Proc. Natl. Acad. Sci. USA
Vol. 93, pp. 8465-8469, August 1996
Ecology
Regulation of herbivore growth by the balance of light and nutrients JOTARO URABE* AND ROBERT W. STERNER Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, MN 55108
Communicated by Eville Gorham, University of Minnesota, St. Paul, MN, April 19, 1996 (received for review November 11, 1995)
proportional to P supply/algal biomass. This ratio implies that above a certain light intensity where P limitation becomes increasingly severe, the algal P/C ratio would start to decline (3, 4). As a result, the algal P/C ratio is expected to reach a low value at high light. Herbivore responses were hypothesized based on ingestion of C and P (Fig. IB). Because algal C content varies only slightly with growth conditions (18), C ingested by the herbivore per unit time (Ic) would be proportional to the rate of ingestion of algal cells, which we took to be a rectilinear functional response (19). P ingestion per unit time (Ip) is equal to Ic multiplied by the algal P/C ratio. The small plateau of Ip in Fig. 1B is due to our assumption that the light level separating light limitation from combined limitation by light and P is less than the light level causing algal biomass to satiate the herbivore's functional response. Depending on the response of algae to given light and nutrients regimes relative to the functional response of herbivores, alternate configurations without a plateau are possible. The critical feature here is that Ip is expected to reach a maximum level at an intermediate light intensity due to the difference in the direction of responseto light intensity between the algal biomass and P/C ratio. The net production of carbon by herbivores is given by the balance of assimilated carbon minus metabolic loss (mainly respiration). However, if Ip is too low compared with Ic, the carbon net production may be lower than otherwise expected. Under such a condition, the carbon net production would be a product of net P intake divided by the P/C ratio of the body tissue. Because the P/C ratio of herbivore biomass is constant (20, 21) and because P excretion approaches zero when herbivores ingest food with low P/C ratio (22), herbivore growth rate in carbon units (G) can be expressed as
ABSTRACT Experiments using planktonic organisms revealed that the balance of radiant energy and available nutrients regulated herbivore growth rates through their effects on abundance and chemical composition of primary producers. Both algae and herbivores were energy limited at low light/nutrient ratios, but both were nutrient limited at high light/nutrient ratios. Herbivore growth increased with increasing light intensity at low values of the light/nutrient ratio due to increases in algal biomass, but growth decreased with increasing light at a high light/nutrient ratio due to decreases in algal quality. Herbivore production therefore was maximal at intermediate levels of the light/nutrient ratio. The results contribute to an understanding of mass transfer mechanisms in ecosystems and illustrate the importance of integration of energy-based and material-based currencies in ecology.
Both light and nutrients are essential in sustaining ecosystems, but very little is known about how relative changes in these abiotic factors extend into food chains (1, 2). Plants use solar radiation to fix carbon while they acquire nutrients at appropriate rates to maintain their biological integrity. However, photosynthesis and nutrient uptake are not perfectly coupled, and thus the contents of bioelements relative to carbon (C) in plant biomass vary within species (3-6). Because foraging and growth of many herbivore species respond to the chemical composition of their diet (7-10), the balance between photosynthesis and nutrient uptake may in turn regulate herbivores through the interplay of food quantity and quality. In this report, we test the hypothesis that herbivore growth is dependent on the light/nutrient balance supplied to laboratory ecosystems. We focused on phosphorus (P) as a limiting nutrient because algal growth is frequently limited by P in freshwater systems (11, 12), and because the algal P/C ratio has been most strongly implicated in regulating planktonic herbivores (1317). We first considered the likely responses of algae to light intensity for a given P supply with a moderate but