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Journal of Plankton Research Vol.23 no.9 pp.999-1008, 2001
© Oxford University Press 2001
Feeding ecology of the mysid, Mesopodopsis wooldridgei, in a temperate estuary along the eastern seaboard of South Africa
Department Of Zoology And Entomology, Rhodes University, Po Box 94 GrahamstowN. 6140. South Africa
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Abstract |
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 TOP Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Feeding ecology of the mysid, Mesopodopsis wooldridgei, was investigated at six stations in the Kariega estuary during summer (November) 1999 using in vitro incubations and the gut fluorescent technique. Three functional feeding groups were examined, adults (>15 mm), immatures (6–8 mm) and juveniles (<4 mm). Individual levels of gut pigment concentration for adults and immatures ranged from 0.8 to 1.4 ng pigment per individual (ind.–1) and between 0.3 and 1.2 ng pigment ind.–1, respectively. Among the juveniles, gut pigment concentrations ranged from 0.1 to 0.3 ng pigment ind.–1. Gut evacuation rates for adults and immatures were 0.90 and 0.98 h–1 respectively. Juvenile gut evacuation rates were equivalent to 1.38 h–1. Losses of pigment during digestion were 53, 67 and 78% for adults, immatures and juveniles. Carbon derived from the consumption of phytoplankton for adults, immatures and juveniles ranged from 1.06 to 1.77, from 1.40 to 3.39 and from 0.19 to 0.56 µg C ind.–1 day–1. Clearance rates of the three size classes feeding on microzooplankton were on average 29, 12 and 6 ml ind.–1 h–1 for adults, immatures and juveniles. These rates correspond to an ingestion rate of between 0.7 and 1.66 µg C ind.–1 day–1 for adults and between 0.21 and 0.63 µg C ind.–1 day–1 for immatures. Total carbon ingested by juveniles feeding on microzooplankton corresponded to between 0.12 and 0.31 µg C ind.–1 day–1. Carbon derived from the consumption of copepod nauplii and copepodids were equivalent to between 0.8 and 3.5 µg C ind.–1 day–1 for adults and between 0.4 and 1.05 µg C ind.–1 day–1 for immatures. Juveniles did not feed on copepod nauplii. Results of the investigation suggest that the diet of M. wooldridgei in the Kariega estuary changes with development. This is probably due to the inability of larger individuals to feed on the small phytoplankton cells (<10 µm) which dominate total chlorophyll a in the estuary.
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INTRODUCTION |
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TOPAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Mysids are a ubiquitous component of the zooplankton assemblages in a variety of aquatic environments (Mauchline, 1980
; Price, 1982; Webb et al. 1987; Grange, 1992; Jerling and Wooldridge, 1995). Although generally representing a small component of the total zooplankton abundance, they may, at times, comprise a significant proportion (up to 20%) of the total zooplankton biomass. Mysids are generally considered omnivorous (Siegfried and Kopache, 1980; Grossnickle, 1982; Jerling and Wooldridge, 1995) although herbivory (Webb et al., 1987) or carnivory (Fulton, 1982) has been reported for some species. There are also data in the literature to suggest that the composition of the mysids' diet changes with developmental stage, with herbivory dominating among juveniles and carnivory among adults (Siegfried and Kopache, 1980).
The Kariega estuary (33°41'S; 26°42'E) is situated on the eastern seaboard of South Africa. As a result of low annual freshwater influx, the estuary is regarded as a homogeneous oligotrophic marine system (Whitfield, 1992; Grange and Allanson, 1995). The limited freshwater influx into the estuary is the result of several factors, including sporadic rainfall, small catchment area (±680 km2) and the presence of several impoundments along the river (Allanson and Read, 1995). Field studies in the estuary have shown that chlorophyll concentrations are low (generally <2 µg l–1) and are dominated by nano- and picophytoplankton (Grange 1992; Grange and Allanson, 1995; Froneman and McQuaid, 1997). The low chlorophyll concentrations are the result of low primary production rates due to nutrient limitation associated with low freshwater influx (Allanson and Read, 1995; Grange and Allanson, 1995).
