Elemental ratios (C:N) and stable isotopic ...

Elemental ratios (C:N) and stable isotopic composition of dominant rotifer species in a tropical eutrophic alkaline–saline Lake Nakuru (Kenya) Emily Jepyegon Chemoiwa, Elijah OyooOkoth, James Mugo-Bundi, Elizabeth W. Njenga, Esther C. Matany, Regina J. Korir, et al. Hydrobiologia The International Journal of Aquatic Sciences ISSN 0018-8158 Hydrobiologia DOI 10.1007/s10750-014-2123-y

1 23

Your article is protected by copyright and all rights are held exclusively by Springer International Publishing Switzerland. This eoffprint is for personal use only and shall not be self-archived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”.

1 23

Author's personal copy Hydrobiologia DOI 10.1007/s10750-014-2123-y

PRIMARY RESEARCH PAPER

Elemental ratios (C:N) and stable isotopic composition of dominant rotifer species in a tropical eutrophic alkaline– saline Lake Nakuru (Kenya) Emily Jepyegon Chemoiwa • Elijah Oyoo-Okoth • James Mugo-Bundi • Elizabeth W. Njenga • Esther C. Matany • Regina J. Korir • Charles C. Ngugi

Received: 16 April 2014 / Revised: 6 November 2014 / Accepted: 24 November 2014 Ó Springer International Publishing Switzerland 2014

Abstract Rotifers dominate zooplankton biomass of many aquatic environments. However, their link to food web biomass has rarely been elucidated in alkaline–saline lakes. Variations in C content, C:N ratio and stable isotope composition (d13C, d15N) of dominant rotifer species were studied from January to December 2008 in alkaline–saline Lake Nakuru to provide insights into their bioenergetics. We established that Brachionus dimidiatus dominated in terms of abundance (80,000–100,000 9 103 ind m-3) and C-biomass. Also B. dimidiatus constituted about 60–75% of the rotifer biomass in the samples. All

the rotifer species exhibited significant (P \ 0.05) seasonal differences in biomass, C and C:N ratios. Rotifers had lower mean d13C than course particulate organic matter, fine particulate organic matter and fish, but higher than mean d13C in the dominant phytoplankton species. In all species, d13C and d15N increased markedly during the rainy season reflecting the feeding onset on allochthonous food sources. The isotopic increase correlated with an increase in their C:N. Our results demonstrate that rotifers respond quickly to any increase in primary production and can cope with changes in its nature and timing.

Handling editor: Judit Padisak

Keywords Carbon biomass  C:N ratios  d13C  d15N  Rotifer biomas  Zooplankton abundance  Rotifer diversity

E. J. Chemoiwa (&)  E. W. Njenga  E. C. Matany  R. J. Korir Department of Biological Sciences, University of Eldoret, P.O. Box 1125-30100, Eldoret, Kenya e-mail: [email protected] E. Oyoo-Okoth  J. Mugo-Bundi Department of Natural Resource, School of Natural Resources and Environmental Studies, Karatina University, P.O. Box 1957-10101, Karatina, Kenya E. Oyoo-Okoth Department of Environmental Biology and Health, School of Environmental Studies, University of Eldoret, P.O. Box 1125-30100, Eldoret, Kenya C. C. Ngugi Mwea Aquafish Farm, P.O. Box 101040-00101, Nairobi, Kenya

Introduction Alkaline–saline lakes occur throughout the arid (25–200 mm annual precipitation) and semi-arid (200–500 mm) basins of the world (Williams, 2002; Jellison et al., 2004). The high alkalinity and salinity coupled with low environmental stability in these lakes render them to characteristically sustain low floral and faunal diversity (Hecky & Kilham, 1973; Finlay et al., 1987; Gilabert, 2001; Harper et al., 2003; Grant, 2006; Oyoo-Okoth et al., 2011; Ong’ondo et al., 2013). Nevertheless, alkaline–saline environments are

123

Author's personal copy Hydrobiologia

rich in terms of species biomass and productivity (Talling et al., 1973; Vareschi, 1982; Agasild & No˜ges, 2007; Oduor & Schagerl, 2007; Okoth et al., 2009). The higher productivity, presumably reflects higher ambient temperatures, higher incident solar radiation throughout the year and unlimited access to inorganic carbon (Melack & Kilham, 1974; Grant, 2006). The zooplankton species composition, abundance and diversity in alkaline–saline lakes aggregate towards dominance by rotifers and/or ciliates in numerical abundance and biomass (e.g. Vareschi, 1982; Vareschi & Vareschi, 1984; Vareschi & Jacobs, 1985; Finlay et al., 1987; Gilabert, 2001; Yasindi et al. 2007; Oyoo-Okoth et al., 2011; Ong’ondo et al., 2013). Generally rotifers thrive well under eutrophic conditions, for example, they may reach population densities as high as 20,000 ind l-1 (Gulati et al., 1992; Ooms-Wilms et al., 1999; Bonecker & Aoyagui, 2005). In the alkaline-lakes, rotifers attain high population densities quickly, especially when large, competitively superior zooplankton species, such as Daphnia, are rare or absent (Gilbert, 1988; Echaniz et al., 2006). However, during a short time period of the year, the size of their population changes rapidly (Echaniz et al., 2006; Oyoo-Okoth et al., 2011). The predominance of rotifers in the eutrophic alkaline– saline lakes concords with their rapid reproductive capacity during optimal conditions, their short generation time and their ability to develop large populations under eutrophic environments (Ghadouani et al., 1998). Also, their role in energy flow in lakes, by acting as primary consumers (Dumont, 1977; Walz, 1995; Garcıˆa et al., 2009; Soares et al., 2010) render them particularly abundant in environment with large abundant primary producers (Bouvy et al., 2001; Leonard & Paerl, 2005). The alkaline–saline lakes in East Africa, have a restricted rotifer composition (Hecky & Kilham, 1973; Nogrady, 1983; Finlay et al., 1987; Gilabert, 2001; Oyoo-Okoth et al., 2011). Previous studies in Lake Nakuru, pointed to the dominant rotifer species of genus Brachionus (Harper et al., 2003; Oyoo-Okoth et al., 2011; Burian et al., 2013). Particularly, OyooOkoth et al. 2011 established that Brachionus dimidiatus, B. plicatilis, Keratella tropica and Filinia longisepa were the dominant rotifers in the lake at all sampling sites, albeit with some rapid changes in a short period of times. Brachionus has a variety of

