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0.2 0.4 0.6 Di ve rs ity (1 -λ ) S- S+ 0.2 0.4 0.6 Di ve rs ity (1 -λ ) N- N+ 40 60 80 Re la tiv e ab un da nc e in d. /g z os te ra )Effects of multiple disturbances and stresses
on a benthic eelgrass community
Stéphanie Cimon & Mathieu Cusson
Université du Québec à Chicoutimi, 555 boulevard de l’Université, Chicoutimi (Québec) G7H 2B1 stephanie.cimon@uqac.ca ; mathieu.cusson@uqac.ca
Results
Context and Objectives
Many ecosystems are facing environmental changes and anthropogenic pressures that may affect communities in terms of both structure and/or function. Disturbances and stresses are commonly co-occurring in nature, however the interaction between them are generally considered as being additive without knowing the true in situ effects. The presence of structuring species may play a major role in the effects that disturbances will have on communities. The inclusion of multiple disturbances and stresses in field experiments in order to assess their potential interactive effects will help disentangle the mechanisms structuring communities following disturbances.
Methods
0 20 40 60 80 100 120 Re la tiv e ab un da nc e (in d. /g z os te ra) Density reduction increased abundances, evenness,
diversity and eelgrass relative growth (fig. 8) There were more periwinkles, isopods and amphipods when
density was reduced 0 0.2 0.4 0.6 0.8 Ev en es s (J ') Z+ Z-0 0.2 0.4 0.6 Di ve rs ity (1 -λ )
The aim of the study is to measure the response of macrobenthic assemblages facing multiple disturbance and stresses. Specifically a
community dominated by a seagrass canopy (Zostera marina) and subjected to density reduction, light reduction and sediment nutrient
enrichment was investigated.
Single effect of density reduction
Week 0 2 4 6 8 Ri ch ne ss (S ) Z+N- Z+N+ Z-N- Z-N+ a b ab ab Expected additive
Single effect of sediment nutrient enrichment or shading Enrichment decreased abundances,
diversity and richness (fig. 5) Polychaeta and bivalves being
affected the most
Interaction effects on community indices and eelgrass density – week 5
Antagonistic effect of density reduction and enrichment on
richness
Antagonistic effect of density and shading on evenness Fig. 2. Average values (±SE; n=20) of invertebrates relative abundance (nb. of individuals/DW zostera (g)) (a), Pielou’s
evenness (J’) (b), and Simpson’s diversity (1-λ) (c) for eelgrass density reduction treatment 2, 5 and 10 weeks after starting the experiment. Z+ normal density; Z- density reduced.
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Shading increased diversity and evenness and decreased eelgrass density (fig. 7) and relative
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Fig. 5. Average values (±SE; n=10) of invertebrates richness (S) for the interaction of eelgrass density reduction (Z-) and enrichment (N+) treatments.
Fig. 6. Average values (±SE; n=10) of invertebrates Pielou’s evenness (J’) for the interaction of eelgrass density reduction (Z-) and shading (S+) treatments. 0 0.2 0.4 0.6 0.8 Ev en ne ss (J ') b ab a a Expected additive W2 W5 W10 W2 W5 W10 W2 W5 W10
Bars with asterisk or different letters are significantly different (p<0.05)
Zostera density reduction compensated the loss in richness by enrichment and the decreasing in dominance by shading
0 0.2 Di ve rs ity ( 0 0.2 Di ve rs ity ( 2D Stress: 0,06 0 20 Re la tiv e ab un da nc e (in d.