Total zooplankton abundance and biomass in the Kariega estuary are generally low, which reflects the low food availability (chlorophyll a) (Grange, 1992). It is also likely that the low abundances of zooplankton in the estuary are the result of the low freshwater influx into the system as increases in the abundance of zooplankton in estuaries along the eastern seaboard of South Africa have been shown to be correlated with freshwater pulses (Wooldridge, 1999). Among the zooplankton, the calanoid copepods Acartia and Pseudodiaptomus dominate, comprising up to 95% of the total abundance (Grange, 1992). Among the larger zooplankton, the mysid Mesopodopsis wooldridgei numerically dominates, although it contribution is generally <5% of total zooplankton abundance. The contribution of M. wooldridgei to total zooplankton biomass may, however, be as high as 20% (Grange, 1992). Despite their importance in the zooplankton assemblages of the Kariega estuary, no feeding studies have been conducted with the mysid. A study conducted in the freshwater-dominated Sunday's River estuary showed that in addition to being an important predator of mesozooplankton, M. wooldridgei also represents an important consumer of phytoplankton production (Jerling and Wooldridge, 1995). The study further indicated that M. wooldridgei preferentially consumes particles >20 µm in size (Jerling and Wooldridge, 1995). In light of previous findings that phytoplankton biomass and production in the Kariega estuary are dominated by picophytoplankton (<2.0 µm) (Grange, 1992; Froneman and McQuaid, 1997), the data suggest that M. wooldridgei will largely be carnivorous. Here, the results of feeding studies conducted with M. wooldridgei in the Kariega estuary are presented.
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MATERIALS AND METHODS |
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TOPAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Feeding ecology of Mesopodopsis wooldridgei was investigated at six stations in the Kariega estuary in summer (November) 1999 (Figure 1
). Three functional feeding groups were examined according to the method described previously (Jerling and Wooldridge, 1995). These were: adults (secondary sexual characteristics fully developed, 15–18 mm), immatures (secondary characteristics not fully developed, 6–8 mm) and juveniles (<4 mm). Feeding ecology of the three groups was examined using two different techniques, the gut fluorescent technique (Perissinotto, 1992) and in vitro incubations (Froneman et al., 2000).
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Gut fluorescent technique
A selection of gut evacuation experiments was carried out through in vitro incubations in particle-free water (obtained by passing surface water through a 0.2 µm Mill Q filtration system) of freshly caught specimens in 20 l polyethylene containers. Specimens were collected using a WP-2 net (nominal mouth size 0.05 m2; mesh size, 60 µm) towed at the surface (approx. 1 m depth). Prior to the incubations, 5–15 specimens of each species were processed within 5 min of collection to monitor initial gut pigment concentrations. The incubations ranged from 1 h to 3 h, with gut fluorescence measured at 10 min intervals for the first hour and every 15 min for 2–3 h thereafter until the end of the experiment. Three to five individuals were collected for each time interval measurement. Gut evacuation rates (k, h–1) were then derived from the slope of regression of the natural logarithm of gut pigment versus time (Perissinotto and Pakhomov, 1996). The analysis was conducted using the computer package, Statgraphics, Version 5.0 (Statistical Graphics Corporation). To determine integrated gut pigment concentrations, specimens of the three feeding groups were collected at 2 h intervals for 24 h. Day and night pigment concentrations were corrected to account for diel feeding patterns.
To estimate the gut pigment destruction efficiency (b') of three functional groups of the mysid, freshly caught animals were incubated in particle-free water to which charcoal particles (<100 µm) were added for 6–24 h to allow the animals to empty their guts of pigments (Perissinotto and Pakhomov, 1996). Specimens (one mysid) were then transferred into one litre polyethylene jars containing natural sea water and incubated for 1 h. The gut pigment destruction efficiency was estimated using the two-compartment (phytoplankton and grazer) pigment budget approach. A comparison of the pigment budgets in the control (without grazers) and experimental treatments was then carried out. Any significant loss in the pigment budget from the experimental treatment was attributed to gut destruction of phytoplankton pigments. Three replicates for each size class were conducted.
