Silting up and development of anoxic conditions enhanced by high abundance of the geoengineer species Ophiothrix fragilis

Author’s Accepted Manuscript Silting up and development of anoxic conditions enhanced by high abundance of the geoengineer species Ophiothrix fragilis A. Murat, Y. Méar, E. Poizot, J.C. Dauvin, K. Beryouni www.elsevier.com/locate/csr

PII: DOI: Reference:

S0278-4343(16)30003-6 http://dx.doi.org/10.1016/j.csr.2016.01.003 CSR3343

To appear in: Continental Shelf Research Received date: 25 June 2015 Revised date: 8 January 2016 Accepted date: 9 January 2016 Cite this article as: A. Murat, Y. Méar, E. Poizot, J.C. Dauvin and K. Beryouni, Silting up and development of anoxic conditions enhanced by high abundance of the geoengineer species Ophiothrix fragilis, Continental Shelf Research, http://dx.doi.org/10.1016/j.csr.2016.01.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Silting up and development of anoxic conditions enhanced by high abundance of the geoengineer species Ophiothrix fragilis A. Murat1,2,3, Y. Méar1,2,3, E. Poizot1,2,3, J.C. Dauvin3,4,5 and K. Beryouni1

1 - Cnam/Intechmer, BP 324 F50103 Cherbourg Cedex, France 2 - Université de Caen Basse-Normandie, LUSAC, EA 4253, Rue Louis Aragon, F50130 Cherbourg-Octeville, France 3 - Normandie Université, France 4 - Université de Caen Basse-Normandie, Laboratoire Morphodynamique Continentale et Côtière, UMR M2C, 24 rue des Tilleuls, F14000 Caen, France 5 - CNRS UMR CNRS 6143M2C, 24 rue des Tilleuls, F14000 Caen

Corresponding author: Name: Poizot E. E-mail: [email protected] Postal address: BP 324 50103 Cherbourg-Octeville. France Phone number: 00 33 (0)2 33 88 73 42

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1

Introduction .......................................................................................................................... 6

2

Study site .............................................................................................................................. 9

3

Environmental data acquisition .......................................................................................... 11 3.1 Data acquisition at sea ................................................................................................. 12 3.2 Laboratory analysis ..................................................................................................... 13 3.3 Statistical analysis ....................................................................................................... 14

4

Results ................................................................................................................................ 14 4.1 Relationships between studied parameters .................................................................. 14 4.2 Environmental situations ............................................................................................. 15 4.2.1 Environmental PCA ............................................................................................. 15 4.2.2 Relationships between Ophiothrix fragilis and sediment fine fraction ................ 16 4.2.3 Relationships between Ophiothrix fragilis and TOC ........................................... 18 4.2.4 Relationships between Ophiothrix fragilis and TS .............................................. 19 4.3 Compact black mud facies .......................................................................................... 20 4.3.1 Relationships between Ophiothrix fragilis and sediment fine fraction ................ 20 4.3.2 Relationships between Ophiothrix fragilis and biogeochemical parameters ....... 20

5

Discussion .......................................................................................................................... 21 5.1 Environmental situations ............................................................................................. 21 5.1.1 Relationships between Ophiothrix fragilis and sediment fine fraction ................ 21 5.1.2 Relationships between Ophiothrix fragilis and TOC ........................................... 23 5.1.3 Relationships between Ophiothrix fragilis and TS .............................................. 24 5.1.4 Synthesis............................................................................................................... 25 5.2 Compact black muds facies ......................................................................................... 27

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5.3 Interactions between environment and Ophiothrix fragilis geoengineer species........ 29 5.3.1 Environmental forcings and development of Ophiothrix fragilis population ...... 29 5.3.2 Effects of geoengineering on rate of silting up and development of anoxic conditions ......................................................................................................................... 30 5.3.3 Geoengineering hypothesis .................................................................................. 31 6

Conclusion .......................................................................................................................... 32

List of tables Table 1: Flood and storm characteristics taken into account for the seven surveys. ____________________________13 Table 2: Correlation coefficients (r) derived from PCA analysis. _________________________________________________15

List of figures Figure 1: Map of the studied site showing main residual tidal currents. _________________________________________ 9 Figure 2: Results of E-PCA projected onto PC1-PC2 plane. A: variable factor map, OPH: Ophiothrix fragilis population density in numbers per 0.25 m2; FF: fine fraction content (< 63 µm) in %; TOC: Total Organic Carbon in %; TS: Total Sulphur in %. B: factor map for individual data points; green open circles: flood samples; blue dots: stability samples, red crosses: storm samples. _______________________________________________15 Figure 3: Relationship between number of Ophiothrix fragilis per 0.25 m2 and fine fraction percentage in flood environment, showing linear regression (dashed green line). _____________________________________________17 Figure 4: Relationship between numbers of Ophiothrix fragilis per 0.25 m2 and fine fraction percentage in a storm environment, showing linear regression (solid red line). The linear regression for the flood environment (dashed green line) is plotted for comparison. _____________________________________________________17 Figure 5: Relationship between number of Ophiothrix fragilis per 0.25 m2 and fine fraction percentage in stability environment, showing linear regression (dot-dashed blue line). The linear regression for the flood environment (dashed green line) is plotted for comparison. _____________________________________________________18 Figure 6: A) Relationships showing significant linear regressions between number of Ophiothrix fragilis per 0.25 m2 and TOC percentage. Flood environment: green open circles; storm environment: red crosses; stability environment: blue dots and dot-dashed blue line; compact black muds: black triangles and solid black line. B)

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Relationships showing linear regressions between number of Ophiothrix fragilis per 0.25 m2 and TS percentage. Flood environment: green open circles; storm environment: red crosses; stability environment: blue dots and dot-dashed blue line; compact black muds: black triangles and solid black line. ________________18 Figure 7: Relationship between number of O. fragilis individuals per 0.25 m2 and fine fraction percentage in black muds, showing linear regression (solid black line). The linear regression for the flood environment (dashed green line) is plotted for comparison. ____________________________________________________________________20

1

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1

Abstract

2

In the English Channel, the brittle-star Ophiothrix fragilis is a common epifaunal species

3

typically found on pebbles in habitats with strong tidal currents. This species forms dense

4

aggregations on the seafloor, supporting populations that can reach up to 7 500 ind.m-2 in the

5

eastern part of the Baie de Seine, offshore from Antifer harbour. Here, O. fragilis occurs in an

6

area with unexpected amounts of fine-grained sediment. Some of these mud deposits are

7

made up of unusually compact black muds, an indication of the development of anoxic

8

conditions in surficial sediments. To highlight a potential link between silting up and dense O.

9

fragilis populations, and identify the interactions between environmental conditions and the

10

population dynamics of this species, we analyse the data from three surveys corresponding to

11

exceptional situations: 1) just after a Seine flood; 2) just after a storm and 3) after a period of

12

ten months without any flood or storm. Four parameters are taken into account: number of

13

brittle stars per 0.25 m2, Fine Fraction percentage, Total Organic Carbon and Total Sulphur.

14

The main environmental forcings appear to be Seine river inflow, regional circulation

15

dependent on tidal currents and the occurrence of storms. O. fragilis is able to geoengineer its

16

environment in various ways and at different rates. Silting up is enhanced by increasing

17

abundance of O. fragilis and takes place at a very fast rate. As a result, floods and storms

18

reflecting instantaneous events give rise to a steady-state situation established between the

19

abundance of this species and the fine fraction percentage. Anoxic conditions are dependent

20

on the degradation of organic matter and require more time to be established. After many

21

months in the absence of any disturbing events, anoxic conditions are developed in non-

22

compacted muddy sediments (stability situation) and represent the normal surficial situation

23

when the sediment becomes compacted (compact black muds). The development of anoxic

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1

conditions is dependent on the abundance of the O. fragilis population and occurs at a very

2

slow rate.

