Journal of Water Resource and Protection, 2013, 5, 405-413
http://dx.doi.org/10.4236/jwarp.2013.54040 Published Online April 2013 (http://www.scirp.org/journal/jwarp)
Response of the Plankton to a Fresh Water Pulse in a Fresh
Water Deprived, Permanently Open South African Estuary
Pierre William Froneman1*, Paul Denzi Vorwerk2
1Coastal Research Group, Department of Zoology and Entomology, Rhodes University, Grahamstown, South Africa
2South African Environmental Observation Network (SAEON) Elwandle Node, Grahamstown, South Africa
Email: *w.froneman@ru.ac.za
Received January 17, 2013; revised February 28, 2013; accepted March 10, 2013
Copyright © 2013 Pierre William Froneman, Paul Denzi Vorwerk. This is an open access article distributed under the Creative
Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original
work is properly cited.
ABSTRACT
This study assessed the influence of a freshwater pulse on selected physico-chemical and biological variables in a per-
manently open freshwater deprived southern African estuary. In the absence of the freshwater pulse a reverse gradient
in salinity was evident with hypersaline (salinity > 40) conditions prevailing in the upper reaches of the estuary. Total
chlorophyll-a (chl-a) concentration during this period ranged from 0.25 to 0.60 µg·l1. The mean total zooplankton
abundance and biomass in the absence of freshwater during the daytime was 666 ind·m3 (SD ± 196) and 12.4
mg·dwt·m3 (SD ± 3.3), respectively. During the night time the mean total zooplankton abundance was 3121 ind·m3
(SD ± 1203) and the biomass 21.8 mg·dwt·m3 (SD ± 196). The total zooplankton abundance during the dry season was
numerically dominated by the copepod nauplii and the calanoid copepod, Pseudodiaptomus hessei, which contributed
up to 76% of the total zooplankton counts. The freshwater pulse was associated with the establishment of a horizontal
gradient in salinity along the length of the estuary and a significant increase in the total chl-a concentration (range from
0.74 to 11.75 µg·l1) and zooplankton biomass (range from 23.7 to 76.6 mg·dwt·m3) (p < 0.05 in both cases). Addition-
ally, there was a marked increase in the total zooplankton abundances and biomass within the estuary. A distinct shift in
the zooplankton community composition was evident with the copepod, Acartia longipatella numerically dominating
the zooplankton counts.
Keywords: Estuary Zooplankton Community; Freshwater Inflow
1. Introduction
Increased population growth coupled with industrializa-
tion has coincided with a decrease in the magnitude of
freshwater flowing into southern African estuaries. The
influence of the reduced freshwater inflow on the biology
of southern African estuaries is now well documented [1,
2]. Reduced freshwater inflow into estuaries has been
linked to a decrease in the phytoplankton size composi-
tion and daily production rates largely because freshwa-
ter inflow represents the primary source of nutrients nec-
essary to sustain the growth of the phytoplankton, par-
ticularly diatoms [1,3,4]. Among the heterotrophic com-
ponents of the estuarine food web, the alteration in the
riverine inflow into estuaries has been linked to changes
in the recruitment, biomass and species composition and
distribution of both inverstebrates and vertebrates within
these systems [2,5,6]. The formation of horizontal salin-
ity gradients in estuaries is also associated with spatial
patterns in the plankton biomass (so-called river-estuary
interface (REI) zone [7] Finally, freshwater inflow into
estuaries has been linked an increased diversity of niches
and food sources available to animals in these systems
[8-10].
The Kariega Estuary on the south-east coast of South
Africa is regarded as a fresh water deprived system and
has been reported as having hypersaline conditions in the
upper reaches in various studies since 1991 [11-13]. The
hypersaline conditions in the upper reaches of the estuary
can be attributed to reduced freshwater inflow as a result
of small catchment area (680 km2), the presence of sev-
eral impoundments along the Kariega River and high
evaporative losses, particularly during the summer months
[12,13]. Localised flooding occurred during September
2006 along the south-east coast of South Africa, resulting
in a horizontal salinity gradient being recorded within the
system for the first time since 1991. The biological res-
*Corresponding author.
