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Zooplankton biomass, distribution and size composition in the KwaZulu-Natal Bight during a wet and dry season

Michelle Pretorius1, Jenny Huggett2,3 and Mark Gibbons1

1 University of the Western Cape, Department of Biodiversity and Conservation Biology, Private Bag X17, Bellville

7535, South Africa 2 Department of Environmental Affairs, Private Bag X2, Roggebaai 8012, South Africa.

3 Marine Research Institute, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa

1. INTRODUCTION Over the past 20 years zooplankton research has focused on the West and South Coasts of South Africa, in the Benguela upwelling ecosystem and on the Agulhas Bank, where South Africa’s commercially important pelagic fish species are located. Considerably less research effort has been directed towards the East Coast zooplankton community except for some surveys completed in the 1960s and some more recent studies on gelatinous zooplankton (Carter, 1977; Schleyer, 1977; Buecher et al. 2005 Thibault-Botha et al. 2004; Thibault-Botha and Gibbons, 2005). This study forms a component of a project entitled Ecosystem Functioning in the KwaZulu-Natal (KZN) Bight, which falls under the African Coelacanth Ecosystem Programme (ACEP), and was developed to investigate the physical oceanographic and geological processes which facilitate the transport of nutrients and sediment across the KZN Bight. The aim of this study is to improve our understanding of the role of zooplankton in ecosystem functioning of the KZN Bight. This paper focuses on the zooplankton biomass, distribution and size composition within the Bight. 2. METHODS Zooplankton samples were collected during two multidisciplinary research cruises in the KZN Bight, in January - February 2010 (during the summer wet season) and July – August 2010 (during the winter dry season). “Synoptic surveys” of zooplankton biomass in the Bight were conducted at the beginning of each cruise, and consisted of 16 transects extending perpendicular to the east coast from south (Durban) to north (Richards Bay) with 59 hydrographic stations. The synoptic surveys were completed within 7-8 days. Vertical bongo nets (200µm mesh) were used to collect zooplankton samples from both cruises; all samples were split once at sea using a Folsom splitter. The first half was fractionated into four different size classes, namely 200-500µm, 500-750µm, 750-1600µm and >1600µm, while the second half was preserved in 5% formalin and seawater. By splitting the samples into two this allowed for two different types of analysis, species identification and enumeration under a stereomicroscope with the formalin preserved samples, and dry weight analysis of the size fractionated samples. To obtain dry weights, samples were filtered onto GF/F filter papers where excess water was removed using a vacuum pump. The filters were weighed before and after sample filtration and then were dried in an oven at 60°C for 24hrs and re-weighed (Lovegrove 1962; 1966). Following the synoptic surveys, “focus site surveys” were conducted at 4-5 fixed locations off Durban, the Thukela mouth and Richards Bay, and consisted of sampling at midday (12h00) and midnight (00h00) over a period of 5-10 consecutive days. Vertical bongo nets were used to collect zooplankton samples for dry weight analysis, following the same procedure as for the synoptic surveys.

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Temperature, salinity and oxygen were profiled during CTD deployments on station, and seawater samples were collected at selected depths for chlorophyll a and nutrient analysis (nitrite, nitrate, silicate and phosphate). Statistica v 6.1 was used to investigate correlations between zooplankton biomass and environmental variables, and analysis of variance (ANOVA) was used to explore the effects of categorical variables such as time (day or night) and season (wet or dry). 3. RESULTS Mean total zooplankton biomass during the wet season was 4.8 mg dry wt m-3. Biomass was most concentrated on the shelf, shallower than 300m (Figure 1a), with highest concentrations off Durban (max = 21.3 mg dry wt m-3). Mean zooplankton biomass was much higher during the dry season (17.1 mg dry wt m-3) compared to the wet season. Highest concentrations of biomass during the dry season were located inshore at Richards Bay (Figure 1b) and offshore slightly farther north (max = 117.8 mg dry wt m-3).

Figure 1: Total zooplankton biomass (mg dry weight m-3) during (a) the wet season synoptic survey (Jan 2010) and (b) the dry season synoptic survey (Aug 2010). Note different scales.

Zooplankton biomass in four different size fractions during the wet season is shown in Figure 2. Biomass was broadly distributed over the shelf throughout the Bight, with a patch of high biomass off Durban for all size fractions. The 200-500µm size fraction comprised small zooplankton such as small copepods and other small zooplankton (Fig 2a), the 500-750µm size fraction comprised medium copepods and other medium zooplankton (Fig 2b), the 750-1600µm size fraction comprised large copepods and other large zooplankton (Fig 2c) and the >1600µm size fraction comprised chaetognaths, euphausiids, salps and jellyfish. The largest size fraction appeared to be the patchiest in terms of horizontal distribution.

