Supplementary information
Text S1: Plankton counts and biomass conversions
Bacterial, protozoan, phytoplankton (>10 µm) and rotifer samples were sampled and counted
separately. Bacterial samples were fixed (5% formalin), stained using the SYBR Gold
technique (Tuma et al., 1998) and counted under a compound microscope (Motic BA 400,
Nikon, Tokyo) equipped with an epifluorescence device. Phytoplankton samples were fixed
(5% formalin) and counted using an inverted microscope (Nikon Diaphot, Nikon, Tokyo)
according to Utermöhl (1958). Two magnifications, one for pico and nanoplankton (1000x)
and one for larger forms (200x) were used. Protozoan samples were fixed with Bouin’s
solution (5%) and stained using the Quantitative Protargol Staining Technique (QPS) by
Montagnes & Lynn (1993) to facilitate counting under a compound microscope. Rotifer
samples were concentrated with a 50 µm sieve (smaller mesh sizes led to clogging) and fixed
with 5% formalin. Individuals and eggs were counted under an inverted microscope (200x).
Biovolumes of the various taxa were estimated using geometric formulae of the shapes of the
respective plankton cells (Sun & Liu, 2003). Group specific conversion factors were used
(Table S1).
Table S1 Conversion factors used to calculate C content of various biota.
Taxa
Conversion factors
Heterotrophic bacteria cell number [ind] : C content [ pg ] = 25
Cyanobacteria
biovolume [mm3] : C content [mg] = 0.22
Protist plankton
log C [pg cell-1] = log -0.665 + 0.939 x log V [µm³]
Bacillariophyceae
log C [pg cell-1] = -0.610 + 0.892 x (log plasma V [µm3])
Rotifers
% C [dry mass] = 7.8 x V [106 µm]-0.37
Reference
Bell (1993)
Ahlgren (1983)
Menden-Deuer and
Lessard (2000)
Strathmann (1967)
After Telesh et al.
(1998)
Text S2: Calibration of mixing models: Stable isotope fractionation
Variation in fractionation factors can substantially affect the outcome of mixing models
(McCutchan et al., 2003), and the use of group-specific rather than general factors is
recommended (Vanderklift & Ponsard, 2003). For N fractionation (Δ15N) in rotifers, only one
laboratory study has been conducted so far suggesting a Δ15N of 2 ‰ for Brachionus plicatilis
feeding on phytoplankton (McClelland & Montoya, 2002). This value agrees with Δ15N
suggested by Vanderklift and Ponsard (2003) for ammonia-excreting invertebrates and we
therefore used it for phytoplankton food sources. As McClelland and Montoya (2002) do not
report uncertainty values of their measurements, we used the standard error of 0.19 as
provided by Vanderklift and Ponsard (2003) instead. Detritus-feeding organisms, however,
are known to have a significantly lower Δ15N than herbivores (Vanderklift & Ponsard, 2003,
Matthews & Mazumder, 2008). Because no data for detritus-feeding rotifers exist, we used
0.5 ± 0.7 ‰ as Δ15N for bacterial and detrital food sources, as suggested for detritus-feeding
organisms (Vanderklift & Ponsard, 2003).
For carbon fractionation (Δ13C) also no rotifer-specific data were available. We therefore used
0.3 ± 0.14 ‰ as suggested by (McCutchan et al., 2003) for invertebrates. This is in line with
values suggested by Grey et al. (2001) for crustacean zooplankton and with other reviews
about variations of Δ13C (Vander Zanden & Rasmussen, 2001, Post, 2002).
Table S2 Isotopic signatures of δ13C, δ15N and C:N ratios of major food web components of
Lake Nakuru (April 7, 2009). Isotopic ratios are given in ‰, C:N ratios in %
group/taxon
DOM
Sediment
Allochthonous matter
Macrophytes
Cyanobacteria ≥40 µm
20<40 µm
2<20 µm
<2µm
Heterotrophic bacteria
Arthrospira fusiformis
Anabaenopsis elenkinii
Brachionus dimidiatus
Brachionus plicatilis
Ephydra sp.
n
6
6
3
3
6
6
6
6
*
*
*
3
3
2
Oreochromis a. grahami 3
Corixidae
6
Leptochironomus deribae 5
Lepidoptera
2
* calculated values, see text
mean δ13C
-17.83
-18.28
-8.79
-12.04
-22.34
-21.40
-20.17
-19.46
-16.83
-23.29
-15.76
-16.47
-20.55
-16.85
-16.80
-15.40
-17.80
-16.90
SD δ13C
0.07
0.48
0.31
0.46
0.06
0.89
0.59
0.60
0.36
0.51
0.14
0.08
0.27
0.21
0.45
0.86
2.44
mean δ15N
6.13
10.63
8.53
-2.78
4.30
4.01
4.42
6.70
9.53
4.24
4.24
10.14
8.65
4.53
6.25
8.01
9.75
-
SD δ15N
0.09
0.32
0.22
0.52
0.23
0.85
0.64
1.16
0.62
0.62
0.91
1.07
0.90
0.47
0.72
0.32
-
mean C:N
13.92
8.37
26.11
19.70
4.91
4..1
5.64
5.50
5.48
5.05
4.31
3.47
4.16
4.48
-
SD C:N
0.24
0.47
3.24
0.91
0.07
0.62
0.25
0.21
0.06
0.13
014
0.09
0.15
0.28
-
Text S3: Separation of cyanobacteria stable isotope signatures via an inverted endmember mixing model
This calculation approach is based on the assumptions that the difference in the cyanobacterial
δ13C values is caused by generic differences between the two cyanobacteria. Vuorio et al.
(2009) showed that there also exist isotopic differences depending on the size of colonies.
However, we believe for two reasons that size is not a main factor for isotopic differences in
our study: First, changes of δ13C with colony size are dependent on changes of the surface/
volume ratio of the colony. But in contrast to spherical colonies, spiral-formed cyanobacteria
increase in length, which results in a minimal change of thfe surface/ volume ratio.
Second, if size was the decisive factor, we would argue that it would, based on the same
physical principle, affect both N and C isotopes. But we found only a significant change in
δ13C values of the two cyanobacteria, suggesting that the observed isotopic differences are
rather derived from real differences between the two algae species and are not dependent on
changes of δ13C values with changes in colony size.
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