123

feeding and life-history strategies e.g. it is regarded as a generalist suspension feeder (Gilbert & Bogdan, 1984; Gilbert & Jack, 1993), which may present them in eutrophic waters with food items of various palatability and nutrition, which may partially explain their dominance in eutrophic Lake Nakuru. Also variety of Branchionus are capable of feeding on microzooplankton, and exploit algae and phytoplankton for their growth and maturation, which in turn is a key link for the trophic transfer of energy and nutrients to top predators (e.g. Agasild & No˜ges, 2007). The variant feeding strategy may provide a crucial link between primary producers and consumers in the food web. Although rotifers play a crucial role in aquatic systems by enhancing nutrient recycling (Anderson et al., 2005) and linking the classical food web to the microbial loop (Arndt, 1993), they are systematically underrepresented in studies dealing with zooplankton dynamics (Chick et al., 2010). This might be due to the fact that rotifers need to be sampled with other sampling methods than crustacean zooplankton (Ejsmont-Karabin, 1978) and that although rotifers frequently dominate abundance wise, their biomass is commonly smaller than that of largersized zooplankton groups (e.g. Gulati et al., 1992) and thus overlooked in terms of their contribution to food web structure in alkaline–saline lakes. The use of stable isotope (SI) analysis is a useful tool for the study of biogeochemical cycles as well as ecosystem structures. We can determine the structure of food webs and the interactions between organisms using distributions and variation in C/N isotope ratios together with their fractions. Variation in the SI ratios of nitrogen (d15N) and carbon (d13C) is well studied for single-feeding processes: d15N is enriched by *3–4% per trophic level (TL) (De Niro & Epstein, 1981; Minagawa & Wada, 1984). Evidence suggests that SIs have the potential to reveal complex interactions, including trophic interactions and energy or mass flow through ecological communities (Peterson & Fry, 1987; Cabana & Rasmusen, 1996). However, precise examinations of trophic fractionation have been mostly limited to freshwater ecosystems (Vander Zanden & Rasmussen, 1999). At present, the magnitude of trophic fractionation of carbon isotopes in alkaline–saline eutrophic ecosystems remains unclear and requires further study with emphasis on kinetic isotope fractionation of dominant primary consumers in those ecosystems.

Author's personal copy Hydrobiologia

Here we report on the temporal variation in abundance, C-biomass, C-dry weight (DW) relationships, C:N ratios and stable isotopic signatures (d13C, d15N) of predominant rotifer species in alkaline–saline Lake Nakuru from January to December 2008. Our main objectives were: (i) to determine the variation in abundance of rotifer species, and to provide precise estimates of their biomass over the study period; (ii) to document the ranges of C:N ratios, d13C and d15N that occur in dominant rotifers and to compare these ranges with those of phytoplankton and fine particulate organic matter (FPOM) and (iii) to test for seasonal differences in the C-DW relationships, C:N ratios and SIs. We hypothesise that there is a significant temporal variation in carbon biomass, C:N ratios and C-DW of rotifers in Lake Nakuru related to changes in phytoplankton and POM.

(17.2 ± 7.9 mg l-1) in the uppermost 50-cm layer during the day (Oduor & Schagerl, 2007). The phytoplankton is dominated by Arthrospira fusiformis (Okoth et al., 2009). During the study, we selected ten sites (Table 1). The geo-referenced sites had been established during a previous Kenya Wildlife Service monitoring programme and also sampled in earlier expeditions (Talling & Talling, 1965; Vareschi, 1982) and recently (see Okoth et al., 2009; Oyoo-Okoth et al., 2011; Ong’ondo et al., 2013). They included inlet sites (Hippopoint, Metal pole, Njoro Shore, Makalia Shore and Nderit Shore), which were closer to the discharge point of the incoming rivers and offshore areas (Njoro East, Jetty West, Jetty Mid, Jetty East and Makalia/ Nderit), which were further away from the point of discharge by the river inlets (Table 1; Fig. 1). Field sampling and samples treatments

Methods Study sites The study was conducted in Lake Nakuru (Longitude: 36°030 22.4000 –36°070 06.1400 E, Latitude: 0°180 31.3400 S– 0°240 34.9100 S) situated within Lake Nakuru National Park approximately 160 km North-West of the Kenyan capital city—Nairobi (Fig. 1). The lake is shallow, with a mean depth of 1.5 m (Williams, 1996), covering an area of 40–60 km2 at an altitude of 1,759 m asl. Three seasonal rivers (Njoro, Makalia and Enderit), the Baharini spring and water from the Nakuru town sewage treatment plant discharge into the lake. The mean temperature varies from 8 to 28°C. The variation in water volume results in large fluctuations of physico-chemical parameters such as water temperature and salinity (Okoth et al., 2009). For example, salinity ranges between 10 and 120% (Williams, 1996) and daily temperature fluctuations result in strong diurnal cycles of stratification and mixing (Melack & Kilham, 1974). Annual rainfall averages 1,000 mm. A bimodal rainfall pattern is normally experienced in the region. Long rains often fall during the months of March–May, while short rains occur during September–October (refer to Okoth et al., 2009 and Oyoo-Okoth et al., 2011 for rainfall patterns). High solar radiation and adequate nutrient supply support the growth of primary producers resulting in a super-saturation of dissolved oxygen

Physico-chemical parameters, namely water temperature, dissolved oxygen concentration, salinity, alkalinity, conductivity and pH were measured in situ at each of the sampling sites, using a calibrated JENWAY 3405 electronic probes (Barloword Scientific Ltd, Essex, UK), with independent probes for each variable. The following chemical analyses were performed: Total-P using a Lambda 20 Perkin ElmerÒ (Waltham MA, USA) spectrophotometer after mineralisation with potassium-peroxodisulfate (NF EN 1189, 1996); DOP using a Lambda 20 Perkin ElmerÒ spectrophotometer (NF EN 1189, 1996); Total-N using a RFA 300 AlpkemÒ (OI Analytical, College Station, TX, USA) continuous-flow analyser after mineralisation with potassium-peroxodisulfate (NF EN ISO 11905-1, 1997); nitrate nitrogen was determined using cadmium reduction methods; and dissolved organic nitrogen by Kjeldahl digestion. All chemical analyses was done following APHA (1998). Sampling of zooplankton species, was performed at monthly intervals from January to December 2008 at the sampling sites. At each site, water were obtained with a diaphragmatic pump at three depths (0, 0.5 m from the surface and 1 m from the surface), producing depth integrated samples at each site. For the rotifers determination at each site, water was filtered through a 20-lm mesh. Samples were treated with CO2 from Alka Seltzer tablets as a narcotic and then preserved in 4% sucrose-buffered formalin (Haney & Hall, 1973).

123

Author's personal copy Hydrobiologia Fig. 1 Map of the Lake Nakuru with the position of stations where zooplankton and POM have been sampled from January to December 2008. The detail of sampling stations is given in Table 1

Samples for the analysis of phytoplankton (n = 120) were obtained at all the sites from January to December, while samples for the determination of FPOM (n = 48), sediment (n = 48) and Course particulate organic matter (CPOM) (n = 48) were obtained at two representative sites from the inlet and offshore sites from January to December. Copepods (Leptochironomus deribae) and fish (Oreochromis alcalicus grahamii) were sampled from 1 representative site for the inlet and offshore sites from January to December. Phytoplankton were collected by plankton net (10-mm mesh), and fixed with Lugol’s solution. One litre quantitative samples were fixed in iodine, sedimented for 24 h and concentrated to 50 ml. Numerical phytoplankton identification was carried

123

out with a compound microscope (Olympus-BHS, Japan; 4009 magnification), using several keys and illustrations as detailed in Okoth et al. (2009). To determine Fine Particulate Organic Matter (FPOM), triplicate 1 l seston samples from 0.5 m of the water column were taken, and filtered on to a preweighed, pre-combusted (450°C) glass fibre filter (Whatman GF/C) of 0.45 lm pore. The course particulate materials (CPOM) was collected 1 m below the surface during the horizontal tow. A total of 1 l of water was obtained and passed through a 1-mm sieve, the sieve was rinsed, weighed to the nearest milligram and transported to the laboratory for further analysis. A total of 100 g of sediment was sampled using Ekmans Grab Sampler as described