3

Graphical abstract

4

5

Research highlights

6

► O. fragilis geoengineers its environment in various ways and at different rates.

7

► Silting up is dependent on the abundance of O. fragilis

8

► Increased silting up leads to anaerobic conditions (preservation of organic matter)

9

► Main environmental/external forcings are floods, tidal currents and storms.

10

► Hypothesis explain relationships between geoengineering species and external forcings.

11

Keywords

12

Mud, TOC, Macrobenthos, Macrotidal, English Channel, Baie de Seine

13 14

1

Introduction

15

In the English Channel, the brittle-star Ophiothrix fragilis is a common vagile epifaunal

16

species typically found on pebbles in benthic habitats subject to strong tidal currents (Warner,

17

1971; Allain, 1974; Cabioch, 1968; Holme, 1984; Ellis and Roger, 2000; Freeman and

18

Rogers, 2003; Lozach et al., 2011). However, in the Baie de Seine, O. fragilis is commonly

19

found on gravel and coarse sandy sediments in relatively large patches in terms of surface

20

area (Gentil and Cabioch, 1997; Lozach et al., 2011). More locally, in the eastern part of the

21

bay (Fig. 1), O. fragilis occurs in an area off Antifer harbour with unexpected amounts of fine

22

sediment (Méar et al., 2006; Dauvin et al., 2013). The Baie de Seine is subject to very strong Page 6 sur 46

1

tidal currents and storms that lead to severe reworking at the water-sediment interface at less

2

than 40 m water depth. As a consequence, it is rather unexpected to find mud deposits in such

3

areas with high-energy hydrodynamics. Furthermore, some of these deposits are made up of

4

unusual compact black muds. This facies is an indication of the development of anoxic

5

conditions in surficial sediments, and was described for the first time in this area by Méar et

6

al. (2006). These anoxic conditions arise from the degradation of organic matter. Oxygen is

7

first consumed by aerobic respiration and then microorganisms utilize sulphate for the

8

mineralization of organic matter. This latter process results in better organic matter

9

preservation and an increase in sulphide contents in sediments (Kasten and Jorgensen, 2000;

10

Lehman et al., 2002; Rullkötter, 2000). A similar occurrence of black mud deposits has been

11

described in the Baie de Mont Saint-Michel (southern coast of the English Channel) by

12

Ehrhold et al. (1998), in areas of high-energy hydrodynamics unfavourable for the settling of

13

particles and mud stability. These authors highlighted a link between this type of sediment

14

and proliferation of the gastropod mollusc Crepidula fornicata. The aggregation of

15

individuals favours the permanent establishment of biogenic muds and radically modifies both

16

the nature and texture of the sediment. Loomis and Van Nieuwenhuyze (1985) studied

17

relationships between the density of Crepidula fornicata populations and various

18

characteristics of the sediments. They concluded that part of the density variance (19%) could

19

be explained by changes in organic matter content (with positive correlation) rather than

20

changes in silt and clay content as proposed by Ehrhold et al. (1998).

21

Due to their suspension feeding activity, O. fragilis plays a major role in the pelago-benthic

22

transfer of fine particles from the water column to benthic habitats (Warner and Woodley,

23

1975; Gounin et al., 1995; Davoult and Gounin, 1995; Allen, 1998; Blanchet-Aurigny et al.,

24

2012). According to Kristensen et al. (2012), O. fragilis is not a bioturbating species since it

25

has no direct or indirect effect on sediment matrices through particle reworking or burrow

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1

ventilation. This species forms dense aggregations on the seafloor (Broom, 1975), supporting

2

population densities that reach up to 7 500 ind.m-2 in the Baie de Seine (Davoult and Migné,

3

2001; Méar et al., 2006; Lozach et al., 2011).

4

The dense O. fragilis aggregations exhibit a high spatial variability both at the local and

5

regional scale (Lozach et al., 2011). Stations with high abundance may be very close to

6

stations without any O. fragilis. Nevertheless, a long-term study (1986-2010) of O. fragilis

7

distribution in the eastern part of the Baie de Seine offshore from Antifer shows the

8

persistence of dense aggregations (Dauvin et al., 2013), and four main patches have been

9

identified in this sector. Three of these patches are located along the 30 m isobath and occupy

10

an area of around 20 km2, while the most seaward patch lies in a deeper zone (36-37 m)

11

occupying around 10 km2. Dauvin et al. (2013) concluded that the development of O. fragilis

12

is flood-dependent since it requires the stabilization of suspensions brought in by Seine river

13

floods. One month after a strong Seine river flood, no significant modification can be

14

observed in the dense O. fragilis aggregations (Dauvin et al., 2013).

15

North-westerly storms induce 1) a south-eastward shift of patches towards the coast, 2) an

16

increase in abundance of the denser patches and 3) a decrease in abundance of the less dense

17

patches. Such events have only a temporary impact.

18

The aim of this study is to highlight the potential link between silting up and the development

19

of dense O. fragilis aggregations, and identify the interactions between environmental

20

conditions and the population dynamics of this species. For this purpose, we make use of the

21

large amount of available data from surveys carried out since 1986. However, under the

22

conditions that applied during most of these surveys, several environmental forcings acted

23

together. To identify the relationships between environmental forcings, the behaviour of O.

24

fragilis aggregations and the sedimentary deposits, we select three surveys corresponding to

25

exceptional situations: 1) just after a Seine flood; 2) just after a storm and 3) after almost ten

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1

months without any flood or storm. We also exclude samples without any O. fragilis. We

2

assume that, when O. fragilis is present, the sediment characteristics are representative of the

3

interaction. On the contrary, in cases where O. fragilis is lacking, we cannot be sure whether

4

the sediment characteristics are representative of this absence. Dense O. fragilis aggregations

5

may shift rapidly (Dauvin et al., 2013) and leave traces of engineered sediments even after

6

their migration. Lastly, four other surveys are used to study the “compact black muds” facies,

7

and compare the data with the results obtained from the three specific situations mentioned

8

above. To carry out this study, four parameters were taken into account (number of brittle star

9

individuals per 0.25 m2, Fine Fraction percentage, Total Organic Carbon and Total Sulphur).

10

2

Study site

11

The Baie de Seine is located off the coast of north-western France, bordering the central part

12

of the English Channel. It forms a quadrilateral with an area of roughly 5 000 km2, measuring

13

~ 50 km from north to south and ~ 100 km from west to east. The eastern Baie de Seine has a

14

maximum depth of 40 m and a mean depth of about 15 m (Fig. 1).

15 16

Figure 1: Map of the studied site showing main residual tidal currents.

17 18

The eastern part of the Baie de Seine is subject to relatively high winds with speeds greater

19

than 10 m.s-1 occurring 10% of the time. The prevailing winds in this part of the Baie de Seine

20

blow from the N and NE, but during storms they are mainly from the SW.

21

The eastern part of the Baie de Seine is also subject to short-period waves (Hmo = 1.5 m and

22

T = 4-5 s) generated by local winds mainly blowing from the NW to the NE (Larsonneur,

23

1972). Longer periods (~ 6-8 s) are the signature of residual swell offshore. Statistics on

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1

rough seas in the Baie de Seine show that the average height of storm waves is 5 m offshore

2

from Le Parfond, but these conditions remain exceptional.

3

Mathematical modelling controlled by in situ measurements allow us to determine the main

4

features of water mass circulation due to the tide. All along the Pays de Caux coast, during

5

both ebb and flood tides, currents are directed towards the NNE (Verhaule current), with

6

velocities reaching more than 5 m.s-1 at spring tide (Le Hir et al., 2001).

