The exceptional Oder Flood in summer 1997 — the fate of nutrients and particulate organic matter in the Baltic Sea

Deutsche Hydrographische Zeitschrift German Journal of Hydrography Volume 50 (1998) Number 2/3

ISSN 0012-0308 �9 BSH, Hamburg und Rostock

The Exceptional Oder Flood in Summer 1997- The Fate of Nutrients and Particulate Organic Matter in the Baltic Sea

C. HUMBORG, G. NAUSCH, T. NEUMANN, F. POLLEHNE, N. WASMUND

Summary

The distribution of dissolved inorganic nutrients, particulate organic matter and phytoplankton pigments in the Oder plume was investigated at peak discharge of the Oder River during the exceptional flood event in summer 1997. Mixing diagrams of dissolved inorganic nutrients (NO3-, PO43-, SiO44-) reveal a nearly complete removal of nitrate during the first steps of estuarine mixing, whereas phosphate and silicate were still available over the entire salinity range. In contrast to silicate and phosphate, the nitrate riverine end-member concentration was about 3-4 times lower than during peak discharge in spring. It appears that during the summer flooding event inorganic nitrogen was not as available as in spring due to a stronger dilution effect and the advanced retention of nitrogen by land vegetation within the drainage area in summer. Therefore, algal biomass production in the Pomeranian Bight was most certainly nitrogen limited and significant removal of other dissolved inorganic nutrients by phytoplankton blooming did not occur.

These observations are consistent with the estuarine distribution of most of the particular organic matter variables (Chl a, POC, PON), which showed conservative mixing patterns during the Oder outflow situation. During the first observation high concentrations of up to 200 IJg/dm 3 Chl a were found near the Swina mouth at low salinities. However, microscopic analyses revealed that these water masses were most certainly washed out from the Szczecin Lagoon, and do not reflect net production in the Pomeranian Bight. Algal pigments (Fucoxanthin, Zeaxanthin) and phytoplankton species analyses indicate that diatoms, which were predominant in the outflowing river water, were mixed conservative, although silicate in the river was readily available. Zeaxanthin differed from the conservatively mixing line due to the usual high summer concentration of pico-cyanobacteria in the open Baltic. --

Model results showed that the Oder flood event had a more regional character. The south-western Po- meranian Bight was affected by the Oder discharges, whereas adjacent coastal seas were not directly influ- enced by increased organic loads. Nevertheless, due to the strong stratification of the water column within the Pomeranian Bight, regionally limited occurrences of anoxia were observed near the bottom.

Die auSergew6hnliche Oderflut im Sommer 1997- Der Verbleib yon N~ihrstoffen und partikul~iren organischen Materials in der Ostsee (Zusammenfassung)

W&hrend des extremen Sommerhochwassers der Oder des Jahres 1997 wurde die Verteilung anorganisch gel6ster NAhrstoffe (NQ-, PO43-, SIO44-), partikulArer organischer Substanz sowie der Phytoplankton- pigmente in der MQndungsfahne der Oder untersucht. Vermischungsdiagramme anorganisch gel6ster N&hr- stoffe weisen auf eine fast vollstAndige Aufnahme des Nitrates wAhrend der ersten Schritte der Vermischung hin, wo hingegen Phosphat und Silikat noch in mel3baren Konzentrationen 0ber den gesamten Salzgehalts- gradienten vorhanden waren. Im Gegensatz zu Phosphat und Silikat war die Nitratausgangskonzentration im Flul3 ca. 3-4 mal geringer als bei einem 0blichen FrL)hjahrshochwasser. Die anorganisch gel6sten Stick- stoffverbindungen waren aufgrund des VerdQnnungseffektes sowie der fortgeschrittenen Fixierung durch Landvegetation im Sommer weniger verf0gbar. Die Biomasseproduktion in der Pommerschen Bucht war daher vermutlich stickstofflimitiert, ein massives Auftreten von PhytoplanktonblQten in der Folge wurde daher verhindert.

Diese Beobachtungen decken sich mit dem ,~,stuarinen Verteilungsmuster der partikulAren organischen Substanz (Chl a, POC, PON), die konservativ eingemischt wurde. WAhrend einer ersten Beprobung in un- mittelbarer N&he der SwinamQndung bei niedrigen Salzgehalten wurden bis zu 200 iJg/dm 3 Chl a gemessen.

