Based on the tests conducted,
no trends with respect to mangrove presence or absence were visible in the data collected
from the tow samples. As seen in Figure 1.7, catch sizes were not significantly different
between cleared and intact areas. Figures 1.8 and 1.9 show no significant differences in
either the numbers of individuals present in represented taxa, or in stages and categories of
decapods found.
Cleared
Intact
250
250
200
Average Abundance
Number of Individuals
200
150
100
150
100
50
50
Figure 1.8. Total number of individuals by taxon in
cleared vs. intact tow samples. (Bars are means 
Std deviation, n=12).
Cr
ab
Cr
a
D
St ec
om ap
at oda
o
Cu po
m da
a
I c
Am sop ea
ph oda
M ip
y o
Ta sid da
Cn
na ace
i
id
Cl dac a
ar
a
ia
ns O do ea
& st cer
Ct rac a
e o
Po nop da
Eu lyc ho
p ha ra
Ch hau ete
ae sia a
to ce
Pl C gna a
a t i r th
yh rip a
el ed
m ia
in
t
A he
Bd car s
e
Co ll ina
m oid
Py atu ea
ro lida
so e
m
id
a
Taxon
bZ
oe
a
me
ga
lop
e
To
tal
Cr
ab
Sh
s
rim
pz
Sh
oe
rim
a
pm
eg
Po
alo
stl
pe
arv
al
sh
rim
To
p
Po
tal
rce
Sh
lla
rim
nid
p
me
ga
lop
Lo
e
bs
te r
lar
va
e
0
0
Stage/Category
Figure 1.9. Breakdown of subtaxa and
developmental
stages
represented
within
Decapoda in cleared vs. intact tow samples.
(Bars are means  Std deviation, n=11).
Frasier 15
Discussion
400
Total Number of Individuals
350
300
Trap vs. Tow Results
250
200
Comparisons of trap versus tow
150
100
samples reveal that trap captures were
50
larger than tow captures – often by
0
Cleared
Intact
Figure 1.7. Total number of individuals in cleared vs.
intact tow samples. (Bars are means  Std deviation,
n=11).
one to two orders of magnitude. This
contrast is not surprising, since the
traps were deployed for an hour, actively attracting organisms in three dimensions during that
time, while the tows were dragged for only a minute, and sampled only those organisms
intercepted along a section of the water column. The trap data provide a longer-term look at
the composition of phototactic zooplankton in the volume of water circulating through the
traps over the course of an hour, while the tow data represent a snapshot of a cross-section of
the water column.
The trap samples were more homogeneous and had more consistent composition due to
the longer periods of exposure, which lessened the influence of small-scale variations in the
water column on the data. In contrast, the tow samples were prone to heavy influence by
local patchiness in the distributions of organisms in the water column due to the brevity of
the exposure. Patchiness in zooplankton distribution has been described on horizontal scales
as small as 10 m (Mackas et al. 1985), resulting in high local variability in zooplankton
abundance and diversity (Yoshoika et al.1985; Rios-Jara 1998; Alvarez-Cadena et al. 1998).
Patchiness is partially explained by swarming behaviors common to many types of tropical
Frasier 16
zooplankton (Haury and Yamazaki 1995), an adaptation linked to predator avoidance, food
source exploitation and mating interactions (Avois-Jacquet et al. submitted). Nonhomogeneous, patchy distributions within a the sampled areas may explain the high standard
deviations in the data and resulting lack of significant trends, especially in the short-exposure
tow samples. Increasing the number of tow samples taken and/or increasing the distance and
time during which the tows were dragged could solve this problem in future studies.
Additionally, trap and tow samples differed in the types of organisms collected. For
instance, cnidarians and ctenophores, as well as comatulids and arrow worms
(Chaetognatha) were more common in the tows than the traps. A few groups, including
Pyrosomida, Bdelloidea, Acarina, and Platyhelminthes, were represented in tow samples but
did not appear in trap samples. These discrepancies are likely a combination of differences
in degrees of phototacticity and mobility between organisms, as well as size considerations:
some of the jellies caught in the tows were too large to enter the traps.