Author's personal copy Hydrobiologia

were placed into an oven at 105°C until constant mass and weighed to the nearest 0.01 g. Phytoplankton, FPOM, CPOM and sediment were analysed directly for element analysis. Copepods were sorted, dried, weighed and placed in tin cups. 1 g of fish muscle was ground to fine powder and used in the isotopic analysis. The samples were kept over an ice bath (4°C) to prevent degradation. For elemental analyses, organisms were filtered, placed on pre-weighed Whatman glass fibre filters (GF/F) and dried at 60°C for 24 h. After drying, GF/F was weighed again for the determination of rotifer dry weight (DW) using a Sartorius ME5 microbalance (precision ?1 lg). The GF/F was burned at 925°C in a Perkin Elmer CHN 2400 Series II Elemental Analyser (CE 440; Vario EL III) (accuracy\0.3% and precision\0.2%) to measure the C and N content of rotifers. For SI analyses, a second subset of rotifers was dried, weighed and placed in GF/F cups as described above. Dried rotifer samples were ground to a fine powder and divided into two to four cups consistent with the maximum detection limit of the SI analyser. Ratios of 13C/12C and 15N/14N were determined using a mass spectrometer (Thermo Finnigan DeltaPlus Advantage) in the continuous-flow mode (ConFlo III), equipped with a high-temperature elemental analyser. SI ratios of C and N are described as a per mil (%) deviation from the respective international standards, using the following equation:   Rsample dX ¼  1  1;000; Rstandard

elsewhere (http://www.for.gov.bc.ca/hts/risc/pubs/ aquatic/lake-stream/lake-stream-05.htm). Samples were transferred directly into a dark isothermal container prior to their transport back to the motorised boat. Laboratory procedures Rotifers were identified in the laboratory to the lowest taxonomic units possible. The keys of Finlay et al. (1987), Curds (1982), Curds et al. (1983), Small & Lynn 1985 and Jersabek et al. (2003) were used for zooplankton identification. At each site, zooplankton sample was preserved in the lake’s water solution with borax-buffered 4% formaldehyde for taxonomic analysis. Counting was made from sub samples taken with a teat pipette and placed in a 1 ml (l = 50, w = 20, h = 1 mm) Sedgwick-Rafter counting cell. Identification and counting were made under an optical microscope (4009) by scanning at least 10 randomly selected longitudinal transects per cell. At least three chambers per sample were analysed. Zooplankton abundance was expressed as the number of individuals per m3. A second zooplankton sample was gently sieved, rinsed and stored in cryovials and frozen at -210°C for elemental and SI analyses. For SI analysis, rotifer species were separated by a multiple subsequent filtration and a series of division chambers based on sedimentation, buoyancy and negative phototaxis. Rotifers were starved in GF/F filtered lake water for 6 h to facilitate gut evacuation. The filtrate for determination of FPOM and CPOM

Table 1 Locations and sampling sites used in the present study Station (Fig. 1)

Station name

Latitude

1

Hippopoint

0°180 53.7500 S 0

Bottom depth (m)

36°060 08.5500 E

0.94

2

Metal pole

0°18 53.75 S

36°060 08.5500 E

1.42

3 4

Njoro Shore Njoro East

0°180 53.7500 S 0°180 53.7500 S

36°060 08.5500 E 36°060 08.5500 E

0.76 1.45

5

Jetty West

0°180 53.7500 S

36°060 08.5500 E

0.91

6

Jetty Mid

0°18 53.75 S

36°060 08.5500 E

1.57

7

Jetty East

0°180 53.7500 S

36°060 08.5500 E

1.19

8

Makalia Shore

0°180 53.7500 S

36°060 08.5500 E

1.34

9

Makalia-Nderit

0°180 53.7500 S

36°060 08.5500 E

1.52

10

Nderit Shore

0

00

Longitude

0

00

00

0°18 53.75 S

0

00

36°06 08.55 E

1.76

Hippopoint, Metal pole, Njoro Shore, Makalia Shore and Nderit Shore were inlet sites while Njoro East, Jetty West, Jetty Mid, Jetty East and Makalia/Nderit represented offshore sites

123

Author's personal copy Hydrobiologia

where X represents 13C or 15N and R is the 13C/12C or 15 N/14N ratio, respectively. L-Glutamic acids (USGS40 and USGS41, US Geological Survey, Reston, VA, USA) calibrated against Peedee-Belemnite and atmospheric N2 were used as international organic references for SI analyses (Qi et al., 2003). Analytical grade acetanilide was used to calibrate the quantification of C and N. Lipids were not extracted prior to SI analyses to prevent any impact on the d15N, which may be altered following the loss of some non-lipid compounds during extraction (e.g. Sweeting et al., 2006). Hence, the d13C of rotifers were corrected for lipids using the mass balance model recommended by Smyntek et al., 2007 for zooplankton, which is increasingly being recognised as a reliable alternative to chemical extraction correction (Logan et al., 2008). The phytoplankton, FPOM, fish, copepods, CPOM and sediments for comparison with rotifers were analysed on the same mass spectrometer as the one used for rotifer measurements and using the same analytical procedure. No post-analysis, correction on the elemental and SI ratios was applied. Data analyses For each tested data set, the assumption of normality prior to ANOVAs was verified using the Shapiro– Wilk test (P \ 0.05). Non-normal distributions were Table 2 Summary of physico-chemical parameters (mean values ? SD) in Lake Nakuru during the period between January and December 2008 Variable

DO concentration (mg l-1) Salinity (%) Alkalinity (meq l-1) Conductivity

Inlet sites (n = 4)

Offshore sites (n = 6)

17.5 ± 3.1

14.5 ± 1.3

8.1 ± 0.6 234.3 ± 19.8

29.7 ± 6.1 398.7 ± 44.3

44.1 ± 3.3

33.0 ± 3.1

9.1 ± 0.5

10.6 ± 0.7

Total phosphorus (mM l-1)

33.2 ± 4.5

87.4 ± 13.2

Dissolved organic P (mM l-1)

16.3 ± 2.1

44.3 ± 6.5

761.5 ± 52.1

895.4 ± 93.1

32.1 ± 4.3

66.3. ± 7.9

311.3 ± 42.5

265.2 ± 42.1

pH

Total nitrogen (mM l-1) Nitrate nitrogen (mM l-1) Dissolved organic N (mM l-1)

123

log- or square-root-transformed in order to reject the null hypothesis that the data were not from a normally distributed population. Differences in the rotifer abundance were analysed using One-Way ANOVA after square-root-transformation of the data to normalise the count data. Significant differences were analysed by post hoc Tukey’s HSD (honestly significant difference) multiple comparison tests. Seasonal differences between mean C:N ratios, d13C and d15N in rotifers were tested using repeated measure Analysis of Variance (Repeated measure ANOVAs). The relationships between d13C and d15N in phytoplankton, POM, CPOM and sediments and the rotifer species were analysed using linear regression analysis after performing an outlier analysis. All statistical analyses and tests were performed at a level of significance a \ 0.05 using the SYSTAT Software Package (V.12, www.systat.com).