7

Le Hir et al. (2001) stressed the complexity of hydrodynamics in the eastern Baie de Seine,

8

showing the existence, among other features, of an anticyclonic eddy situated in the vicinity

9

of the studied area off the Pays de Caux coast northwest of Antifer harbour (Cugier and Le

10

Hir, 2002) (Fig. 1).

11 12

Since 1941, the annual average discharge of the Seine at Poses is about 500 m3.s-1

13

(Guézennec, 1999). The Seine discharge is controlled by the seasonal periodicity of winter

14

rainfall events. However, the fluvial regime is interspersed by episodes of short-duration

15

floods (> 1 000 m3.s-1) that occur 6% of the time. Daily sediment loads range from a few 100s

16

of tonnes during low water up to 30 000 tonnes in flood stage, representing an annual mean of

17

600 000 to 700 000 tonnes per year.

18

According to the hydrodynamic regime described above, significant material transfers occur

19

between the Seine estuary, considered as the main source of particulate matter, and the

20

adjacent areas (Dupont et al., 1991). When there are suitable tidal and fluvial flow conditions,

21

the maximal turbidity plume formed in the Seine estuary is expelled offshore (Garnaud et al.,

22

2002, 2003; Lesourd et al., 2003). Along the coast of the “Pays de Caux”, the turbid plume

23

coming from the Seine estuary develops over a width of a kilometre, even in the absence of

24

waves (Brylinski et al., 1991). This remarkable coastal water mass is sometimes referred to as

25

the "fleuve côtier" (Dupont et al., 1991). Garnaud et al. (2003) showed that expelled sediment

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1

carried by the Seine is also transported toward the south-eastern part of the Baie de Seine.

2

Hydrological measurements in the Baie de Seine show that resuspension of the fine fraction

3

by storm waves is sufficient to produce higher turbidity than during Seine floods.

4

Experiments using radioactive tracers show that the silt deposits are unconsolidated, and are

5

thus easily resuspended by tidal currents (Avoine, 1986; Lesourd et al., 2003; Ubertini et al.,

6

2012).

7

The English Channel seafloor is covered by highly heterogeneous sediments ranging from

8

very fine sands, silts and muds to gravels and pebbles. A sedimentary gradient from West to

9

East has been highlighted by Vaslet et al. (1978) and Larsonneur et al. (1982), with abundant

10

bioclastic material (0.5 to 2.5 mm grain size) and gravels in the western part of the channel

11

and sand and gravelly sand (0.15 to 0.5 mm grain size) predominating in the eastern part

12

(Lesourd et al., 2001).

13

At some locations near the coast, i.e. in the Baie des Veys, off Ouistreham and in the Seine

14

estuary, high percentages of fine fraction are recorded, ranging from 10% to more than 50%.

15

Off Antifer harbour, an unusual deposit of fine sediments is found in a very high-energy

16

hydrodynamic context. These fine-grained sedimentary deposits occur as diffuse patches up to

17

a few kilometres in size (Méar et al., 2006; Dauvin et al., 2013) at water depths of more than

18

30 m. Two types of muddy lithofacies are distinguished in the study area: (1) heterolithic

19

oxidized sediments, grey-beige in colour, with high water content, and (2) compact black

20

muds formed under reducing conditions (Méar et al., 2006).

21

3

22

The Seine discharges are obtained from the GIP Seine Aval database (http://seine-

23

aval.crihan.fr/web/pages.jsp?currentNodeId=150).

Environmental data acquisition

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1

These data include the daily discharge of the Seine at the Poses weir from 1 January 1941. In

2

this study, a flood situation is defined when the peak discharge of the Seine is at least 1 000

3

m3.s-1.

4

Data on swell conditions, which are therefore related to storm events, are derived from the

5

results of numerical simulations extracted from the digital atlas of sea states (ANEMOC:

6

Digital Atlas of State Oceanic and Coastal Sea), which was established for the French coast

7

through a partnership between EDF R&D-LNHE and CETMEF. The database is available at

8

http://anemoc.cetmef.equipement.gouv.fr/anemoc/

9

Wave conditions are obtained from the TOMAWAC numerical model of sea states. Results

10

are validated using buoy measurements (CETMEF), using wind data from the European

11

Centre for Medium-Term Forecast (ECMWF) (wind fields given at 10 m with a time step of 6

12

h and 0.5° grid-point spacing) under steady state hydrodynamic conditions (constant water

13

level). The Hmo parameter is used to characterize sea state, being defined as the mean wave

14

height from trough to crest of the largest third of the waves (H1/3 or a third of the significant

15

wave height). The geographical coordinates of the station COAST_2621 considered in this

16

study are 0°00.48 W and 49°43.02 N (WGS84), with a water depth of about 29 m.

17

3.1

18

From October 1989 to February 2009, seven surveys (Table 1) were carried out. Depending

19

on the sampling survey considered, different positioning systems were used (SYLEDIS, GPS

20

SA, GPS or DGPS). Thus, for each survey, sampling localisation can be considered as being

21

accurate to at least 100 m. For each survey, the boat was moored above an O. fragilis patch

22

such as those illustrated by Méar et al. (2006). Surface sediment samples were taken by

23

SCUBA divers using a 100 × 100 cm (1 m²) plastic frame divided into four equal subsets of

24

0.25 m². O. fragilis individuals were sampled from each of these rectangles, packed in plastic

Data acquisition at sea

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1

bags and returned to the boat for counting. The final value used here is the mean of the four

2

subsamples. Then, the uppermost 2 cm of the sediment was carefully scraped off with a

3

trowel and gently introduced into a 1 000 ml plastic container, avoiding as far as possible any

4

resuspension or loss of sediment. According to the available time remaining for the diver,

5

other sampling operations were carried out with a spacing of at least 2 m between samples.

6

Considering the sea conditions, it can be assumed that all the samples from a given survey are

7

positioned within a radius of 150 m around the sampled patch.

8

Table 1: Flood and storm characteristics taken into account for the seven surveys. Seine flood

Storm

Sampling date

Maximum flow 3 -1 (m .s )

Volume 3 (m )

From the last flood (days)

Hmo (m)

From the last storm (days)

1991

July 1991

1512

2358720000

160

4.1

169

1998

September 1998

1359

1952640000

119

4.7

147

1999

May 1999

1338

1560384000

15

4.0

112

SA09

February 2009

1490

1167091200

30

4.3

43

Flood

April 1999

1560

4103827200

5

4.5

197

Storm

February 1990

1207

684633600

428

4.1

3

Stability

October 1989

1207

684633600

289

4.6

661

Black muds

Between 1990 and 2009

Surveys

9 10

3.2

Laboratory analysis

11

First of all, the sediment samples were freeze-dried. Then, they were weighed (w1) and wet

12

sieved on a screen with a mesh of 63 μm. Both the fine (< 63 μm) and the coarse (> 63 μm)

13

fractions were freeze-dried again and weighed (yielding w2 and w3, respectively). The coarse

14

fraction percentage was derived from w3/w1. The fine fraction was then sub-sampled,

15

crushed and homogenized for geochemical analysis.

16

Total carbon contents and total sulphur content were measured by combustion in a LECO CS

17

300 carbon sulphur analyzer. Three replicates of dried and homogenized sediment (50 mg)

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1

were analysed per sample. Samples were heated to 1 200 °C, and the amounts of CO2 and SO2

2

released were measured by infrared absorption. For the analysis of Total Organic Carbon

3

(TOC), sediment samples were acidified by H3PO4 (1M) to remove carbonates, dried on a hot

4

plate at 50 °C and then analysed using the same procedure.