Differences Between Samples
Considering only the light trap data set, a few trends appear in the collected
information. First, larger numbers of individuals per sample were collected in cleared areas
than in intact areas. Decapods in general, and particularly shrimp, were found in larger
numbers in the cleared areas, and shrimp, in particular among the decapods, were more
common in cleared areas than intact. This was unexpected, and contradicts Krishnamurthy’s
Frasier 17
hypothesis that marine organisms actively recruit to mangroves and use them as nurseries
(Krishnamurthy, 1982; also Dennis, 1992).
To my knowledge this is the first report that zooplankton abundances differ between
cleared and intact mangrove forests. A few studies have compared zooplankton communities
among different coastal habitats, but these have revealed mixed results. Sheridan (1996)
found that mangroves supported higher densities of microscopic individuals than mudflats or
seagrass beds (also Kieckbusch 2004 et al.), and Laegdsgaard and Johnson found mangroves
to be important habitats for zooplankton survival (1995). A 2005 study by Bloomfield and
Gillanders showed the opposite trend, finding lower invertebrate abundances in mangroves,
in accordance with the findings of the present study, but their samples were not limited to
zooplankton.
The majority of research on mangroves as potential nurseries has focused on larval fish
(Pinto and Punichihewa, 1996; Robertson and Blaber 1992). It has been repeatedly
suggested that young fish use mangroves as nursery habitats, benefiting from various
characteristics including the complex root systems, reduced light penetration, and decreased
risks of predation compared to more open, less complex systems (Cocheret de la Morinière et
al. 2002). However, few have quantified this difference in the field. Research has shown
habitat selection by fish to be species specific, with certain coastal and reef fishes using
sheltered coastal habitats as nurseries, while pelagic species are less prevalent (Harris et al.,
2001). The findings in the present study suggest that these patterns of habitat selection do
Frasier 18
not extend to zooplankton. Instead, zooplankton densities are positively affected by the
conditions in cleared areas.
A number of factors could explain this trend:
1. Decreased water circulation in root-dense areas. Toffart (1983) suggested that
increasing root density results in a strong decrease in species diversity with increased
distance from the open-water edge of mangrove forests. Since accessibility and flow
to sheltered, higher density regions is limited, increased active locomotion is required
in order for zooplankton to reach more sheltered areas (Toffart, 1983). This effect of
reduced circulation may also have caused the traps to sample a smaller volume of
water in intact areas than cleared areas where water circulates more freely.
2. Higher predation rates in intact areas. Fish are important predators on zooplankton
(Williamson et al., 1994; Columbini et al., 1996), and studies show that many species
choose to live within the mangroves rather than in unvegetated coastal habitats
(Chong et al. 1990; Sedberry and Carter 1993). This could result in higher predation
rates in the intact areas due to increased fish presence in comparison to cleared areas.
3. Higher abundances of phytoplankton in cleared areas due to higher light availability.
Although sampling was conducted at night, during daylight there is much more light
available in unsheltered cleared areas compared to that underneath the intact
mangrove canopies. Phytoplankton, the prey of zooplankton, are highly influenced by
light availability (Tillman et al. 1982), and reduced light availability in intact areas
may reduce the amount of food available for zooplankton. Residual effects of this
Frasier 19
higher diurnal phytoplankton productivity may be partially responsible for the higher
abundance of zooplankton in cleared areas.
4. Reduced competition/higher resource availability in cleared areas. The reduced
presence of adult invertebrate populations in cleared areas compared to intact areas,
linked to the lack of suitable substrate, may lead to less competition for resources in
cleared areas.
5. Differences in effective volume sampled. The root structure in intact areas may have
limited the visibility of the light source, reducing volume of water sampled in
comparison to cleared areas, where light diffusion was unobstructed.