Results Physico-chemical parameters in Lake Nakuru An overview of the physical and chemical variables in Lake Nakuru observed at the inlet and offshore sites is shown in Table 2. The values of physical and chemical parameters are comparable to previously reported values (see Melack, 1988; Harper et al., 2003; Oduor & Schagerl, 2006; Oduor & Schagerl, 2007; Okoth et al., 2009; Oyoo-Okoth et al. 2011). Salinity was observed to sometimes exceed 35%. Dissolved oxygen (DO) was clearly oversaturated with a concentration ranging between 15 of 19 mg O2 l-1 during the day at a 22°C water temperature. Also, Lake Nakuru had very high concentrations of soluble reactive phosphorus, ammonia, nitrate and nitrite. Generally, inlet sites sampled had lower values of salinity, alkalinity, pH, TP, dissolved organic P, NO3 and TN. Rotifer abundance and seasonal variation in Cbiomass of dominant rotifers Table 3 provides information of the abundance (ind*103 m-3) and the carbon biomass (g C m-3) of dominant rotifer species in Lake Nakuru during the period between January and December 2008 at the inlet and offshore sites. Brachionus dimidiatus was the most abundant (&60–75%), with peak abundance

Author's personal copy Hydrobiologia Table 3 Summary of abundance and the carbon content (mean ± SD) of dominant rotifer species in Lake Nakuru during the period between January and December 2008 at the inlet sites (n = 60) and offshore sites (n = 60) Abundance (ind*103 m-3)

B. dimidiatus

Carbon content (g C m-3)

Inlet (n = 60)

Offshore (n = 60)

Inlet (n = 60)

Offshore (n = 60)

86,400 ± 9,210

56,000 ± 9,096

45.7 ± 16.72

31.5 ± 10.99

B. hexartha

2,754 ± 400

2,064 ± 379

23.4 ± 5.42

16.6 ± 5.61

B. calciflora

2,769 ± 225

1,848 ± 211

8.8 ± 1.83

5.5 ± 1.76

647 ± 62

455 ± 53

6.2 ± 2.17

4.4 ± 1.11

Keratella spp.

Short rainy season

Long rainy season

100.0

100.0

B. dimidiatus idi d atus

75.0

50.0

50.0

25.0

25.0

0.0

Carbon content (g C m-3)

Carbon content (g C m-3)

B. hexartha arth t a

30.0 20.0 10.0 0.0 12.0

B. calyciflora ycif iflora

9.0 6.0 3.0

R2 = 0.9457

36.0 30.0 24.0 18.0 12.0 6.0 0.0

40

80

120

B. hexartha y = 0.7442x + 5.4274 R2 = 0.9576

0 18.0

10

20

30

40

B. calyciflora

12.0 y = 0.7085x + 0.5059 R2 = 0.9117

6.0 0.0 0

0.0 8.0

Keratella lla l spp.

5

10

15

20

Keratella spp.

6.0

y = 0.6575x + 0.0976 R2 = 0.8594

4.0 2.0

Dec

Nov

Oct

Sep

Aug

Jul

Jun

May

Apr

Mar

Jan

0.0

Feb

10.0 8.0 6.0 4.0 2.0 0.0

y = 0.7711x + 12.234

0

0.0 40.0

B. dimidiatus

75.0

0

2

4

6

8

10

12

-3

Dry weight (g DW m )

2008 Fig. 2 Time series of C content of rotifer species (mean ± SD) at the inlet sites (dashed line n = 5) and offshore sites (solid line n = 5) in Lake Nakuru in 2008. The vertical ray bars depict the standard deviations. Long rains often fall during the months of March–May, while short rains occur during September–October

(80,000–100,000 9 103 ind m-3) in the samples. The mean C for B. dimidiatus was significantly (One-Way ANOVA; n = 3, P \ 0.05) the highest among the rotifer species followed by B. hexartha while the least C-biomass was Keratella spp. The precise estimates of the temporal variation in C-biomass for B. dimidiatus, B. hexartha, B. calyciflora and Keratella spp. are shown in Fig. 2. The most abundant species was B. dimidiatus represented 52–60% of the C-biomass. Brachionus

Fig. 3 Linear regressions of C content against DW for the rotifers community. The regressions include all the data obtained for the entire study period from January to December 2008

hexartha accounted for 24% and 26–30% of the total abundance and C-biomass, respectively. There were significant temporal variation in abundance of all rotifer species (Repeated measure ANOVA; n = 12 for each rotifer species; P \ 0.05). During the long rainy season between March and May, the C content from all rotifer species in Lake Nakuru increased significantly. Also the C of all rotifer species was significantly (P \ 0.05) higher at the inlet sites compared to the offshore sites.

123

Author's personal copy Hydrobiologia Table 4 C:N ratio (%) and isotopic ratios of d13C and d15N of rotifers (n = 120) together with major food web components of Lake Nakuru in 2008 Variable

n

Mean C:N (%)

SD C:N (%)

Mean d13C (%)

SD d13C (%)

Mean d15N (%)

SD d15N (%)

Branchionus dimidiatus

120

10.05

1.03

-18.57

0.15

9.14

0.90

B. hexartha

120

9.41

1.04

-20.41

0.17

8.31

1.12

B. calyciflora

120

8.37

1.05

-22.53

0.13

7.52

0.09

Keratella spp.

120

8.12

1.01

-23.11

0.15

7.48

1.12

Leptochironomus deribae

24

5.51

0.97

-18.22

0.92

9.78

0.44

Arthrospira fusiformis

48

12.98

0.31

-24.01

1.03

4.29

0.71

Anabaena circinalis

48

13.11

0.51

-24.82

1.79

3.63

0.41

Oreochromis alcalicus grahamii

24

6.74

1.24

-16.99

0.21

6.79

0.23

Sediment

48

11.37

1.51

-17.31

0.54

10.94

0.38

FPOM

48

13.97

1.24

-16.99

0.07

6.11

0.08

CPOM

48

25.11

2.11

-8.71

0.49

9.33

0.25

Isotopic ratios are given in %, C:N ratios in %. Phytoplankton were sampled at all sites. FPOM, sediment and detritus were analysed based on two representative sites from the inlet and offshore sites while copepod (Leptochironomus deribae) and fish (Oreochromis alcalicus grahamii) were sampled from 1 representative site for the inlet and offshore sites from January to December

B. dimidiatus (77%) followed by B. hexartha (74%) while Keretella spp. had the least C content (65%).

16 O. gra

δ15N(‰)

12 B. hex Ker B. cal

8

L. der

SED CPOM

B. dim FPOM

Variations of C:N ratios and stable isotopes in rotifers and POM

A. fus

4 A. cir

0 -28

-23

-18

-13

-8

δ13C (‰) Fig. 4 d13C versus d15N plot of major food web components of L. Nakuru. B. dim = Brachionus dimidiatus, B. hex = Brachionus hexartha; B. cal = Brachionus calyciflora; KER = Keratella spp., L. der = Leptochironomus deribae; A. fus = Arthrospira fusiformis; A. cir = Anabaena circinalis; Det = Detritus; POM = Particulate organic matter. O. gra = Oreochromis alcalicus grahami

Accordingly, the seasonal variation of rotifer C-biomass exhibited an apparent bimodal pattern, with a noticeable increase during the long rainy season between April and May and during short rainy season between October and November. During all seasons in 2008 in Lake Nakuru, the percentage of C contained in DW were significantly (One-Way ANOVA; P \ 0.05) different among the rotifer species (Fig. 3). There was an increased C content recorded in the rotifer species with increased DW at all seasons. The highest mean C content was measured in the