constant loss rate (Fig. 1A). Here, we expressed the response of algae by a rectilinear form to show the essence of trends and qualitative differences along the light gradient. Precise response to light and nutrients would depend on the identity of the algal species and other environmental factors. At low light, algal growth should be limited by irradiance such that algal biomass increases with light intensity. At high light, algal growth should be limited by finite P and algal biomass should reach a plateau. At extremely high light, algal growth may decrease due to photoinhibition, but Fig. 1A assumes light is below the photoinhibition point. The response of algal P/C ratio is also shown in Fig. 1A. At low light, the algal P/C ratio is expected to be high, close to the Redfield ratio (0.0094 by atoms), because algal growth is limited by irradiance alone and thus P supply is sufficient relative to algal biomass. Because the P supply rate is constant, per capita P availability depends on algal biomass and the algal P/C ratio is expected to be
G = min[Ic x ac - ,3, Ip x
ap/Zp/c]
[1]
where ac and ap are production efficiencies for C and P, Zp/c is the P/C ratio of the herbivore, and C is the metabolic loss rate of C (respiration). As an example, we show the response of G to light intensity by setting 0.8 for ac and ap and 5% of a maximum Ic for X3 (Fig. 1B). Eq. 1 suggests that herbivore growth will decrease with increasing light if they cannot compensate for decreased algal P/C by increasing P production efficiency, which of course must be the case at 100% production. Thus, herbivore growth may be maximal at intermediate light intensity at the point where algal composition becomes deficient in P relative to herbivore demands. Furthermore, the light intensity where herbivores show maximal growth rate may decrease with decreasing P supply rate, because the algal P/C ratio at a given light intensity is expected to be lower at lower P supply rate. Abbreviations: C, carbon; P, phosphorus; N, nitrogen. *To whom reprint requests should be sent at the present address: Center for Ecological Research, Kyoto University, Shimosakamoto
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
4-1-23, Otsu, 520-01, Japan. e-mail:
[email protected].
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Proc. Natl. Acad. Sci. USA 93
A Light
Nutrient limitation
limitation
Light intensity FIG. 1. Qualitative model showing responses of algal biomass and P/C ratio (A) and herbivore ingestion and growth rate (B) to changes in light intensity. We assumed that all chemical elements besides P are not limiting and that algae suffer from a moderate but constant loss rate. The model does not incorporate feedbacks from the herbivore to algal biomass or physiology. The scale of they axis differed among the parameters and was not specified. Herbivore growth was found using Eq. 1 with ac = ap = 0.8, and 5% of a maximum Ic for 13-
MATERIALS AND METHODS To test this set of predictions, we performed semibatch culture experiments using the alga Scenedesmus acutus (Chlorophyta) as a resource and Daphnia obtusa (Crustacea, Cladocera) as an
herbivore. S. acutus and D. obtusa were obtained from stock cultures maintained for >4 yr under constant lab conditions (23). We used several P concentrations and light intensities typically found in freshwater lakes (24). The semibatch cultures were initiated by inoculating the algae into flasks containing 1 liter of growth medium. We used COMBO mediumt, which supports long-term growth of both phyto- and zooplankton. Nutrient concentrations were adjusted by adding the desired concentration of P as K2HPO4 and nitrogen (N) as NaNO3. The N/P ratio was held at 80:1 (molar), so that N was sufficient relative to P. Light was provided by cool-white fluorescent bulbs, and light intensity was adjusted by a black window screen placed over the bulbs. Light intensity was measured using a Li-Cor (Lincoln, NB) quantameter (LI-1000 with 2ir collector) placed just outside of the experimental flasks. Flasks were shaken once per day by hand to homogenize the culture suspension. Every 2 days, 25% of the culture suspension was replaced by fresh growth medium. Algal density reached near maximum level within 6 to 8 d, after which 20 neonates of D.
*Kilham, S., Annual Meeting of the American Society of Limnology and Oceanography, June 11-15, 1995, Nevada.