In contrast, dense root structure, shelter, availability of organic material, and local
presence of spawning adults, (especially in the case of crabs and lobster), do not positively
affect the abundance and diversity of the zooplankton supply.
Despite potential issues with experimental design, these trends indicate significant
differences in population size and composition between area types and definite relationships
between zooplankton communities and coastal habitat. Impacts on, and modifications of food
chains in these open areas seem likely.
Similarities Between Samples
Some observations can be made based on trends that were tested for, but not seen, in
the data sets. Most prominently, there was no difference in species diversity between
sampled areas. All species were represented in both types of environments, though there was
Frasier 20
a trend towards higher diversity in intact areas. Looking more closely at decapod
representation, the numbers of individuals in crab and lobster populations were not
significantly different between cleared and intact areas.
Lastly, there was no difference between site types in the number of developmental
stages represented in shrimp and crab populations. The stages seen were early
developmental forms, thus for these individuals, pre-settlement survival may be independent
of environment type. This raises questions concerning the point at which environment does
become an important factor in invertebrate survival and presence.
The similarities between zooplankton communities in intact and cleared areas
elucidated by this study suggest similar incoming zooplankton supply or some amount of
homogenizing circulation between the two types of areas, since overall composition was
similar. This idea is supported by other studies, which have found that many organisms have
open populations of widely dispersed planktonic stages (Young, 1995). Young’s work shows
that large-scale dispersal makes populations resistant to localized environmental damage and
maintains diversity (1995).
Conclusion
The results of this study show that conditions in cleared mangrove environments
positively influence zooplankton abundances, and do not strongly affect diversity.
Frasier 21
According to this information, small scale mangrove removal is does not seem to damage
zooplankton communities.
Future Directions for Study
The lack of distinct differences between intact and cleared sites raises questions as to the
distances over which variability could begin to be seen in this type of study. Would areas
with larger cleared swaths, ranging from 500 to 1000 m for example, show similar trends? In
addition, the traps and tows were distributed in a linear pattern, parallel to shore. No data
were collected on variability in relation to distance from shore, distance from open water (in
intact mangrove areas), or water depth. Comparisons perpendicular to shore would help to
further understand coastal zooplankton distribution patterns.
This study also raises questions regarding the impacts of zooplankton distribution
trends on conservation and management practices. Determining the dimensions of variability
and dispersal of zooplankton populations is important for better understanding their patterns
of distribution. This knowledge will be useful for marine reserve design and for re-stocking
depleted or damaged populations.
Extending and modifying this study to include fish larvae is another important
direction to pursue. Are young fish more selective in their choice of mangrove habitat? By
modifying the traps to include greater light intensity and larger points of entry, a similar type
of study could address these issues in regards to fish populations.
Frasier 22
Limited work has been done to determine the role of mangroves as habitats for marine
flora and fauna. Long considered as primarily terrestrial systems, they have only recently
begun to be studied from a marine perspective. As mangroves continue to disappear, and
become increasingly threatened, it is critical to develop a clearer picture of the impacts that
this destruction will have on the biology of the world’s oceans.
Frasier 23
Chapter 2:
A comparison of zooplankton populations between mangrove systems in
Panama and Aldabra
Introduction
Mangroves, loosely defined as “woody plants that grow at the interface between land
and sea in tropical and subtropical latitudes” (Kathiresan, 2001), are found in 122 countries
worldwide and cover an estimated 18 million hectares of coastline (Spalding et al., 1997).
Their distribution is primarily circumtropical, between 30 N and 30 S, with ranges
influenced primarily by temperature (Duke, 1992), moisture (Saenger and Snedaker, 1993),
and large-scale current patterns (De Lange and De Lange, 1994). Throughout this extensive
distribution, mangroves experience a wide range of challenging environmental conditions,
including high salinity, variable tidal exchange, winds, high temperatures and muddy,
anaerobic soils. As a result, these ecosystems, which originated approximately 114 million
years ago (Duke, 1992), have developed numerous unique physical adaptations allowing
them to cope with their challenging settings. These adaptations include specialized
mechanisms of gas exchange, salt exclusion, and aerial root structures (Kathiresan, 2001).