5

3.3

6

As each variable in the datasets used here can contain errors (Laws and Archie, 1981; Sokal

7

and Rohlf, 2012), we need to apply the Model II regression to determine correlations between

8

the studied variables. Such a model allows us to avoid underestimating the regression slope

9

(Riker, 1973). Regression analysis was performed using the lmodel2 package from R

10

language (R Development Core Team, 2013). The statistical level of significance was defined

11

at p < 0.05 (Spearman’s rank correlation test). Principal Components Analysis (PCA) of the

12

normalized variables was performed to extract significant Principal Components (PCs) and

13

reduce the contribution of variables with minor significance. Four parameters (number of

14

brittle star individuals per 0.25 m2: OPH; Fine Fraction percentage: FF; Total Organic

15

Carbon: TOC; Total Sulphur: TS) were taken into account. All the data processing was

16

performed using FactoMineR (Husson et al., 2007; Lê et al., 2008) in R language.

17

4

18

4.1

19

PCA was performed on data from surveys providing samples characteristic of three specific

20

enrironments (flood, storm and stability). The respective PCAs are denoted F, S and SB (for

21

Flood, Storm and StaBility). A single PCA (denoted E for Environment) was conducted

22

considering all three environments together. The fifth PCA (noted B for Black mud) was

23

performed with a specific sediment facies termed “compact black mud” by Méar et al. (2006).

Statistical analysis

Results Relationships between studied parameters

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1

Finally, one PCA (denoted A for All the dataset) incorporated the entire dataset. Table 2

2

reports the numbers of individuals/samples used and the correlation coefficients (r) obtained

3

from each of these six PCAs.

4 5

Table 2: Correlation coefficients (r) derived from PCA analysis. F Flood

S Storm

SB Stability

E Environment

B Black muds

A All data

N

32

64

39

133

56

189

OPH

FF

0.95 (VHS)

0.76 (VHS)

0.81 (VHS)

0.84 (VHS)

0.31 (S)

0.64 (VHS)

OPH

TOC

0.22 (NS)

0.08 (NS)

0.87 (VHS)

0.40 (VHS)

0.84 (VHS)

0.45 (VHS)

OPH

TS

0.37 (S)

-0.10 (NS)

0.60 (VHS)

0.33 (VHS)

0.39 (HS)

0.31 (VHS)

FF

TOC

0.18 (NS)

-0.14 (NS)

0.68 (VHS)

0.46 (VHS)

0.33 (S)

0.68 (VHS)

FF

TS

0.34 (NS)

0.06 (NS)

0.44 (HS)

0.47 (VHS)

0.34 (S)

0.67 (VHS)

TS

TOC

-0.04 (NS)

0.04 (NS)

0.63 (VHS)

0.65 (VHS)

0.37 (HS)

0.80 (VHS)

NOTES.- N: number of samples; OPH: number of Ophiothrix fragilis per 0.25 m2; FF: fine fraction percentage; TOC: Total Organic Carbon; TS: Total Sulphur. p > 0.05 i.e. non-significant (NS); p between 0.05 and 0.01, significant (S); p between 0.01 and 0.001, highly significant (HS); and p < 0.001, very highly significant (VHS).

6 7 8

By comparing the correlations obtained in the different datasets, we are able to investigate the

9

influence of different environmental conditions.

10 11

4.2

Environmental situations

4.2.1 Environmental PCA

12

Figure 2 shows the results of the E-PCA (this PCA noted E for Environment was conducted

13

considering all three environments together) projected onto the 1-2 plane for the studied

14

parameters (A) and samples (B).

15 16

Figure 2: Results of E-PCA projected onto PC1-PC2 plane. A: variable factor map, OPH: Ophiothrix fragilis

17

population density in numbers per 0.25 m2; FF: fine fraction content (< 63 µm) in %; TOC: Total Organic Carbon

Page 15 sur 46

1

in %; TS: Total Sulphur in %. B: factor map for individual data points; green open circles: flood samples; blue dots:

2

stability samples, red crosses: storm samples.

3 4

The PC1-PC2 plane accounts for about 88% of the total variance of the data point cloud,

5

which allows a reliable interpretation. The four studied parameters contribute positively to the

6

PC1 axis (from 22% for TS to 30% for FF). TS and TOC contribute positively to the PC2 axis

7

(31% and 24%), while OPH and FF contribute negatively (29% and 16%, respectively).

8

Based on this PCA, two relationships can be identified between the studied parameters (Fig.

9

2A). The fine fraction percentage and the population density of brittle stars are strongly

10

correlated (r = 0.84, N = 133, very highly significant, Table 2). The two biogeochemical

11

parameters TOC and TS are also correlated, but the correlation coefficient is lower (r = 0.65,

12

N = 133, very highly significant, Table 2). Although these two groups of variables are

13

positively correlated together, their link is weaker (r ranging from 0.33 to 0.47, N = 133, very

14

highly significant, Table 2).

15

The identification of the three environments (flood, storm and stability) highlights a specific

16

pattern (Fig. 2B). First, the fields of data points scarcely overlap, and each environment is

17

discriminated by the studied parameters. Data points converge when the O. fragilis population

18

density is low and diverge when it increases. The stability environment is associated with high

19

TOC and TS contents, while the storm environment has the lowest values of these parameters

20

and the flood environment is intermediate.

21

To understand the patterns of data point clouds, it is necessary to consider the raw dataset and

22

the variability of relationships according to the studied environments.

23

4.2.2 Relationships between Ophiothrix fragilis and sediment fine fraction

24

The flood environment represents samples taken in the Antifer patches area, five days after

25

an exceptional flood peak of the Seine (1 560 m3s-1), while the last storm occurred more than Page 16 sur 46

1

six months previously. Silting up increases linearly with O. fragilis abundance, showing a

2

high correlation (r = 0.95, N = 32, very highly significant, Table 2). This increase in fine

3

fraction ranges from 5% for one individual of O. fragilis per 0.25 m2 to 50% for abundances

4

of about 750 individuals per 0.25 m2, which represents more than 3 000 individuals per m2

5

(Fig. 3).

6 7

Figure 3: Relationship between number of Ophiothrix fragilis per 0.25 m2 and fine fraction percentage in flood

8

environment, showing linear regression (dashed green line).

9 10

The storm environment represents samples taken three days after a storm (Hmo > 4 m) in

11

the area of patches located off Antifer harbour, after a period of more than one year (428

12

days) during which no Seine flood was recorded. In this environmental context, silting up also

13

increases with O. fragilis abundance (Fig. 4). The correlation is lower than in the case of the

14

flood-influence situation (r = 0.76, N = 64, very highly significant, Table 2). No correlation is

15

found at low abundances (less than 250 individuals per 0.25 m2), (r = 0.15, N = 41, non-

16

significant) and the fine fraction percentage varies from 3% to about 20%, regardless of the

17

abundance of the population. On the contrary, in the case of high abundances (more than 250

18

individuals per 0.25 m2, i.e. 1 000 brittle stars per m2) the correlation coefficient becomes

19

high (r = 0.84, N = 23, very highly significant).

20

For population density > 250 O. fragilis per 0.25 m2, flood and storm fine fraction

21

percentages are significantly different (Student test, p-value < 0.005, i.e. 0.001665). So, at an

22

abundance of about 700 individuals per 0.25 m2, the fine fraction percentage difference is

23

about 5% (45% for flood conditions and 40% after a storm) and is statistically significant.

Page 17 sur 46

1

Figure 4: Relationship between numbers of Ophiothrix fragilis per 0.25 m2 and fine fraction percentage in a storm

2

environment, showing linear regression (solid red line). The linear regression for the flood environment (dashed green

3

line) is plotted for comparison.