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Deutsche Hydrographische Zeitschrift- German Journal of Hydrography

Mikroskopische Untersuchungen ergaben jedoch, dab diese Wassermassen vermutlich ausgewaschenes Haffwasser darstellten und nicht eine Biomasseproduktion in der Pommerschen Bucht widerspiegeln. Pig- mentmessungen (Fucoxanthin und Zeaxanthin) sowie Phytoplanktonuntersuchungen zeigten, dab Diato- meen, die die Phytoplanktonbiomasse im Oderwasser dominierten, konservativ eingemischt wurden, obwohl Silikat hinreichend zur VerfQgung stand.

Modellberechnungen zeigen, dab die Oderflut eher regionalen Charakter hatte. Die sedwestliche Pom- mersche Bucht war haupts&chlich von einem erh6hten Eintrag organischen Materials betroffen, wohingegen angrenzende KL)stengebiete nicht direkt von diesen Eintr&gen beeinfluBt wurden. In dem am st&rksten be- troffenen Gebieten der Pommerschen Bucht traten als Folge einer Kombination von hohen Eintr&gen organischen Materials und einer starken Schichtung der Wassers&ule anoxische Bedingungen im BodenwasserkSrper auf.

Introduction

in high and mid latitudes, river peak discharges in spring normally carry maximum annual loads of dissolved and suspended matter to the coastal seas (MEYBECK [1993]). Long-term measurements in the Swina channel, where about 80% of the Oder discharges enter the Baltic Sea, show that the highest concentrations of dissolved inorganic nutrients ap- pear during the spring peak discharge period (MO- HRHOLZ et al. [1998]). Nitrate and ammonium concentrations peak in February/March with values up to 100 #mol/dm 3 and 70 #mol/dm 3, respectively, which is one order of magnitude higher than the summer minimum concentration. Ortho-phosphate reaches about 4 iJmol/dm 3 compared to less than 0.5 IJmol/dm 3 in summer. In contrast, highest concentrations of organic matter enter the Pomeranian Bight in summer. Typical organic nitrogen and phos- phorus values in the Swina are about 120 iJmol/dm 3 and 3 IJmol/dm 3, respectively, when dissolved inorganic constituents are at their minimum. Before the Oder water enters the Baltic Sea, it passes the Szc- zecin Lagoon, where nutrients are transformed into particulate organic matter during the phytoplankton growth season (JOST AND POLLEHNE [in press]). Pre- requisite for the elevated biomass observed (Chl a up to 200 IJg/dm 3) during the entire growth season from March to September is a mean water residence time of about two month and sufficient light due to the shallowness of the Lagoon. In contrast, in the Pomeranian Bight, elevated phytoplankton biomass is only observed in spring when nutrient inputs via the three outlets of the Lagoon and nutrient input from the sediments due to complete vertical mixing of the water column as well as sufficient light condi-

tions favour algal blooms. These blooms consist mainly of diatoms. Phytoplankton biomass in the Bight is one order of magnitude lower (10-20 #g/dm 3 Chl a) compared to the Lagoon. In summer, normally low biomasses prevail (< 3 IJg/dm 3 Chl a) due to the nutrient depletion by the preceding spring bloom and minimum nutrient inputs through the Szc- zecin Lagoon outlets. Diatoms and blue green algae are the predominant species during this season (POLLEHNE et al. [1995]).

The summer flood 1997 produced different patterns of water and nutrient discharges compared to "normal" peak discharges in spring. The main Oder discharges entered the Baltic Sea via the eastern part of the Lagoon and the Swina within a few days directly (MOHRHOLZ et al. [1998]). The short residence times combined with the turbidity of the water prevented significant primary production and nutrient removal in the river and the eastern part of the Lagoon. This situation was expected to change when the Oder freshwater reached the Pomeranian Bight, forming a shallow freshwater lens above the dense seawater. Under these circumstances particles were expected to settle out improving light cli- mate in the surface layer and thus providing favour- able conditions for a primary production. If the nutrient concentrations were one order of magnitude higher than the admixed seawater as is typical for peak discharge in spring, substantial phytoplankton development was expected.

No detailed studies on the effects of exceptional floods on biogeochemical conditions in summer, when organic matter turnover is rapid, in the Baltic Sea are available so far. In this paper, the response of an already established primary productivity regime (summer system) on the irregular nutrient dis-

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Volume 50 (1998) Number 2/3

charges into the Pomeranian Bight will be outlined. Furthermore, the observed effect of the enhanced organic matter supply on oxygen conditions in the bottom waters of the Bight as well as modelled spa- tial sedimentation patterns will be presented. An estimate of the ecological effects of the summer flood on the basic environmental parameters of the coastal marine system will be given.