Mangrove Communities
Associated with mangrove stands, one finds entire mangrove forest communities,
including associated microbes, fungi, plants and animals (Kathiresan, 2001). Bacteria are
thought to be important in controlling the chemistry of the within-mangrove environment
Frasier 24
through such processes as nitrogen fixation. Microalgae and macroalgae within mangrove
communities are key in supporting higher trophic levels - namely invertebrates and fish
(Robertson and Blaber, 1992).
The fauna associated with mangrove communities is highly diverse and variable.
Zooplankton can be an important component, but their abundances are variable relative to
offshore waters (see previous chapter). Macroscopic mangrove-associated invertebrates,
occupy various niches within mangrove systems. Submerged root surfaces often support
populations of sponges, hydroids, anemones, polychaetes, bivalves, barnacles, bryozoans and
ascidians (Kathiresan, 2001). These organisms are generally found in distinct vertical
zonations on mangrove prop roots, influenced by factors including desiccation, wave action,
temperature and salinity (Rützler, 1995; Farnsworth and Ellsion, 1996). Sands and sediments
in mangroves support higher densities of benthic organisms than sands and sediments
elsewhere (Edgar 1990; Sasekumar and Chong, 1998). A 1997 study by Sheridan identified
over 300 benthic taxa represented in Rhizophora mangle stands in South Florida. In addition,
numerous studies have shown that higher abundances of crustaceans, including shrimp and
crabs, may be positively correlated with the occurrence of mangrove areas in close proximity
(Saskumar et al., 1992; Kathiresan et al., 1994; Vance et al., 1996).
Variations in Community Composition
Due to the wide variety of environmental conditions experienced globally by
mangroves, associated organisms could be expected to colonize and utilize these systems
Frasier 25
differently in different locations. This study focuses on differences in zooplankton presence
in mangroves on opposite sides of the globe, comparing zooplankton communities found
along the Caribbean coast of Panama to those found in the Seychelles archipelago in the
Indian Ocean. Both locations are dominated by red mangrove forests, with Rhizophora
mangle as the primary species in Panama, and Rhizophora mucronata the primary species in
the Seychelles.
Key environmental differences make these two sites worthy of a closer look and useful for
comparison:
First, they may have unique native taxa and population structures, simply as a result of
the great distance between them.
Second, although these mangroves occupy similar environments in terms of water
temperatures and geological conditions, they experience very different submergence patterns.
The Panamanian mangrove systems are characterized by very low tidal exchange, on the
order of 15 to 50 cm. At low tide, conditions provide at least 50 cm of water within the roots
at the sites studied. As a result, the prop root communities in Panamanian sites are distinctly
zoned. At the top of the roots, individuals experience regular exposure and completely
emerged conditions, while individuals attached to the bottom of the roots are never emerged.
This is not the case in Aldabran sites, where the tidal exchange is much more extreme - on
the order of 2 to 3 m (Aldabra Marine Programme) - leaving the root structure completely
exposed during low tides. As a result, no purely subtidal organisms can survive on the roots.
While Panamanian mangroves exhibit rich and diverse prop-root communities, invertebrates
Frasier 26
and algae are sparse on Aldabran mangrove root surfaces. This difference impacts the ability
of organisms to use mangrove root systems as viable and consistent sources of shelter and
habitat. Based on these differences, I hypothesized that mangroves in Aldabra are less
hospitable for zooplankton than Panamanian mangroves, and thus that zooplankton would be
less abundant and less diverse in Aldabra.
The intent of this study was to examine the differences in the composition of the
zooplankton populations found in Panama and Aldabra, looking specifically at total catch
sizes, the types of organisms present, respective abundances of represented organisms, and
overall diversity.
Materials and Methods
Samples collected in two locations, the island of Bocas del Toro, Panama (90.0' N,
8000' W), and the Aldabra atoll in the Seychelles (92.4' S, 46.20' E), were compared by
sample size, composition, and orders represented. All samples were collected within the root
structures of intact mangrove sites – defined as areas where the trees had not been damaged
by human activity.
