-RESEARCH ARTICLE

I. Kibirige Æ R. Perissinotto Æ X. Thwala

A comparative study of zooplankton dynamics in two subtropicaltemporarily open/closed estuaries, South Africa

Received: 20 July 2005 / Accepted: 28 October 2005 / Published online: 7 December 2005� Springer-Verlag 2005

Abstract This study aims at comparing the zooplanktonabundance/biomass of two South African estuaries af-fected to a different degree by sewage pollution. Inparticular the hypothesis that microzooplankton mayincrease relative to mesozooplankton, as eutrophicationincreases, is tested for the first time in African estuaries.The survey was carried out during a whole-year cycle,including both open and closed phases. Results showthat zooplankton abundance and biomass are higher inthe Mdloti than in the Mhlanga, during both the closed(U=1, P<0.001, U=16, P<0.001) and the open phase(U=101, P<0.05, U=88, P<0.01). There were nosignificant differences in abundance/biomass betweenthe different reaches of each estuary, during either theiropen or closed phase (1-way ANOVA, P>0.05). Thedifferent levels of dominance of the calanoid copepodPseudodiaptomus hessei, which accounted for 53–64 and86–97% of the total abundance in the Mdloti and theMhlanga, respectively, suggests a shift in the zooplank-ton community structure. There was a lower microzoo-plankton abundance/biomass contribution to the totalzooplankton in the Mhlanga (59.6–15.8%), compared tothe Mdloti (99.1–96.5%). The highest microzooplanktoncontribution in the Mdloti was observed during theclosed phase, while in the Mhlanga this occurred duringthe open phase. This suggests that eutrophication in theMhlanga may impact negatively on the microzoo-plankton community. On the other hand the less affected

Mdloti exhibits an opposite trend with an increasedmicrozooplankton component.

Introduction

Zooplankton constitutes a central component of marineecosystems, providing a link between phytoplanktonproduction and higher trophic levels, such as commer-cially exploited fish stocks (Allen et al. 1995; Blaber1997; Harris et al. 2000; Little 2000). Zooplankton is,therefore, very important from both ecological andeconomic perspectives (Harris et al. 2000; Little 2000).Several zooplankton studies have been conducted in thepermanently open estuaries of South Africa, but veryfew in its temporarily open/closed estuaries (TOCEs).Yet TOCEs constitute the vast majority of South Afri-can estuaries, accounting for about 71% of the total(Reddering and Rust 1990; Whitfield 1992, 2000; Peris-sinotto et al. 2000; Nozais et al. 2001; Kibirige andPerissinotto 2003a). During periods of low rainfall (dryseason), TOCEs exhibit a sand bar at the mouth thatcuts off any connection with the sea. Conversely, duringhigh rainfall periods (wet season), the sand bar is brea-ched and a connection with the sea is established(Whitfield 1992; Perissinotto et al. 2000; Nozais et al.2001).

The few studies conducted in these estuaries so farhave shown that zooplankton abundance/biomassexhibits marked variations between daytime and night-time, as well as between the closed and the open phase(Whitfield 1980; Perissinotto et al. 2000; Kibirige andPerissinotto 2003a, b; Froneman 2004).

The Mdloti and the Mhlanga estuaries are only 7 kmapart on the KwaZulu-Natal north coast (Fig. 1).However, the Mdloti catchment area is five times largerthan that of the Mhlanga (Begg 1984; Cooper 1991). Onthe other hand, the Mhlanga Estuary receives20 Ml day�1 of treated sewage waters, which is 2–3times more than the 8 Ml day�1 reported for the Mdloti

Communicated by J.P. Thorpe, Port Erin

I. Kibirige Æ R. Perissinotto (&) Æ X. ThwalaSchool of Biological and Conservation Sciences,University of KwaZulu-Natal, Howard College Campus,G. Campbell Building, 4041 Durban, South AfricaE-mail: perissinottor@ukzn.ac.za

Present address: I. KibirigeSchool of Education, Discipline of Mathematics,Science and Technology Education (DMSTE) Old-R Building,University of Limpopo, Private Bag X1106, Sovenga, South Africa

Marine Biology (2006) 148: 1307–1324DOI 10.1007/s00227-005-0175-2

Estuary (W. Pfaff, Ethekwini Municipality, personalcommunication). Apart from the inherent nutrientloading (eutrophication), the sewage effluents also causedifferences in the mean flow rates of the two estuaries,and hence likely affect both variations in zooplanktonabundance/biomass and composition.

There is a controversy as to whether the smallerfraction of the estuarine zooplankton (microzooplank-ton) increases or decreases with an increase in eutro-phication (Marshall and Park 2000). A few studiesconducted in lake systems have indicated that largespecies may replace smaller ones, following an increasein eutrophication (Gliwicz 1969). Other studies have alsoshown that microzooplankton and mesozooplankton

relative importance change with increased eutrophica-tion (Gannon and Stemberger 1978; Bays and Crisman1983, 1989). Pace (1986) found an increase in the totalzooplankton biomass with increased eutrophication,while no changes were observed in the microzooplank-ton assemblage.

Previous studies in the Mhlanga Estuary have shownthat higher zooplankton abundance/biomass occurduring the closed phase, compared to the open phase(Whitfield 1980). Recently, similar observations havebeen reported from another two South African TOCEs:the warm-temperate Nyara Estuary (Perissinotto et al.2000); and the subtropical Mpenjati Estuary (Kibirigeand Perissinotto 2003b). These studies have shown that

Fig. 1 Map of the Mdloti andthe Mhlanga estuaries, SouthAfrica, showing stations wherezooplankton samples werecollected during the study

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TOCEs zooplankton abundance and biomass are mainlydominated by calanoid copepods. Generally, copepodsoccupy a fundamental level in estuarine food webs thateventually leads up to fishes of commercial importance(Miller et al. 1996). It is also well documented thatmysids and other macro-zooplankton feed primarily onestuarine copepods, like Acartia natalensis and Pseudo-diaptomus hessei (Wooldridge and Bailey 1982). Thesecopepods are also among the main food sources forfishes in estuaries (Wooldridge and Bailey 1982; Allenet al. 1995; Blaber 1997; Harris et al. 2000; Little 2000;Froneman and Vorwerk 2003). For example, in theMhlanga Estuary Monodactylus falciformis, Ambassisproductus and Gilchristella aestuaria are reported to feedmainly on zooplankton (Whitfield 1980; Harrison andWhitfield 1995).

The fish communities of the Mdloti and the Mhlangahave been investigated previously (Blaber et al. 1984;Harrison and Whitfield 1995), and yet zooplanktonstudies from these two estuaries are very scanty. Thisstudy, therefore, aims to answer the following questions:(a) What is the effect of closed and open phases on thezooplankton abundance/biomass of the Mdloti andMhlanga estuaries? (b) Do microzooplankton taxa(<90 lm) increase relative to mesozooplankton(>200 lm) as eutrophication increases from the Mdlotito the Mhlanga systems? (c) Are there differences inzooplankton abundance/biomass between the Mdlotiand the Mhlanga estuaries?

Materials and methods

Study area

The Mdloti Estuary

The Mdloti Estuary (29�38¢S, 31�05¢E, Fig. 1) is located27 km north of Durban and has a catchment area of550 km2 (Begg 1984), most of which is used for sugarcane plantations. The estuary is river-dominated (Coo-per et al. 1999) and the river is 88 km long. The estuaryhas an average depth of 2.5 m and exhibits low nutrientsconcentrations during the closed phase (Nozais et al.2001). The Hazelmere Dam is located 20 km upstreamof the estuary, with a capacity of 24·106 m3. The MdlotiEstuary currently receives 8 Ml day�1 of treated sewage,which is equivalent to a capping flow of 0.092 m3 s�1

(W. Pfaff, personal communication). The Health Indexof the estuary has recently been estimated as aboveaverage, with moderate to good water quality and areasonable appearance (Cooper et al. 1993).