C
opyright © 2013 SciRes. JWARP
P. W. FRONEMAN, P. D. VORWERK
406
ponse to the resumption in freshwater inflow into the
estuary has been presented previously [14,15]. Results of
these studies indicate that the freshwater pulse was asso-
ciated with the re-establishment of a viable population of
the endangered River pipefish, Sygnathus watermeryi,
[16], elevated biomass of plankton and fish larvae [15]
and increased recruitment of two important sport fishery
species in the Kariega Estuary [14]. This manuscript pre-
sents the main findings of an investigation that examined
the response of the zooplankton community to the fresh-
water pulse in the freshwater deprived Kariega Estuary.
2. Study Area
The Kariega Estuary (Figures 1(a) and (b)) is a perma-
nently open marine dominated system on the southeast
Port Alfred
Seafield
Eastern Cape
Kenton-on-Sea
Indian
Ocean
South
Africa
East
Kleinemonde
Great
Fish
0 5
Kasouga
Kariega
km
Scale:
N
33
o
3655.5S
26
o
521.4E
Port Alfred
Seafield
Eastern Cape
Kenton-on-Sea
Indian
Ocean
South
Africa
South
Africa
East
Kleinemonde
Great
Fish
0 5
Kasouga
Kariega
NN
km
0 5
Scale:Scale:Scale: kmkm
33
o
3655.5S
26
o
521.4E
(a)
33o391.2S
26o3849.8E
N
0 1500m
33o391.2S
26o3849.8E
N
0 1500m
33o391.2S
26o3849.8E
N
0 1500m
(b)
Figure 1. The study area showing the location of the study estuary along the Eastern cape coast (adapted from Walton 1984).
Copyright © 2013 SciRes. JWARP
P. W. FRONEMAN, P. D. VORWERK 407
coast of South Africa (33˚40'46.6"S; 26˚40'57.9"E) [14]
The estuary is approximately 18 km long, has a depth
range of 2.5 m to 3.5 m and varies in width between 40
m and 100 m [7,13] The estuary is predominantly marine
with hypersaline conditions being recorded in the upper
reaches since 1991 [3,11,13]. The dominance of the ma-
rine environment within the system is indicated by the
106:1 ratio of tidal prism volume to river flow volume
[17]. The middle reaches of the estuary are characterised
by extensive salt marshes comprising the high marsh
plants Sarcocornia perennis, Chenolea diffusa and Spar-
tina maritima. The marine dominance of the system has
also contributed to the eelgrass, Zostera capensis, ex-
tending its distribution along the length of the estuary
[7,13].
3. Materials and Methods
Sampling of the zooplankton and related physico-che-
mical variables was conducted during November 2005
and November 2006. The November 2005 sampling re-
presents the flow rates at present (low flow rates, design-
nated dry season) within the system, while the November
2006 sampling represents a wet season for the Kariega
Estuary. Sampling was conducted at 10 stations along the
length of the Kariega Estuary during daylight and night
time during low tide.
3.1. Physico-Chemical Variables
Temperature and salinity (practical salinity units) at each
station were measured using a Horiba U10 water sampler
at both the surface and bottom of the water column with-
in the estuary.
3.2. Chlorophyll-a (chl-a)
Chlorophyll-a concentrations were determined for sur-
face and bottom waters for each station by collecting 200
ml water sample using an 8 L Niskin bottle. These sam-
ples were vacuum filtered (<5 cm Hg) through GF/F fil-
ters which were subsequently extracted in 90% acetone
for 24 hr in the dark at 20˚C. The total chlorophyll-a
concentrations were then determined using a 10AU-Tur-
ner flourometer before and after acidification according
to the method of Holm-Hansen and Riemann [18].
3.3. Zooplankton
Zooplankton at each station was collected during three
replicate surface tows (depth 0.5 m) conducted during
the day and night using a WP-2 Net (50 cm diameter,
100 μm mesh) fitted with a General Oceanics flow meter
to allow for volumetric standardisation of the samples.