123

The mean C:N ratio, d13C and d15N of rotifer species relative to phytoplankton, POM sediment and CPOM is shown in Table 4 and Fig. 4. Generally rotifers had lower mean d13C than FPOM, CPOM and fish, but higher than mean d13C in the dominant phytoplankton species i.e. Arthrospira fusiformis and Anabaena circinalis. The d13C content of B. dimidiatus was higher than other rotifer species. Concerning nitrogen assimilation, rotifers had higher d15N than phytoplankton, FPOM, CPOM and sediments. Only a copepod (Leptochironomus deribae) and Oreochromis alcalicas grahamii had higher d15N. As for C:N ratio, all rotifers had a lower C:N ratio as compared to the phytoplankton, FPOM, sediments and CPOM but higher C:N ratio than copepod and fish. It is worth noting that the striking patterns were observed in March–May, when quasi-concomitant shifts from relatively low-to-high values of C:N, d13C and d15N was observed in every rotifer species (Fig. 5). These shifts, were also found to be, significantly different (P \ 0.05) between mean seasonal elemental or stable isotopic ratios of the individual rotifer species. Linear

Author's personal copy Hydrobiologia Short rainy season

Long rainy season

15 12

C:N (%)

BRA_D BRA_H BRA_C KER

9 6 3 0 -15 -17

δ13C (‰)

-19

BRA_D

-21

BRA_H BRA_C

-23

KER

-25

δ15N (‰)

-27 12 11 10 9 8 7 6 5

BRA_D BRA_H BRA_C KER

Jan Feb Mar Apr MayJun Jul Aug Sep Oct Nov Dec

2008 Fig. 5 Time series of C:N ratios, d13C and d15N for each of the studied rotifer species in the Lake Nakuru from January to December 2008. This data set represents pooled results from analyses on inlet and offshore areas in order to present the mean isotopic signal of rotifer species that dominated the biomass. The vertical grey bars depict the standard error of the mean

regression plots between d13C and d15N in phytoplankton, FPOM, CPOM and sediments and the rotifer species in Lake Nakuru is shown in Fig. 6. There were contrasting patterns of elemental enrichment of d13C and d15N in rotifers relative to the phytoplankton, FPOM, sediments and CPOM. Enrichment of d13C in all rotifer species significantly (regression analysis; P \ 0.05, R2 [ 0.56) increased with increasing enrichment of d13C in phytoplankton and FPOM. On the other hand enrichment of d15N in all species of rotifers increased significantly (P \ 0.05, R2 [ 0.80) in concomitance with increased d15N in sediments and CPOM.

Discussion Rotifers are important members of zooplankton communities in several aquatic ecosystems and play an

important role in energy flow in the aquatic ecosystem as primary consumers (Walz, 1995; Dumont, 1977; Garcıˆa et al., 2009; Sartori et al., 2009). In eutrophic, saline–alkaline lakes, they appear to thrive well, where, for example, they may reach population densities as high as 20,000 ind l-1 (Gulati et al., 1992; Ooms-Wilms et al., 1999; Bonecker & Aoyagui, 2005). Such population dynamics may render them useful contributor to the trophic food web in their environments. Yet, in many alkaline–saline environments, the contribution of rotifers to carbon and nitrogen sources remains less studied. In the present study, rotifer occurred in high abundance, with peak abundance up to 80,000–100,000 9 103 ind m-3 comparable to values obtained earlier in the sample lake (Oyoo-Okoth et al., 2011). The high rotifers abundance and biomass measured in Lake Nakuru in 2008 was probably the result of the enhanced primary production and stable surface temperature as previously reported (Oyoo-Okoth et al., 2011). Rotifers are also able to reach high population densities quickly, especially when large, competitively superior zooplankton species, such as Daphnia, are rare or absent (Gilabert, 2001). Brachionus was the most dominant rotifer in the lake during the study period. In the tropical environment, Brachionus species dominate and can make up 50–95% of the zooplankton community (Harper et al., 2003). Dominance of Brachionus species is consistent with the observations in most saline waterbodies worldwide (Ruttner-Kolisko, 1974), mainly because they are generalist suspension feeders and are therefore presented with wide food sources such as microalgae (Gilbert & Bogdan, 1984). the elevated abundance of phytoplankton in Lake Nakuru (Okoth et al., 2009), may contribute to the higher abundance of Brachionus in the lake. However, there was a rapid shift in rotifers abundance has been reported previously in this lake (Oyoo-Okoth et al. 2011) and this was linked to inorganic nutrients (Harper et al., 2003) as a result of long rainy and short rainy periods and also due to onset of phytoplankton production in the lake. The feeding spectrum of Brachionus is assumed to be concentrated on smaller size classes (Rothhaupt, 1990). Studies of temporal populations of B. plicatilis defined an optimum range of food size between 5 and 10 lm (Hansen et al., 1997), suggesting nanoplanktivory. However, it is not clear how much of the carbon and nitrogen nutrients obtained from the phytoplankton are assimilated by the rotifers.

123

Author's personal copy Hydrobiologia

δ15N

δ13C B. dimidiatus

0 -22 -5

-21

15 -20

-19

-18

-17

-16

R2 = 0.350

-10 -15

R2 = 0.398

-20

R2 = 0.871 2 R = 0.778

-15

12

R2 = 0.928

SED CPOM FPOM

9

R2 = 0.866

A_CIR A_FUS

3

R2 = 0.825

-25

B. hexartha -20.5

-19.5

-18.5

R2 = 0.318 R2 = 0.349

-10 -15 -20

R2 = 0.683 R2 = 0.605

-25

R2 = 0.729

-17.5 SED CPOM FPOM A_CIR A_FUS

-21

R2 = 0.273

-15

R2 = 0.157

-20

R2 = 0.917 R2 = 0.273

6

-19

-23.5

-22.5

R2 = 0.183 2

R = 0.173 R2 = 0.793

-20 -25

7

8

9

10

11

12

SED R2 = 0.815

CPOM R2 = 0.845

9 6

R2 = 0.212 R2 = 0.129

3

R2 = 0.087

FPOM A_FUS A_CIR

R2 = 0.717

-21.5

7

8

9

10

Keretalla spp.

15

R2 = 0.579

FPOM A_FUS A_CIR

0

Keretalla spp.

-15

R2 = 0.073

B. calyciflora

6

-10

CPOM

R = 0.123

12

-30

-24.5

12

2

3

SED CPOM FPOM A_CIR A_FUS

R2 = 0.759 R2 = 0.568 R2 = 0.660

0 -25.5 -5

11

0

-10

-25

10

SED

15

-20

9

9

B. calyciflora -22

A_FUS A_CIR

R2 = 0.930

6

-23

8

12

-30 0 -24 -5

FPOM

B. hexartha

15

-21.5

CPOM

0 7

0 -22.5 -5

SED

R2 = 0.306 R2 = 0.079 R2 = 0.140

6

-30

δ13C or δ15N (‰) in rotifer food web

B. dimidiatus

12

SED CPOM FPOM

R2 = 0.850

SED CPOM

9

A_CIR A_FUS

3

R2 = 0.747 R2 = 0.234 R2 = 0.179 R2 = 0.073

FPOM A_FUS A_CIR

6

0 6

-30

δ C (‰) in rotifers 13

7

8

9

10

δ N (‰) in rotifers 15

Fig. 6 Linear regression plots between d13C (left) and d15N (right) in phytoplankton (A_FUS and A_CIR), fine particulate organic matters (FPOM), course particulate organic matters (CPOM) and sediments (SED) and the rotifer species in L.