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Figure 2: Zooplankton biomass (mg dry wt m-3) during the wet season for four size fractions: (a) 200-500µm, (b) 500-750µm, (c)

750-1600µm and (d) >1600µm.

Zooplankton biomass in the four different size categories during the dry season is shown in Figure 3. Biomass was distributed more patchily compared to the wet season. All size categories except the 750-1600µm fraction showed highest biomass to the north of the Bight, either inshore off Richards Bay or offshore farther north. The smallest size fraction (mainly small copepods) showed secondary biomass peaks off the Thukela River mouth, south of Durban and off Scottburgh. Highest biomass in the 750-1600µm fraction (mostly large copepods) was located inshore to the north of Durban and farther south off Scottburgh.

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Figure 3: Zooplankton biomass (mg dry wt m-3) during the wet season for four size fractions: (a) 200-500µm, (b) 500-750µm, (c)

750-1600µm and (d) >1600µm. Note different scales.

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Figure 4: Total zooplankton (mg dry wt. m-3) biomass during the wet season, recorded approximately every 12 hours over consecutive days at the focus sites in the Bight. The symbols represent sampling during the day (sun) and night (moon).

Figure 4 shows the daily and diel variability in zooplankton biomass at the focus sites during the wet season cruise. A clear diel signal was only evident at the Durban Eddy site, which was the deepest site (200m), while the other shallower focus sites showed no clear indication of diel vertical migration.

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Figure 5: Total zooplankton biomass (mg dry wt. m-3) during the wet season, recorded every 12 hours over consecutive days at

the focus sites in the Bight. The symbols represent sampling during the day (sun) and night (moon).

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Figure 5 shows the daily and diel variability in zooplankton biomass at the focus sites during the dry season cruise. During the dry season mean zooplankton biomass was significantly higher at the Richards Bay North and Thukela Mouth sites than during the wet season (Fig 6a). The diel signal for the Durban Eddy site was not as marked as during the wet season, but for both surveys combined mean biomass at night was significantly higher than during the day at this relatively deep site (Fig 6b).

Figure 6: Mean zooplankton biomass at the focus sites during (a) wet and dry seasons, and (b) during day and night sampling. Focus site abbreviations are as follows: DE = Durban Eddy, TM = Thukela Mouth, MS = Mid-shelf, RS = Richards Bay South,

RN = Richards Bay North.

Table 1: Linear correlations between environmental variables and zooplankton biomass (total biomass and four different size fractions) during both wet and dry season cruises. Significant correlations are indicated in red, and the percentage variance explained is also shown.

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biomass 200-500µm 500-750µm 750-1600µm >1600µm

SST p=0.000001 r2=0.1816

p=0.000004 r2=0.16

p=0.0005 r2=0.098

p=0.00005 r2=0.1333

p=0.0004 r2=0.1031

Salinity p=0.2788 r2=0.01

p=0.3582 r2=0.0072

p=0.4089 r2=0.0058

p=0.3741 r2=0.0068

p=0.3036 r2=0.0091

Chl a p=0.00006 r2=0.1289

p=0.00003 r2=0.1409

p=0.0038 r2=0.0692

p=0.0008 r2=0.0935

p=0.0072 r2=0.0606

Integrated Chl a p=0.5957 r2=0.0024

p=0.4560 r2=0.0048

p=0.5797 r2=0.0026

p=0.8234 r2=0.0004

p=0.3972 r2=0.0062

Highly significant correlations (p<0.0001; Table 1) were found between zooplankton biomass (total biomass as well as all size fractions) and sea surface temperature (negative correlations) and surface chlorophyll a (positive correlations), but the variance explained was generally low. Interestingly, there was no correlation between zooplankton biomass and integrated chlorophyll a.

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Figure 7: Box plot showing a comparison between wet and dry season and day and night samples.

Multi-factorial ANOVA showed that total zooplankton biomass was significantly higher at night than day during the dry season (p<0.0001; F=19.47) but not during the wet season (Fig 7). 4. REFERENCES

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thesis, University of Natal, South Africa. Lovegrove T.1962.The effect of various factors on dry weight values. Rapp P (eds) Reun. Cons. Int. Explor. Mer.

153: 89-91.In: ICES Zooplankton Methodology Manual. Harris R et al. (ed) Academic Press, London. Lovegrove T.1966. The determination of dry weight of plankton and the effect of various factors on the values

obtained. In: Some contemporary studies in Marine Science. Barnes H (ed) George Allen &Unwin, London.

Thibault-Botha D, Lutjeharms JRE,Gibbons MJ.2004.Siphonophore assemblages along the east coast of South

Africa; mesoscale distribution and temporal variations. Journal of Planktonic Research 26(9): 1115-1128. Thibault-Botha D, Gibbons MJ.2005. Epipelagic siphonophores off the east coast of South Africa. African Journal

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