Methods

Sampling strategy

Water samples were collected in the Pomerani- an Bight during a cruise of RV Prof. Albrecht Penck. The location of stations are given in SIEGEL et al. [1998]. For the demonstration of nutrient and particulate organic matter distributions within the Oder plume only data of those sampling occasions were used, where grid stations covered the entire plume and samples were collected over most of the salinity gradient. These samplings of the grid stations (04/05 August and 11/12 August) were performed during peak discharge of the Swina channel and at low wind speeds when the plume propagated along the Usedom coast (SIEGEL et al. [1998]). During 04/05 August (first sampling) only the inner stations

ing a Secchi disk. The depth of the photic zone (Zp; 1% incident light level) was calculated by multiplying the Secchi depth by 3.24. The factor was determined for the estuarine area during several previous cruises (SIEGEL [unpubl. data]) and corresponds to the slope of correlated Secchi disk readings vs. direct Zp measurements using a quantameter (LI-Cor, 193 SA).

Inorganic nutrient measurements (NO3-, NO2-, NH4 +, PO43-, SiO44-) were carried out on board with an autoanalyzer (ALLIANCE INSTRUMENTS) using standard colorimetric methods according to GRASSHOFF et al. [1983]. Oxygen concentrations were determined employing the Winkler method (described in GRASSHOF et al. [1983]) using a TITRI- NO 916 (METHROM). Samples for particulate organic carbon (POC), particulate organic nitrogen (PON) and phytoplankton pigments were filtered im- mediately after sampling on WHATMAN GF/F glass fibre filters and were stored deep frozen. POC and PON were measured by CHN-Analyzer (CARLO ERBA/FISONS 1108) after GRASSHOFF et al. [1983]; Chlorophyll a was measured by a fluorometer (TURNER INSTRUMENTS) after LORENZEN AND JEFFREY [1978] and phytoplankton pigments (Fu- coxanthin and Zeaxanthin) were determined by HPLC (MERCK) method after (KRAAY etal. [1992]). Phytoplankton species composition was determined

close to the Swina mouth were sampled. Sampling. by inverted microscopy (UTERMOHL [1958]). occurred quasi-synoptically within less than 24h. In addition, continuous measurements of temperature, salinity, oxygen at two water depths (3 and 12 m) are available every 10 min from the automatic ODAS (oceanographic data acquisition system) station Oder Bank in the Pomeranian Bight (position also shown in SIEGEL at al. [1998]).

Analytical methods

Salinity and temperature were determined with a profiling CTD (SEABIRD, SBE 911+). Continuous measurements of temperature, salinity and oxygen were collected by the ODAS at the Oder Bank station using a sealogger (SEABIRD, 20-03). The water column light penetration was measured employ-

Mixing diagrams

Mixing diagrams (property-salinity plots) are widely used to investigate the general distribution patterns of dissolved constituents within estuaries. They indicate whether an element behaves conservatively, is removed or added during the mixing of waters of different salinities. Some authors empha- size the restricted applicability of this concept, because the end member concentrations are set con- stant in time (LBs [1976], WOLLAST AND DUINKER [1982], SHARP et al. [1984]). Linear (conservative) property-salinity distributions are only expected when the time scale of variability for the river mixing member is greater than the hydrodynamic resi-

171


dence time of the estuary (CIFUENTES et al. [1991]). The use of this approach as a basis for estimating removal of solutes, for instance by primary production, during estuarine mixing, as presented e.g. by BOYLE et al. [1974], OFFICER [1979] and KAUL AND FROELICH [1984], is questionable in outer estuaries like the Pomeranian Bight due to the complex spa- tial and temporal scales of mixing processes as well as to the multiple fresh water sources of the Szcze- cin Lagoon. Therefore, in addition to the distribution of dissolved inorganic nutrients (NO3- , PO4 3-, SIO4 4-) also particulate organic matter (POC, PON and phytoplankton pigments) was considered to give com- plementary information on nutrient patterns (HUM- BORG [1997]). Ammonium and nitrite as potentially important constituents of the dissolved inorganic nitrogen pool are not shown, because very small concentrations compared to nitrate were found. In Fig. 1 the theoretical distribution of nutrients and particulate matter is shown, when nutrients are transformed into particles during mixing.