The Panamanian samples used for the comparison were a subset of those collected
using larval light traps in Chapter 1. The subset included six samples collected from three
different intact areas over the course of 6 consecutive nights centered around the full moon.
Frasier 27
These samples were collected in mid-May, 2004. (See Chapter 1 for details on collection
methods). The Indian Ocean samples were collected in red mangrove forests with the same
light traps and methods during late April to mid May 2005, on six different nights within a
one-month period. The sampling approaches were kept as consistent as possible between the
two locations. Factors including time of night, sampling period, mangrove type, as well as
collection, preservation, and analysis techniques, were all kept uniform throughout both
sampling efforts.
Statistical Analysis
To test the null hypothesis that there was no difference between the zooplankton
communities of the two sites, I used 2 sample t-tests. Measures included total abundance,
number of taxonomic groups, proportion of individuals in each of the taxonomic groups,
number of larval stages of shrimp and crabs and differences in proportion of shrimp and
crabs in each developmental stage.
Copepods were excluded from the
Total Number of Individuals
1800
1600
analysis due to their pervasive presence
1400
and disproportionately large abundances.
1200
1000
800
600
Results
400
200
0
Aldabra Bocas del Toro
Figure 2.1. Total number of individuals in Aldabra
vs. Bocas del Toro. (Bars are means  std
deviation, n=6).
Mean abundance was larger in
Panamanian than in Aldabran mangroves,
Frasier 28
as shown in figure 2.1 (p-value = .038). The mean Panamanian sample size was 988
individuals per sample (standard deviation = 670.845), while the mean for Aldabran samples
was 225 individuals per sample (standard deviation = 240.848).
At the taxonomic level, cumaceans stood out, accounting for a larger percentage of the
sample composition in Panamanian samples than in Aldabran samples (p-value = 0.018)(Fig
2.2). Ostracods were also more prevalent in the Panamanian samples (p-value = 0.066)(Fig
2.2). Tanaids, jellies, barnacles, and comatulids were present only in the Panamanian
samples, and were not represented in any of the Aldabran samples.
In contrast, decapods represented a larger proportion of the total number of individuals
present in Aldabran samples (p-value = 0.009), although in terms raw abundance, there were
fewer in Aldabra. In addition, among the decapods, shrimp megalope were less prevalent in
Aldabran mangrove waters than in Panama (borderline p-value = 0.057)(Figure 2.3).
No difference in overall taxonomic diversity was detected between the two sites.
Frasier 29
Intact
500
450
450
400
400
Number of Individuals
500
350
300
250
200
150
350
300
250
200
150
100
50
50
0
0
C
ra
b
C
ra
b
zo
m
ea
Po
e
st gal
la
rv ope
al
c
To rab
s
ta
S lc
Sh hrim rab
s
rim
p
zo
p
Po
ea
m
e
st
la gal
rv
o
p
al
S e
To hr
im
Po tal
p
rc Sh
rim
el
la
p
Lo nid
bs
cr
a
te
rl b
ar
va
e
100
D
St ec
om ap
at od
o a
C po
um d
a a
I c
Am sop ea
p od
M hip a
ys od
Ta i d a
C
na ac
ni
e
da
C idac a
ria
la e
ns O do a
& st ce
C ra ra
te c
Po nop oda
Eu lyc ho
p h ra
C hau aete
ha s a
et iac
og e
a
C nat
C irr ha
om ip
at edia
ul
id
ae
Number of Individuals
Cleared
Stage/Category
Taxon
Figure 2.2. Total number of individuals by taxon in
Aldabra vs. Bocas del Toro. (Bars are means  std
deviation, n=6).
Figure 2.3. Stages and categories of decapods
represented in Aldabra vs. Bocas del Toro. (Bars are
means  std deviation, n=6).
