® •

THE MEASUREMENTS OF NUTRIENT FLUXES AT THE

WATER-SEDIMENT INTERFACE: INCUBATION METHODS

(IN LABORATORY AND IN SITU) ; GRADIENT METHOD

(CALCULATED FLUXES) ; BENTHIC ECOSYSTEM TUNNEL

M. FEUILLET-GIRARD(1), D. GOULEAU(1), W. ZURBURG(2),

A.C. SMAAL(3), N. DANKERS(4), M. HERAL(5)

(1) CNRS-IFREMER, Crema L'Houmeau B.P. 5, 17137 L'Houmeau (France). (2) Delta Institute for hydrobiological Research, Vierstraat 28, 4401 EA Yerseke (Netherlands) (3) Ministry of Transport and Public Works, Tidal Waters Division, P.O. Box 8039, 4330. EA Middelburg (Netherlands). (4) mN TEXEL (Netherlands). (5) LABEIM-IFREMER, B.P . 133, 17390 La Tremblade (France).

- 6S -

THE MEASUREMENTS OF NUTRIENT FLUXES AT THE WATER­SEDIMENT INTERFACE : INCUBATION ~1ETHODS (IN LABORATORY • " AND IN-SITU ; GRADI ENT METHOD (CALCULATED FLUXES) BENTHIC ECOSYSTEM TUNNEL.

, . ' ' , ' .

M. FEUILLET-GIRARD (1), D. GOULEAU (1), V. ZURBURG (2) , A.C. SMAAL (3), N. DANKERS (4), M.HERAL (5).

(1) CNRS-IFREMER, Crema l'Houmeau B.P. 5 17137, , l'Houmeau FRANCE.

(2) Delta 1 nsti tute for hydrolog ical Research, Vierstraat 28, 4401 EA Yerseke, The NETHERLANDS. (3) Ministry of Transport and Public Works, Tidal Wl\ters Division, P.O. Box 8039, 4330. EA ~liddleburg,

The NETHERLANDS. (4) 1 B N. TEXEL, THE NETHERLANDS.

_ (5) Labeim-IFREMER B.P. 133, 17390 La Tremblade FRANCE.

INTRODUCTION

Many flux measurements methods have been described in the literature. The purpose of our study was to compare results obtained by these different methods : Gradient Method, Incubation ~Iethod (Laboratory and In Si tu), Benthic Ecosystem Tunnel. The field experiment has been performed in the Marennes Ol éron Bay since May 1991 (contrat C.E.E. FAR) .

DIAGRAM OF THE APPARATUS DESIGNED FOR MEASUREMENT OF FLUXES AT SEDIMENT-WATER INTERFACE

1. - GRADIENT METHODS : calculated fluxes w i th F ick' first law

.-

dc ~ = -n D IlM m-2 h-l

dz

~ OVCII,in9 w~ltr ,-__ ~~~ ____ ==-==-____ ~ C, •

NU I,,«"1 conCtnlf~tlon

Lnel 0 ----- ----.. ----~-- '. ~ c, _.· .• . , . , .~ ,~ ~ . . . - .

, ; " " . ".

: ~ . . "

Sêdimenl

O z Oillus,on cocll ,( ,lnl

n: Poros , l y

Port .... .. I tl

t~ll .. cllon t

C"

!Jc~C , ·c.

'luIIHI: n! ( ooccnlr.l,on o 0.5 cm

2.- I NCUUi\T ION ~IETIIOVS

r 6em

6em

l

-66-

Li\BOIIATORY (in the dark)

slirr\! r waler

sedimtnt sUllace : 8.1 en"

sedtme:nlcOfe

•

wat t t - billh wilh 12 COfe5 ., plug TO in situ

Short incubation .J-c. h

ill SITU (in tlle dark)

P,V, C.

j 6,8 em

Sedi ment A'ea : JG,J2crn'

[

- 67 -3. - IlENTIlI C ECOSYSTHI TUNNEL

RESULTS

~

l: N

É ::;: ::l ..,.

l Z

Yzcm

Neopre ne Ski rt ~

Reinforcement bottom surf ace = 8,5 ml

Benthic Ecosystem Tunnel

(BESn

The: BEST struclu re usc:d 10 obt:l.in nUACS of mltCfI:Jh Jeron J.J1 opler retr.

