1 / 17

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

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. Nutrient stoichiometry N or P limitation?. Trophic cascades. Abundance, biomass and community structure.

johnna
Télécharger la présentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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 !

More Related