Title
Measuring biomass transfer to herbivorous zooplankton
By
Dörthe C. Müller-Navarra
Key words
Food quality, method, secondary production, aquatic systems, estuarine systems
1
Abstract:
Here I present an approach by which the biomass transfer between seston and a secondary
consumer can be measured under standardized conditions (i.e. temperature, light, condition of
animals). The approach could be applied to different seasons in different systems. Results
show that the ability of a certain seston concentration to sustain secondary production varies
to a large extent, i.e. seston food quality differs greatly. In accordance to the general
knowledge, seston biomass (carbon concentrations) increases with higher trophic state
(nutrient concentrations) of the system. However, the share of carbon which can be used for
growth of the consumer decreased steeply, especially during summer. This points to the fact
that food quality rather than food quantity is important for biomass transfer in nutritious
systems.
When comparing four habitats in an estuary system (San Fransisco Delta), it becomes obvious
that there are large differences among the habitats to which extent sestonic carbon can
promote consumer growth. Most strikingly, river channel habitats partly not even sustain
threshold food concentrations, whereas the natural predominant habitat, the tidal marches,
could provide maximal consumer growth (Müller-Solger et al. 2002). Thus shallow water
habitats, as tidal marshes, have a key role for biomass transfer across the pant animal interface
in those estuarine freshwater systems, with impact on higher trophic levels.
2
Introduction:
Food quality determines the efficiency with which a certain amount of food is converted in
consumer biomass, thus production (i.e.,  biomass / time). Especially between primary and
secondary producers there exist large differences in the efficiency by which organic carbon,
and thus energy is transferred within aquatic food webs, which leads to low predictability of
biomass and energy fluxes to higher trophic levels. The phenomenon is already known for
decades and described as food quality differences, especially important to herbivorous
zooplankton, i.e. at the plant-animal interface.
The composition of a particle - the smallest food unit for phagotrophic organisms as
metazooplankton - can vary, depending on the origin of the particle. Hence, it can be of
different food quality, which can be defined to be the degree of the match (or mismatch)
between the animals’ requirement for potential limiting resource(s), their accessibility and
abundance in the food particle.
In the field variation of food quantity and its quality can take place at the same time and both
can compensate each other in their effect on zooplankton. In addition, other factors which can
co-vary in the field (e.g. temperature) also have an impact on zooplankton performance, like
their growth rates. Therefore, laboratory experiments under standardized conditions are
necessary to separate those forcing variable and be able to estimate and quantify food quality
effects, thus carbon transfer efficiency.
Material and Methods:
3
Sampling and analyses are described in the respective articles (Müller-Navarra and Lampert
1996 for Schöhsee, Müller-Navarra et al. 2000 for Stonegate Pond, Müller-Solger et al. 2002
for San Francisco Delta).
The experiments were run in a temperature controlled room at 20 ºC under constant dim light
condition. In addition to seston, Scenedesmus obliquus was fed as a reference food alga. It
was grown in a chemostat to ensure physiological constant conditions of the food algae.
Daphniids were used, which were obtained from one pathenogenetic female. Thus clonal
variation can be excluded within one experimental trial. The daphniids were raised on S.
obliquus for several generations in the laboratory. Juveniles were used for the experiments
which were not more than 8 hours apart in age and grown on S. obliquus of unlimited food
concentrations for their first 3 days. Thereafter they were transferred to the experimental
vessels. An aliquot of the juveniles were withdrawn to measure the initial length and dry
weight (Wi) of the animals set into experiments. During the experiments, daphniids were fed
either Scenedesmus or seston as food solutions. Seston was collected fresh from the field
every day of experiments. To remove large zooplankton it was screened through a 100 µm
mesh. The flow rate in the flow through vessels was set to 50 ml/h, which is sufficiently high
that the overall filtration of the daphniids in the vessel does not exceed 10% of the total flow
rate. Thus food concentrations in the vessel were nearly constant. Exactly after 4 days of
experiments, the animals were withdrawn. Their lengths were recorded and their dry weight
(Wt) taken on an electronic microbalance, after drying at 60 ºC for at least 12 hours. From the
increase of dry weight over time (t) growth rates (g) were determined for seston fed and
Scenedesmus fed animals, respectively:
g = (ln Wt – ln Wi) / t (day-1)
4
Experiments with Scenedesmus as food were run at various food concentrations to obtain a
growth response curve (see also Müller-Navarra and Lampert 1996). Asymptotic regressions
were fitted to relate the carbon food concentration to the growth rates (STATGRAPHICS).