The Mhlanga Estuary

The Mhlanga Estuary (29�42¢S, 31�05¢E, Fig. 1) is only18 km north of Durban and is part of the Mhlanga

Nature Reserve, currently managed by EzemveloKwaZulu-Natal (KZN) Wildlife. The catchment area is118 km2 and most of it is also used for sugar caneplantations. The Mhlanga River covers a length of28 km2. The average depth of the estuary is generallyabout 1.5 m when it is closed with a continuous sandbarrier, although depths of >3.0 m have also been re-ported (Begg 1984; Whitfield 1980; Cooper 1999). TheMhlanga Estuary has no dams in its catchment andconstantly receives 20 Ml day�1 of treated sewageeffluents, which are equivalent to 0.23 m3 s�1 of cappingflow, i.e. 2.5 times that of the Mdloti Estuary (W. Pfaff,personal communication). Its Biological Health Indexhas been regarded as moderate and its nutrients levelshave indicated a potential for eutrophication (Cooperet al. 1993). During a preliminary survey undertaken in1992–1993, dissolved inorganic nitrogen, DIN(NO3

�+ NH4+), and dissolved inorganic phosphorous,

DIP (orthophosphate), were found to be an order ofmagnitude higher here than in the Mdloti Estuary(Cooper et al. 1993).

Physico-chemical parameters and trophic environment

In both estuaries, trophic and environmental parametersmeasured during the study included phytoplankton andmicrophytobenthic biomass, temperature, salinity, dis-solved oxygen (DO) and nutrients, such as DIN as wellas DIP. Water-column chlorophyll-a (chl-a) sampleswere collected 5–10 cm below the surface, filtered ontoGF/F filters, and then placed in 10 ml of 90% acetonefor the extraction of pigments. Microphytobenthic chl-adetermination were made using a perspex twin-corer of20 mm internal diameter. Three core samples were takenon each occasion and at each station. The top first cm ofthe sediment was cut and placed in a 100 ml polyethyl-ene bottle containing 30 ml of 90% acetone for theextraction of pigments (Nozais et al. 2001; Perissinottoet al. 2002). After 24 h of incubation in the dark and atlow temperature (2�C), the pigments were measuredfluorometrically with a Turner Designs 10-AU fluo-rometer, using the narrow-band method with no acidi-fication (Welschmeyer 1994; Nozais et al. 2001).Temperature (�C), salinity (ppt) and DO were recordedduring each survey using a YSI 6920 water logger. Watersamples for nutrients analysis were collected from mid-water at both the Mdloti and the Mhlanga estuariesusing 500 ml acid pre-washed polyethylene bottles. DINand DIP concentrations were determined by the Ana-lytic Laboratory of the CSIR-Environmentek, Durban,using a Technicon Autoanalyzer II system and followingthe methods of Mostert (1983). Mouth state (open orclosed) of each estuary was monitored daily by fieldrangers of KZN Wildlife and residents in the area.Rainfall data were obtained from the Mt Edgecombeweather station of the South African SugarcaneResearch Institute (SASRI).

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Zooplankton abundance and biomass

Zooplankton samples were collected during daytime atthree stations, representing the lower, middle and upperreaches of each estuary at monthly intervals, fromMarch 2002 to March 2003. A UNESCO WP-2 net(Tranter and Fraser 1968) of 90 lm mesh and 57 cmmouth diameter, fitted with a General Oceanics (GO)flowmeter, was used to collect mid-water samples.Horizontal tows were made from a flat-bottom boat ata speed of 1–2 knots, keeping the upper part of the net5–10 cm below the surface. The mesh size used in thisstudy is suitable for the quantitative collection of co-pepods and other micro- and mesozooplanktonorganisms (Jerling and Wooldridge 1991, 1992; Peris-sinotto et al. 2000). However, abundance and biomassdata obtained in this way for the larger mysids andpenaeid shrimps may be under-estimated due to netavoidance (Gal et al. 1999; Bradford et al. 1999). Inorder to account for the diel vertical migration of adultzooplankton, a suprabenthic sled of 200 lm mesh wasused to collect samples from the sediment/water inter-face of each estuary. The sled was allowed to settle atthe bottom of the estuary and then dragged for 50 mbefore it was retrieved.

To estimate the sampling efficiency of the sled and therepresentivity of vertical migrators collected with thisgear during daytime, a comparison was made with datacollected at the same time using the WP-2 net in thewater-column. The ratio of catches collected with thetwo different gear types was then compared withthe ratio of day versus night catches obtained using onlythe WP-2 net in a previous survey of a similar estuaryin the area (Kibirige and Perissinotto 2003b). Thecopepod P. hessei was used as ‘‘indicator species’’ in thiscomparison, as it is a typical diel migrator and one of thedominant taxa in South African TOCEs. The two typesof gear were also used to test the hypothesis that there isa shift in size structure of zooplankton community, withchanges in levels of eutrophication. Thus, the mainlymicrozooplankton and mesozooplankton componentswere collected using the WP-2 net (>90 lm) and thesled (>200 lm), respectively. All zooplankton sampleswere preserved in 4% hexamine-buffered formalin.

In the laboratory, subsamples for identification andenumeration were drawn off the samples, after resus-pension in 1–5 l solution, depending on the concentra-tion of organisms. Using a 20 ml plastic vial attached toa rod, subsamples were withdrawn from the mid-depthof the suspension, while stirring constantly to preventsettlement (Perissinotto and Wooldridge 1989; Jerlingand Wooldridge 1995). The coefficient of variation be-tween sub-samples was always less than 10%. Total dryweight, in mg (DW) m�3, was measured by oven-drying(60�C, 24 h) half of each zooplankton sample, afterremoving detrital particles under a dissecting micro-scope. The DW of the dominant zooplankton specieswas obtained by drying at least 100–200 individuals ofeach taxon. In order to obtain the total biomass of these

dominant taxa, the average weight of one individual wasmultiplied by the total abundance of that taxon in eachsample.

Statistical analyses

A one-way analysis of variance (ANOVA) was used totest for differences in zooplankton abundance and bio-mass between estuarine phases (open or closed) andstations, for both the Mdloti and the Mhlanga estuaries.A two-way ANOVA was also applied to the total zoo-plankton abundance in order to test for differences be-tween dates (temporal patterns), sampling stations(spatial patterns) and interactive effects. To equalisevariance and normalise distribution, all data used in theANOVA were log10 (x+1)-transformed. Where signifi-cant differences in the ANOVA were detected, a Tukey’shonestly significantly different (HSD) test was applied toidentify sources of variation. Zooplankton diversity in-dexes were calculated using the Shannon–Wiener equa-tion (Shannon 1948):

H0 ¼ �

Xðpi ln piÞ;

whereH’ is the Shannon–Wiener diversity index, pi is thepopulation size of species i divided by the populationsize of all species combined (ni/N). The zooplanktoncommunity structure and diversity indexes of the MdlotiEstuary were compared to those of the Mhlanga Estuaryin order to identify any differences or similarities be-tween the two estuaries.

Spearman rank correlation analyses were performedon all environmental data to identify any potentialrelationship between these and zooplankton abundance/biomass in each estuary. A correlation-based principalcomponents analysis (PCA) was also used to identifythose environmental factors that may have played themost significant role in the variability observed in thezooplankton abundance/biomass of the two estuaries. Inorder to identify the relationships between eutrophica-tion and zooplankton community structure, chl-a andnutrient concentrations, DIN and DIP, were regressedagainst zooplankton biomass for each estuary. All val-ues were log10(x+1)-transformed for data normality.Regressions were also used between the percentagebiomass of the dominant zooplankton taxa and themean annual un-transformed data for chl-a, DIN andDIP, respectively.