4 5

The stability environment is an exceptional situation occurring during a period without a

6

recent flood or storm, i.e.: 289 days after a Seine flood and 661 days after a storm. The fine

7

fraction percentage increases with the abundance of O. fragilis, as observed with the flood

8

situation presented above (Fig. 5). However, the correlation coefficient is slightly lower (r =

9

0.81, N = 39, very highly significant, Table 2). The linear regressions for the stability (slope =

10

0.0602) and flood (slope = 0.0571) situations are nearly parallel, with a systematic enrichment

11

(about 5%) of the fine fraction percentage in the stability situation regardless of population

12

density.

13 14

Figure 5: Relationship between number of Ophiothrix fragilis per 0.25 m2 and fine fraction percentage in stability

15

environment, showing linear regression (dot-dashed blue line). The linear regression for the flood environment

16

(dashed green line) is plotted for comparison.

17 18

4.2.3 Relationships between Ophiothrix fragilis and TOC

19

During a flood situation, the TOC remains almost constant (1.5% to 2.0%) regardless of the

20

density of the O. fragilis population (Fig. 6A). The correlation coefficient r is positive, but

21

very low and non-significant (r = 0.22, N = 39, p > 0.05 i.e. non-significant, Table 2).

22 23

Figure 6: A) Relationships showing significant linear regressions between number of Ophiothrix fragilis per 0.25 m2

24

and TOC percentage. Flood environment: green open circles; storm environment: red crosses; stability environment:

25

blue dots and dot-dashed blue line; compact black muds: black triangles and solid black line. B) Relationships

Page 18 sur 46

1

showing linear regressions between number of Ophiothrix fragilis per 0.25 m2 and TS percentage. Flood environment:

2

green open circles; storm environment: red crosses; stability environment: blue dots and dot-dashed blue line;

3

compact black muds: black triangles and solid black line.

4 5

After a heavy storm (Fig. 6A), the TOC also remains constant regardless of the O. fragilis

6

population density, but falls to lower values (0.8 to 1.7%). Since the correlation coefficient r

7

is very low and non-significant (r = 0.08, N = 64, non-significant, Table 2), the two

8

parameters can be considered as independent.

9 10

During a period of stability, TOC increases with the O. fragilis population density (Fig. 6A),

11

from 1.5% for 1 individual per 0.25 m2, to 2.5% for 700 individuals per 0.25 m2. The

12

correlation coefficient r is high and very highly significant (r = 0.87, N = 39, very highly

13

significant, Table 2).

14

4.2.4 Relationships between Ophiothrix fragilis and TS

15

During floods, the TS percentage remains constant and low (0.13% to 0.35%) regardless of

16

the abundance of O. fragilis (Fig. 6B). The correlation coefficient r is significant but low (r =

17

0.37, N = 32, significant, Table 2).

18

After a heavy storm, the TS percentage remains unchanged regardless of the O. fragilis

19

population density, but falls to slightly lower values (0.06 to 0.30%) (Fig. 6B). Since the

20

correlation coefficient r is very low and non-significant (r = -0.10, N = 64, non-significant,

21

Table 2), the two parameters can be considered as independent.

22

In both situations, during floods and after a heavy storm, the TOC and TS percentages can be

23

considered as independent (respectively r = - 0.04, N = 32, non-significant and 0.04, N = 64,

24

non-significant, Table 2).

Page 19 sur 46

1

During a stability period, TS increases with the O. fragilis population density (Fig. 6B) and

2

the relationship shows a relatively high correlation coefficient (r = 0.60, N = 39, very highly

3

significant, Table 2). However, the correlation is lower than in the case of the population

4

density vs TOC relationship (r = 0.87, N = 39, very highly significant, Table. 2). For

5

abundances lower than 350-400 brittle stars per 0.25 m2, the TS percentage remains constant

6

and low (0.21% to 0.34%), as observed for flood and storm situations.

7

4.3

8

Compact black mud facies

4.3.1 Relationships between Ophiothrix fragilis and sediment fine fraction

9

The compact black muds correspond to samples collected during four surveys under different

10

environmental conditions (Table 1). The common characteristic of all these samples is their

11

sedimentary facies: compact mud-rich deposits associated with anaerobic degradation of

12

organic matter and formation of sulphide (black sediment colour and H2S odour).

13

The fine fraction percentage increases linearly with O. fragilis population density (Fig. 7).

14

Although there is a correlation between these two parameters, its significance is weak (r =

15

0.31, N = 56, significant, Table 2).

16 17

Figure 7: Relationship between number of O. fragilis individuals per 0.25 m2 and fine fraction percentage in black

18

muds, showing linear regression (solid black line). The linear regression for the flood environment (dashed green line)

19

is plotted for comparison.

20

4.3.2 Relationships between Ophiothrix fragilis and biogeochemical parameters

21

In the case of compact black muds, the TOC percentage increases with the O. fragilis

22

population density (Fig. 6A), rising from 2.0% for less than 10 individuals per 0.25 m2 to

Page 20 sur 46

1

2.9% for more than 850 individuals per 0.25 m2. The correlation coefficient r is high (r = 0.84,

2

N = 56, very highly significant, Table 2).

3

The TS percentage increases slightly with the O. fragilis population density (Fig. 6B). The

4

correlation coefficient r is positive but weak (r = 0.39, N = 56, highly significant, Table 2).

5

However, although the correlation is more significant in the case of the stability environment,

6

the compact black mud samples yield higher TS percentages ranging from 0.4% to 1.2%.

7

5

8

5.1

9

Discussion Environmental situations

5.1.1 Relationships between Ophiothrix fragilis and sediment fine fraction

10

In flood situation (Fig. 3), silting up increases linearly with O. fragilis abundance, showing a

11

high correlation (Table. 2). The supply of suspended matter related to the flood can be

12

considered as homogeneous over the entire study area, which small (radius of about 150 m) is

13

compared with the distance to the river mouth (30 km) and the size of the Seine plume. This

14

suggests that the difference in silting up between samples is mainly due to the variability of O.

15

fragilis abundance.

16

The influence of the O. fragilis population could be the result of two processes: (1) the mode

17

of feeding of O. fragilis, by collecting suspended particles with their raised arms, followed by

18

the release of particles as faeces. These faecal pellets (Migné et al., 2012) are weakly

19

consolidated and packed in a framework of mucus which limits their destruction by erosional

20

processes, (2) the physical impact of the O. fragilis population at the sediment-water

21

interface. By attributing an average diameter of about 5 cm to an individual O. fragilis, we

22

can assume that the area covered by this individual on the sea bottom is around 0.002 m2. As

23

a consequence, considering a maximum abundance of between 400 and 500 individuals per

Page 21 sur 46

1

m2, the brittle stars are necessarily tangled and overlapping. This causes a reduction in the

2

bottom boundary current which, accordingly, leads to the deposition of suspended particulates

3

brought in by the turbid flood plume. In both cases, the efficiency of trapping and silting up is

4

proportional to the abundance of the population.

5

In the storm environment, a relatively high amount of silting up (up to 20% fine fraction)

6

occurs (Fig. 4) at very low population densities (less than 30 O. fragilis individuals per 0.25

7

m2). These samples probably reflect previous sedimentation conditions under higher O.

8

fragilis abundance; the storm could have dispersed the population and fine sediments could

9

have been partly eroded. However, for high population densities, there is a slight but

10

significant reduction (about 5%, Fig. 4) of the fine fraction percentage for the same O. fragilis

11

abundance in the flood environment. This can be interpreted either as due to an increased

12

particulate input during the previous flood or erosion of the sea bed by waves during storms.