Both, growth rates on Scenedesmus food (gS) and with seston as food (ga) were determined.
Growth rates obtained with seston (ga) food were related to the Scenedesmus standard growth
curve (r2 = 0.99) to determine a carbon food concentration equivalent to Scenedesmus carbon
(Cs) according to Müller-Navarra and Lampert 1996).
Cs = 0.014 – [ln – ga / 0.44)/ 4.3]
Cs is therefore the Scenedesmus carbon concentration which permits the same growth
measured with seston as food at that time. By relating Cs to the actual sestonic carbon (Ca),
i.e. Cs:Ca, the seston food quality can be quantified (Müller-Navarra and Lampert 1996).
This was done for every growth rate determined with seston food. Afterwards mean seasonal
values were calculated for each system tested.
Results:
Results from three studies are compared, which are: mesotrophic Schöhsee (Fig. 1; MüllerNavarra and Lampert 1996), hypereutrophic Stonegate Pond (Fig. 2; Müller-Navarra et al.
2000), and four habitats in the San Fransisco Delta, and estuary system (Fig. 3 and Fig. 4;
Müller-Solger et al. 2002).
For all permanent aquatic systems, seasonality in respect to transferred carbon can be detected
with higher values during spring and fall/winter than summer. In mesotrophic Schöhsee high
values, between 0.45-0.65 mg CScen/L corresponding to around 100% of seston carbon, were
measured in spring and fall, and 0.22 mg CScen/L (around 55% of seston carbon) during
summer (Fig. 1). Similarly trends were obtained for the shallow eutrophic Mildred Station (a
flooded Island) for which high values (0.55 mg CScen/L and 1.32 mg CScen/L at ca. 100%
5
seston quality) were recorded in spring and fall, respectively (Fig. 4a). Low values (0.11 mg
CScen/L relating to 15%), below the threshold for egg production, were measured over
summer. Even more pronounced seasonal differences were present in the hypereutrophic
Stonegate Pond with values around the Incipient limiting level (ILL) of 0.68 mg CScen/L (19%
of seston carbon) in spring to saturating levels (1.0 mg CScen/L; 59% of the seston carbon) in
fall/winter and very low values (0.08 mg CScen/L; 1% of the carbon present), below the
threshold for egg production, during summer (Fig. 2). Comparing hypereutrophic Stonegate
Pond, eutrophic Mildred Pond, and mesotrophic Schöhsee reveals that summer values were
especially different between the systems of different trophy. A several fold increase of seston
carbon could be observed (0.4 mgC/L in Schöhsee, 0.75 mgC/L at Mildred, and 6.87 mgC/L
in Stonegate Pond), but food limitation due to insufficient food quality of the seston becomes
increasingly important during summer with higher trophy of the system. In contrast, tidal
marsh habitats of the San Francisco Delta (S.F. Delta), although of high seston carbon (2.7
mgC/L in summer), could always sustain daphnids growth close to the ILL (0.6 mg CScen/L in
summer and fall) or above (1 mg CScen/L in winter and spring) (Fig. 3b). In contrast, seston
from river channel habitats could mostly not even sustain egg production despite high seston
carbon concentrations of around 1 mg C/L. Only weak seasonal differences, between 0.06 mg
CScen/L (7% of the carbon) in summer and 0.19 mg CScen/L (17% of the carbon present) in
spring and fall, were detected in the river channel habitats of the S.F. Delta (Fig. 3a).