Nakuru. A_FUS = Arthrospira fusiformis; A_CIR = Anabaena circinalis; DET = Detritus; POM = Particulate organic matter

In the present study, the most abundant species was B. dimidiatus representing 52–60% of the C-biomass (20–80 g C m-3). The pattern of carbon biomass changes appear to reflect the changes with season. During the long rainy season (March–May) and short rainy seasons (September–October) there were significantly higher carbon content in the rotifers, The C

appear to be from the phytoplankton as confirmed by SI analysis, performed on other samples in Lake Nakuru, which disclosed cyanobacteria to be one source of nutrition for suspension feeding rotifer (Burian et al., 2013). During the long rainy season between April and May, the C content from all rotifer species in Lake Nakuru increased significantly over

123

Author's personal copy Hydrobiologia

time. The C of all rotifer species was significantly higher at the inlet sites compared to the offshore sites perhaps due to increased input of allochthonous particulate matter from the incoming rivers. It is is possible that the observed differences in rotifer density between inflow and offshore sites may be due to the differences in water quality parameters at these sites (refer to Oyoo-Okoth et al., 2011). Accordingly, the seasonal variation of rotifer C-biomass exhibited an apparent bimodal pattern, with an increasing trend during the long rainy season between March and May. The simultaneous response of rotifers to increased rainfall and sustained productivity in the long rainy season suggested their overall capacity to cope with environmental changes. This aptitude is presumably a consequence of their inherent plasticity in dealing with various physical gradients and food regimes (e.g. Garcı´a-Roger et al., 2006; Weithoff, 2007; Pagano, 2008; Hartwich et al., 2010). Thus, external factors like rain or strong winds, leading to an input or a resuspension of non-biotic particulate matter, seem to be tightly connected to rotifer peaks. This confirms assumptions made by Vareschi & Jacobs, 1985 that CPOM materials may play an important role in rotifer nutrition. The only potential pelagic predator of rotifers is the omnivorous soda tilapia, Oreochromis alcalicus grahami Boulenger, which normally feeds on filamentous phytoplankton but at times of smallersized algae dominance, it might adopt its feeding habits (Vareschi, 1982) and prey increasingly on rotifers, leading to a top down control of zooplankton. The isotopic fractionation in organism tissue varies according to food consumed from different sources (Adam & Sterner, 2000). In the present study, the isotopic fractionation was quite distinct among species at the different sampling stations. Brachionus dimidiatus displayed a significantly higher level of d13C than other rotifers. Concomitant with this rain-induced transition, marked increase and subsequent declines in the d13C occurred in every rotifer species. The rapid increase in d13C observed in early March to May perhaps reflected the timing of feeding on allochthonous food sources, since the isotopic d13C signatures in rotifers reflected the d13C in phytoplankton and FPOM. This suggests that all rotifers species exploited the new carbon organic matter produced by dominant phytoplankton, FPOM and to some extend allochthonous input of materials into the lake. Also due to increased inflow of inorganic carbon from the

incoming water from the allochthnous sources may enhance uptake of dissolved inorganic carbon by the rotifers from the phytoplankton and particulate organic matters (e.g. Hinga et al., 1994). Alternatively, active metabolic activities, production of tissue and excretion/egestion by rotifers during enhanced growth periods might have the effect to enrich their bodies with carbon (Vanderklift & Ponsard, 2003; Tamelander et al., 2006). The concomitant isotopic enrichment of rotifers in rainy season thus supports the notion that rotifer are adapted to match their growth with pulses of energy, from phytoplankton and FPOM. As a result, the peaks in the C:N ratio of rotifers in late May/early June were presumably a result of the buildup of body reserves following the phytoplankton production. However, the drop in their C:N over June suggests that the rotifer lost part of their reserves during the period of low phytoplankton biomass. A stepwise increase of d15N in aquatic food webs is usually 3–4% per trophic level (Michener & Schell, 1994). Here, the fractionation of d15N in rotifers appeared to concord with changes in nitrogen derived from FPOM and phytoplankton, which may suggest that nitrogen is derived from phytoplankton and old recycled organic materials in the waterbody. Nitrate inflow into the aquatic ecosystem from anthropogenic sources is known to induce a progressive 15N enrichment of the nitrogen pool (Sigman & Casciotti, 2001). The present result therefore suggest that rotifers obtain their carbon and nitrogen sources through different sources in the lake. In SI field studies, zooplankton bulk samples are analysed instead of species specific samples because of methodical difficulties and limited resources. Our study clearly revealed that even very closely related zooplankton species may assign very distinct ecological niches, leading to considerably different SI ratios. The changes in the elemental and stable isotopic composition of rotifers observed in this study reflected variance food sources and their importance in the alkaline saline food web. We thus provide specific relationships between C content and DW of these dominant rotifers species. This should enable more precise estimates of their contribution to C and biomass in the future. In addition, the substantial d13C and d15N variations measured in rotifer illustrated their rapid response to pulsed phytoplankton production. It is likely that such episodic enrichment in the isotopic composition of key zooplankton species

123

Author's personal copy Hydrobiologia

will be transferred to higher trophic levels. Future analyses of food web interactions in the Lake Nakuru ecosystem should therefore be cautious and consider these marked variations in the estimation of trophic levels. Rotifers have been reported to strongly influence abundance and biomass of microorganisms in experiments with natural phytoplankton communities (Agasild & No˜ges, 2007). They cause cascading structural effects of the food chain by stabilizing bacterial and picophytoplankton biomass and reducing numbers of bacterivorous grazers. Even B. dimidiatus, which has been shown to rely at times to a considerable degree on bacterial carbon on its own (Hartwich et al., 2010), has been shown to cause a significant impact in this respect. Overall, the nutritional characteristics of rotifer communities of alkaline–saline lakes might be essential elements in supporting organisms of the higher trophic levels in such ecosystems. We conclude that dominant rotifers in Lake Nakuru may continue to benefit from a longer season of pelagic primary production as induced by changes in phytoplankton biomass, FPOM and allochthonous inputs into the lake and optimal year round environmental conditions that favour the growth and reproduction of the rotifers. However, the monitoring of key zooplankton species is a critical task as thresholds associated with more intense interspecific competition or negative shifts in the food quantity/quality are possible in the context of ongoing rapid changes. Acknowledgments We are especially grateful to the Global Livestock Collaborative Research Support Program (GL-CRSP) partially funded by the United States Agency for International Development (USAID) who funded this project under Grant No. LAG-G-00-96-90015-00. The lead author benefited from additional funds from the Kenyan government through the National Council for Science and Technology (NCST). This work is a joint contribution to the research on Sustainable Management of Watershed (SUMAWA) and Global Livestock Collaborative Research Support Program.

References Adam, T. S. & R. W. Sterner, 2000. The cost of dietary nitrogen content on trophic level 15N enrichment. Limnology and Oceanography 45: 601–607. Agasild, H. & T. No˜ges, 2007. Cladoceran and rotifer grazing on bacteria and phytoplankton in two shallow eutrophic lakes: in situ measurement with fluorescent microspheres. Journal of Plankton Research 27: 1155–1174.