13

In any case, a high abundance of O. fragilis severely limits but does not prevent erosion of the

14

underlying sediments. This is in agreement with the observations of Méar et al. (2006), who

15

showed that heavily silted sediments in the eastern Baie de Seine affected by strong wave

16

activity loose up to 50% of their fine fraction. The same kind of sediment, when covered by a

17

dense population of brittle stars, loses only 10% to 15% of its fine fraction. Based on the

18

temporal variability of the O. fragilis patches, Dauvin et al. (2013) have shown that storms

19

induce only temporary impacts, mainly in zones with low abundances. Although storms have

20

an erosional impact, the effect is limited if the O. fragilis population density is high.

21

In the stability environmental context, silting up also depends on the number of O. fragilis

22

individuals at the water-sediment interface. Population density remains the key factor

23

controlling silting up. For the same population density, the fine fraction percentage is higher

24

during a period of stability than during a flood, irrespective of the number of brittle stars per

25

0.25 m2. In the absence of a recent flood, there is no reduction in silting up if no storm occurs

Page 22 sur 46

1

during the considered period. By comparing the data characteristic of flood and stability

2

situations (Fig. 5), we can infer that the duration of the stability period is a key parameter

3

controlling silting up in the environment of the benthic population.

4 5

5.1.2 Relationships between Ophiothrix fragilis and TOC

6

During a flood situation, the constant TOC percentage (Fig. 6A) probably reflects the

7

influence of terrigenous organic matter present in the suspended matter brought in by the

8

Seine flood.

9

After a heavy storm, strong swell may induce an increase of dissolved oxygen concentration

10

in the water column, leading to reoxygenation of the upper part of the sediment. This gives

11

rise to a higher activity of aerobic bacteria and therefore a decrease in the amount of organic

12

matter preserved in the sedimentary deposits (Fig. 6A). At the sediment-water interface,

13

brittle stars form a protective mat trapping the fine particles, thus preventing erosion, but they

14

do not impede the establishment of vertical water exchange during storms.

15

In both situations, during floods and after a heavy storm, the TOC percentage reflects the

16

instantaneous event and is not influenced by the O. fragilis population density.

17

During a stability period, system remains undisturbed by any event (flood or storm) for a long

18

time. This stability allows the population of brittle stars to influence its environment. When

19

the abundance of the population increases, both the TOC and the fine fraction increase in

20

parallel (Fig. 5 and Fig. 6A). This increase in TOC percentage could be related to: (1) increase

21

of faeces along with the number of individuals, (2) the thickness of the layer of brittle stars,

22

which would limit the exchange of water and thus the renewal of dissolved oxygen consumed

23

during the degradation of organic matter by aerobic bacteria, (3) a decrease in the porosity of

24

the sedimentary deposits linked to silting up, which would have the same effect (the TOC vs

Page 23 sur 46

1

FF correlation is high, r = 0.68, Table 2), or which might also be related to the natural

2

consolidation of fine-grained deposits.

3

These processes would result in: 1) an insufficient amount of dissolved oxygen available for

4

aerobic oxidization of the organic matter, and 2) development of the less effective sulphate

5

reduction mechanism, resulting in an increase in the preservation of organic matter (TOC

6

percentage) (Lehmann et al., 2002). During the stability situation, dense O. fragilis

7

aggregations directly affect their environment by excreting organic matter, and also as a

8

function of their abundance, by limiting the renewal of dissolved oxygen at the sediment

9

surface. They also have an indirect influence by causing silting up, which has the same effect

10 11

as limiting the oxygen supply. 5.1.3 Relationships between Ophiothrix fragilis and TS

12

During floods (Fig. 6B), the consistently low values of TS probably reflect the homogeneity

13

of suspended matter brought in by the Seine flood. Sulphur is mainly present as sulphate. No

14

sulphate reduction occurs because the sediment-water interface is not in a steady state, as a

15

result of the high sediment supply. The analysed TS contents reflect the source rather than the

16

sedimentary deposit under the brittle star-rich layer.

17

After a heavy storm, strong swell may cause an increase of dissolved oxygen concentration in

18

the water column, leading to reoxygenation of the upper part of the sediment. Enhanced

19

activity of aerobic bacteria restricts sulphate reduction as well as TS content (Fig. 6B).

20

In both situations, during floods and after a heavy storm, the TOC and TS percentages are

21

independent. This lack of correlation support the hypothesis of sulphate reduction deficiency.

22

During a stability period, the weakness of the correlation between TS and population density

23

regarding TOC vs population density correlation (Table. 2) is the result of the restriction of

24

sulphate reduction at low population densities. Moreover, for low density O. fragilis

Page 24 sur 46

1

population, the TS percentage remains constant and low as observed for flood and storm

2

situations in relation with an oxic respiration mode of organic matter degradation. The

3

Ophiotrix abundance vs. TS correlation becomes stronger and more widespread only at high

4

abundances when sulphate reduction appears in the uppermost 2 cm of the sediment

5

associated with TS contents higher than 0.35%. As in the case of the TOC and the fine

6

fraction percentages, TS percentage is dependent on O. fragilis abundance during a stability

7

period. The TS results corroborate the development of anoxic conditions in the sediment

8

under the brittle star-rich layer.

9

5.1.4 Synthesis

10

A comparative analysis of the three studied environments (flood, storm and stability) shows

11

that, for the Antifer patches, silting up is linked with the O. fragilis population density. An

12

overview of O. fragilis and sedimentary facies relationships in the Baie de Seine indicates that

13

this species is not dependent on silting up, since it is commonly found on gravel and coarse

14

sediments (Gentil and Cabioch, 1997; Lozach et al., 2011). On the contrary, we may infer that

15

silting up is controlled by both O. fragilis population density and the length of the population

16

stability period.

17

The E-PCA reveals the impact of O. fragilis population density:

18

(1) With low-abundance aggregations, the three studied environments (flood, storm,

19

stability) are convergent (Fig. 2B), and the fine fraction, TOC and TS contents are

20

low. Hence, O. fragilis aggregations do not impact their environment.

21 22

(2) With high-abundance aggregations, the three environments are divergent (Fig. 2B). Dense O. fragilis aggregations impact their environment.

23

Page 25 sur 46

1

This divergence may be explained by different impacts of the O. fragilis-rich layer in the case

2

of each environment. In the flood environment, silting up is controlled solely by the high

3

abundance of O. fragilis. The sediment-water interface is not in a steady state, and the

4

sediment characteristics reflect the instantaneous event. During a storm, the O. fragilis-rich

5

layer forms a protective mat for fine particles, thus limiting the erosion. But heavy swell may

6

lead to an increase of dissolved oxygen in the water column producing reoxygenation of the

7

upper part of the sediment. Enhanced activity of aerobic bacteria restricts sulphate reduction

8

and limits the amount of organic matter preserved in sedimentary deposits. Again, we find

9

that high abundances of O. fragilis alone control the silting up.

10

Floods and storms are events of short duration, whereas dense O. fragilis aggregations require

11

some time to produce geochemical modifications in their environment. Therefore, in a

12

stability situation, high-abundance O. fragilis aggregations are able to modify their

13

environment, producing increased silting up along with anaerobic conditions and enhanced

14

organic matter preservation. SB-PCA (SB stand for StaBility environment) in most cases

15

yields high correlations, whatever parameters are compared (r > 0.6). Thus, in a stability

16

situation, O. fragilis adapts its behaviour to the surrounding conditions: offshore from Antifer

17

harbour, the occurrence of high-density patches allows individuals to survive high-energy

18

hydrodynamic conditions (strong tidal currents and storms). This efficient strategy means that

19

O. fragilis can act as a geoengineer species, which influences depositional and/or erosional

20

processes in the sediment, excluding bioturbation. O. fragilis is a vagile epifaunal suspension

21

feeder. In cases of high population density, this geoengineer species changes the depositional

22

processes directly by collecting suspended particles with their raised arms and indirectly by

23

reducing the speed of bottom boundary currents. Accordingly, this has the effect of enhancing

24

the deposition of suspended particles. Moreover, brittle stars directly modify the erosional

25

processes by producing faecal pellets and indirectly by forming a protective mat.