In all the experiments, gut pigments were extracted in 10 ml polyethylene tubes (1 individual per tube) with 8–10 ml of 90% acetone and stored at -20°C for 12 h. After centrifugation at 5000 r.p.m. (1745 x g), the pigment content of the acetone was measured before and after acidification using a Turner 10AU Model fluorometer (Mackas and Bohrer, 1976). Pigment contents were expressed in terms of chlorophyll a (Chl a) equivalents per individual according to Strickland and Parsons (1968), as modified by Conover et al. (Strickland and Parsons, 1968; Conover et al., 1986). Where ratios of chlorophyll to phaeopigment in the gut contents were higher than 0.25, total pigment levels were corrected according to Baars and Helling (Baars and Helling, 1985).
Daily ingestion rates (I, ng pigment individual–1 day–1; ng pigm. ind.–1 day–1) were then estimated from the equation of Perissinotto (Perissinotto, 1992):
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). In vitro incubations
To investigate the importance of microzooplankton in the diet of M. wooldridgei, a series of in vitro incubations was conducted using individuals of the three feeding groups. Prior to the onset of the incubations, triplicate samples from each grazing station were prepared in 600 ml polycarbonate bottles filled with natural sea water. Sea water had been passed through a 200 µm mesh to exclude metazoan grazers. The bottles were allowed to stand for 2 h to allow for the stabilization of the plankton community in the incubation bottles (Gifford, 1993). For each experiment, one individual per 600 ml container was used. For controls, containers containing only natural sea water were employed. The controls and treatments were then incubated for 12 h under ambient conditions. Each container was gently inverted every 2 h to prevent the settlement of plankton. At the beginning of the experiment, a 100 ml subsample was removed from each container to determine the initial concentrations of protoplankton. This procedure was repeated at the end of the incubations to determine final protoplankton concentrations. All samples were fixed in 10% Lugol's solution (Froneman and McQuaid, 1997). Microzooplankton densities were determined within 1 month of collection using the Utermöhl settling technique after sedimentation in a 10 ml settling chamber. For the analysis, all dinoflagellates were considered as phytoplankton and ciliates were assumed to be microzooplankton (Jerling and Wooldridge, 1995).
Grazing by mysids on microzooplankton was estimated by employing a modification of Frost's equation (Frost, 1972):
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To estimate the amount of carbon derived from consuming microzooplankton, biovolume : carbon ratios were employed (Jerling and Wooldridge, 1995). Bio-volumes were estimated by measuring average microzooplankton prey size (n = 50) at each grazing station using an eye-piece micrometer. Ciliate volumes were converted to carbon using the factor 1 µm3 = 0.19 pg C (Putt and Stoecker, 1989). Results were expressed as ng C consumed ind.–1 h–1.
Predation impact on copepod nauplii and copepodid stages
Contribution of copepod nauplii and copepodid stages to total daily carbon intake of the three functional feeding groups of M. wooldridgei was estimated using in vitro incubations. Copepodids and nauplii were collected at each grazing station using a WP-2 net (mesh size 60 µm) towed during the day. Samples were collected during the day as previous studies have shown that copepodid stages and nauplii of the genera Acartia and Pseudodiaptomus, numerically dominate zooplankton samples during daylight hours (Grange, 1992). Samples collected at each station were gently transferred into a 4 l carboy containing natural, metazoa-free sea water. Metazoans were removed by passing estuarine water through a 60 µm mesh. Each water sample was then divided into equal sub-samples using a Folsom plankton splitter. Incubations were conducted in 2 l polycarbonate bottles For the experiments, a single mysid was added to each incubation bottle and allowed to feed for 24 h. For the control, incubations were conducted in bottles containing only zooplankton prey. Incubation bottles were inverted every 2 h to prevent the settlement of plankton.
Three 100 ml subsamples of sea water were collected from each incubation bottle using a syringe at the beginning of the experiment to determine initial prey biomass. The procedure was repeated at the end of the incubation for the determination of final prey biomass. All samples collected were preserved in 10% buffered (hexamine) formalin. Total dry weight of prey in each sample was then determined using a Sartorious microbalance after oven drying at 60°C for 36 h. No correction factor for the loss of tissue for samples preserved in formalin was applied. To estimate the amount of prey consumed, total dry weight of sample at the beginning of the experiment was subtracted from dry weight at the end of the experiment. Carbon content of prey was assumed to correspond to 40% of dry mass (Jerling and Wooldridge, 1995). Data were then expressed as µg C consumed ind.–1 day–1.