Ratios of sled to daytime WP-2 net zooplanktonabundances from both the Mdloti and the Mhlangaestuaries were tested for potential differences with ratiosof daytime over the night-time WP-2 samples previouslycollected from the Mpenjati TOCE (Kibirige andPerissinotto 2003b). A t test was applied after convertingratios to log10(x+1)-transformed values.

The level of pollution in the two estuarieswas assessed using partial dominance curves with thePlymouth Routines In Marine Ecological Research

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(PRIMER) statistical package, version 5 (Clarke andWarwick 2001).

Results

Environmental variables

During the study period, the Mdloti Estuary was openfor 37% of the total time, while the Mhlanga was open50% of the time. The Mdloti remained closed duringMay–July 2002 and November 2002–March 2003. TheMhlanga was closed in March, September and October2002 as well during January–March 2003. Mean salinityvalues ranged from 0 to 24& in the Mdloti and from 0.1to 25& in the Mhlanga. During the open phase, hori-zontal and vertical salinity values were highest in thelower reaches than elsewhere, in both estuaries. Duringthe closed phase, there were no horizontal salinity gra-dients in either estuary, but there were vertical salinitygradients in all three reaches of both systems. In theMdloti, the salinity gradient ranged from <1& at thesurface to 12& at the bottom, while at the Mhlangavalues ranged from <1& to 24% (Thomas et al. 2005).Water temperature ranged from 15 to 30�C in theMdloti and from 14 to 29�C in the Mhlanga. Dissolved

oxygen values ranging from 0.2 to 11.9 mg l�1 and from0.8 to 8.1 mg l�1 were recorded for the Mdloti and theMhlanga, respectively.

Average phytoplankton biomass (chl-a) in the Mdlotiranged from 0.88 to 96 mg m�3 in September andDecember 2002, respectively, while in the Mhlanga val-ues ranged from 1.12 mg m�3 in April 2002 to179 mg m�3 during August 2002. Phytoplankton chl-abiomass was always dominated by the nano-size class,which accounted for 75–90% of the total chl-a biomassin each estuary (Thomas et al. 2005). Microphytobenthicchl-a concentrations ranged from 1.3 to 391 mg m�2 inthe Mdloti and from 1.7 to 313 mg m�2 in the Mhlanga,with the highest values observed during the open phaseof each estuary (Iyer 2004). In both systems, the lowestmean microphytobenthic biomass was observed in thelower reaches. The highest values were observed in theupper reaches at the Mdloti (December 2002), but in thelower reaches at the Mhlanga (June 2002).

In the Mdloti Estuary, DIN ranged from 0.14 to123 lM and DIP from 0.09 to 4.77 lM. In the Mhlanga,DIN and DIP ranged from 13.5 to 418 lM and from 4.7to 81 lM, respectively (Fig. 2). In the Mdloti, thehighest DIN values were observed during March 2002,in the lower reaches and during the closed phase. Thehighest DIP values were observed during October 2002,

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in the middle reaches and during the open phase. In theMhlanga, the highest values for both DIN and DIP wererecorded in July 2002, in the upper and middle reaches,respectively, and during the open phase.

Zooplankton abundance and biomass

Total zooplankton abundance (WP�2 and sled com-bined) were higher in the Mdloti than in the MhlangaEstuary. In the Mdloti, total zooplankton abundanceranged from 1.01·102 to 1.83·106 ind m�3 (mean2.45·105±9.01·105 ind m�3 SD), while Mhlanga val-ues ranged from 1.1·102 to 5.30·106 ind m�3 (mean1.48·104±3.37·104 ind m�3 SD) (Fig. 3a, b). In theMdloti, total zooplankton abundance varied between3.61·104 and 1.83·106 ind m�3 (mean 3.6·105±1.09·106 ind m�3 SD) during the closed phase,and between 1.02·102 and 1.3·103 (mean 6.3·102±4.91·102 ind m�3 SD) during the open phase (Fig. 3a).At the Mhlanga, total zooplankton abundance valuesranged from 2.98·102 to 9.23·104 ind m�3 (mean2.67·104±4.68·104 ind m�3 SD) during the closedphase, and from 1.08·102 to 5.06·104 ind m�3 (mean4.46·104±6.65·104 ind m�3 SD) during the open phase(Fig. 3b). The highest zooplankton abundances werefound in the lower and upper reaches of both estuaries(Tables 1, 2). Considering the average zooplanktonabundances collected using the sled and the WP-2 netseparately, sled samples were consistently 1–2 times ri-cher than those collected using the WP�2 net in bothestuaries (Fig. 4a–d).

Considering the two estuaries separately, there weresignificant differences in both biomass/abundance be-tween their closed and open phases (Mdloti: U=1,P<0.001, U=16, P<0.001 and Mhlanga: U=101,P<0.05, U=88, P<0.01). Overall, there was a signifi-cant difference in total zooplankton abundance betweenthe two estuaries (two-way ANOVA, F1,69=4.99,P<0.05). Concerning differences between the differentreaches of each estuary, there were no significant dif-ferences in abundance/biomass between the threereaches at either the Mdloti or Mhlanga (two-wayANOVA: F2,36=0.9, P>0.05, F2,36=0.2, P>0.05) and(F2,36=0.01, P>0.05, F2,36=0.28, P>0.05). There werealso no significant interactions between estuary andstations in either estuary (F1,69=0.41, P>0.05). Whenopen and closed phases were tested separately using aMann–Whitney U-test, there were no significant differ-ences in the zooplankton abundance during the openphase of the two estuaries (U=92, P>0.05), but therewere significant differences in their biomass (U=68,P<0.05). Conversely, during the closed phase, therewere significant differences in abundance (U=107,P<0.01), but not in biomass (U=173, P>0.05) betweenthe two estuaries. The microzooplankton componentcontributed 59.6% of the total abundance (copepodnauplii 42.0%, rotifers 17.6%) and 15.8% of total bio-mass (copepod nauplii 13.8%, rotifers 1.96%) in theMhlanga Estuary. At the Mdloti, however, the contri-bution of microzooplankton was 99.1% of total abun-dance (copepod nauplii 16.5%, rotifers 82.6%) and96.5% of biomass (copepod nauplii 35.8%, rotifers60.7%). The highest microzooplankton contribution in

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the Mdloti was observed during the closed phase, whilein the Mhlanga this occurred during the open phase.However, the percentage partial dominance curves ofthe two estuaries did not differ significantly (Fig. 5a, b).

When total zooplankton densities derived from WP-2samples were compared to those derived from sled col-lections for the entire study period, no significant dif-ferences were found between them for the MdlotiEstuary, (t test=�0.19, P> 0.05). There were, however,significant differences between WP-2 and sled densitiesfor the dominant zooplankton species, P. hessei(t test=�3.50, P<0.01). At the Mhlanga, zooplanktondensities obtained from the sled samples were signifi-cantly higher than those derived using the WP-2 net(t test=�3.57, P<0.01). However, there were no sig-nificant differences between the dominant zooplanktongroup (brachyuran larvae) collected with the WP-2 andthe sled (t test=1.22, P>0.05). For both estuaries,P. hessei abundances derived from sled data were con-sistently 1–3 orders of magnitude higher than thoseobtained from the WP-2 net. Nauplii abundances col-lected using the WP-2 net were maximum 2–3 weeksafter major rainfall events, suggesting a possible delayedresponse of copepod eggs to freshwater pulses (Fig. 6a, b).

