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

2. 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

3. 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

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. Study site: Experimental history of lake 227:
: N and P lake fertilization at a molar ratio of 29:1
= increased phytoplankton biomass, non-nitrogen
fixing cyanobacteria
: 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)

6. 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)

7. 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)

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. 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

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 !