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

Ecological stoichiometry: Nutrient inputs (external load) and food-web structure: key forces governing the structure and function of lake ecosystems Nutrient stoichiometry N or P limitation? Trophic cascades Abundance, biomass and community structure Ecological stoichiometry: study of the balance of energy and multiple chemical elements in ecological interactions internal nutrient cycling Ecological stoichiometry: theoretical framework for studying how trophic dynamics and biogeochemistry interacts in regulating lake ecosystem dynamics Structure and function of lake ecosystems

differential storage, loss Whole lake food-web manipulation Trophic cascade Stoichiometric Mechanisms differential storage, loss and recycling of N and P C:N:P changes ? Before this sitiuation. Their aim is to show that manipulating the food web will not only result in a trophic cascade but that it also afects stoichiometric mechanisms which alter the internal nutrient cycling and thereby ecosystem dynamics

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

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 Aerial view of Lake 227 in 1994 1993: introduction of northern pike (60) 1994: additional 140 (areal density of 26kg ha-1)

Methods: Parameters determined: 1992 – 1996 from May/June until August/september 7 – 10 days interval epilimnion (mixed sample from three depths) Sampling scheme: 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)

Data analysis: comparing data 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)

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)

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).

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

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

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

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

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.

Zooplancton Seston Pike fish Pike fish 92-95 96 invertebrates 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-fix cyano) N:P and C:P high Seston N:P and C:P low N-limited system P-limited system

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?)

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 !