Similar to abundance patterns, total zooplanktonbiomass values were low during the open phase, whilehigh biomass characterised the closed phase at all threereaches in both estuaries. At the Mdloti, zooplanktonbiomass ranged from 0.34 to 705 mg (DW) m�3 (mean126.54±173.14 mg (DW) m�3 SD) (Fig. 7a), while inthe Mhlanga mean values ranged from 0.56 to 431 mg(DW) m�3 (mean 51.6±192.87 mg (DW) m�3 SD)(Fig. 7b). Biomass values obtained with the sled and theWP-2 net also differed, with higher values generallycollected with the former gear (Fig. 8a–d). There was,however, no significant difference in total zooplanktonbiomass between the two estuaries during the period ofthe survey (one-way ANOVA, F1,68=1.15, P>0.05).

For the Mdloti, Spearman rank correlation analysisof total zooplankton abundance/biomass show thatthere were significant correlations with rainfall andphytoplankton as well as microphytobenthos (MPB),but not with other physico-chemical parameters. Thedominant taxon, P. hessei, was negatively correlatedwith temperature and rainfall. Ceriodaphnia sp., rotifersand copepod nauplii also exhibited negative correlationswith MPB (Table 3). For the Mhlanga, again there werepositive significant correlations between total zoo-

Table 1 Composition of the most abundant taxa at the Mdloti Estuary during: a the closed phase; and b the open phase

Stations Lower reaches Middle reaches Upper reaches

Taxon Ind m�3±SD Percentage Ind m�3±SD Percentage Ind m�3±SD Percentage

a Mdloti Estuary during the closed phasePseudodiaptomus hessei 2840.7±3602.4 73.6 1423.3±1911.9 50.3 1213.1±1300.6 38.6Acartia natalensis 594.6 ±1426.8 15.4 600.4 ± 1550.9 21.2 293.6±660.8 9.3Harpacticoids 0.0±0.0 0.0 09±2.4 0 3.2±7.1 0Cyclopoids 5.3±7.6 0.2 14.6±18.7 0.5 48.6±58.2 1.5Brachyuran larvae 1.5±3.4 <0.1 5.0±13.4 0.2 2.2±3.1 0.1Caridean larvae 5.2±8.3 <0.1 18.0±31.9 0.6 61.3±106.7 1.9Mesopodopsis africana 0.3±0.8 <0.1 <0.1±0.1 <0.1 0.0±0.0 0Amphipods 8.4±12.2 0.2 0.7±1.4 <0.1 1.1±2.6 <0.1Ostracods 5.3±7.6 0.1 14.6±18.7 0.5 48.6±58.2 1.5Chydorus sphericus 0.1±0.2 <0.1 0.0±0.0 <0.1 1.4±3.0 <0.1Hydra sp? 63.7±168.5 1.6 0.3±0.8 <0.1 0.0±0.0 0Fish larvae 1.6±3.2 <0.1 10.5±28.8 0.4 13.8±33.8 0.4Fish eggs 0.1±0.4 <0.1 0.0±0.0 0 0.0±0.0 0Insect larvae <0.1±0.1 <0.1 0.2±0.4 <0.1 1.6±2.7 <0.1Ceratonereis keiskama 0.0±0.0 0.0 0.0±0.0 0 0.2±0.4 <0.1Other polychaetes 27.1±71.6 0.7 1.2±2.8 <0.1 0.5±1.3 <0.1Total 3253.7±4331.8 100.0 2229.6±2069.5 100 2843±3404.4 100

b Mdloti Estuary during the open phasePseudodiaptomus hessei 193.6±228.9 84.6 11.7±12.7 35.4 4.9±3.4 7.4Oithona plumifera 5.3±9.6 2.3 6.1±9.2 17.5 18.8±36.7 28.3Other cyclopoids 2.8±5.6 1.2 0.1±0.2 0.2 0±0 0Brachyuran larvae 0.7±1.1 0.2 0.24±0.4 0.7 0±0 0Caridean larvae 8.9±13.9 3.9 4.5±6.0 12.5 3.6±4.1 5.4Mesopodopsis africana 0.1±0.2 <0.1 0.1±0.2 0.2 0±0 0Ostracods 5.3±9.6 2.3 6.2±9.2 17.5 18.8±36.7 28.3Ceriodaphnia producta (?) 1.51±1.7 0.7 3.4±5.6 9.7 14.2±27.7 21.3Chydorus sphericus 1.4±1.7 0.6 1.6±1.2 4.5 3.3±5.9 5Hydra sp. 0±0 0 0±0 0 0.1±1.6 0.1Fish larvae 0.1±0.2 <0.1 0.1±0.2 0.2 0±0 0Fish eggs 4.9±10.4 2.6 0.24±0.5 0.7 0±0 0Insect larvae 0.2±0.3 0.1 0.2±0.3 0.1 0.7±1.4 1.1Ceratonereis keiskama 0.5±0.9 0.2 0±0 0 0.2±0.3 0.2Other polychaetes 2.7±5.2 1.2 1.0±1.3 2.7 1.9±3.4 2.9Total 228.9±368.7 100 35.4±22.6 100 66.5±116.2 100

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plankton abundance/biomass and phytoplankton as wellas MPB, but not with other physico-chemical parame-ters. Also, P. hessei and brachyuran larvae were posi-tively correlated with phytoplankton (Table 4).

Community structure

A total of 27 zooplankton taxa were recorded at theMdloti Estuary and 32 at the Mhlanga Estuary duringthe entire study period. At the Mdloti, 23 and 25 taxawere recorded during the closed and open phaserespectively (Tables 5, 6). At the Mhlanga, there were 24taxa during the closed and 31 during the open phase. Atthe Mdloti, P. hessei consistently dominated the assem-blage, accounting for 63.5 and 52.5% of the total catchin the lower reaches of the estuary during the closed andopen phase, respectively. There were also high numbersof rotifers during February 2003, when they accounted

for 83% of the total WP-2 net catch (Table 5a, b). At theMhlanga, the zooplankton community was also domi-nated by the calanoid copepod P. hessei, which ac-counted for 97.4 and 86.1% of the total catch in thelower reaches of the estuary during the closed and openphase, respectively. During the closed phase, brachyuranlarvae were very abundant, contributing 46% of thetotal abundance in the middle reaches of the estuaryduring February 2003 (Table 6a, b).

Shannon–Wiener diversity indexes of the loge-trans-formed means of zooplankton abundance ranged from0.05 to 1.96 for the Mdloti and from 0.13 to 2.04 for theMhlanga (Fig. 9). Generally, lower diversity indexeswere observed during the closed phase than during theopen phase in both estuaries (Fig. 9). A Student’s t testapplied to the diversity indexes obtained for the Mdlotiand the Mhlanga revealed that there were significantdifferences between the two estuaries (t test=4.91,P<0.001).