Page 26 sur 46

1

5.2

Compact black muds facies

2

A comparison of the results obtained from the compact black muds and the three

3

environmental situations discussed above shows that this facies displays a weak correlation

4

between fine fraction percentage and O. fragilis population density. Compact black muds

5

always contain a high percentage of fine sediment fraction (> 30%), even at low population

6

densities (less than about 380 O. fragilis per 0.25 m2). Under these conditions, silting up is not

7

closely linked to population density.

8

On the opposite, the correlation between TOC percentage and O. fragilis population

9

abundance is high. Thus, we may consider that the O. fragilis population controls the amount

10

of preserved organic matter. A comparison between situations associated with stability and

11

compact black muds shows similar results when considering high abundances of brittle stars

12

(> 430 individuals per 0.25 m2). The processes invoked to explain this correlation during a

13

period of stability remain valid for the compact black muds. At lower abundances, however,

14

TOC percentage is always higher in the compact black muds. This difference may be linked

15

either to the higher fine fraction percentage of the compact black muds and/or their higher

16

degree of compaction. In both cases, the diffusion of dissolved oxygen is restricted and the O.

17

fragilis abundance cannot be considered as a limiting parameter.

18

The compact black muds represent the only sedimentary facies with normal marine TS/TOC

19

ratios (mean value = 0.29) (Rullkötter, 2000). The three environmental situations discussed

20

here are associated with much lower mean TS/TOC ratio values (storm: 0.11; flood: 0.14;

21

stability: 0.16). Such low values are generally interpreted as being characteristic of fresh

22

water sedimentary deposits. In the Baie de Seine, the environment is clearly oxic with normal

23

marine conditions, and these low values reflect organic matter degradation due essentially to

24

aerobic respiration rather than sulphate-limited mechanisms. The studied samples were taken

25

in the uppermost 2 cm of the sediment. In the storm and flood environments, this 2-cm-thick Page 27 sur 46

1

surface layer is fully oxic, but, in the stability environment and in the case of high O. fragilis

2

population density, the oxic-anoxic interface moves upward and reaches the uppermost layer

3

of the sediment. Due to the consolidation of compact black mud facies, oxygen penetrates

4

only a few mm into the sediment, and the uppermost 2 cm of the sediment becomes mainly

5

anoxic, with sulphate reduction being the main mechanism of organic matter degradation.

6

The normal marine TS/TOC ratio observed in compact black muds indicates that

7

concentrations of organic carbon and sulphur have reached a quasi-steady state. The

8

development of compact black mud facies results from the persistent stability of patches with

9

highly abundant O. fragilis, and may be interpreted as a final stage in the establishment of the

10

stability environment. In this situation, dense O. fragilis aggregations are able to geoengineer

11

their environment. Their activity induces an increase of silting up, leading to anaerobic

12

conditions and the preservation of organic matter in sediments containing up to 50% fine

13

fraction, 3% TOC and 1% TS. At the same time, as compaction is time-dependant,

14

environmental stability leads to the compaction of muddy sediments and, consequently, a

15

decrease in water content and oxygenation of the upper part of the sediment, along with a

16

higher resistance to erosion. Even if the O. fragilis population density is decreasing, compact

17

mud offers a higher resistance to erosional processes than non-compacted sediment. Thus,

18

relative to the fine fraction percentage, the compact black muds facies is less dependent on O.

19

fragilis population density.

20

Page 28 sur 46

1 2

5.3

Interactions between environment and Ophiothrix fragilis geoengineer species

5.3.1 Environmental forcings and development of Ophiothrix fragilis population

3

The spread of compact black mud facies is a consequence of O. fragilis geoengineering

4

despite high-energy hydrodynamic conditions. It remains to address the question of why O.

5

fragilis dense aggregations are so stable in such an environment.

6

Unfortunately, we have no information on the size distribution structure of the Ophiothrix

7

population during our surveys. Nevertheless, Lefbevre et al. (2003) have shown that

8

persistence of the O. fragilis population in the eastern Baie de Seine could be ensured by self-

9

recruitment and more or less permanent fluxes of larvae coming from populations in the

10

western Baie de Seine and off the Pays de Caux. In this latter area, Muths et al. (2010) have

11

shown that recruitment events occur several times a year but in pulses of small numbers of

12

settlers. Therefore, 2-year old adults form mainly the O. fragilis population. As in other

13

populations, juveniles and adults are closely associated with one another, and both the adult

14

morphology and behaviour of juveniles play an important role in long-term stability of the

15

eastern Baie de Seine population. This population could be maintained by favourable

16

hydrodynamic conditions (Morgan and Jangoux, 2004, 2005).

17

We can assume that brittle stars find excellent conditions for their development in the area

18

offshore from Antifer. Food is provided either by suspended particulates rich in organic

19

matter coming from Seine floods or by locally higher pelagic productivity controlled by

20

nutrient supply from the Seine (Dauvin and Ruellet, 2008). The total number of ophiurids

21

should be dependent on total food supply, which is consistent with the observations of Dauvin

22

et al. (2013) showing that two years without a flood causes a decline in O. fragilis population.

23

Paradoxically, the high-energy hydrodynamic conditions in this area represent an advantage.

24

In calm conditions, the long-term stability of dense O. fragilis aggregations will result in

Page 29 sur 46

1

severe anoxic conditions within the sediment and the release of H2S towards the surface layer

2

containing the living O. fragilis. However, more than anything else, O. fragilis needs oxic

3

conditions to survive. Episodic storms, and probably semi-permanent tidal currents as well,

4

will enhance the vertical mixing of water, thus leading to the renewal of dissolved oxygen

5

consumed by the aerobic bacterial degradation of organic matter. These processes ensure the

6

almost unchanging geographical location of the patches. The main environmental forcings

7

appear to be Seine river input as well as regional circulation controlled by tidal currents and

8

storms.

9 10

5.3.2 Effects of geoengineering on rate of silting up and development of anoxic conditions

11

Ophiothrix fragilis is able to geoengineer its environment in various ways and develop effects

12

with different rates of response. Silting up is dependent on the abundance of O. fragilis and

13

occurs at very fast rate, which means that floods and storms reflecting an instantaneous event

14

result in a steady-state situation with respect to the fine sediment fraction percentage.

15

Anoxic conditions are dependent on the degradation of organic matter and require more time

16

to become established. Such conditions appear after many months without any major

17

disturbing event in uncompacted muddy sediments (stability situation), then representing the

18

normal surficial situation when the sediment becomes compacted (compact black muds). This

19

process is dependent on O. fragilis population density and takes place at a very slow rate.

20

In this part of the Baie de Seine, the great spatial and temporal variability of the sediment

21

characteristics is due to: 1) changing intensity of geoengineering activities; 2) environmental

22

events (flood discharge, storms) with irregular frequency and 3) the possible temporal

23

displacement of patches with highly abundant O. fragilis. A credible hypothesis can be

24

proposed to explain the different situations encountered.

Page 30 sur 46

1 2 3

5.3.3 Geoengineering hypothesis Figure 8 illustrates the different stages of the hypothesis.

4 5 6 7 8

(1) In the initial stage, coarse sediments are present containing only a few percent of fine fraction and low numbers of O. fragilis. (2) A flood event supplies fine particles as well as organic matter. When the O. fragilis aggregation density is sufficient, an increase of the silting up occurs.

9

(3) After a long period without a storm, the O. fragilis aggregation density increases along

10

with silting up (b); if the aggregation density is very high (a), anoxic conditions may

11

exist within the sediment close to the surface.

12

(4) A storm event occurs: in the case of low or moderate aggregation densities of O.