All samples collected were stored in 10% buffered for-
malin for later identification in the laboratory. For each
replicate sample the zooplankton dry biomass was de-
termined by filtering a 1/2 to 1/32 sub-sample, obtained
using a Folsom plankton splitter, through a pre-weighed
GF/F filter and then oven drying at 60˚C for a period of
24 hr. The dry biomass was calculated as the difference
between the filter weight and the combined dry weight.
All zooplankton were counted to allow for density esti-
mates which were then averaged across the three repli-
cate samples. Abundance and biomass values were ex-
pressed as ind·m3 and mg·dwt·m3, respectively.
3.4. Numerical Analyses
The average community abundance data for each station
was entered into the PRIMER (Plymouth Routines in
Multivariate Ecological Research version 5.2.4 software
package for comparison between the stations [19]. The
data was transformed (log(x + 1)) to minimise the effect
of less abundant species and input into a group-averaged
Bray-Curtis Similarity analysis from which a cluster dia-
gram was generated. The SIMPER routine was then em-
ployed to identify which species were contributing to the
differences between the groupings identified with the nu-
merical analysis [19].
3.5. Statistical Analyses
To test for differences between wet and dry seasons for
chlorophyll concentrations, zooplankton biomass and
zooplankton densities, a Lilliefors test for normality was
used prior to inputting the data into an ANOVA. These
statistical tests were run in the STATISTICA for Win-
dows package [20].
4. Results
4.1. Flow Data
The monthly flow volumes in the four months preceding
the dry season sampling (from June to September) in
2005, never exceeded 0.026 × 106 m
3 with a maximum
occurring in June 2005 (Figure 2). The monthly flow
volumes increased from 0.012 × 106 m3 in June 2006 to
4.45 × 106 m3 and 2.9 × 106 m3 in August and September,
respectively (Figure 2) (Department of Water Affairs
and Forestry Flow Data).
4.2. Physico-Chemical Variables
The temperature profiles recorded during both the wet
and dry seasons were similar, with a temperature gradient
evident from the mouth to the head of the system (Figure
3). Temperatures during the dry season (November 2005)
ranged from 19.3˚C near the mouth to 23.4˚C at the head
of the estuary, while during the wet season (November
2006) the lower reach temperatures were 17.4˚C with a
maximum of 24˚C at the head. The recorded salinity
Copyright © 2013 SciRes. JWARP
P. W. FRONEMAN, P. D. VORWERK
408
Month
JuneJulyAugust September
Monthly Flow Volume (million m3)
0.00
0.01
0.02
0.03
2.50
3.00
3.50
4.00
4.50 2005
2006
Figure 2. The monthly flow volume (million m3) in the months preceding the sampling during 2005 and 2006 (drawn from
Department of Water Affairs and Forestry Flow Data).
20
20
21
21
22
22
22
22
23
23
Depth (m)
0.0
0.5
1.0
1.5
18
18
19
19
20
20
21
21
23
23
23
22
22
Distance from Mouth (km)
2 4 6 810121416
Depth (m)
0.0
0.5
1.0
1.5
A
B
(b)
(a)
Figure 3. Contour plots of the temperature (˚C) in the Kariega Estuary during the dry season, November 2005 (a); and the
wet season, November 2006 (b).
demonstrated a reverse salinity gradient during the dry
season, with a minimum salinity of 35 near the mouth of
the estuary and a maximum of 36.5 near the head of the
system (Figure 4). Conversely, during the wet season, a
normal salinity gradient was evident, with salinities of 34
recorded at the mouth and 4.7 at the head of the estuary
(Figure 4). A salt wedge was also evident in the upper
reaches of the system during the wet season, with salini-
ties on the bottom less than 12.
4.3. Chlorophyll-a
The total chlorophyll-a concentration during the dry sea-
son was relatively uniform along the length of the estuary,
and ranged from 0.25 μg·L1 to 0.61 μg·L1 (Figure 5).
No apparent horizontal or vertical patterns were evident
in the total chlorophyll-a concentration during the dry
season. During the wet season, the total chlorophyll-a
concentration was significantly higher (p < 0.001; F =
34.08), ranging from 0.74 μg·L1 to a maximum of 11.75
μg·L1. During the wet season the maximum chlorophyll-
a concentration occurred in the mixing zone between oli-
gohaline and polyhaline conditions approximately 14 km
from the mouth (Figures 4 and 5).