Table 2 Composition of the most abundant taxa at the Mhlanga Estuary during: a the closed phase; and b the open phase

Stations Lower reaches Middle reaches Upper reaches

Taxon Ind m�3±SD Percentage Ind m�3±SD Percentage Ind m�3±SD Percentage

a Mhlanga Estuary during the closed phasePseudodiaptomus hessei 8510±11152 98.3 2073.9±3373.5 91.8 3386.9±711.9 97.8Cyclopoids 15±24.6 0.2 7.9±8.0 0.3 3.2±3.1 0.1Acartia natalensis 0.4±0.8 <0.1 2.2±4.8 0.1 1.2±2.7 <0.1Brachyuran larvae 64.2±102.7 0.7 54.8±121.3 2.4 33.1±73.5 0.1Caridean larvae 24.9±36.3 0.3 11.5±19.3 0.5 17.0±32.3 0.5Mesopodopsis africana 12.9±23.9 0.1 0.7±1.0 <0.1 0.0±0.0 0Ostracods 22.2±29.1 0.3 9.4±15.4 0.4 5.85±9.3 <0.1Ceriodaphnia producta (?) 0.0±0.0 0 1.1±2.4 <0.1 0.0±0.0 0Chydorus sphericus 0.0±0.0 0 0.0±0.0 0 0.2±0.3 <0.1Fish larvae 2.9±4.6 <0.1 17.7±29.3 0.8 9.3±13.3 0.3Fish eggs 1.7±3.7 <0.1 0.4±0.8 <0.1 0.0±0.0 0Ceratonereis keiskama 0.0±0.0 0 0.0±0.0 0 0.1±0.1 <01Other polychaetes 1.9±4.3 <0.1 6.7±11.4 2.9 6.3±11.4 0.2Chaetognaths 0.8±1.8 <0.1 0.0±0.0 0 0.0±0.0 0Amphipods 1.3±2.8 <0.1 2.5±5.7 0.1 0.0±0.0 0Oligochaetes 0.0±0.0 0 9.0±20.0 0.4 0.0±0.0 0Total 8658.3±11064.1 100 2257.8±3474.2 100 3463.2±7098.9 100

b Mhlanga Estuary during the open phasePseudodiaptomus hessei 108.1±219.0 79.8 412.4±659.7 88.1 75.3±122.0 77.4Acartia natalensis 4.6±3.5 3.4 7.4±10.7 1.6 5.3±8.2 5.4Oithona plumifera 0.2±0.4 0.7 0.0±0.0 0 0.0±0.0 0Other cyclopoids 0.5±0.5 0.4 0.1±0.2 <0.1 0.05±0.1 <0.1Brachyuran larvae 0.2±0.4 0.1 0.0±0.00 0 0.0±0.0 0Caridean larvae 5.3±9.5 3.9 4.3±5.9 0.9 1.5±2.2 1.5Mesopodopsis africana 0.0±0.00 0 0.05±0.1 <0.1 0.0±0.0 0Ostracods 6.4±5.8 4.7 1.5±2.5 0.3 1.3±3.5 0.3Ceriodaphnia producta (?) 7.1±9.7 5.3 8.8±11.1 1.9 5.1±4.6 5.2Chydorus sphericus 1.7±3.6 1.3 1.3±2.8 0.3 0.7±1.2 0.7Hydra sp. 0.0±0.00 0 0.0±0.00 0 0.1±0.4 0.1Fish larvae 0.6±0.5 0.4 3.1±8.1 0.7 0.2±0.5 0.2Fish eggs 0.0±0.00 0 0.5±1.2 0.1 0.1±0.4 0.1Insect larvae 0.2±0.4 7.4 22.4±57.5 6 7.5±17.7 2.5Ceratonereis keiskama 0.1±0.4 0.1 0.6±1.2 0.1 0.1±0.2 0.1Other polychaetes 1.6±2.4 0.7 2.41±6.4 0.6 0.6±1.3 0.2Hirudineans 0.4±0.8 0.3 0.0±0.0 0 0.0±0.0 0Nematods 0.2±1.1 0.2 0.0±0.0 0 0.3±0.7 0.3Euphausiids 0.0±0.00 0 0.9±2.4 0.2 0.0±0.0 0Total 135.4±240.0 100 465±648.3 100 96.9±118.5 100

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Using mean values for the environmental variablesof each estuary, a PCA ordinance was performed tosummarise the contribution of the most importantfactors to zooplankton variance (Fig. 10a, b). In theMdloti, three components account for 57.7% of thevariance. Component I represents 25.2% of the vari-ance and includes DIN, rainfall and mouth state.Component II represents 17.7% of the variance andincludes temperature and dissolved oxygen (DO). Thethird component accounts for 14.7% of the varianceand is associated with phytoplankton and MPB bio-mass (Fig. 10a). In the Mhlanga Estuary, the threecomponents account for 67% of the total variance.Component I represents 31% of the variance and in-cludes phytoplankton biomass and DO. Component IIshows that rainfall and temperature account for 18%of the variance, while the third component contributes17% of the variance and is associated with samplingstation and salinity (Fig. 10b).

In the Mdloti, the percentage dominant zooplanktonbiomass increased with an increase in DIN and DIPconcentrations, while the opposite trend was observedin the Mhlanga. Results of the regression analysis(Mdloti, r=0.129–0.427; Mhlanga, r=0 .026–0.278)show an increase in the abundance/biomass of allpercentage dominant zooplankton biomass with in-creased chl-a concentrations. Total zooplankton bio-mass was significantly correlated with chl-aconcentration (Mdloti: P<0.05, Mhlanga: P<0.001),but not with DIN and DIP concentrations in eitherestuary. P. hessei biomass was also significantly corre-lated with chl-a in the Mhlanga (P<0.001), but not inthe Mdloti.

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Discussion

Both Mdloti and Mhlanga estuaries exhibited higherzooplankton abundance/biomass during their closedphase, compared to their open phase (Figs. 3, 7). This isconsistent with reports from other TOCEs in the region(Perissinotto et al. 2000; Kibirige and Perissinotto2003a). In particular, a study conducted at the MpenjatiEstuary concluded that the opening and closing of theestuary is the main factor controlling zooplanktonabundance and biomass in this TOCE (Kibirige andPerissinotto 2003b). An extended period of mouth clo-sure is required for zooplankton to respond to an in-crease in microalgal availability and convert this intonumber and biomass growth (Whitfield 1980; Kibirigeand Perissinotto 2003b). While the Mdloti Estuaryexhibited prolonged periods of mouth closure during thestudy period, with a maximum duration of 67 consecu-tive days during the dry season, the Mhlanga exhibitedregular and frequent breaching, never experiencing ex-tended closure of more than 15 consecutive days(Perissinotto et al. 2004). In the Mdloti Estuary, thisperiod most likely allowed the zooplankton communityenough time to utilise the available food sources andbuild-up higher biomass, while the same did not occur inthe Mhlanga Estuary. Similar observations have beenreported from the Neuse River estuary, USA, where lowtidal amplitude was associated with long water residencetime in the system, thus allowing the use of DIN by

phytoplankton (Monbet 1992; Christian and Thomas2003, and references therein). Also, the general rela-tionships between residence time and zooplanktondevelopment observed here are in agreement with whathas been reported from other subtropical waters, e.g.Kaneohe Bay, Hawaii (Calbet et al. 2000) and also intemperate coastal environments (Gotsis-Skretas et al.2000).

The high frequency of breaching observed at theMhlanga resulted in the regular flushing of zooplanktonand their phytoplankton food out to sea. The waterresidence time in the Mhlanga may be insufficient toresult in high biomass build-up of zooplankton withinthe estuary itself, although their regular export out tosea probably contributes significantly to the productivityof the adjacent coastal zone. This observation is inagreement with a reduction in copepods and mysidsabundance/biomass reported from the Mpenjati Estuaryafter similar breaching events (Kibirige and Perissinotto2003b). The Mhlanga Estuary currently breaches evenduring periods of low or no rainfall, a clear indicationthat the volume of treated sewage waters discharged intothe estuary is sufficient to maintain elevated flow rateswithin the system, despite its small catchment area andlow freshwater run-off. It is estimated that the regularsewage discharge contributes a capping flow of about0.23 m3 s�1 to the estuary (Perissinotto et al. 2004). Astudy conducted over 20 years ago in the MhlangaEstuary showed far fewer breaching events (Whitfield1980) than what was observed in this investigation.

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Indeed, Whitfield (1980) reported the occurrence of amaximum peak in zooplankton biomass during thewinter closed phase of the estuary, the duration of whichwas estimated at about 6 months in 1978 (Whitfield1980). The situation of the Mhlanga has, therefore,changed quite dramatically during the last 20 years andthis has affected its entire ecosystem functioning, possi-bly including the food web structure and biodiversity ofthis estuary (Perissinotto et al. 2004).