Discussion:
The experimental trials gave important information about biomass transfer between primary
producers and consumers in freshwater systems, including seston food quality. Seasonal
trends in the degree to which seston carbon can sustain consumer production exist,
irrespectively of the productivity of the system. Summer seston was always of lower quality
than seston from winter or spring, with values lower than the ILL. Differences in the seston
6
quantity but also quality exist mainly during summer. At increasing nutrient and seston
carbon levels transferred carbon doesn’t necessary increase accordingly but even decreases
substantially leading to levels lower than the threshold for egg production. For example,
comparing mesotrophic Schöhsee and hypereutrophic Stonegate Pond revealed that during
summer, the actual carbon concentration was almost 20 fold higher in Stonegate Pond but the
carbon which could be converted into zooplankton growth was not even 30% from that
measured in Schöhsee. Interestingly, during spring, fall and winter trophic transfer was
similar among those systems irrespectively of higher particulate organic carbon
concentrations in the more eutrophic systems. In mesotrophic Schöhsee seston carbon
concentrations (i.e. food quantity) constrained carbon transfer during spring and fall. As
seston carbon was higher in the more productive systems, food quality constrained trophic
transfer even between fall and spring, leading to lower relative carbon transfer.
When comparing lacustrine habitats it becomes obvious that they are very different in
sustaining secondary production. River channel habitats could hardly reach food levels
necessary for egg production of daphniids. In contrast, the native shallow water habitats as
tidal marshes and flooded areas could reach saturating concentrations of transferred carbon.
Those overall differences are larger than observed among lake systems of very different
trophy.
The presented comparison shows that the method is applicable to measure biomass transfer
between respective trophic levels. It gives a descriptive measure in that sense that it includes
all features which can affect food quality, like ingestiblility, assimilation properties,
nutritional properties and toxic effects of seston.
7
The growth rates used to calculate the food quality have to come from standardized growth
experiments in which natural food (seston) is used as food. The following considerations are
made to underpin strengths, prerequisites and potential pitfalls in the application of the
approach.
a) Standardization
Standardization of the experiments is extremely important to obtain results which can be
compared among several trials. Especially constant temperatures are of primary importance as
food quality is retrieved from a vital rate measurement, i.e. growth rate measurements. A
constant food concentration has also to be ensured as food quality and trophic transfer is
determined from growth rates which also depend on food quantity. The flow through
experimental design is therefore necessary to study food quality effects. In addition,
standardization of parameters like pre-feeding conditions, age, time in the life cycle, and light
conditions is also important.
Not only the experimental conditions but also the experimental organism (reference alga and
consumer) has to be standardized. The seston growth rates were related to a standard growth
curve of a specific algae (Scenedesmus) to determine the measure of food quality (Cs:Ca).
Hence they cannot be used as absolute values. Food quality could also be related to any other
alga which can be cultivated and be used as food in growth experiments. Seston food quality
above 100% even shows that Scenedesmus is not the most nutritious algal food. However, as
most of the existing data on Daphnia feeding, growth and life history were performed with
Scenedesmus as food, the results can well be compared to that existing body of literature.
Daphnia is suitable as the reference consumer as it is a not actively selective filter feeder and
can easily been cultured. Its role as a key stone species in many freshwater systems is relevant
for the applicability of the results to the field situation. The parthenogenetic mode of
8
reproduction even enables genetically identical experimental animals. The applicability to
zooplankton from other groups with different life history adaptations, e.g. copepods still has
to be studied in more detail.
As the reference measured variable we choose growth rates obtained from dry weight
increases over the experimental duration. Growth rates have several advantages, e.g. over egg
production as a measure of daphniids response to changing food conditions. Below the
threshold for egg production, growth rates determined from dry weight changes can still be
determined. In addition, dry weight measurements may be more accurately achieved than egg
production measures (clutch size determinations). A connection between growth rates
obtained that way and fitness could be shown by Trubetchova and Lampert 19??.
b) Applicability to field situation
Standardization of the experiments is important but may constrain the applicability of the
obtained results to the field situation. In contrast to the Daphnia cohort used in the
experiment, a natural Daphnia population exists of animals of different clons, age and feeding
pre history. In addition, experimental data can only give integrated measures of the food
situation, as they can only average over the experimental duration. Hence, it is advised to
keep the experimental duration as short as possible to avoid large variation in seston during a
single growth rate determination. Temperature may also be a factor altering zooplankton
growth in the field, especially when considering that zooplankton migrates vertically thus
experiencing a wide range of temperatures. However, it should not affect the food quality
determination as both Cs and Ca are obtained at the same temperature, unless temperature
changes Cs and Ca differently. Overall, however, it could be shown that the growth rates and
egg production obtained in the experiments feeding on seston well correlated with clutch sizes
obtained from field daphnids collected during the same time period (r2 = 0.53 for growth rates
9
and r2 = 0.41 for egg production, respectively; Fuhlendorf 1991). Thus when feeding seston
under laboratory conditions growth and egg production reflects the food situation experienced
from field animals to a large extent, also emphasizing the importance of food limitation as a
predominant factor zooplankton is facing in the field.