4.4. Zooplankton
Daytime dry season densities of zooplankton ranged
from 432 ind·m3 to 936 ind·m3, while during the wet
season total zooplankton densities varied between 76
Copyright © 2013 SciRes. JWARP
P. W. FRONEMAN, P. D. VORWERK 409
35.4
35.4
35.1
35.1
36.0
36.0
35.7
35.7
36.0
36.0
36.3
36.3
Depth (m)
0.0
0.5
1.0
1.5
30
30
30
30
25
25
20
20
15
15
10
10
Distance from Mouth (km)
246810 12 14 16
Depth (m)
0.0
0.5
1.0
1.5
A
B
(a)
(b)
Figure 4. Contour plots of the salinity (practical salinity units) in the Kariega Estuary during the dry season, November 2005
(a); and the wet season, November 2006 (b).
0.30
0.35
0.30
0.35
0.30
0.55
0.50
0.45
0.40
0.30
0.40
0.40
0.35
0.30
Depth (m)
0.0
0.5
1.0
1.5
2
2
6
6
4
46
6
6
84
8
8
6
6
10
88
Distance from Mouth (km)
246810121416
Depth (m)
0.0
0.5
1.0
1.5
A
B
(a)
(b)
Figure 5. Contour plots of the total chlorophyll-a concentration (μg·L1) in the Kariega Estuary during the dry season, No-
vember 2005 (a); and the wet season, November 2006 (b).
ind·m3 and 11180 ind·m3 (Figures 6(a) and (b)). The
night time densities during the dry season were signifi-
cantly lower than those recorded for the wet season (p <
0.03; F = 6.39), with mean densities of 3121 ind·m3
(±1203 SD) recorded during the dry season and 14,270
ind·m3 (±13900.53 SD) recorded during the wet season
(Table 1). The horizontal trends demonstrated relatively
uniform densities throughout the estuary during the dry
season and higher densities in the lower and middle rea-
ches during the wet season (Figures 6(a) and 7(b)). The
observed peak in densities during both day and night
time sampling during the wet season occurred at a poly-
haline salinity and at the surface chlorophyll maximum.
Daytime total zooplankton biomass demonstrated no
significant differences (p < 0.05) between seasons, rang-
ing between 9.0 mg·dwt·m3 and 16.7 mg·dwt·m3 during
the dry season and from 5.8 mg·dwt·m3 to 73.8
mg·dwt·m3 during the wet season (Figures 6(c) and (d)).
The night time biomass, however, demonstrated a sig-
nificant increase from dry to wet season (p < 0.01; F =
11.34). The mean dry season night time biomass was
21.75 mg·dwt·m3 (±3.33 SD), while the mean wet sea-
son biomass was estimated at 50.16 mg·dwt·m3 (±26.46
SD) (Table 1). During the dry season the total zooplank-
ton biomass was relatively uniform throughout the estu-
ary during both day and night samples, while during the
wet season the biomass was highest in the lower and
middle reaches of the system (Figures 6(c) and 7(d)).
Copyright © 2013 SciRes. JWARP
P. W. FRONEMAN, P. D. VORWERK
410
Dry Season
Distance from Mouth (km)
024681012141618
Biomass (mg dwt m-3)
0
10
20
30
40
Day
Night
Wet Seaso
Distance from M
n
outh (km)
0246810
Density (ind.m-3)
0
1000
2000
3000
4000
5000
6000
7000
Day
Night
0
10000
20000
30000
40000
50000
Day
Night
12141618
0
20
40
60
80
100
120
140
160
180
Day
Night
A
CD
B
(a) (b)
(c) (d)
(ind·m
3
)
(mg·dwt·m
3
)
Figure 6. The mean zooplankton biomass and density recorded during day and night surveys. Error bars represent the stan-
dard deviation. (a) = dry season zooplankton biomass; (b) = wet season zooplankton biomass; (c) = dry season density; (d) =
wet season density. Note the different scales on the y-axes.
Table 1. The mean zooplankton biomass (mg·dwt·m3) and density (ind·m3) during the day and night for both seasons.
