; Reference )

In situ determination of the impact of press and pulse

disturbances on spatial variability of a St. Lawrence rocky intertidal

ecosystem assemblages

Annie Séguin, Philippe Archambault

Annie.Seguin@UQAR.QC.CA

Institut des sciences de la mer de Rimouski, 310 Allée des Ursulines, C.P. 3300, Rimouski, Québec G5L 3A1 Canada

ICES CM 2008/J:20

METHODOLOGY

RESULTS

CONCLUSIONS

ACKNOWLEDGMENTS

This project is supported by the Natural Sciences and Engineering Research Council of

Canada (NSERC) to Philippe Archambault and a NSERC scholarship to Annie Séguin.

Special thanks to Pauline Robert and Francis Jacques for their help on the field, to

Robert Chabot for specimens identification, to Diane Bérubé and Yves Paquin for their

advice and experience in nutrients analysis.

INTRODUCTION

Manipulation was carry out along a 770 m of shore in a rock pool system, located in the Lower

St. Lawrence estuary, Quebec (Fig. 1). Experiment was conducted between May 26th and

August 31th 2008. Perturbations started at the week 7 of the experiment, the first six weeks

allowing determining natural biodiversity and variability of each rock pool. Overall, 35 rock

pools were chosen (length between 0.9 & 2.3 meters ; volume between 8 & 60 liters.

Press disturbance simulations were applied every diurnal low tide. Pulse disturbance

simulations, were done only 5 times, randomly chosen, among the 8 weeks disturbances

lasted.

Two different disturbance types were applied: Disturbance through dessiccation, by draining

5 rock pools (Fig. 2), and through nutrients enrichment in 5 others. Enrichment were done in

order to increase the rock pools nutrients concentration to 18 μM of inorganic nitrogen as

NH4NO3 and 1.125 μM of inorganic phosphorus as H3PO4, highest concentrations recorded

throughout the year in the study area. 5 rock pools acted as controls, which allow to reveal

the impact of manipulations and finally, 5 rock pools acted as reference, which did not

receive any perturbation nor manipulation (Fig. 3).

Sampling was done weekly in each rock pool, three 15 x 15 cm quadrats were randomly

disposed. The percentage cover of algae and sessile organisms were estimated with a non-

destructive method.

Fig 1. Experimental field representation. Numbers represent location of the 35 rock pools studied.

Natural and anthropogenic disturbances are observed within every ecosystems,

sometimes being an important component of them, structuring the aquatic assemblages

structure1. However, the environment also has to deal more and more frequently with

anthropic disturbances. A better understanding of the impact of anthropogenic activities can

help to identify biodiversity changes, as well as the goods and services they provide.

According to the Intermediate Disturbance Hypothesis2, the highest biodiversity level should be

observed at intermediate disturbance intensity or frequency. But this hypothesis, based on

means comparisons, does not reveal any information about disturbance influences on the

spatial and temporal variability of assemblages. Regarding that, Caswell and Cohen (1991)3

built a model, which reveals that variability would be greater on disturbed sites when

compared with control sites. This conclusion has been confirmed in many studies, but this has

not been validated in all studies.

An explanation could be found in the distinction between the different disturbance types. In

fact, pulse disturbance type, which occurs occasionally (i.e. floods or oil spills) would induce a

greater spatial variability on disturbed sites than on controls. Press disturbance type is defined

as being continued through time (i.e. sewage). They would induce a smaller spatial variability.

Due to the technical complexity relative to field manipulations, a few in situ studies have

experimented the impact of perturbations. To circumvent this problem, rock pool systems are

suggested to study ecological models. They have defined boundaries, are easy to manipulate

and have been well studied4.

OBJECTIVES

The aim of this study is to determine in situ the impact of press and pulse disturbances on

spatio-temporal variability of biodiversity. More precisely, we will precise the Caswell and

Cohen’s model.

Fig 2. A rock pool before and after a press disturbance simulation by draining.

REFERENCES

Overall, this study will help to identify general patterns of ecosystem responses to different types of disturbance and improve

management and conservation approaches in cases of disturbed sites. It shows that conclusions should not be made only on

the basis of means comparisons, but spatial variability of assemblages should be regarded as important as well.

Because disturbances also have repercussions on ecosystem functioning, through biodiversity modification, the next step will

be to define the impact of the different disturbance types on primary and secondary production. Furthermore, new variability

indexes will be considered and studied in order to reveal not only the intra-group variability, but also to allow comparisons

between the different groups.

1- Glasby, TM et Underwood, AJ, 1996. Environmental Monitoring and Assessment 42: 241-252

2- Caswell H and Cohen, JE, 1991. In: Ecological heterogeneity, Springer-Verlag, New York, 320 p.

3- Connell, JH, 1978. Science 199: 1302–1310 4- Metaxas, A et Scheibling, RE, 1993. Marine Ecology Progress Series 98: 187-198

After 11 weeks, 43 taxa were observed. Mytilus edulis was the most abundant

species and Ralfsia verrucosa, the most abundant algae recorded. Both were

present in every rock pool.

A multivariate analysis showed that the interaction Frequency x Treatment x

Period does not neither have any significant impact on the benthic

assemblages present in rock pools. (Pseudo-F2;540= 0.8235, p= 0.7162).

However, when looking at the spatial variability between weeks 8 to 10, a

smaller variability was observed in rock pools that received a nutrient

enrichment following a press frequency, whereas in reference rock pools,

variability stayed more or less similar. Because only 1 of the 5 pulse

disturbances was done after 11 weeks, rock pools seem, in this case, to react

like reference rock pools. Variability does not neither seem to become smaller

nor larger than before the disturbance (Fig. 4).

Further similarity analysis applied on data from weeks 1 to 11 showed a

tendency toward an increased of similarity in rock pools disturbed at a press

frequency once the disturbance started (Fig. 5). In those disturbed at a pulse

frequency, the intra-group similarity showed a greater week-to-week

variability than before the beginning of the disturbance, as we were

expected.

Source: Department of Fisheries and Oceans.

Time

Perturbation

Time

Perturbation

BEFORE AFTER

NH4NO3H3PO4

H2O

1 2 3 4 5

Fig 3. Experimental design representation. Note that rock pools acting as reference were non included within the experimental

design.

NH4NO3H3PO4

H2O

BEFORE AFTER

21 22 23 24 25

Frequency

Treatment

Rock pool

Period

Time

Fig 4. nMDS showing spatial variability between benthic assemblages in rock pools disturbed by press type disturbance, pulse

type and references rock pools at A) week 8, B) week 9, C) week 10.

A) B) C)

Fig 5. Intra-group similarity in rock pools

disturbed by nutrients enrichment from weeks

1 to 11. The black dotted line represents the

beginning of the disturbance. (Legend:

Press

Rimouski

Sept-Iles

; Pulse

Weeks

%

St.John