Pelagic C:N:P Stoichiometry in a

Eutrophied Lake: Response to a

Whole Lake Food-Web ManipulationElser et al. 2000 (Ecosystems)

Aline Frossard & Silke Van den Wyngaert

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

internal nutrient cycling

Structure and function of lake ecosystems

Ecological stoichiometry:study of the balance of energy and multiple chemical

elements in ecological interactions

Whole lake food-web manipulation

C:N:Pchanges ?

C:N:Pchanges ?

Trophic cascade

Stoichiometric Mechanisms

differential storage, loss and recycling of N and P

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

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.

Aerial view of Lake 227 in 1994

Experimental history of lake 227:

1970-1974: N and P lake fertilization at a molar ratio of 29:1

= increased phytoplankton biomass, non-nitrogenfixing 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

Study site:

1993: introduction of northern pike (60)1994: additional 140 (areal density of 26kg ha-1)

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)

• 1992 – 1996 from May/June until August/september

• 7 – 10 days interval

• epilimnion (mixed sample from three depths)

Sampling scheme:

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.

Pike fish

Zooplancton

(Daphnia) N:P high

Seston(N-fix cyano) N:P and C:P high

Minnow fish

Pike fish

Zooplancton(Daphnia => P sink)

N:P low

SestonN:P and C:P low

Nutrient availability low, TIN:TDP low Nutrient availability increased, TIN:TDP higher

92-95 96

invertebrates

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?)

• 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

Discussion points II

Anyway, ecological stoichiometry and trophic cascade theoryare useful fur the understanding of ecosystem dynamicsbut not sufficient for predicting ecosystem dynamics !