Standard deviations, minimum and maximum recorded values are also presented.
Dry Wet
Day Night Day Night
Biomass Mean 12.42 21.75 22.49 50.16
SD 2.65 3.33 19.80 26.46
Max 16.67 29.06 73.79 89.64
Min 8.96 18.29 5.79 7.86
Density Mean 666.20 3121.20 2891.52 14270.37
SD 196.34 1203.89 4276.58 13900.53
Max 964.0 5780.0 11180.15 39962.84
Min 432.0 1864.0 76.32 436.60
4.5. Numerical Analyses
The numerical analyses of the zooplankton density data
resulted in a significantly different separation (p = 0.001;
R = 0.861) between the wet and dry seasons at approxi-
mately 45% similarity level (Figure 7). A further separa-
tion was evident during the dry season between the lower
reach stations (designated Marine Dry Group) and the
remainder of the estuary at approximately 75% similarity.
ANOSIM indicated differences between the two group-
ings were significant (p < 0.05). The separation between
the lower reach stations and the remaining stations could
largely be ascribed to the increased contribution of ma-
rine species to the total counts including copepods from
the genera Oithona, Eucalanus and Calanus (Table 2).
During the wet season a longitudinal separation oc-
curred with the upper reach stations separating from the
remaining stations at approximately 50% similarity (Fig-
ure 7). Again, ANOSIM indicated differences between
the two groupings were significant (p < 0.05). SIMPER
analysis indicated that the separation of the upper sta-
tions from the remaining station could largely be as-
cribed to reductions in the abundances of the numerically
dominant species rather than the presence of individual
species.
5. Discussion
Despite the permanently open nature of the Kariega Es-
tuary, the system is currently highly impacted due to
fresh water deprivation resulting from impoundments
along the Kariega River [4,17]. The recorded normal
Copyright © 2013 SciRes. JWARP
P. W. FRONEMAN, P. D. VORWERK 411
Figure 7. The numerical analyses of the night time zooplankton communities at each site during the wet and dry seasons. The
dotted line represents a 45% similarity.
Table 2. Mean abundances of the five most numerically abundant zooplankton accounting for up to 78% of the similarity
within each grouping identified with the hierarchical cluster analysis.
Marine-dry Middle and upper reaches-dry Freshwater-wet Middle and lower reaches-wet
Species
Average
abundance
(ind·m3)
Species
Average
abundance
(ind·m3)
Species
Average
abundance
(ind·m3)
Species
Average
abundance
(ind·m3)
Oithona nana 33.3 Copepod nauplii 1897.1 A.longipatella 131.5 A.longipatella 13379.5
Eucalanus sp. 14.7 P.hessei 909.4 P.hessei 62.1 P.hessei 2607.8
Calanus simillimus 31.3 A.longipatella 343.1 Halicyclops sp. 105.3 Copepod nauplii 1313.4
Pseudodiapto mus h essei 450.0 Halicyclops sp. 81.7 Copepod nauplii30.5 Ostracod 289.6
Copepod nauplii 1267.7 Tortanus capensis34.9 Ostracod 6.1
flow rate of 0.003 m3·s1 (Department of Water Affairs
and Forestry Flow Data) regularly results in hypersaline
conditions predominating in the upper reaches of the
estuary [11,13,21]. This study examined the response of
the plankton to a fresh water pulse resulting from pro-
longed heavy rainfall within the catchment area over a
two-month period. Both the temperature and salinity pro-
files demonstrate a well-mixed marine dominated lower
to middle reaches of the estuary during both the dry and
wet seasons (Figures 4 and 5). The difference in the wa-
ter column characteristics between the dry and wet sea-
son was evident in the upper reaches of the system. Dur-
ing the wet season, a distinct salt wedge was evident in
the headwaters of the estuary while hypersaline condi-
tions predominated in the upper reaches during the dry
season (Figure 4).