Contrary to the abundance/biomass pattern, a higherzooplankton taxonomic diversity was found in theMhlanga than in the Mdloti (Tables 5, 6; Fig. 9). Thiscould be related to the influx of freshwater, but mainlymarine neritic species, during the open estuarine phase.It has been shown that during this period many marine

zooplankton groups enter the estuary and utilise localfood sources (Kibirige and Perissinotto 2003b). As theMhlanga exhibited sixteen breaching events during thestudy period, compared to only nine recorded at theMdloti, the impact of non-typical estuarine species onthe total taxonomic diversity of this estuary would havebeen substantially higher than in the Mdloti.

Apart from the sustained high flow, the discharge oftreated sewage waters in the Mhlanga, and to a muchlesser extent in the Mdloti, is also the main cause ofeutrophication in these estuaries. During the last decadethere has been a general tendency worldwide towardselevated nutrient concentrations in estuaries. Typicalexamples of this trend are the situations reported fromthe Vilaine River in France (Moreau et al. 1998), the San

Table 3 Spearman rank correlation analysis between physico-chemical parameters, abundance, biomass of total zooplankton and theirdominant components at the Mdloti Estuary (n=36)

Variable Total abundance P. hessei Ceriodaphnia sp. Rotifers Copepod nauplii

AbundanceTemperature �0.22 �0.53** �0.11 0.21 �0.14Salinity �0.03 0.14 �0.37* �0.11 �0.27Dissolved oxygen �0.06 0.28 �0.16 �0.20 �0.16Rainfall �0.54** �0.51** �0.25 �0.21 �0.27Microphytobenthos 0.46** 0.16 0.40* 0.41* 0.49**Phytoplankton 0.47** �0.04 0.16 0.76** 0.20

Variable Total biomass P. hessei Ceriodaphnia sp. Rotifers Copepod nauplii

Biomass of total zooplanktonTemperature �0.29 �0.56** �0.11 0.21 �0.14Salinity 0.04 0.21 �0.37* �0.11 �0.27Dissolved oxygen 0.00 0.29 �0.16 �0.20 �0.16Rainfall �0.58** �0.53** �0.25 �0.21 �0.27Microphytobenthos 0.41* 0.08 0.40* 0.41* 0.49**Phytoplankton 0.36* �0.12 0.16 0.76** 0.20

* Significant at P<0.05; ** Significant at P<0.01

Table 4 Spearman rank correlation analysis between physico-chemical parameters, abundance, biomass of total zooplankton and itsdominant components at the Mhlanga Estuary (n=36)

Variable Total abundance P. hessei Brachyuran larvae Rotifers Copepod nauplii

AbundanceTemperature 0.23 0.07 0.59** 0.44** 0.11Salinity 0.27 0.32* 0.02 0.18 0.30Dissolved oxygen 0.05 0.21 0.01 �0.41** �0.06Rainfall �0.49** �0.22 0.02 �0.03 �0.53**Microphytobenthos 0.38* 0.01 0.33* 0.18 0.37*Phytoplankton 0.35* 0.33* 0.60** 0.25 0.02

Variable Total biomass P. hessei Brachyuran larvae Rotifers Copepod nauplii

Biomass of total zooplanktonTemperature 0.23 0.07 0.47** 0.37* 0.15Salinity 0.14 0.29 0.26 0.28 0.25Dissolved oxygen 0.14 0.21 0.04 �0.39* �0.10Rainfall �0.30 �0.20 �0.16 �0.12 �0.51**Microphytobenthos 0.30 0.02 0.22 0.13 0.42**Phytoplankton 0.45** 0.36* 0.43** 0.16 0.06

* Significant at P<0.05; ** Significant at P<0.01

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Joaquin River (Kratzer and Shelton 1998) and the lowerMississippi River (Rabalais et al. 1996) in the UnitedStates. Other important reports from estuarine systemsinclude those from the north-eastern of the UnitedStates (Jaworski et al. 1997; Conley 2000), the coastalwaters of Romania (Mee 1992), the Baltic Sea (Nehring1992) and the Irish Sea (Allen et al. 1998).

The partial dominance curves produced for theMdloti and the Mhlanga (Fig. 5a, b) suggest that bothestuaries may be polluted, since these curves take intoaccount the possible artifacts of recruitment and ofdominance of one or two species (Clarke 1990; Little

2000; Clarke and Warwick 2001). Nutrient levels duringthe study period fluctuated widely from open to closedphase, but a clear and consistent pattern was the orderof magnitude higher concentrations of both DIN andDIP recorded at the Mhlanga, compared to the Mdloti(Fig. 2). This is a reflection of the average volume ofsewage effluents discharged daily in the two estuaries, i.e.8 Ml for the Mdloti versus 20 Ml for the Mhlanga(W. Pfaff, Ethekwini Municipality, personal communi-cation). The Nutrient values obtained for the MdlotiEstuary on this occasion are in the range of whatwas previously reported (Nozais et al. 2001). Negative

Table 5 Zooplankton species contribution in samples collected using different gear types at the Mdloti Estuary during March 2002–March 2003 (a open phase and b closed phase)

Gear Sled WP2 Total (Sled + WP2)

Taxa Mean ± SD % contribution Mean ± SD % contribution Mean ± SD % contribution

a Open phasePseudodiaptomus hessei 70.1±213 63.57 172±183 33.10 242±262 38.50Oithona spp. 10.1±21.4 9.16 15.9±23 3.05 26±41.5 4.12Other cyclopoids 0.96±3.21 0.87 0.00±0.00 0.00 0.96±3.21 0.15Harpacticoids 0.00±0.00 0.00 8.14±9.17 1.57 8.14±9.58 1.29Copepod nauplii 0.00±0.00 0 200±284 38.50 200±297 31.70Caridean larvae 5.65±8.59 5.13 10.4±14.3 2.01 16.1±22.51 2.55Brachyuran larvae 0.27±0.68 0.24 1.01±2.04 0.19 1.28±2.46 0.20Fish larvae 0.05±0.12 0.05 0.00±0.00 0.00 0.05±0.12 0.00Fish eggs 2.06±6.17 1.87 0.00±0.00 0.00 2.06±6.17 0.33Rotifers 0.00±0.00 0 56.1±85.8 10.80 56.1±85.8 9.00Mesopodopsis africana 0.05±0.12 0.05 0.00±0.00 0.00 0.05±0.12 0.01Ceratonereis keiskama 0.21±0.57 0.19 2.98±5.28 0.57 22.7±42.3 3.60Ostracods 10.1±21.4 9.16 0.09±0.3 0.02 10.19±21.36 1.62Nematodes 0.00±0.00 0.00 1.97±3.9 0.25 1.97±4.07 0.31Ceriodaphnia sp. 6.37±15.89 5.78 16.3±28.8 3.14 22.7±42.34 3.60Chydorus sp. 2.12±3.37 1.93 19.7±24.4 3.80 21.87±26.66 3.47Hydra sp? 0.03±0.09 0.02 0.00±0.00 0.00 0.03±0.09 <0.01Prionospio sp. 0.93±2.63 0.84 7.05±15.7 1.36 7.98±18.97 1.27Other polycaetes (4 spp.) 0.93±1.85 0.01 1.29±4.45 0.25 2.21±5.19 0.35Oligochaetes 0.00±0.00 0.00 0.07±0.24 0.01 0.07±0.24 0.01Chironomid larvae 0.29±0.66 0.26 4.18±5.67 0.80 4.48±5.89 0.71Zygopteran larvae 0.05±0.18 0.05 2.43±5.3 0.47 2.49±5.64 0.39Total 110±221 100 520±390 100 630±491 100