X t- .o

t___

r 0 C 0 0

. . . . "~-~. < distribution of particulate ~ ~ . " " " - . , , , organic matter

\ \ " , ~ ~ - - conservativ~, " \ \ \ " ~ x i n g \\ \

\ \ \ ~ \\

x ~ distribution of >'" ~,~ ~ il

nutrients " - ~"--,..,.____.......'~_

Fig. 1 :

0 7.5 Salinity

Theoretical distribution of inorganic solutes (e.g. DIN) and corresponding particulate organic matter (e.g. PON) when dissolved constituents are transformed into particles during mixing, for an estuary in which there is a single source of river and seawater (after LIss [1976]); the negative de- viation of solutes from the conservative mixing line points towards a removal, the positive devia- tion of particles indicates apparent production during estuarine mixing.

Results and Discussion

Hydrographic conditions and light regime

Between 27 July and 15 August a continuous outflow of Oder river water through the Swina channel was observed (MOHRHOLZ et al. [1998]). The prevailing easterly winds from 30 July onwards propagated the Oder water masses along the Usedom coast. Directly prior to the first cruise the most intense outflow of about 2300 m3/sec occurred on 02/03 August (SIEGEL et al. [1998]). On 04/05 August a sharp salinity front about 10 nautical miles in front of the Swina mouth was observed. One week later the horizontal salinity gradient was smoother and an extensive plume of about 50 nautical miles extended up to the east coast of RQgen. The water column was stratified and the vertical extension of the plume was about 5.7 m (04/05 August) and 4.5 m (11/12 August) on average. In Tab. 1 the relationship between the mixed layer depth (SL) and the depth of the photic zone (Zp) in the Oder plume calculated by Secchi disk data is shown.

a) 04/05 August 1997 b) 11/12 August 1997

St. S L Zp St. S L Zp no. [ml [m] no. [m] [m]

1 4.5 - 1 1.0 5.2

2 5.0 4.5 2 2.5 4.9

OB4 6.0 4.5 OB4 6.0 3.6

3 6.5 3.9 3 4.0 4.2

4 5.0 3.9 10 5.0 3.9

5 8.0 5.2 164 6.0 4.2

10 5.5 2.6 14 6.5 3.6

164 5.0 4.2 130 4.5 3.6

14 12.0 4.5 O l l 6.0 4.9 (outside

the plume Arkona 20.0 9.7

(outside the plume)

SL, surface layer depth (within the river plume the 5.5 psu isoline served as the surface layer criterion)

Zp, depth of the photic zone

Tab. 1: Mixing depth and light conditions in the Pomera- nian Bight during the Oder flood (position of stations given in SIEGEL et al. [1998]).

172


The average depth of the photic zone corresponds to about 72% and 92% of the average depth of the surface layer on 04/05 and 11/12 August, respectively. Net production in estuaries already starts, when the photic zone corresponds to about 20% or more of the mixed layer depth (COLE AND CLOERN [1984, 1987], CLOERN [1987]). It appears that no station within the Oder plume showed a potential light limitation for primary productivity during the observations. Also the residence time of the Oder plume along the Usedom coast of more than one week (SIEGEL et al. [1998])is theoretically sufficient to achieve a measurable increase in phytoplankton biomass (WOLFF [1980]).

Mixing diagrams of nutrients and particulate organic matter

In Fig. 2 the mixing diagrams of nutrients during the two sampling occasions are shown. Only for nitrate and phosphate river end-member data from the Swina mouth are available. These data were taken from MOHRHOLZ et al. [1998] and obtained by MIR (Morski Instytut Rybacki, Gdynia, Poland). All other variables were sampled on board the RV Prof. Albrecht Penck.