300

200

100

0

· 100

·200

~OO

HOW NEASUREHENTS OF FLUXES ARE FUNCTION OF THE USED HETHODS ?

AMMONIUM FLUXES

642

1.

~ '- L 1 . 1. 1 Il~ .... • .. • 1

1 May 1 1 Oclober 1

13 15 21 23 8 10 22 24

Sampling Dais

1_ Calculated _ LDbOIBIOry _ ln Situ

50

~ a ,

.s:

'" -50

è -100

:; ·150 :J

'" -200 0 Z -250

-300

- 68 -

NITRATE FLUXES

1 Oclober 1

13 15 21 23 B 10 22 24

Sampling Dale

1 D Calculated _ Laooralory _ ln Situ

Pore water concentrations of NH4+ and N03- ranged from about 10 to 120 p~l and 0.42 to 20 pM. Calculated Ammonium Fluxes were higher than the measured ones. This difference could be explained by the decrease of molecular diffusion on ;the sediment surface (see picture 2).

[NCUBA TI ON MErl/OOS

- 69 -

AMMONIUM FLUXES

40

JO

~ 20

1-1-. ~ N la-E 0 ::;: -la :J ..,. -20 I

~ 190 Z -30

1 May 1 1 Oclober -40

-50 13 15 21 23 6 la 22 24

Sampling Date

1_ Labolalory _ ln Silu

Both measurements, in laboratory and in situ ,!(ave weak flux intensities compared with previous measurements. Short incubations in laboratory (4h) showed that ammonium fluxes were dominated by a weak re lease in ~Ia)'. Reverse fluxes were observed in October. In the case of in situ incubation, we fo~nd r everse fluxes in May, and releases in October. Differences in sampling might account for the discrepancies between the results of both techniques.

NITRATE FLUXES

50~--------------------------------------,

-\------r~· ~ -5:-:1-' E -100 ::;: :J cry o z

-150

-200

-250

13 15 21 23 6 10

Sampling Date

1_ Laboralory ~ ln SUu

22 24

In the case of in situ incubation using micro benthic chambers Nit ra te fl ux intensities were hir:;he r than those obtained by the laboratory incubation method . In October differences were l owe r . III almost a IL cns~s. we obsened a reverse flux that could be thE' .-esul t of nutrient uptake by nitrif r in" bacteria u r bv denitrificntion.

I-\IVIIVIUI\lIUIVI rLUI\Cù _ 70 _

Tunnel experiment 13-14.05.-1991 For a tidal cycle. 92-95 caeL

15 16 ,. 2 J 4 5 6 7 Hours

Here 1-Ie anly show results obtained in spring tide over an oyster bed, we cou ld not obtain eithe r uptake o r release wi th the control tunnel. By another way Ammonium flux calcu lated with Ammonium excretion ra te found in ~IIlY in laborato r y, a<:reed wi th fluxes found in the tunnel and r eae hed 400 I.fl m-2 h-1.

il

NH4 EXCRETIOI\j RÂ TE by oysters in laborator y

Moy

2 .3 4 5 6 7 8 9 10 11 12

l,lOIS

•

CONCLUSION - 71 -

The non-ag reement bet" een flu xes estimated t'rom pore water profiles and exchange measurements is apparent for Ammonium fluxes. This disagreement could be explained by the presence of Il biof ilm on the sedimen't surface which hides the diffusion. The fluxes measured by the SEST permitted us to measure an Ammonium release from oyster bed and the mussel bed, but the control tunnel did not take into account the flux coming from the sediment, because the resident time was tao short.