Discussion
# of Taxonomic Orders
Present
14
12
10
The overarching trend in this comparison
8
6
was the difference in the total abundance of
4
2
zooplankton found at the two locations with
0
Aldabra
Bocas del Toro
Figure 2.4 Number of taxa represented
in Aldabra vs. Bocas del Toro. (Bars are
means  std deviation, n=6).
Panamanian abundances almost 4 times larger
than Aldabran abundances. Although these
mangrove forests are based on similar plants
and are both tropical, the two systems have very different abundances of zooplankton. This
difference may be the result of the different tidal regimes in these two locations. In Panama,
Frasier 30
sea levels in mangrove systems change relatively little, and standing water is always present.
Such conditions may favor large planktonic communities. In contrast, the mangrove habitats
of Aldabra may be less favorable for zooplankton. These mangroves experience highly
variable tidal conditions, fluctuating from submerged to completely emerged conditions with
each tide cycle. As a result, the Aldabran mangroves have no water column for zooplankton
to reside in during low tide, meaning that zooplankton must leave and return with each tide.
No previous research has been done on the effects of tidal exchange on zooplankton
populations, thus there is little other information with which to compare these results.
Trends Among Taxonomic Orders
Overall, the abundances of most taxa were similar between locations and there was no
difference in diversity. Decapod larvae were the major type of meroplankton found in the
samples collected in Aldabra, possible linked to the fact that certain species of decapods have
developed mechanisms for dealing with the extreme tidal exchange. In contrast, shrimp
megalope were proportionally less common in Aldabran samples, suggesting that shrimp are
either less abundant, or not well-suited to the variable tide conditions. Overall, decapods
seem to be able to use the Aldabran mangroves as habitat despite the harsh conditions.
Holoplankton, however, were more negatively affected by the environmental
conditions at the Aldabran sites. In particular, cumaceans and ostracods were less prevalent
in the samples collected than in Panama. Some taxa, including Tanaidacea, Cnidarians &
Ctenophora, Cirripedia, and Comatulidae, were not represented at all in Aldabra. The
Frasier 31
variability of the conditions in Aldabra may be unfavorable for these taxa, which require
completely subtidal habitats.
Lastly, samples from both sites had variable taxonomic composition, and Aldabran
samples had higher variability relative to their sample size. These samples were highly
influenced by spotty, localized occurrences of certain types of individuals. There was not a
consistent pattern of organisms present in each sample. Samples collected in Panama
generally had more consistent composition. This may be partially due to the rapidity of the
tidal flux and the relative brevity of the period of submersion in Aldabra, which may not
allow complete mixing of individuals between mangrove sites. Effects of the lunar cycle
may also have influenced sample composition in Aldabra, since catches were made over the
span of a month.
Conclusion
As shown by this study, mangroves host very different marine communities on a global
scale. Mangrove environments are highly variable and their communities are strongly
influenced by the physical factors to which they are exposed. As a result, mangroves in
different locations may serve invertebrate communities differently and have varying impacts
zooplankton.
Frasier 32
This study raises further questions regarding invertebrate life-histories and mangrove
use. First of all, if zooplankton are not found in mangroves in Aldabra, where are they
found? Future research comparing pelagic populations to nearshore catches in both Panama
and Aldabra might clarify the overall presence of invertebrates in these areas. Are the
Seychelles biologically poorer in plankton biomass? If so, is this an effect of limited suitable
mangrove habitat? Or do the patterns of plankton distribution differ between these two
areas?
The answers to these questions apply directly to conservation and ocean management
practices. Understanding how populations use coastal habitats is important in choosing
functional approaches to reserve planning, species protection, and deforestation policies. The
plankton populations in question are both sources of juvenile individuals that maintain
macroscopic invertebrate populations, as well as food sources for upper trophic levels.
Given their central role in marine food webs, it is essential to understand the conditions that
influence and maintain zooplankton communities. Toward this end, the impacts of varying
mangrove environments on zooplankton merits further consideration.
Frasier 33
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