b Closed phasePseudodiaptomus hessei 2278±3444 52.50 247±541 0.07 2429±3740 0.66Cyclopoids 18.72±35.1 0.42 131±241 0.04 147±237 0.04Acartia natalensis 440±1166 10.15 6.71±19.6 <0.01 429±1142 0.12Harpacticoids 1.45±4.37 0.03 204±831 0.06 206±831 0.06Copepod nauplii 0.00±0.00 0.00 60133±128315 16.51 60133±128315 16.32Caridean larvae 24.37±58.46 0.56 16.4±33.7 <0.01 40±81.1 0.01Brachyuran larvae 2.57±58.46 0.06 5.71±28 <0.01 8.17±28.3 <0.01Fish larvae 7.69±22.4 0.18 0.37±1.5 <0.01 7.74±21.9 <0.01Fish eggs 0.04±0.2 <0.01 0.00±0.00 0.00 0.04±0.2 <0.01Grandidierella lignorum 2.92±7.35 0.07 0.00±0.00 0.00 2.92±35 <0.01Rotifers 0.00±0.00 0.00 3011192±1088129 82.70 301192±1088129 81.7Mesopodopsis africana 0.11±0.44 <0.01 0.00±0.00 0.00 0.1±0.43 <0.01Ostracods 18.7±35.1 0.43 3.13±9.95 <0.01 21.1±34.5 0.01Nematods 0.03±0.13 <0.01 0.00±0.00 0.00 0.03±0.13 <0.01Ceriodaphnia sp. 1515±2399 34.91 2342±3609 0.64 3794±5682 1.03Chydorus sp. 0.32±155 0.01 11.9±30.8 <0.01 12.2±31.1 <0.01Hydra sp? 18.7±91 0.45 0.00±0.00 0.00 18.7±91 0.01Ceratonereis keiskama 0.06±0.23 <0.01 0.07±034 <0.01 0.13±0.4 <0.01Prionospio sp. 2.15±8.56 0.05 1.32±5.05 <0.01 3.38±9.49 <0.01Other polychaetes (2 spp.) 6.6±40 0.15 0.06±0.27 <0.01 6.38±30.3 <0.01Chironomid larvae 0.97±2.36 0.02 13.7±22 <0.01 14.6±22.1 <0.01Zygopteran larvae 0.13±0.48 <0.01 1.9±7.5 <0.01 2.03±7.49 <0.01Total 4339±4824 100 364311±20764776 100 368469±1090128 100

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correlations between the percentage of dominant zoo-plankton biomass and DIN and DIP, such as thoseobtained for the Mdloti, may suggest that the estuary isoligotrophic or only slightly eutrophic (Park and Mar-shall 2000). Conversely, the positive correlation between

percentage contribution of dominant zooplankton taxaand chl-a, DIN and DIP obtained for the Mhlanga, isindicative of eutrophic conditions. Further evidence ofthe eutrophication state of this estuary is provided bythe dominance of one species, the copepod P. hessei.

Table 6 Zooplankton species contribution in samples collected using different gear types at the Mhlanga Estuary during March 2002–March 2003 (a open phase and b closed phase)

Gear Sled (200 lm) WP2 (90 lm) Total (WP2 + Sled)

Taxa Mean ± SD % contribution Mean ± SD % contribution Mean ± SD % contribution

a Open phasePseudodiaptomus hessei 199±417 86.10 100±217 2.15 290±458 6.50Copepod nauplii 0.00±0.00 0.00 3953±65016 94 3953±6516 88.70Rotifers 0.00±0.00 0.00 14.8±34.1 0.35 14.8±34.1 0.30Caridean larvae 3.79±6.49 1.64 7.21±14.3 0.17 11±20.3 0.25Brachyuran larvae 0.05±0.21 0.02 5.59±22.9 0.13 5.64±22.9 0.13Fish larvae 1.27±4.66 0.55 0.6±2.22 0.01 1.87±5.12 0.04Fish eggs 0.2±0.72 0.09 0.33±1.42 0.01 0.53±1.55 0.01Ostracods 0.67±1.07 0.29 1.59±4.89 0.04 2.25±5.35 0.05Acartia natalensis 0.20±0.37 0.09 2.91±12.7 0.07 3.11±13.0 0.07Cyclopoids 5.64±7.74 2.45 81.7±146 1.93 87.4±145 2.00Oithona sp. 0.05±0.21 0.02 0.00±0.00 0.00 0.05±0.21 <0.01Harpacticoids 0.00±0.00 0.00 18.5±35.0 0.44 18.5±35.0 0.42Grandidierella lignorum 0.00±0.00 0.00 5.83±21.8 0.14 5.83±21.8 0.13Oligochaetes 0.00±0.00 0.00 0.79±2.65 0.02 0.79±2.65 0.02Hirudeneans 0.11±0.49 0.05 0.91±3.59 0.02 1.01±3.6 0.02Ceratonereis keiskama 0.31±0.72 0.14 0.00±0.00 0.00 0.31±0.72 0.01Nematodes 0.29±0.77 0.12 10.2±14.5 0.24 10.5±14.7 0.23Chaetognaths 0.00±0.00 0.00 0.31±1.23 0.01 0.31±1.23 0.01Euphausiids 0.3±1.39 0.13 0.00±0.00 0.00 0.3±1.39 0.01Ceriodaphnia sp. 6.84±8.58 2.97 2.81±8.96 0.07 9.65±13.5 0.22Chydorus sp. 1.15±2.6 0.5 6.84±19.1 0.16 8.0±18.9 0.18Hydra ? 0.06±0.22 0.03 0.00±0.00 0.00 0.06±0.22 <0.01Prionospio sp. 0.2±0.47 0.02 0.00±0.00 0.00 0.2±0.47 <0.01Mesopodopsis africana 0.02±0.07 0.01 0.00±0.00 0.00 0.02±0.07 <0.01Other polychaetes (3 sp.) 9.92±34.1 0.04 4.14±10.6 0.10 14.1±36.5 0.32Gastropod viligers 0.00±0.00 0.00 6.32±29 0.15 6.32±29.0 0.14Chironomid larvae 0.9±3.69 0.41 7.62±30.3 0.18 8.56±30.4 0.19Zygopteran larvae 0.05±0.15 0.02 1.67±4.44 0.04 1.71±4.5 0.04Mud prawns 0.00±0.00 0.00 0.01±0.04 <0.01 0.01±0.04 <0.01Total 4252±7236 100 4225±6468 100 4456±6652 100