Silicate concentrations in the Oder water masses showed exceptional high values during the flood due to a wash out effect of soils. Phosphate concentrations were comparable to "normal" peak discharge conditions, whereas during the ship cruises nitrate concentrations in the river were about 3-4 times lower (MOHRHOLZ et al. [1998]). The low nitrate concentrations can be attributed to a dilution effect, because the summer flood was about twice the volume of a normal peak discharge in spring, as well as to the advanced retention of nitrogen by land vegetation in summer. In contrast to nitrate which derives mainly from diffusive sources, phosphate originates mainly from point sources such as sew- age plants and fertilizer industries, of which many were submerged during the flood. Hence, a dilution effect was probably compensated by these addition- al sources. As a consequence the N/P ratio of the water masses entering the Pomeranian Bight (river

35

3 0 -

2 5 - < l=

20- 0 E .=,. 15- o" Z

1 0 -

5 -

0

0

(a)

4.0

3 . 5 -

3 . 0 - r - , - i r

2 . 5 -

o 2 .O- F

=3,

" 1 . 5 - 0 r

1.0

0 . 5 "

0.0

0

(b) i

1

�9 NO 3 04105 Aug O NO 3 11/12 Aug

00 0

0 0

i O i ~

3 4 5 6 7

Salinity [PSU]

�9 PO 4 04105 Aug 0 PO 4 11/12 Aug

I I

2 3

0 �9 0

I I I I

4 5 6 7

Salinity [PSU]

140

120 -

,_, lOO- <

E 8 o -

0 s �9 ~. 6 0 -

q r

o ,,,,,,

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(c) o

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| @ 0 0 @

@ Q~or

8 o ~ o ~

, ,

I I I I ! 1 I

1 2 3 4 5 6 7

Salinity [PSU]

Fig.2: (a) Nitrate, (b) phosphate and (c) silicate vs. salinity within the Pomeranian Bight during the Oder flood 1997.

173


end-member) of about 7 and 8 during both observations were much lower than the N/P demand of marine phytoplankton (16) pointing towards a N-limitation of primary production in the outflowing Oder plume. Whereas phosphate and silicate could be determined in measurable quantities over the entire salinity range, nitrate was depleted at salinities >4.5-5.5 (Fig. 2 (a)). In conservative mixing the nitrate data should be positioned close to a straight line connecting the end members of the mixing series (salinity between 0 and 7.5). At salinities between 4.5 and 5.5, where nitrate depletion already occurred, they should theoretically amount to about 5-10 IJM. Applying the compositional N/P ratio of 16 (REDFIELD et al. [1963]) and Si/N of 1.12 (BRZEZINSKI [1985]) of marine phytoplankton and diatoms, respectively, a removal of about 0.3-0.6 IJM phosphate and 6-11 IJM silicate can be expected. For phosphate at higher salinities this removal might be indicated by the mixing curves showing no further decrease >5.5, for silicate no such removal can be observed, indicating that phytoplankton species other than diatoms were responsible for the nitrate uptake. However, the extent as well as the patterns of the nutrient removal in the Pomeranian Bight during the flood were not different compared to a typical summer situation (POLLEHNE et al. [1995], JOST AND POLLEHNE [in press]) due to the N-limitation of the phytoplankton biomass production, which was not raised but enhanced during the exceptional summer discharges.

In Fig. 3 the mixing diagrams of POC and PON are shown. Mixing process of particles are much more complex compared to solutes due to multiple physico-chemical reactions taking place (DUINKER [1980]) and therefore some more scatter in the values can be expected. However, all measurements exhibit more or less conservative mixing patterns. Also Chl a and Fucoxanthin, a marker pigment for diatoms, showed conservative mixing patterns (Fig. 4 (a), (c)). In contrast the marker pigment for blue green algae, Zeaxanthin, showed non-conservative mixing patterns (Fig. 4 (b)). This is due to the fact that a large part of pelagic autotrophic biomas during summer in the open Baltic consists of pico-cyanobacteria with high specific content of zeaxan-

thin. Due to the ambient light conditions in the river waters, however, filamentous cyanobacteria from this source contained only minor amounts of this light-protective pigment. This led to the fact that the zeaxanthin output was small as compared to the standing stock in the Bight, and mixing of this class of autotrophs was not indicated by that specific com- pound.

350.,

3 0 0 -

m 2 5 0 - E

0 E 2OO-

o 0 1 5 0 - a.

1 0 0 -

50

0

50 ,

(a) I

1

�9 POC 04105 Aug O POC 11/12Aug

O

oO

o O

�9 %

! I I !

2 3 4 5

Salinity [PSU]

1 I

6 7

4 5 -

4 0 -

3 s - E

3 0 - =o .~. 25 - Z 0 2 0 - a.

1 5 -

1 0 -

Fig.3:

�9 PON 04/05 Aug O PON 11/12Aug

(b)

O 0

0~ , %

0 0

db

c~

I I I I I I I

0 1 2 3 4 5 6 7

Salinity [PSU]

(a) Particulate organic carbon, (b) particulate organic nitrogen vs. salinity within the Pomeranian Bight during the Oder flood 1997.