Methods
Fig. 1. Scheme of the experiment
Z+ N-S- S+ N+ S- S+ Z- N-S- S+ N+ S- S+ 5 replicates/treatment Eelgrass density Sediment enrichment Shading 80 % reduction in Z-70 % reduction PAR in S+ 75 g N m-2 in N+ Experiment:
• Treatments were applied in 1 x 1 m plots and all measures were taken in their centers • Experiment took place from early July to mid-September 2015
• Shading and enrichment were added two weeks after density reduction • Shading took place for 19 days
Variables measured:
• Abundances: number of individuals collected with mesh bag (200 µm; 18 cm diameter) • Community was sampled three times : after 2, 5 and 10 weeks
• Community indices were calculated based on number of individuals / Zostera dry mass • Zostera density was measured three times : before, at week 5 and at week 10 • Zostera relative growth was evaluated once from week 2 to 5 during shading
Antagonistic effect of enrichment and shading on
eelgrass density. The increased density by enrichment was cancelled
by shading
Additive effect of density reduction and shading on
relative growth
Conclusions
• Eelgrass density reduction affected community characteristics through time.
• Twenty days were enough to induce a community response to sediment enrichment and shading, though the effects were gone 5 weeks later. This demonstrated a potential of resilience of the eelgrass system.
• Antagonistic effects between treatments were observed on community indices and eelgrass density.
• Our results suggest that eelgrass bed can be resistant to multiple disturbances and stresses as no effect was observed on measured variables from the community and plant when all our treatments were applied.
Acknowledgments
We want to thank the people who helped us during field work: C. Valcourt, G. Grosbois, M. Wauthy, S. Pelletier, D. Villeneuve and Y. Valcourt.
Multivariate results
Fig. 4. nMDS of estimate 95% region of bootstrap averages (n=150) from replicates (Bray-Curtis; dispersion weighted and square-root transformed) of control and (a) density reduced plots (week 10), (b) enriched plots (week 5), and (c) shaded plots (week 5). Bootstrapping performed in m=10 dimensional nMDS space.
Fig. 3. Average values (±SE; n=20) of invertebrates relative abundance (a) and Simpson’s diversity (1-λ) (b-c) for sediment enrichment (a-b) and shading (c) treatments before (W2), 3 (W5) and 8 (W10) weeks after applying the treatment (dash line). N- no enrichment; N+ enriched; S- natural light; S+ light reduced.
eelgrass density (fig. 7) and relative growth (fig. 8) Generally amphipods and isopods were more abundant under shading W2 W5 W10 Week 0 Z+S- Z+S+ Z-S- Z-S+ 0 2 4 6 8 10 Zo st er a de ns ity (x 10 ² p la nt m -²) N-S- N-S+ N+S- N+S+ a b c bc Expected additive
Fig. 7. Average values (±SE; n=10) of eelgrass density for the interaction of enrichment (N+) and shading (S+) treatments. 0 1 2 3 4 Zo st er a re la tiv e gr ow th (m m d ay -1) Z+S- Z+S+ Z-S- Z-S+ Expected additive
Fig. 8. Average values (±SE; n=10) of eelgrass relative growth (mm day-1) for the interaction of eelgrass
density reduction (Z-) and shading (S+) treatment. 2D Stress: 0,1 2D Stress: 0,12 PERMANOVA p=0.008 PERMDISPp=0.016 PERMANOVAp=0.036 PERMANOVA p=0.001
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The studied site is located on the Manicouagan Peninsula near the city of Baie-Comeau on the
north shore of the St. Lawrence Estuary Quebec, Canada.
W2 W5 W10 W2 W5 W10
2D Stress: 0.06 2D Stress: 0.1 2D Stress: 0.12
Density reduction affected community structure over the course of the entire experiment (10 weeks). Enrichment and shading individually affected community structure after 3 weeks (week 5) but the
effects disappeared by week 10. For details see single effect results in Fig. 2 and 3.
A total of 29 invertebrates and 3 fishes taxa were identified :
5 gastropods, 5 bivalves, 2 mysids, 1 shrimp, 8 amphipods, 3 isopods, 4 polychaete and 1 crab
Montréal Québec
Rimouski Baie-Comeau
Our results show the importance of field experiments that include multiple disturbances and stresses and their interactions in order to estimate the impacts on community assemblages and the fact that interactions are not always additive and therefore impossible to predict.
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