(1996)
obtusa born within a 12-hr time span from the second clutch of the maternal individuals were placed into each flask. Neonate dry mass was 1.80 ,ug (SD, 0.05). Because D. obtusa initiates reproduction within 1 week under preferable food conditions and because it is difficult to quantify individual growth rate (somatic growth + reproduction rates) after release of offspring, we incubated Daphnia for 6 days and then measured their dry mass for estimation of growth rate. During the 6-day run, algae continued to be diluted every 2 days but animals were not diluted. Potential complications that could invalidate our results include the possibility for interference or blocking in food collection by very high food density (25) or a direct inhibitory effect of high light on animal performance. To overcome such difficulties, a second experiment was performed with an "adjusted" treatment and effects of food quality and quantity were separately assessed. In this experiment, semibatch algal cultures with six different nutrient concentrations (N/P ratio = 80:1) were established and maintained under the high (260 ,uE m-2.s-) and low (12 ,uE m-2.s-) irradiance as mentioned above. These treatments were used as controls. In parallel with these treatments, algal suspensions of adjusted treatments were made every 2 days by adding 100 ml of algal suspension from the high-light treatments to 900 ml COMBO medium without P and N, and placed at the low irradiance. Thus, animals in the adjusted treatments were offered food of similar composition to the high-light treatment, but algal biomass was reduced 10-fold. Twenty neonates born within 12 h were introduced to each treatment and body mass on day 6 was measured. C and P contents of algae were examined using 250-ml culture suspensions collected for replacement at 2-day intervals while D. obtusa was incubated. Known aliquots of the suspension were filtered onto precombusted glass fiber filters and analyzed for algal P content by spectrophotometric means after oxidation by persulfate (26), and analyzed for C content using a Perkin-Elmer model 2400 CHN analyzer. Animals in each treatment were pooled into samples of 3 to 5 individuals, placed in preweighed aluminum boats, and dried at 60°C overnight. Dry mass was measured with a Mettler model UMT2 microbalance.
RESULTS AND DISCUSSION Both algae and herbivores showed responses consistent with our predictions (Fig. 2). In all nutrient concentrations, algal abundance increased with light intensity, and at high light, algal abundance also increased with increasing phosphorus concentration. The algal P/C ratio was high at low light, being similar to or somewhat higher than the Redfield ratio at high P supply, and decreased with increasing irradiance. These quantitative and qualitative changes in the algae show that with increasing light intensity, factors limiting algal growth shifted from light, which controls C uptake through photosynthesis, to P availability, which limits P uptake. At all P concentrations, body mass of 6-day-old D. obtusa was greatest at intermediate light/phosphorus ratios (Fig. 2). In our experiments, body mass is a good indicator of production rate because in no cases had animals released any offspring. Both maximum body mass and the ligh-t intensity at which this maximum was reached were greater in higher nutrient treatments. Low growth rates of herbivores at low light can be explained by low algal biomass. However, we cannot explain lowered herbivore growth at high light intensity in the same way, because algal biomass was similar to, or even higher than that at, intermediate light. Previous studies estimated a threshold food P/C ratio of =0.0032 below which net production of Daphnia would be P limited even if they assimilate 100% of the P in the food (17). Daphnia in the experiments showed a decreased growth rate when the algal P/C ratio was less than this threshold. Thus, it
Proc. Natl. Acad. Sci. USA 93 (1996)
Ecology: Urabe and Sterner 3 0
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FIG. 2. Responses of algae and herbivores to varying light intensity at different concentrations of phosphorus. Algal biomass (0) and P/C atomic ratio (a) are mean values during the incubation of the herbivore consumer. Herbivore growth is given as body mass at age 6 days (O). Error bars indicate standard deviation. The dashed horizontal line is the food P/C ratio below which the growth rate of the herbivore is expected to be limited by P rather than by C (17). Algal biomass, algal P/C ratio, and herbivore body mass differed significantly among the light intensities in all nutrient concentrations (P < 0.001, data log transformed). Maximum body mass at each nutrient concentrations was also significantly different among the nutrient concentrations (F3,12 = 37.6, P < 0.001).