13

fragilis (b), there is a high degree of patches dissociation and the dispersion of

14

individuals, as well as erosion of fine sediment. For the denser aggregations (a), which

15

are more resistant to wave action, the erosion is limited. However, irrespective of the

16

aggregation density, reoxygenation occurs in the surficial deposit.

17

(5) After a long period of stability in a patch with highly abundant O. fragilis, the fine

18

fraction reaches values of 30% to 50% and the sediment becomes compacted, while

19

reoxygenation during storm events is limited and anoxic conditions are fully

20

established. Compact black muds are formed.

21

(6) An exceptional storm event displaces the patch with high abundance of O. fragilis.

22

(7) Some of the previously deposited compact black muds are now devoid of or only

23

support scarce O. fragilis aggregations, and are no longer protected from erosional

Page 31 sur 46

1

processes. This erosion results in heterogeneous sediments (gravels mixed with sands

2

and mud clasts).

3

(8) A new cycle begins.

4 5

Figure 8: Geoengineering scenario. Black arrows indicate the main stages in the development of the proposed

6

Ophiothrix-sediment relationships and sediment characteristics (see corresponding item in the text for more details).

7

Green arrows indicate reversible changes.

8 9

6

Conclusion

10

In the eastern Baie de Seine, in a high-energy environment off Antifer harbour, silting-up and

11

the development of anoxic conditions lead to high abundances of the vagile epifaunal species

12

Ophiothrix fragilis. The temporal stability of these dense aggregations results in the

13

development of mud patches. Modifications of the environmental situation, such as during an

14

exceptional storm, are able to induce a spatial shift of the patches with high abundance of

15

Ophiothrix fragilis. As a consequence, mud patches are no longer protected from erosional

16

processes and the sediment characteristics show a high spatial and temporal variability

17

ranging from muds to gravelly sands.

18

All the recorded environmental modifications result from the occurrence of high-density

19

patches where Ophiothrix fragilis is able to geoengineer its own environment. The term

20

“geoengineer species” used here is distinct from previous descriptions in terms of “ecosystem

21

engineer organisms” (Jones et al., 1994; 1997), a concept developed by Guttiérrez and Jones

22

(2006), Badano and Cavieres (2006) and Kristensen (2008). Geoengineer species only act on

23

depositional and/or erosional processes affecting the sediment, excluding bioturbation.

Page 32 sur 46

1

High-energy shelves are continuously swept clear of fine sediment particles. Wave action

2

leads to the suspension of fine particulates and tidal currents export this sediment load to

3

greater depths or along the coast towards sheltered areas. The pattern of sedimentation in the

4

English Channel is compatible with such processes. However, we demonstrate that muds can

5

be deposited and buried even under these high-energy conditions. Muddy deposits are linked

6

to the geoengineering activity of epifaunal species such as Ophiothrix fragilis. In ancient shelf

7

environments, the occurrence of fine-grained sediments should no longer be systematically

8

considered as characteristic of a low-energy depositional environment.

9 10

Acknowledgements

11

The authors are grateful to the crews of R/V Côtes de Normandie and R/V Côtes de la

12

Manche for their help during sampling. This study was undertaken as part of the Seine-Aval

13

programme (COLMATAGE project), co-coordinated by the Seine Aval Public Interest Group

14

(GIPSA) and the Haute-Normandie region of France. The authors wish to thank Mike

15

Carpenter for correcting the English style and grammar.

16

Page 33 sur 46

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O. fragilis geoengineers its environment in various ways and at different rates.

13

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14

Increased silting up leads to anaerobic conditions (preservation of organic matter)

15

Main environmental/external forcings are floods, tidal currents and storms.

16

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17

Table 1 Flood and storm characteristics taken into account for the seven surveys.

F Flood

N

SB

E

B Black

A All

Stability

Environment

muds

data

133

56

189

0.84 (VHS)

0.31 (S)

S Storm

32

64

39

0.95

0.76

0.81

OPH FF (VHS)

(VHS)

0.64

(VHS)

(VHS)

0.87 OPH TOC 0.22 (NS)

0.08 (NS)

0.45 0.40 (VHS)

(VHS)

0.84 (VHS) (VHS)

Page 44 sur 46

F Flood

E

B Black

A All

Stability

Environment

muds

data

-0.10

0.33 (VHS)

0.39 (HS)

0.31

0.60

0.37 (S)

OPH TS

FF

SB S Storm

(NS)

(VHS)

-0.14

0.68

(VHS) 0.68

TOC 0.18 (NS)

0.46 (VHS) (NS)

0.33 (S) (VHS)

(VHS)

0.67 FF

0.34 (NS)

TS

0.06 (NS)

0.44 (HS)

0.47 (VHS)

0.34 (S) (VHS)

-0.04 TS

TOC

0.80

0.63 0.65 (VHS)

0.04 (NS) (NS)

0.37 (HS) (VHS)

(VHS)

1

NOTES.- N: number of samples; OPH: number of Ophiothrix fragilis per 0.25▒m2; FF: fine

2

fraction percentage; TOC: Total Organic Carbon; TS: Total Sulphur. p > 0.05 i.e. non-

3

significant (NS); p between 0.05 and 0.01, significant (S); p between 0.01 and 0.001, highly

4

significant (HS); and p < 0.001, very highly significant (VHS).

5 6 7

Table 2: Correlation coefficients (r) derived from PCA analysis. Seine flood Surveys

Sampling date Maximum flow (m3.s1)

Storm Volume

From the

Hmo

From the

(m3)

last flood

(m)

last storm

(days)

(days)

1991

July 1991

1512

2358720000 160

4.1

169

1998

September

1359

1952640000 119

4.7

147

1338

1560384000 15

4.0

112

1998 1999

May 1999

Page 45 sur 46

Seine flood Surveys

Sampling date Maximum flow (m3.s1)

Storm Volume

From the

Hmo

From the

(m3)

last flood

(m)

last storm

(days) SA09

February

(days)

1490

1167091200 30

4.3

43

2009 Flood

April 1999

1560

4103827200 5

4.5

197

Storm

February

1207

684633600

428

4.1

3

1207

684633600

289

4.6

661

1990 Stability

October 1989

Black

Between 1990

muds

and 2009

1

Page 46 sur 46

Figure1

Figure2a

Figure2b

Sediment fine fraction (%)

Figure3

FLOOD 60 50 40 30 y = 0.0571x + 3.968

20 10 0 0

200

400

600

O.fragilis abundance (individuals / 0.25 m²)

800

Figure4

STORM 60

Sediment fine fraction (%)

50 40 30 y = 0.0620x −2.485 20 10

Linear regression line for > 250 O. fragilis per 0.25 m²

0 0

200

400

600

O.fragilis abundance (individuals / 0.25 m²)

800

Sediment fine fraction (%)

Figure5

STABILITY 60 y = 0.0602x + 8.434 50 40 30 20 10 0 0

200

400

600

O.fragilis abundance (individuals / 0.25 m²)

800

TOC (%)

Figure6a

3.5

3.0

A y = 0.0009x + 2.003

2.5 y = 0.0015x + 1.494 2.0

1.5

1.0

0.5

0.0 0

200

400

600

800

O.fragilis abundance (individuals / 0.25 m²)

1000

Figure6b

1.5

B y = 0.0007x + 0.443

TS (%)

1.0

0.5 y = 0.0003x + 0.196

0.0 0

200

400

600

800

O.fragilis abundance (individuals / 0.25 m²)

1000

Sediment fine fraction (%)

Figure7

COMPACT BLACK MUDS 60 y = 0.0252x + 30.315 50 40 30 20 10 0 0

200

400

600

800

O.fragilis abundance (individuals / 0.25 m²)

1000

Figure8