174


Fig.4:

50

4 0 -

< E 3 0 -

~ 2 0 - 0

1 0 -

i e Chla04/05Aug O Chl a 11/12Aug

(a)

0 , ~~

~ o �9 0 0 0

d> o�9149

0 I I I ! I I I

0 1 2 3 4 5 6 7

4 -

< E

, .__,

, , =

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1-

0 0

Salinity [PSU]

I �9 Zea 04/05 Aug O Zea 11/12 Aug

. . . . . .

(b)

O

O0 O0 �9 �9 0 0

O0 0 �9 II 0

@0 0

I I I

3 4 5

Salinity [PSU]

I I

6 7

1 4 -

1 2 -

r < 1 0 - E

.~. 8 -

,Ic e~ 6 -

X 0 0 = 4 -

ml_

2 -

(c) 0 - ~ T

0 1

l �9 Fuco 04/05 Aug O Fuco 11/12 Aug

%

0 0 0

1 l I T ~

3 4 5

Salinity [PSU]

O �9 % �9

0 �9

et ~149 o T ~ r - ' -

6 7

(a) Chlorophyll a, (b) Zeaxanthin and (c) Fu- coxanthin vs. salinity within the Pomeranian Bight during the Oder flood 1997.

During the first observation at three station close to the Swina mouth (St 002,003 and 004) exceptional high Chl a concentrations of 100-200 pg/I were observed. Microscopic analyses of phytoplankton species composition revealed that these extreme values were brought about by the blue-green algae Microcystis spec (Fig. 5 (a)). Mi- crocystis is the typical bloom forming species in the Szczecin Lagoon in summer. The western part of the Szczecin Lagoon was more or less unaffected during the observations. Here the typical summer conditions with salinities of about 2 and high phytoplankton biomass was observed (MOHRHOLZ et al. [1998]). Parts of these water masses were probably washed out during the first part of the observations (04/05 August) by the pulse-like outflow event after the short-term dam up of water masses in the La- goon directly before the measurements (SIEGEL et al. [1998]). The extreme Chl a (see Station 003 in Fig. 5(a)) values observed during the 04/05 August most certainly do not represent the real Oder river end-member, they reflect the premixed Lagoon water and therefore these stations are not included into the mixing diagrams (also not for the Zeaxanthin mixing diagram). At all other stations the diatoms Melosira granulata, Skeletonema subsalsum and other small (10 pM) centric forms were-prevailing (Fig. 5). These diatoms, dominating the real Oder river lagoon end-member species composition, were more or less diluted, which is indicated also by the mixing diagrams of silicate and fucoxanthin, without net production due to the N-limitation of the pelagic productivity during the flood.

Sedimentation of particulate organic matter and oxygen conditions

The ship measurements give only insights of the ecological effects of the exceptional Oder discharges in the water column. Detailed investigations on the effects on the benthic community in the Pomeranian Bight were carried out on later cruises, but these samples are still not completely analysed. However, to estimate the long-term effect of the high organic matter supply to the sediments a model approach was used.

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Deutsche Hydrographische Zeitschrift - German Journal of Hydrography

30000 250

25000

•E' 20000

= 15000

10000

5000

(a) r '~-- ' lOthers

Chlorophyceae Bacillariophyc.

I'PPm Cyanobacteria ~ ,-l~Chl.a

, , , , , , , . . . . . . . . . . . . . .

010 OB4 003 164 014

Station

30000 (b)

25000

Others

Chlorophyceae

200

" o

150 ~. m >,

100 ~o. o

5O

14,,- ,

20000 ol .=.

15000 ._~

10000

50OO

10 OB4

Bacillariophyc. Prrm Cyanobacteria

~ Chl.a

164 14 130 TF11 Arkona

Station

01 :=1. =..=

lO

6 2 o

2 0

Fig.5" Microscopic analyses of phytoplankton species composition of selected stations in the Pomerani- an Bight (position of stations given in SIEGEL et al. [1998]) during the Oder flood 1997 (a) 04/05 Au- gust 1997, (b) 11/12 August 1997.