is most likely that the lower herbivore growth at high light was due to low P content in the food. In the second experiment, in addition to low- and high-light treatments, we measured Daphnia growth in an "adjusted" treatment where algae from the high-light treatments were diluted and offered to the herbivores at the low-light level. Regardless of nutrient concentrations, algal abundance in the adjusted treatment was close to the low-light treatment (Fig. 3A). On the other hand, the algal P/C ratio in the adjusted treatment was much lower than that in the low-light treatment but was almost the same as the high-light treatment even after 2 days incubation (Fig. 3B). We found little or no difference in animal mass at any nutrient concentrations between the highlight and the adjusted treatments (Fig. 3C), indicating that the interference effect of high algal biomass was inconsequential. Furthermore, these results reject the possibility of direct effects of light intensity on animal performance due to the fact
that animals at low light, given food with low P/C ratio, still showed low production rates. Again, herbivore growth was high in foods with a P/C ratio greater than the calculated threshold for P limitation. In contrast, herbivore growth was low in foods with a P/C ratio below this threshold. Thus, quality was far more important than quantity in determining animal production at high food biomass. However, when the P/C ratio was above the threshold, chemical substances other than P may be important in determining food quality because animal mass in the two highest nutrient concentrations in the low light treatment (Fig. 3C, solid circles) was less than at the same nutrient concentrations in the high light treatment (Fig. 3C, open circles). This may be due to differences in C quality in the algal cell, such as the relative composition of cellulose and essential fatty acids. Our experiments demonstrate for the first time that there is an optimum light intensity relative to nutrient supply to
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Ecology: Urabe and Sterner
Proc. Natl. Acad. Sci. USA 93
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P supplied [ pM ] FIG. 3. Biomass (A) and P/C ratio (B) of S. acutus and body mass of D. obtusa at age 6 days at various nutrient concentrations in high-light treatment (260 ,uE m-2.s-1; 0), low-light treatment (12 ,uE m-2.s-l; *), and adjusted treatment (OI) where biomass of high-light algae was diluted to 10% and incubation was made at the low light (mean ± SD). The dashed horizontal line in B is the threshold food P/C ratio (17). In all nutrient concentrations, algal biomass in the high-light treatment was significantly higher than the other two treatments (P < 0.001), whereas algal P/C ratio in the low-light treatment was significantly higher than the other two treatments (P < 0.001). The body mass in the low-light treatment was significantly higher (P < 0.01) at the lower three nutrient concentrations but lower (P < 0.001) at the higher two nutrient concentrations than the high-light and adjusted treatments.
maximize production of animal biomass. At low light relative to nutrients, herbivore production is governed by production of autotrophs. Such systems are clearly best thought of as energy limited, both in primary and secondary production (1, 2). In contrast, beyond a certain point of light/nutrient balance, increases in light energy input for given nutrient supply actually reduce animal growth rates. These systems are nutrient limited, and surprisingly, increasing influx of energy to the system is detrimental to herbivore production. Thus, the decoupling of nutrient uptake and photosynthesis by plants modulates a balance between energy and nutrients that influences food chain dynamics in ways that at first seem paradoxical from the view of energy input, but are reasonable from a standpoint of energy/nutrient balance.
(1996)