123

Anderson, N. J., E. Jeppesen & M. Sondergaard, 2005. Ecological effects of reduced nutrient loading (oligotrophication) on lakes: an introduction. Freshwater Biology 50: 1589–1593. APHA, 1998. Standard Methods for the Examination of Water and Wastewater. 20th ed. American Public Health Association, American Water Works Association, Water Environment Federation. United Book Press Inc., USA. Arndt, H., 1993. Rotifers as predators on components of the microbial web (bacteria, heterotrophic flagellates, ciliates)—a review. Hydrobiologia 255(256): 231–246. Bonecker, C. & A. S. M. Aoyagui, 2005. Relationships between rotifers, phytoplankton and bacterioplankton in the Corumba´ reservoir, Goia´s State, Brazil. Hydrobiologia 546: 415–421. Bouvy, M., M. Pagano & M. Trousellier, 2001. Effects of a cyanobacterial bloom (Cylindrospermopsis raciborskii) on bacteria and zooplankton communities in Ingazeira reservoir (northeast Brazil). Aquatic Microbial Ecology 25: 215–227. Burian, A., M. Schagerl & A. W. Yasindi, 2013. Microzooplankton feeding behaviour: grazing on the microbial and the classical food web of African soda lakes. Hydrobiologia 710: 61–72. Cabana, G. & J. B. Rasmusen, 1996. Comparison of aquatic food chains using nitrogen isotopes. Proceedings of the National Academy of Sciences, USA 93: 10844–10847. Chick, J. H., A. P. Levchuk, K. A. Medley & J. H. Havel, 2010. Underestimation of rotifer abundance, a much greater problem than previously appreciated. Limnology and Oceanography-Methods 8: 79–87. Curds, C. R., 1982. British and Other Freshwater Ciliated Protozoa, Part I. Ciliophora: Kinetofragminora. Cambridge University Press, Cambridge: 387 pp. Curds, C. R., M. A. Gates & M. C. L. Roberts, 1983. British and Other Freshwater Ciliated Protozoa, Part II: Oligohymnephora and Polyhymnephora. Cambridge University Press, Cambridge. 474 pp. De Niro, M. J. & S. Epstein, 1981. Influence of diet on the distribution of nitrogen isotopes in animals. Geochimica et Cosmochimica Acta 45: 341–351. Dumont, H. J., 1977. Biotic factors in the population dynamics of rotifers. Archives of Hydrobiology 8: 98–122. Echaniz, S. A., A. M. Vignatti, S. Jose´ De Paggi, J. C. Paggi & A. Pilati, 2006. Zooplankton seasonal abundance of South American Saline Shallow Lakes. International Review of Hydrobiology 91: 86–100. Ejsmont-Karabin, J., 1978. Studies on the usefulness of different mesh-size plankton nets for thickening zooplankton. Ekologia Polska 26: 479–490. Finlay, B. J., C. R. Curds, S. S. Bamforth & J. M. Bafort, 1987. Ciliated protozoa and other micro-organisms from 2 soda lakes (Lake Nakuru and Lake Simbi, Kenya). Archiv fu¨r Protistenkunde 133: 81–91. Garcı´a-Roger, E. M., A. Martı´nez & M. Serra, 2006. Starvation tolerance of rotifers produced from parthenogenetic eggs and from diapausing eggs: a life table approach. Journal of Plankton Research 28: 257–265. Garcıˆa, C. E., S. Nandini & S. S. Sarma, 2009. Seasonal dynamics of zooplankton in Lake Huetzalin, Xochimilco (Mexico City, Mexico). Limnologica 39: 283–291.

Author's personal copy Hydrobiologia Ghadouani, A., B. Pinel Alloul, Y. Zhang & E. E. Prepas, 1998. Relationships between zooplankton community structure and phytoplankton in two lime-treated eutrophic hardwater lakes. Freshwater Biology 39: 775–790. Gilabert, J., 2001. Seasonal plankton dynamics in a Mediterranean hypersaline coastal lagoon, the marine Menoroplankton. Journal of Plankton Research 23: 207–217. Gilbert, J. J., 1988. Suppression of rotifer populations by Daphnia: a review of the evidence, the mechanisms, and the effects on zooplankton community structure. Limnology and Oceanography 33: 1286–1303. Gilbert, J. J. & K. G. Bogdan, 1984. Rotifer grazing: in situ studies on selectivity and rates. In Trophic interactions within aquatic ecosystems. AAAS Selected Symposia, 85, AAAS National Annual Meeting Toronto, Canada. Westview, Boulder: 97–133. Gilbert, J. J. & J. D. Jack, 1993. Rotifers as predators on small ciliates. Hydrobiologia 255(256): 247–253. Grant, W. D., 2006. Alkaline environments and biodiversity in extremophiles. UNESCO/Eolss Publishers, Oxford. Gulati, R. D., A. L. Oomswilms, O. F. R. Van Tongeren, G. P. Postema & K. Siewertsen, 1992. The dynamics and role of limnetic zooplankton in Loosdrecht Lakes (the Netherlands). Hydrobiologia 233: 69–86. Haney, J. F. & D. J. Hall, 1973. Sugar-coated Daphnia: a preservation technique for Cladocera. Limnology and Oceanography 18: 331–333. Hansen, B., T. Wernberg-Møller & L. Wittrup, 1997. Particle grazing efficiency and specific growth efficiency of the rotifer Brachionus plicatilis (Muller). Journal of Experimental Marine Biology and Ecology 215: 217–233. Harper, D. M., R. B. Childress, M. M. Harper, R. R. Boar, P. Hickley, S. C. Mills, N. Otieno, T. Drane, E. Vareschi, O. Nasirwa, W. E. Mwatha, J. P. E. C. Darlington & X. Escute-Gasulla, 2003. Aquatic biodiversity and saline lakes: Lake Bogoria National Reserve, Kenya. Hydrobiologia 500: 259–276. Hartwich, M., A. Wacker & G. Weithoff, 2010. Changes in the competitive abilities of two rotifers feeding on mixotrophic flagellates. Journal of Plankton Research 32: 1727–1731. Hecky, R. E. & P. Kilham, 1973. Diatoms in alkaline lakes: ecology and geochemical implications. Limnology and Oceanography 18: 53–71. Hinga, K. R., M. A. Arthur, M. E. O. Pilson & D. Whiticar, 1994. Carbon isotope fractionation by marine phytoplankton in culture: the effects of CO2 concentration, pH, temperature, and species. Global Biogeochemical Cycles 8: 91–102. Jellison, R., Y.S. Zadereev & P. A. DasSarma, 2004. Conservation and management challenges of saline lakes: a review of five experience briefs. Lake Basin manage initiative. Thematic paper 1–28. Jersabek, C. D., H. Segers & J. Morris, 2003. An illustrated online catalog of the rotifera in the Academy of Natural Sciences of Philadelphia (version 1.0). http://rotifer. acnatsci.org/rotifer.php. Accessed 8 April 2003. Leonard, J. A. & H. W. Paerl, 2005. Zooplankton community structure, micro-zooplankton grazing impact, and seston energy content in the St. Johns river system, Florida as influenced by the toxic cyanobacterium Cylindrospermopsis raciborskii. Hydrobiologia 537: 89–97. Logan, J. M., T. D. Jardine, T. J. Miller, S. E. Bunn, R. A. Cunjak & M. E. Lutcavage, 2008. Lipid corrections in