The sedimentation loss of particulate organic matter was simulated by a coupled physical-chemical-biological model (FENNEL AND NEUMANN [1996]). The model describes a nitrogen cycle with the variables dissolved inorganic nitrogen, phytoplankton, zooplankton and detritus. At the sea floor a sediment boundary condition with a sediment layer ac- cumulating sinking detritus is used. In the sediment layer detritus is remineralized and released as dissolved nitrogen. By exceeding a threshold value of bottom shear stress detritus can be resuspended. The model is forced by meteorological parameters obtained with the ODAS buoy located in the Pome- ranian Bight. The river runoff of the Oder is simulated by fresh water input with containing of chemical and biological variables according to the measured discharge.

The model approach is used here to get an im- pression of the impact of the flood on the near bottom layer. From the amount of accumulated organic nitrogen in the sediment layer a first approximation of oxygen depletion and the influence on the benthic community is made. In Fig. 6 the modelled net changes in nitrogen compounds (dissolved and particulate) during the flood in the sediment layer is shown. It can be seen that mainly the south-western part of the Pomeranian Bight was affected by high organic particle fluxes. This is consistent with the satellite and ship observations of the main plume drift direction during the flood (SIEGEL et al. [1998]). The organic matter supply is estimated between 100-500 mmol N/m 2 during the entire flood period (10 July - 14 September) compared to > 1000 mmol N/m 2 in the Szczecin Lagoon.

The sediment of the Pomeranian Bight consists mainly of sandy material (NAUSCH [1997]). In summer, the benthic oxygen demand of sandy sediments in this region is estimated of about 15-20 mmol OJm2d (BALZER [1984], WASMUND [1993]). This corresponds to a nitrogen remineralization rate of about 1.74-2.32 mmol N/m2d when Redfield ratio is applied. Thus, the flood derived material is theoretically respired within about half a year. This conservative estimate exceeds probably the real degradation time of organic matter, because the reported oxygen demands were measured under steady state conditions and enhanced organic matter supply leads to a rapid benthic response with several times higher oxygen consumption rates (GRAF et al. [1983]). Moreover, this estimate does not include the further distribution by resuspension caused by deep currents, which lead to a more rapid remineralization.

During the ship observations at the beginning of August no anoxia could be detected. Oxygen concentrations in the water masses below the plume decreased from about 4.4 cm3/dm 3 (04/05 August) to 2.2 cm3/dm 3 (11/12 August). A similar reduction of oxygen concentrations show also the time series data of automatic oxygen measurements from the ODAS Oder Bank (Fig. 7). Oxygen concentrations decreased here from about 4 cm3/dm 3 on the 7 August to concentrations below the detection limit on the 19 August. The

176


Fig.6: Modelled changes of nitrogen inventories in the sediment layer of the southern Baltic Sea (Pomeranian Bight, Arkona Sea and Bernholm Sea) during the Oder flood.

177

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: : : ~ ~ : : l ~ , a , 1 3 m m , a ! i i 2 m t ' i 2 : : : : : : : " ' . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . :

( b ) : ! ! . : : i : : i ' : : �9 : : " ! : : . . . : : , : . . : : . : : : :

, , ; ; ; ........ ; . . . , .......... ; . . . . , ..... ; ...... ; ........ ; , ; . . , , ; , , ;,,,

i

. . . . �9 : . . . . . ; . . . . : . . . . . : - . . . . : . . . . - , . . . . . ; . . . . : . . . . . ,,, . . . . : . . . . - : . . . . . ; . . . . .: . . . . . ,,, . . . . : . . . . - : . . . . . ; . . . . ; . . . . . : . . . . . : . .

i i i i i ! i i i i f ' ~ o ~ : 3 ~ , ........ ' ~ : : : ~ o ~ : i 2 m ) i . . . . " . . . . . :. . . . . i . . . . . .:. . . . . i . . . . .: . . . . . .: . . . . �9 . . . . . " . . . . i . . . . | - ~ , - - t i t . 3 r n , t i t . 1 2 m .... | . . . . !..

" " ' ' " ~ . . . . . ': . . . . : ' " ~ . . . . . : . . . . "'i i i : ~ ' i " " : ..~ . . . . " . . . . . i:" . . . . ! ' "

'i'= i ! i i ! i ! i i i

10 . . . . . -: . . . . . :- . . . . ~ . . . . -: . . . . . :- . . . . ~ . . . . ~ . . . . -: . . . . . :- . . . . - . . . . -: . . . . . ~ -i . . . . -: . . . . . :- . . . . . . . . . . :- . . . . .

