Pelagic C:N:P Stoichiometry in a Eutrophied Lake: Response to a Whole Lake Food-Web Manipulation

1. Pelagic C:N:P Stoichiometry in a Eutrophied Lake: Response to a Whole Lake Food-Web Manipulation Elser et al. 2000 (Ecosystems) Aline Frossard & Silke Van den Wyngaert

2. Nutrient stoichiometry N or P limitation? Trophic cascades Abundance, biomass and community structure internal nutrient cycling Ecological stoichiometry: study of the balance of energy and multiple chemical elements in ecological interactions Structure and function of lake ecosystems Nutrient inputs (external load) and food-web structure: key forces governing the structure and function of lake ecosystems

3. Trophic cascade C:N:P changes ? Stoichiometric Mechanisms differential storage, loss and recycling of N and P C:N:P changes ? Whole lake food-web manipulation

4. Hypothesis: Changes in the C:N:P stoichiometry of the planktonic food web are important mechanisms involved in altered ecosystem dynamics after changes in food-web structure. 1. C:P and N:P ratios of zooplankton biomass decrease (P-rich Daphnia) 2. zooplankton P-pool becomes an important internal component 3. sedimentation losses of P increase disproportionately 4. relative availability of N increases 5. cyanobacterial dominance decreases 6. contribution of N fixation to the lake's N budget diminishes

5. Aerial view of Lake 227 in 1994 1993: introduction of northern pike (60)1994: additional 140 (areal density of 26kg ha-1) Study site: Experimental history of lake 227: 1970-1974: N and P lake fertilization at a molar ratio of 29:1 = increased phytoplankton biomass, non-nitrogen fixing cyanobacteria 1975- 1985:N and P lake fertilization at a molar ratio of 11:1 (P-loading rate constant) = increase N-fixing cyanobacteria (but variable) 1990:N fertilization terminated, P-loading rate constant = monospecific blooms of N-fixing cyanobacteria Zooplankton biomass low, dominated by copepods, small cladocera and rotifers

6. Sampling scheme: • 1992 – 1996 from May/June until August/september • 7 – 10 days interval • epilimnion (mixed sample from three depths) Methods: Parameters determined: • Zooplankton: abundance, biomass, taxonomy, C:N:P • Seston: C:N:P • Dissolved N and P, TDN, TDP (0.2 um filtrate) • Sedimentation rates of C, N and P (sediment traps)

7. In addition: • Assesment of minnow abundance • Phytoplankton biomass and species composition (ELA records) biomass of N-fixing cyanobacteria Data analysis: comparing data • Two data bins per month: • observations within each half month interval were averaged „Summertime mean“ (average of the averaged observations)

8. Results – Fishes Decrease of minnow fishes (planktivorous) after the introduction of pike fishes (piscivorous) No minnow fishes after 1995 (high survival rate of introduced pike fishes)

9. Results - zooplankton Increase of zooplankton biomass visible after 4 years (1996). Higher biomass of Daphnia Deacrease of N:P in the zooplankton: increase of Daphnia abundance (P-rich) compare to Copepod (low-P).

10. Results - Seston 92-95: C:P and N:P ratios high. 96: decrease of C:P and N:P, total seston, phytoplankton bacteria, carbon Low C:P and N:P reflects rapid growing phytoplankton

11. Results – Phytoplankton community composition 92-95: biomass of phytoplankton high, N-fixing cyanobacteria important 96: biomass of phytoplankton lower, due to Daphnia invasion. N-fixing cyanobacteria absent

12. Results - Sedimentation 96: lower residence time for particulates C and P (=>loss), but sedimentation rate constant and less particles in the water column Stoichiometric aspects of sedimentation: C:P and N:P of sedimenting particles low in 95/96

13. Results – nutrient availability in water 92-95: low and constant, TIN:TDP low 96: concentration of dissolved nutrients increased, TIN:TDP increase

14. Summary Effects of pike fishes introduction: • Zooplankton biomass more P rich (dominance of Daphnia) • Importance of zooplankton as a nutrient pool in the water column increase greatly. > less P available for the phytoplankton (TIN:TDP increase) • Increase in zooplankton => increase nutrient availability larger for N than for P => N-fixing cyanobacteria no more important • Creation of low N:P sink in the lake through the elimination of planktivorous fishes.

15. 92-95 96 Nutrient availability increased, TIN:TDP higher Nutrient availability low, TIN:TDP low Pike fish Pike fish invertebrates Minnow fish Zooplancton (Daphnia => P sink) N:P low Zooplancton (Daphnia) N:P high Seston N:P and C:P low Seston (N-fix cyano) N:P and C:P high N-limited system P-limited system

16. Discussion points I • Interesting experiment in a whole lake system, integrating all compartment of the food chain, integrating theories. • By manipulating the foodweb, stoichiometry of pelagic compartments can change, thereby altering ecosystem dynamics. • Effect only clearly visible in 96 after 4 years of “no real effect”. => No explanation for the delayed responses • „Summertime mean“: arguable if this is a good solution for expressing and comparing data. (late spring and summer are different situations?)

17. Discussion points II • 97 and 98: despite the absence of planktivorous fishes, zooplankton biomass low, Daphnia rare, dense cyanobacterial bloom again. (see previous years) • - Is 96 a “special” year? • Alternative stable states? • Effects of intensive experimental history of the lake! • NOT enough discussion on that point • Anyway, ecological stoichiometry and trophic cascade theory • are useful fur the understanding of ecosystem dynamics • but not sufficient for predicting ecosystem dynamics !