Conclusions and Application:
A coupled lab field approach is necessary to estimate biomass transfer as food quality cannot
directly measured in the field directly. The standardized approach could show that biomass
transfer is different, depending on the season and the trophy of the system, especially during
summer. In addition, habitat characteristics are important for the extent to which seston
biomass is transferred to higher trophic levels. For example, in the San Fransisco Delta only
the shallow water habitats of the tidal marshes or flooded areas could sustain saturating food
conditions, especially during spring. This points to the predominant role of those habitats for
trophic transfer in lacustrine/riverine systems.
10
References:
Fuhlendorf, D.C. 1991. Measuring seasonalpatterns of food limitation in Daphnia – an
experimental study. Verh. Internat. Verein. Limnol. 24: 2792-2793.
Müller-Navarra, D.C. & W. Lampert. 1996. Seasonal patterns of food limitation in
Daphnia galeata: separating food quantity and food quality effects. J. Plankt.
Res. 18: 1137-1157.
Müller-Navarra, D.C, M.T. Brett, A.M. Liston & C.R. Goldman. 2000. A highly
unsaturated fatty acid predicts carbon transfer between primary producers and
consumers. Nature 403: 74-77.
Müller-Solger, A., A.D. Jassby & D.C. Müller-Navarra. 2002. Seasonal and habitatspecific variation in the nutritional value of particulate organic matter in
the Sacramento-San Joaquin Delta for Daphnia growth. Limnol.
Oceanogr. 47: 1468-1476.
Trubetskova, I. & W. Lampert. 1995. Egg size and egg mass of Daphnia magna: response to
food availability. Hydrobiologia 307: 139-145.
11
Figure 1: Seasonal averages of seston carbon concentrations (dark grey) and the Scenedesmus
equivalent carbon concentration (light grey) from Schöhsee 1988. Data are from MüllerNavarra and Lampert 1996.
Figure 2: Seasonal averages of seston carbon concentrations (dark grey) and the Scenedesmus
equivalent carbon concentration (light grey) from Stonegate Pond 1997. See Y-axis is
different from Fig. 1. Data are from Müller-Navarra et al. 2000.
Figure 3: Seasonal averages of seston carbon concentrations (dark grey) and the Scenedesmus
equivalent carbon concentration (light grey) from San Francisco Delta habitats 2000. 3a
(above) denotes the river channel habitats and 3b (below) the tidal marsh habitats. Data are
from Müller-Solger et al. 2002.
Figure 4: Seasonal averages of seston carbon concentrations (dark grey) and the Scenedesmus
equivalent carbon concentration (light grey) from San Francisco Delta habitats 2000. 4a
(above) denotes a shallow lake habitat (flood area) and 4b (below) flood plains which are only
aquatic during winter and spring. Data are from Müller-Solger et al. 2002.
12
SCHÖHSEE
Carbon Conc. (mg*l-1)
1
0,5
0
Spring
Early
Summer
Summer
Fig. 1
13
Fall
STONEGATE
Carbon Conc. (mg*l-1)
8
6
4
2
0
Spring
Summer
Fall/Winter
Fig. 2
14
RIVER
-1
Carbon Conc. (mg*l )
3
2
1
0
Winter
Spring
Summer
Fall
Fig. 3a
TIDAL MARSH
-1
Carbon Conc. (mg*l )
3
2
1
0
Winter
Spring
Summer
Fig. 3b
15
Fall
MILDRED LAKE
-1
Carbon Conc. (mg*l )
3
2
1
0
Winter
Spring
Summer
Fall
Fig. 4a
FLOOD PLAIN
-1
Carbon Conc. (mg*l )
3
2
1
0
Winter
Spring
Summer
Fig. 4b
16
Fall