~ 5 -

c

-10 -

-20 , , i , , , , , , , , , , , , , , ~, , ~ , ,

5.8 9 .8 13 .8 17 .8 2 1 . 8 25 .8 29 .8 2 .9 6 .9 10 .9 14.9

F ig .7 : O D A S O d e r B a n k a u t o m a t i c m e a s u r e m e n t s of (a) t e m p e r a t u r e , (b) sa l i n i t y , (c) o x y g e n ( c r o s s l a b e l l e d v a l u e s

i n d i c a t e t i t r a t e d o x y g e n m e a s u r e m e n t s ) in 3 a n d 12 m w a t e r d e p t h a n d (d) w i n d in t h e P o m e r a n i a n B i g h t (po -

s i t i on of t h e b u o y g i v e n in SIEGEL et al. [ 1 9 9 8 ] ) d u r i n g t h e O d e r f l o o d 1 9 9 7 (05 A u g u s t - 14 S e p t e m b e r ) .

1 7 8


missing oxygen data in Fig. 7 (c) between the 19 Au- gus t - 02 September werecaused by the appearance of hydrogen sulphide and the resulting destruction of the oxygen sensor. This is in agreement with direct measurements of the environmental agency Meck- lenburg Vorpommern (LAUN) which reported an extreme oxygen depletion and formation of hydrogen sulphide in front of the Usedom coast between the 19 and 25 August. From the 29 August onwards, the in- creasing wind stress led to a vertical mixing and the break down of the water column stratification, which is indicated by the convergence of deep and surface layer temperature and salinity curves (Fig. 7 (a), (b)). Oxygen concentration in the deep water approached surface values several days later. However, the enor- mous organic matter supply in summer together with the strong stratification of the water column for a period of about one month, of which two weeks with the formation of hydrogen sulphide, led most certainly to an elimination of higher benthic life forms in the western part of the Pomeranian Bight. The formation of hydrogen sulphide rich bottom layers in parts of the Po- meranian Bight as a result of a 3 week calm weather period was also reported for summer 1994. A recov- ery of a higher benthic community varied between a few weeks and two years. The differences in rate of re-colonization were referred to the severity of oxygen deficiency and the distance from undisturbed recruit- ment areas (KUBE AND POWlLLEIT [1997]). This situation was reinforced during the summer flood in 1997 by the much higher organic matter supply. Direct bal- ances between carbon-supply and oxygen demand are, however, not possible, as lateral exchange could not be taken into account and the largest fraction of biomass that decompose under suboxic and anoxic conditions (the resident macrofauna biomass with a mean of about 4 mol C m -2 (KUBE [1996]) and an according potential of about 4 000 mmol 02 uptake) could not be quantified.

Conclusion

The ecological effects of the exceptional Oder summer flood in 1997 on the Baltic Sea ecosystem were not as dramatic as expected. The conse-

quences of the flood were mitigated by the low nitrate concentrations in the outflowing water masses. The typical pattern of nitrogen limitation for biomass production in the Pomeranian Bight in summer was not changed by the Oder discharges. However, as could be demonstrated, phosphate passed the Po- meranian Bight with only minor transformations. Off- shore the Bight one of the most intense blooms of Nodularia spumigema in several years, was observed (Algaline 1997 see LEPP,&NEN et al. [1994]). This bloom developed in the open Baltic and was transported to the coast by easterly winds. Nodular- ia is capable of fixing elementary nitrogen and therefore Nodularia blooms in the Baltic Sea are often phosphate limited (GRANELI et al. [1990]). Phos- phate derived from the Oder flood might have stim- ulated these blooms, although direct investigations were not carried out. Silicate was most certainly further diluted without apparent biological utilization by diatoms. The most severe ecological effect was the increased particulate organic matter supply which, in combination with stratification of the water column, led presumably to an enhanced degradation of the benthic community in large areas of the Pomer- anian Bight.

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Submitted: 18. 04. 1998

Accepted: 19. 10. 1998

Address of corresponding author'.

C. Humborg* Institut fEir Ostseeforschung Warnem#nde SeestraBe 15 18119 Rostock

* present affiliation: Department of Systems Ecology Stockholm University S- 10691 Stockholm e-mail: christoph @ system.ecology.su.se

A complete list of authors is given on page 289.

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Documents

The exceptional Oder Flood in summer 1997 — the fate of nutrients and particulate organic matter in the Baltic Sea