carbon and nitrogen stable isotope analyses: comparison of chemical extraction and modelling methods. Journal of Animal Ecology 77: 838–846. Melack, J. M., 1988. Primary producer dynamics associated with evaporative concentration in a shallow, equatorial soda lake (Lake Elmenteita, Kenya). Hydrobiologia 158: 1–14. Melack, J. M. & P. Kilham, 1974. Photosynthetic rates of phytoplankton in East African alkaline, saline lakes. Limnology and Oceanography 19: 743–755. Michener, R. H. & D. M. Schell, 1994. Stable isotope ratios as tracers in marine aquatic food webs. In Lajtha, K. & R. H. Michener (eds), Stable isotopes in ecology and environmental science. Blackwell Scientific Publications, Oxford: 138–157. Minagawa, M. & E. Wada, 1984. Stepwise enrichment of I5N along food chains: further evidence and the relation between 15N and animal age. Geochimica et Cosmochimica Acta 48: 1135–1140. Nogrady, T., 1983. Succession of planktonic rotifer populations in some lakes of the Eastern Rift-Valley, Kenya. Hydrobiologia 98: 45–54. Oduor, S. O. & M. Schagerl, 2006. Temporal trends of ion contents and nutrients in three Kenyan Rift Valley saline– alkaline lakes and their influence on phytoplankton biomass. Hydrobiologia 584: 59–68. Oduor, S. O. & M. Schagerl, 2007. Phytoplankton primary productivity characteristics in response to photosynthetically active radiation in three Kenyan Rift Valley saline–alkaline lakes. Journal of Plankton Research 29: 1041–1050. Okoth, E. O., M. Mucai, W. A. Shivoga, S. N. Miller, J. Rasowo & C. C. Ngugi, 2009. Spatial and seasonal variations in phytoplankton community structure in alkaline–saline Lake Nakuru, Kenya. Lakes and Reservoirs: Research & Management 14: 57–69. Ong’ondo, G. O., A. W. Yasindi, S. O. Oduor, S. Jost, M. Schagerl, B. Sonntag & J. Boenigk, 2013. Ecology and community structure of ciliated protists in two alkaline– saline Rift Valley lakes in Kenya with special emphasis on Frontonia. Journal of Plankton Research 35: 759–771. Ooms-Wilms, A. L., G. Postema & R. D. Gulati, 1999. Population dynamics of planktonic rotifers in Lake Loosdrecht, the Netherlands, in relation to their potential food and predators. Freshwater Biology 42: 77–97. Oyoo-Okoth, E., M. Muchiri, C. C. Ngugi, E. W. Njenga, V. Ngure, P. S. Orina, E. C. Chemoiwa & B. K. Wanjohi, 2011. Zooplankton partitioning in a tropical alkaline–saline endorheic Lake Nakuru, Kenya: Spatial and temporal trends in relation to the environment. Lakes Reservoir: Research & Management 16: 35–47. Pagano, M., 2008. Feeding of tropical cladocerans (Moina micrura, Diaphanosoma excisum) and rotifer (Brachionus calyciflorus) on natural phytoplankton: effect of phytoplankton size–structure. Journal of Plankton Research 30: 401–414. Peterson, B. J. & B. Fry, 1987. Stable isotopes in ecosystem studies. Annual Review of Ecological System 18: 293–320. Qi, H., T. B. Coplen, H. Geilmann, W. A. Brand & J. K. Bo¨hlke, 2003. Two new organic reference materials for delta 13C and delta 15N measurements and a new value for the delta 13 C of NBS 22 oil. Rapid Communication in Mass Spectrometry 17: 2483–2487.

123

Author's personal copy Hydrobiologia Rothhaupt, K. O., 1990. Changes of the functional responses of the rotifers Brachionus rubens and Brachionus calyciflorus. Limnology and Oceanography 35: 24–32. Ruttner-Kolisko, A., 1974. Plankton rotifers. Biology and taxonomy. English translation of Die Binnengewiisser vol 26, part 1, 146 p, DM46.80. Sartori, L. P., M. G. Nogueira, R. Henry & E. M. Moretto, 2009. Zooplankton fluctuations in Jurumirim Reservoir (Sa˜o Paulo, Brazil): a three-year study. Brazil Journal of Biology 69: 1–18. Sigman, D. M. & K. L. Casciotti, 2001. Nitrogen isotopes in the ocean. In Steele, J. H., S. A. Thorpe & K. K. Turekian (eds), Encyclopedia of ocean sciences. Academic Press, New York: 1884–1894. Small, E. B. & D. H. Lynn, 1985. Phyllum Ciliophora. Doflein. In Lee, J. J., S. H. Hutner & E. C. Bovee (eds), An Illustrated Guide to Protozoa. Society of Protozoology, Lawrence, Kansas: 593–604. Smyntek, P. M., M. A. Teece, K. L. Schulz & S. J. Thackeray, 2007. A standard protocol for stable isotope analysis of zooplankton in aquatic food web research using mass balance correction models. Limnology and Oceanography 52: 2135–2146. Soares, M. C. S., M. Lu¨rling & V. L. M. Huszar, 2010. Responses of the rotifer Brachionus calyciflorus to two tropical toxic cyanobacteria (Cylindrospermopsis raciborskii and Microcystis aeruginosa) in pure and mixed diets with green algae. Journal of Plankton Research 32: 999–1008. Sweeting, C. J., N. V. C. Polunin & S. Jennings, 2006. Effects of chemical lipid extraction and arithmetic lipid correction on stable isotope ratios of fish tissues. Rapid Communication in Mass Spectrometry 20: 595–601. Talling, J. F., A. W. Wood & M. W. Prosser, 1973. The upper limit of photosynthetic productivity by phytoplankton: evidence from Ethiopian soda lakes. Freshwater Biology 3: 53–76.

123

Talling, J. F. & B. Talling, 1965. The chemical composition of African lake waters. International Review of Hydrobiology 50: 1–32. Tamelander, T., P. E. Renaud, H. Hop, M. L. Carroll, W. J. Ambrose Jr & K. A. Hobson, 2006. Trophic relationships and pelagicbenthic coupling during summer in the Barents Sea Marginal Ice Zone revealed by stable carbon and nitrogen isotope measurements. Marine Ecology Progressive Series 310: 33–46. Vander Zanden, M. J. & J. B. Rasmussen, 1999. Primary consumer d15N and d13C and the trophic position of aquatic consumers. Ecology 80: 1395–1404. Vanderklift, M. A. & S. Ponsard, 2003. Sources of variation in consumer-diet d15N enrichment: a meta-analysis. Oecologia 136: 169–182. Vareschi, E., 1982. The ecology of Lake Nakuru (Kenya) III. Abiotic factors and primary production. Oecologia 55: 81–101. Vareschi, E. & J. Jacobs, 1985. The ecology of Lake Nakuru (Kenya) VI. Synopsis of production and energy flow. Oecologia 65: 412–424. Vareschi, E. & A. Vareschi, 1984. The ecology of Lake Nakuru (Kenya) IV. Biomass and distribution of consumer organisms. Oecologia 61: 70–82. Walz, N., 1995. Rotifer populations in plankton communities: energetics and life history strategies. Experientia 51: 437–454. Weithoff, G., 2007. Dietary restriction in two rotifer species: the effect of the length of food deprivation on life span and reproduction. Oecologia 153: 303–308. Williams, W. D., 1996. What future saline lakes? Environment 38: 12–20. Williams, W. D., 2002. Environmental threats to salt lakes and the likely status of inland saline ecosystems in 2025. Environmental Conservation 29: 154–167. Yasindi, A. W., W. D. Taylor & D. H. Lynn, 2007. The trophic position of planktonic ciliates in the food webs of some East African lakes. African Journal of Aquatic Sciences 31: 53–62.