Light/nutrient regimes can now be seen to affect ecological transfer efficiency from plant to herbivore. Existing theories have pointed out that ecological transfer efficiency is a key parameter in regulating trophic dynamics and exploitation (27-29), but few studies have attempted to elucidate the factors determining such transfer efficiencies. In Lake Biwa, the largest lake in Japan, ecological transfer efficiency from primary producers to zooplankton was higher for P than for C because of elemental imbalances (30). Under such situations, changes in abiotic factors increasing C fixation rates alone could lead to further reductions in C transfer efficiency because herbivores must eliminate a greater amount of C to maintain their own rather strict homeostasis. We induced these changes in chemical composition of algae through direct manipulations in the light regime. However, we can speculate that increases in C fixation by plants due to globally increased availability of CO2 might have similar effects, such that ecological transfer efficiencies may either increase or decrease, depending upon the stoichiometric balance between producers and consumers for C, nutrients, and energy. For example, in a system where primary production is limited by nutrient supply rate, increase in the availability of CO2 is likely to decrease mineral/C ratio in producers and may result in lowered quality of food for consumers. In addition, increased light transmission owing to the reduction in deoxycholate in lakes and streams affected by acid rain might be expected to show similar responses. Early writers concerned with applying thermodynamic principles to ecological systems wrote optimistically of how laws of energy and mass transfer would lay bare the workings of ecosystems (31-33). Since Lindeman's (34) classic work, efforts to understand ecosystems based solely or primarily on energy flows have been made, but their success and impact have arguably been limited (35-37). Our study demonstrates that a greater integration of energetics and nutrient-based analyses may provide more predictive and explanatory power to mass transfer and trophic structure. We thank N. George for technical assistance, and J. Elser, A. Galford, T. Hara, S. Kilham, E. Litchman, S. Naeem, and D. Tilman for comments and suggestions. This work was supported by grants from the National Science Foundation. 1. Hill, W. R., Ryon, M. G. & Schilling, E. M. (1995) Ecology 76, 1297-1309. 2. Wootton, J. T. & Power, M. E. (1993) Proc. Natl. Acad. Sci. USA 90, 1384-1387. 3. Goldman, J. C., McCarthy, J. J. & Peavey, D. G. (1979) Nature (London) 279, 210-215. 4. Sommer, U. (1989) in Plankton Ecology: Succession in Plankton Communities, ed. Sommer, U. (Springer, Berlin), pp. 57-106. 5. Harrison, P. J., Thompson, P. A. & Calderwcrod, G. S. (1990) J. Appl. Phycol. 2, 45-56. 6. Rhee, G.-Y. & Gotham, I. J. (1981) Limnol. Oceanogr. 26, 635-648. 7. Checkley, D. M., Jr. (1980) Limnol. Oceanogr. 25, 430-446. 8. Dale, D. (1988) in Plant Stress-Insect Interactions, ed. Heinrichs, E. A. (Wiley, New York), pp. 35-110. 9. Sterner, R. W. (1993) Ecology 74, 2351-2360. 10. McNaugliton, S. J. (1988) Nature (London) 334, 343-345. 11. Schindler, D. W. (1977) Science 195, 260-262. 12. Elser, J. J., Marzolf, E. R. & Goldman, C. R. (1990) Can. J. Fisheries Aquatic Sci. 47, 1468-1477. 13. Hessen, D. 0. (1992) Am. Nat. 140, 799-814. 14. Urabe, J. (1993) Arch. Hydrobiol. 126, 417-428. 15. Elser, J. J. & Hassett, R. P. (1994)Nature (London) 370,211-213. 16. Sterner, R. W. & Hessen, D. 0. (1994)Annu. Rev. Ecol. Systemat.
25, 1-29. 17. Urabe, J. & Watanabe, Y. (1992) Limnol. Oceanogr. 37,244-251. 18. Goldman, J. C. & McCarthy, J. J. (1978) Limnol. Oceanogr. 23, 695-703.
Ecology: Urabe and Sterner 19. Sterner, R. W. (1989) in Plankton Ecology: Succession in Plankton Communities, ed. Sommer, U. (Springer, Berlin), pp. 107-170. 20. Andersen, T. & Hessen, D. 0. (1991) Limnol. Oceanogr. 36, 807-814. 21. Hessen, D. 0. & Lyche, A. (1991)Arch. Hydrobiol. 121, 355-363. 22. Olsen, Y., Jensen, A., Reinertsen, H., B0rsheim, K. Y., Heldal, M. & Langeland, A. (1986) Limnol. Oceanogr. 31, 34-44. 23. Sterner, R. W., Hagemeier, D. D., Smith, W. L. & Smith, R. F. (1993) Limnol. Oceanogr. 38, 857-871. 24. Wetzel, R. G. (1983) Limnology (Saunders, Philadelphia), 2nd Ed. 25. Porter, K. G., Gerritsen, J. & Orcutt, J. D., Jr. (1982) Limnol. Oceanogr. 27, 935-949. 26. Strickland, J. D. H. & Parsons, T. R. (1972) Bull. Fish. Res. Bd. Canada 167, 1-311. 27. Oksanen, L. (1988) Am. Nat. 131, 424-444.
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28. DeAngelis, D. L. (1992) Dynamics of Nutrient Cycling and Food Webs (Chapman & Hall, New York). 29. Power, M. E. (1992) Ecology 73, 733-746. 30. Urabe, J., Nakanishi, M. & Kawabata, K. (1995) Limnol. Oceanogr. 40, 232-242. 31. Hutchinson, G. E. (1959) Am. Nat. 43, 145-159. 32. Morowitz, H. J. (1968) Energy Flow in Biology (Academic, New York). 33. Odum, E. P. (1969) Science 164, 262-270. 34. Lindeman, R. L. (1942) Ecology 23, 399-418. 35. Mansson, B. A. & McGlade, J. M. (1993) Oecologia 93, 582-596. 36. DeAngelis, D. L. (1995) in Linking Species and Ecosystems, eds. Jones, C. G. & Lawton, J. H. (Chapman & Hall, New York), pp. 263-272. 37. Hairston, N. G. & Hairston, N. G. (1993)Am. Nat. 142,379-411.