b Closed phasePseudodiaptomus hessei 4141±7252 97.40 39.5±118 0.18 4180±7338 15.60Copepod nauplii 0.00±0.00 0.00 6944±9836 30.8 6944±9836 25.90Rotifers 0.00±0.00 0.00 4824±11971 21.4 4824±11971 18.01Caridean larvae 17.0±26.1 0.40 109±263 0.48 125±271 0.47Brachyuran larvae 42.4±87.3 1.00 10357±43314 46 10399±43364 38.80Fish larvae 8.55±17.1 0.20 34.1±50.4 0.15 42.7±51.1 0.05Fish eggs 0.71±2.03 0.02 0.03±0.14 <0.01 0.74±2.02 <0.01Ostracods 2.64±4.42 0.06 0.62±1.43 <0.01 3.26±4.38 0.01Acartia natalensis 1.13±2.84 0.03 0.03±0.13 <0.01 1.16±2.95 <0.01Cyclopoids 7.37±13.8 0.17 151±356 0.67 159±355 0.59Grandidierella lignorum 1.08±3.27 0.03 838±33.1 0.04 9.46±36.0 0.04Oligocaetes 2.49±10.6 0.06 6.29±17.39 0.03 8.74±19.5 0.03Mesopodopsis africana 3.77±12.97 0.09 0.00±0.00 0.00 3.77±13.0 0.01Hirudeneans 0.21±0.90 <0.01 0.00±0.00 0.00 0.21±0.90 <0.01Nematodes 0.72±2.03 0.02 1.52±3.82 0.01 2.27±4.14 0.01Chaetognaths 0.23±0.98 0.01 0.00±0.00 0.00 0.23±0.98 <0.01Ceriodaphnia sp. 1.12±2.27 0.03 0.04±0.18 <0.01 1.17±2.7 <0.01Chydorus sp. 0.12±0.29 <0.01 5.63±18.3 0.02 5.75±18.4 0.02Prionospio sp. 20.1±74.7 0.47 0.00±0.00 0.00 20.1±74.7 0.08Ceratonereis keiskama 0.02±0.08 <0.01 13.0±26.5 0.06 13.0±26.5 0.05Other polychaetes (3 spp.) 0.78±2.22 0.02 0.04±0.12 <0.01 0.82±2.32 <0.01Chironomid larvae 0.00±0.00 <0.01 4.96±6.93 0.02 4.69±6.93 0.02Zygopteran larvae 0.52±1.59 0.01 18.8±48.0 0.08 19.4±49.4 0.07Total 4252±7236 100 22538±46058 100.00 16019±17174 100

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This concurs with what has been observed in ClydeEstuary, Scotland (Pearson et al. 1986; McLusky andElliott 2004). The high nutrient values observed in theMhlanga were an order of magnitude higher than whatthey were in 1992–1993, when a preliminary studyindicated a potential for eutrophication (Cooper et al.1993). At the Mdloti, however, nutrient levels have in-creased only slightly compared to the 1992–1993 levels.

The total micro- and mesozooplankton abundance/biomass decreased from the Mdloti to the Mhlanga,following the eutrophication gradient. Also, the per-centage contribution of microzooplankton abundances/biomass was higher in the Mdloti than in the Mhlanga,again possibly reflecting the higher level of nutrientpollution encountered in the latter estuary. This con-clusion is in agreement with the observations made inthe Chesapeake Bay, where zooplankton decreased withincreased nutrient concentration (Park and Marshall2000). It also concurs with the observations made byCloern (2001, and references therein) that eutrophica-tion in coastal waters can, on a long-term basis, cause adecline in meso-, macrozooplankton and fish popula-tions. Albeit a generalised decline of similar proportionshas not yet been confirmed in this study, it is a matter ofgreat concern worldwide (Nixon 1995) and may indeedaffect the Mhlanga Estuary in the near future. Forinstance, the Tongati Estuary located some 17 and10 km from Mhlanga and Mdloti, respectively, has been

reported to exhibit impoverished fish population as aresult of the sewage effluents discharged there (Blaber1997). There is, however, no general pattern yet emerg-ing from these few studies and some results are actuallyin open contrast to what has been presented above (Parkand Marshall 2000). For instance, our findings from theMhlanga Estuary are in contrast to observations fromthe Florida lakes and Perdido Bay (north of Gulf ofMexico), where zooplankton biomass has increased withan increase in eutrophication (Bays and Crisman 1983;Pace 1986; Livingstone 2001). There are still too fewsimilar studies carried out in estuaries, particularly in thesouthern hemisphere, and as such, meaningful compar-isons are difficult to make. This is the first study of thistype undertaken in a temporarily open/closed estuaryand others are clearly needed in order to establish theextent of eutrophication in these ecosystems and its ef-fects on the structure and functioning of their trophicwebs.

From the PCA ordinances (Fig. 10a, b), it can beconcluded that out of ten parameters, three, i.e. the stateof the mouth, chl-a and nutrients, play the most signifi-cant role in the zooplankton community structure ofboth estuaries (Fig. 10a, b). This further supports thehypotheses that mouth-state together with nutrientenrichment may largely determine zooplankton dynam-ics in South African TOCEs (Cloern 2001; Kibirigeand Perissinotto 2003b). The numerous significant

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Fig. 9 Shannon–Wienerdiversity indexes for thezooplankton community of: athe Mdloti; and b the Mhlangaestuaries. Gray horizontal barsindicate periods of mouthclosure

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correlations between total zooplankton abundance andchl-a (Tables 3, 4) also highlight the importance ofphytoplankton/microphytobenthic biomass in both theMdloti and the Mhlanga estuaries. The water-column ofthese estuaries is mainly dominated by nanophyto-plankton of 2–10 lm size, which is easily ingested by thezooplankton of these estuaries (Kibirige et al. 2003). Thisobservation has also been reported from temperatecoastal environments, where this specific cell size ofphytoplankton is a major food source for zooplankton(Miller et al. 1991; Gotsis-Skretas et al. 2000; Dittel et al.2000; Mousseau et al. 2001).

Finally, the issue of a suitable sampling design andstrategy for TOCEs zooplankton has been raisedrecently, particularly in view of the difficulties encoun-tered when trying to obtain representative collections inextremely shallow and obtstacle-laden conditions. To

effectively sample vertical migrating species that gener-ally dominate the zooplankton community, night-timesampling would be most suitable (Wooldridge 1999;Kibirige and Perissinotto 2003b). However, this is gen-erally problematic in TOCEs and alternative optionshave been investigated. Daytime suprabenthic collec-tions, using a sled-mounted net, have been tested in thisstudy against both diurnal and nocturnal collectionsmade with a standard WP-2 net (Tables 5, 6; Fig. 4).The dominant copepod species P. hessei was chosen astest species because of its strong diel migratory behav-iour and large abundance/biomass it exhibits in mostTOCEs (Perissinotto et al. 2000; Froneman 2004). Whenthe abundance ratios of sled to WP-2 net for the Mdlotiand the Mhlanga were compared separately with theratios of night/day WP-2 net samples collected fromthe Mpenjati Estuary (Kibirige and Perissinotto 2003b;

Mdloti

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Fig. 10 Principal componentsordinance of physico-chemicaland biological parameters for: aMdloti Estuary; and b MhlangaEstuary. (DIN, dissolvedinorganic nitrogen; DIP,dissolved inorganicphosphorous; DO, dissolvedoxygen)

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Kibirige et al. 2003), there were no significant differencesbetween the two sets (t test=1.42, P>0.05, t test=0.74,P>0.05). Also, the ratio of 1(sled):3 (WP-2) obtainedfor P. hessei abundance is similar to that obtained fromthe Mpenjati Estuary between daytime and night-timeWP-2 catches (Kibirige and Perissinotto 2003b). All thissuggests that samples collected during the day near thebottom with the sled method can be used as adequaterepresentatives of night-time water-column densities inthese estuaries. It may, however, be necessary to carryout more, similar tests whenever dealing with dominantspecies that burrow deep into the sediment during theday, such as the various mysid species that often occur inSouth African TOCEs (Kibirige et al. 2003; Kibirige andPerissinotto 2003a).

Acknowledgements This study was funded by the Water ResearchCommission (WRC, Pretoria) and a Joint Venture Project of theNational Research Foundation (NRF, Pretoria) and Marine andCoastal Management (MCM, Cape Town). The University ofKwaZulu-Natal (Durban) also provided funds and facilities. Wewish to thank the Marine Section of KwaZulu-Natal Wildlife forproviding logistic support and for supplying data on the mouthconditions of the two estuaries. Thanks also to Derek Watt of theSouth African Sugarcane Research Institute (SASRI), for provid-ing rainfall and other meteorological data from the Mt EdgecombeStation. Finally, we are grateful to Mark Olbers, Saras Mundree,Kogilam Iyer and Cheryl Thomas for their invaluable assistance inthe field and in the laboratory.

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