Daily rations
To estimate the contribution of the different food types to the total daily ration of the three functional feeding groups, the dry weights of six individuals from each group were determined using the method described above and a carbon content of 40% dry weight was assumed (Jerling and Wooldridge, 1995). Results were then expressed as percentage body carbon consumed per day.
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RESULTS |
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TOPAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Gut fluorescent technique
Average individual levels of gut pigment concentration for adult Mesopodopsis wooldridgei varied considerably during the study, ranging from 0.8 to 1.4 ng pigm. ind.–1 (Figure 2). Immature and juvenile gut pigment concentrations were lower, ranging between 0.3 and 1.2 and between 0.1 and 0.3 ng pigm ind.–1, respectively (Figure 2). There were no significant differences in the gut pigment concentration between day and night samples for any of the three functional feeding groups (P >0.05) (Figure 2).
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In all experiments a negative linear model provided the best fit in the change in gut pigment concentration over time (P <0.05 in all cases) (Figure 3
). The values obtained for the gut evacuation rate (k) and gut passage time(1/k) for adults were 0.9 h–1 and 1.11 h–1, respectively. For immatures and juveniles the gut evacuation rates were estimated at 0.98 and 1.38 h–1, respectively. These rates correspond to a gut passage time of 1.02 h –1 for immatures and 0.72 h–1 for juveniles. Losses of gut pigment fluorescence by the three feeding groups were 53% (SD = 11) for adults, 78% (SD = 5) for immatures and 84% (SD = 3) for juveniles.
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Assuming a Chl a : carbon ratio of 50, total carbon derived from the consumption of phytoplankton for adults and juveniles ranged from 1.06 to 1.77 µg C ind.–1 day–1 and between 1.40 and 3.38 µg C ind.–1 day–1, respectively (Table I
). Daily ingestion of carbon for juveniles ranged from 0.19 to 0.56 µg C ind.–1 day–1 (Table I).
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Microzooplankton feeding experiments
The results of in vitro experiments conducted to estimate the importance of microzooplankton in the diet of M. wooldridgei in the Kariega estuary are shown in Figure 4. Clearance rates of adults and immatures during the incubations ranged from 18.7 to 41.2 ml ind.–1 h–1 and from 7.8 to 11.8 ml ind.–1 h–1, respectively (Figure 4). These rates correspond to a carbon ingestion rate between 29.1 and 69.0 ng C ind.–1 h–1 for adults and between 8.6 and 26.3 ng C ind.–1 h–1 for the immatures (Figure 4). Clearance rates of juveniles ranged from 3.8 to 7.9 ml ind.–1 h–1 which corresponds to a carbon ingestion rate of between 4.8 and 12.9 ng C ind.–1 h–1 (Figure 4).
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Predation on copepodids and nauplii
In all experiments, nauplii and copepodid stages of the genera Acartia and Pseudodiaptomus numerically dominated, comprising >90% of all zooplankton counted. Total biomass of the nauplii and copepodid stages during the study ranged from 15.8 to 53.3 µg l–1. Throughout the investigation, predation by juveniles did not result in a decrease in prey biomass during the incubations. Adult M. wooldridgei consumed between 2.05 and 8.78 µg dry weight prey ind.–1 day–1. These predation rates correspond to an average ingestion rate equivalent to between 0.8 and 3.5 µg C ind.–1 day–1 (Figure 5). Consumption of copepod nauplii and copepodid stages by immatures ranged from 0.98 to 2.45 µg dry weight prey ind.–1 day–1 which corresponds to an average carbon ingestion rate of between 0.4 and 1.05 µg C ind.–1 day–1 (Figure 5).
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Daily rations
Average dry weight of the adult, immature and juvenile M. wooldridgei was 238, 139 and 48.1 µg, respectively. Total daily ration of the adults and immatures corresponded to between 3.1 and 6.7% body carbon and between 5.1 and 7.5% body carbon, respectively. Total average daily ration for juveniles was equivalent to 3% body carbon (range 1.7 to 3.4%).