The total chlorophyll-a concentration during the wet
season (range 0.74 to 11.5 µg·l1) was significantly
higher than during the dry season (range 0.25 to 0.60
µg·l1) (p < 0.05). The increase in the total chl-a concen-
tration in freshwater dominated estuaries is largely
thought to be result of elevated phytoplankton production
rates resulting from increased macronutrient availability
[1,3,17]. Alternatively, it is also possible that the ele-
vated total chlorophyll-a concentrations recorded during
the wet season may have be derived from riverine input
or the resuspension of microphytobenthic algae [4,13,
17].
The total zooplankton abundances and biomass at-
tained the highest levels during the wet phase (Figure 3).
Shifts in the total zooplankton abundances and biomass
within southern African estuaries and indeed estuaries
Copyright © 2013 SciRes. JWARP
P. W. FRONEMAN, P. D. VORWERK
412
worldwide, have been reported to be related to the inter-
active effects of temperature and food availability [2,5,17]
The influence of temperature can largely be discounted
as water temperatures were broadly similar during the
two seasons The significant increase in zooplankton den-
sity and biomass recorded from dry to wet season during
this study is, therefore, likely to be the result of elevated
food availability. While the dry season biomass and den-
sities values are in the range reported for Kariega Estuary,
the wet season densities and biomass are substantially
higher and are in the range reported for permanently
open southern African estuaries with sustained freshwa-
ter inflow [2-4,13]. Results of the hierarchical cluster
analyses indicated that the wet and dry seasons were cha-
racterised by distinct zooplankton communities (Figure
7). The species which demonstrated the greatest increase
in numbers between dry and wet season was the copepod,
Acartia longipatella, which contributed 8.5% of the
total abundance during the dry season, but represented
75% of the total abundance during the wet season. Al-
though the actual abundances of Pseudodiaptomus hessei
increased from dry to wet season, the percentage contri-
bution of the total abundance decreased from 25% to
15%. Successional patterns of copepods within southern
African estuaries are largely driven by alterations in sa-
linity [2]. Acartia longipatella reportedly attains the high-
est abundances and biomass during periods when oligo-
haline conditions prevail [2,5,21]. Conversely, the calan-
oid copepod, P. hessei can be considered as a pioneer
species able to tolerate a high variance in salinity and
temperature [2,21,22]. The observed shift in the numeri-
cally dominant copepod species from the dry to wet phase
can therefore be attributed to a change in the salinity re-
gime within the estuary resulting from the freshwater
pulse.
Results of the numerical analyses conducted during the
dry and wet season indicated the presence of a longitu-
dinal gradient in the zooplankton assemblages within the
Kariega estuary. During the dry season, those stations oc-
cupied in the lower reaches of the estuary were distinct
from the stations within the middle and upper reaches of
the system. The clear separation of the two groupings
could largely be attributed to the increased contribution
of marine species (copepods of the genera Oithona, Eu-
calanus and Calanus) to the total counts at stations in the
lower reaches reflecting the influence of the marine en-
vironment on the estuary. On the other hand, during the
wet season, the upper reach stations separated from those
occupied within the middle and lower reaches of the es-
tuary. SIMPER analyses indicated that the separation
could be largely ascribed to reductions in the numerical
abundances of the dominant copepods within the upper
reaches of the estuary. The reduced abundances within
the upper reaches can probably be ascribed to the inflow
of freshwater which would prevent the build up of zoo-
plankton biomass within the region.
Results of this study indicate that the freshwater pulse
into the Kariega Estuary was associated with an increase
in the zooplankton biomass and a shift in the zooplankton
species composition. The horizontal patterns in zooplank-
ton community structure and biomass can be ascribed hy-
drodynamics of the estuary, reflecting both the magni-
tude of freshwater inflow into the system and the influ-
ence of the marine environment on the lower reaches of
the estuary. The increase in the phytoplankton biomass
associated with the freshwater inflow is also likely to be
associated with a change in the food web structure from a
detrital food web to one where the classical food web
predominates [23]. Additionally, the outflow of estuarine
water is also likely to be associated with elevated pri-
mary and secondary production rates in the near shore
marine environment [24].
6. Acknowledgements
The authors would like to thank the National Research
Foundation (NRF) of South Africa and the South African
Observation Network (SAEON) Elwandle Node for pro-
viding funds and facilities tocomplete this study.
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