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DISCUSSION |
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TOPAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Studies on the biota of the Kariega estuary have largely focused on examining the interaction between water column characteristics, phytoplankton biomass (Allanson and Read, 1995
; Grange and Allanson, 1995) and the distribution of invertebrates and vertebrates (Grange, 1992; Paterson, 1999). Data on the trophic interaction in the planktonic food web are limited. Grange showed that the copepods were able to clear up to 40% of the water column per day (Grange, 1992). Similarly, Froneman and McQuaid showed that microzooplankton were able to graze up to 65% of the phytoplankton production per day and as a consequence, probably represented the dominant herbivores in the system (Froneman and McQuaid, 1997). The present study represents the first attempt to examine the feeding ecology of an individual zooplankton species, the mysid Mesopodopsis wooldridgei.
The gut passage times of adult, immature and juvenile M. wooldridgei during this study are in the same range as those reported in the literature (Grossnickle, 1982; Webb et al., 1987) For example, the gut evacuation of Mesopodopsis slabberi in Algoa Bay, South Africa, was estimated at 1.5 h–1 (Webb et al., 1987). Similarly, the gut passage time for the freshwater mysid, Mysis relicata, ranged from 0.5 to 1.75 h–1 (Grossnickle, 1982). The decrease in gut passage time for the smaller M. wooldridgei is consistent with results obtained for other pelagic crustaceans and can probably be related to the higher metabolic rates of the smaller individuals (Pakhomov et al., 1997; Gurney, 2000). Although M. wooldridgei undertake vertical migrations (Wooldridge, 1999), no diel patterns in gut pigment content were observed for the three feeding groups during this study (Figure 2
). The Kariega estuary is a well-mixed system, characterized by the even distribution of chlorophyll a throughout the water column (Grange, 1992; Grange and Allanson, 1995; Froneman, in press). Clearly under these under these conditions the mysids would be able to feed continuously. Data on the gut pigment destruction rates for mysids are not available in the literature. However, the estimated gut pigment destruction rate for the three size classes of mysid obtained here, compares well with results obtained for similar sized pelagic crustaceans (Pakhomov et al., 1997; Gurney, 2000). Finally the clearance rates of the three functional mysid groups feeding on microzooplankton during this study are in agreement with a similar study conducted in the Sundays River estuary (Jerling and Wooldridge, 1995).
The contribution of phytoplankton to total carbon intake was highest for juveniles (38–70% of total) and lowest for the adults (range 21–38% of the total) (Figure 6). Immatures obtained between 49 and 80% of their total carbon intake by feeding on phytoplankton (Figure 6). A study conducted with the mysids, Mesopsis relicta and M.mercedis showed that phytoplankton was an important component in the diets of adults only when large phytoplankton cells (>50 µm) dominated total chlorophyll a (Siegfried and Kopache, 1980). According to Jerling and Wooldridge (Jerling and Wooldridge, 1995) adult M. wooldridgei preferentially consumes phytoplankton cells >20 µm. During this study, total chlorophyll a was dominated by small picophytoplankton (<2 µm) which comprised up to 60% of the total (Froneman, 2000). The low contribution of phytoplankton to the total daily carbon intake of adults during this study is therefore, not surprising. In contrast, the smaller juveniles and immatures were able to feed on small phytoplankton cells, as evident from the higher contribution of phytoplankton to their total daily carbon ration (Figure 6). This result is consistent with feeding studies conducted with the opossum shrimp, Neomysis mercedis which showed that it was predominantly herbivorous when small whereas carnivory increased in importance in larger individuals (Siegfried and Kopache, 1980).
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Predation on copepodid stages and copepod nauplii was most pronounced for adult M. wooldridgei, where it comprised between 29 and 54% of the total carbon ingested (Figure 6
). It is worth noting that copepods comprised up to 84% of the total energy intake of the adult mysid, N. mercedis (Siegfried and Kopache, 1980). The predominance of carnivory by adult mysids has been demonstrated in several previous studies and appears to be related to food availability (Siegfried and Kopache, 1980). Generally, in regions where chlorophyll is dominated by phytoplankton cells too small to be efficiently fed on by adults, mysids consume metazoan prey to meet their energetic requirements. Alternatively, the adults may optimize net energy per unit feeding time by selecting the largest prey available. In agreement with previous studies examining the feeding ecology of mysids, juvenile M. wooldridgei did not consume copepod nauplii during this study (Siegfried and Kopache, 1980). The results of the in vitro incubations suggest that adult M. wooldridgei can probably be regarded as the key predator of copepods in the Kariega estuary.
Total average daily carbon rations, including both autotrophic and heterotrophic carbon, for juvenile and immatures were equivalent to 3.0 and 6.2% body carbon, respectively. Adult carbon rations were equivalent to between 3.1 and 6.8% (mean 4.7%) body carbon per day. The values obtained here are in agreement with the values reported in the literature for mysids, deep-sea and pelagic crustaceans (Evans, 1984; Norte-Campos and Temming, 1994; Maynou and Cartes, 1998; Pakhomov et al., 1999). For example, the daily ration for the brown shrimp, Crangon crangon, in the shallow waters of the Wadden Sea was estimated at between 6.4 and 16% body weight (Norte-Campos and Temming, 1994). Following this, Bowers and Vanderploeg estimated the daily ration of the mysid M. relicta to be equivalent to between 2 and 7% body weight (Bowers and Vanderploeg, 1982). It should be pointed out that the estimates of daily ration should be regarded with caution as there are several sources of error which may have resulted in the underestimation of carbon ingested by the three functional feeding groups of M. wooldridgei. For example, the estimates of carbon consumed by feeding on microzooplankton were based on biovolume to carbon ratios. Studies have shown that cell shrinkage of as much as 25% may occur in samples preserved in Lugol's solution (Leakey et al., 1994). Furthermore, we have assumed a chlorophyll to carbon ratio of 50. Clearly this value varies considerably in different estuarine environments. Detritus is known to contribute significantly to the diet of several mysid species (Mauchline, 1980). Unfortunately, the importance of detritus in the diet of the mysid was not examined during this study. Despite these possible sources of error, the results of the study provide an indication of the relative importance of the different food types to the diet of M. wooldridgei in the Kariega estuary.
It is well documented that influx of fresh water into estuaries promotes primary production largely through increased water column stability and nutrient availability (Mallin and Paerl, 1994). According to Whitfield (Whitfield, 1992), the Kariega estuary can be classified as a homogeneous marine system largely due to low freshwater influx. As a consequence of the limited freshwater inflow, total production is low and is dominated by small phytoplankton cells (Allanson and Grange, 1995; Froneman and McQuaid, 1997; Froneman, in press). The results of the feeding studies suggest that the diet of M. wooldridgei shifts from principally ominiverous feeding to a carnivorous feeding mode with development. The low contribution of phytoplankton to total carbon intake for adults (generally <25%) appears to be related to the size structure of the local phytoplankton which is too small to be efficiently grazed by the adults. In the absence of large phytoplankton cells, adult M. wooldridgei consume microzooplankton and copepods. These data suggest that reductions in freshwater influx into the estuary may have resulted in a shift in diet of the dominant mysid in the Kariega estuary through its effect on the phytoplankton community size structure. Although the data presented here represent the summer situation, data on the phytoplankton size structure in the estuary indicate little seasonality (Grange, 1992). These facts suggest that the results obtained here probably reflect the feeding ecology of M. wooldridgei throughout the year.
In conclusion, the study indicates that, owing to the predominance of small nano- and picophytoplankton, copepods and or microzooplankton represent the most important consumers of phytoplankton production in the temperate Kariega estuary. However, mysids, in particular the adult and immature stages, can be regarded as important predators of copepods and microzooplankton in the estuary. As a consequence, mysids may play an important role in regulating the feeding impact of herbivorous zooplankton on phytoplankton production in the Kariega estuary. To gain a better insight into the role of M. wooldridgei in the food web of the Kariega estuary, their role as a food source for the larger predators including juvenile fish and gelatinous zooplankton needs to be investigated.
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Acknowledgments |
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Funding for this study was obtained from the Joint Research Council, Rhodes University. I would like to thank Dr Angus Paterson from the JLB Smith Institute of Ichthyology for providing valuable assistance in the field.
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Received on August 6, 2000
; accepted on April 10, 2001
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