Biogeochemical processes in Curonian lagoon: state of the art

Images: R. Paskauskas, R. Pilkaitytè

Prepared by M. Zilius and A. Razinkovas-Baziukas

Introduction

Curonian Lagoon - one of the largest lagoons in Europe• Surface ~1600 km2

• Shallow non-tidal• Small saltwater intrusions from the Baltic• Freshwater input from nutrient rich Nemunas river (27000 ton N year-1)• Hypertrophic - cyanobacterial blooms, fish death, dystrophy-• Benthic nitrogen cycling poorly studied• Limited light penetration due to turbidity/resuspension

Aims

To assess the role of the benthic system for metabolism and nitrogen cyclingTo provide data for the setup and calibration of the benthic part of the NPZD model

-Flux of dissolved oxygen and nutrientsat deep and shallow sites within the Lagoon

-Role of benthic microalgae as regulators of processes and exchange rates at the sediment-water interface

-Fraction of river-associated nitrogen load removed by surface sediments via dissimilative NO3

- reduction.

Study Area

Sedimentary environmentsSedimentary environment Littoral sand zone Open lagoon muddy sand Open lagoon mud

Relative area in the lagoon (%) <1 ~54 ~44

Study site K_1 VE_5 V_2 AR_3 N_4

Water depth (m) 1 1 1.7 2.5 3.5

Sediment type Fine sand Fine sand Fine muddy sand

VE_5 K_1 V_2 AR_3N_4

• Four field campaigns (March, May, July and October 2009), just after ice cover melting and in full summer till autumn.

• Measurements done in intact cores (5 replicates, one station per site ) simulating in situ conditions.

• Oxygen microprofiling in dark (3 replicates per site)• Light and dark fluxes (Dalsgaard et al., 2000).• Denitrification (revised IPT, Risgaard-Petersen et al. 2003).

Material and methods

Summarized data

TOTAL  OTOTAL  O22 UPTAKE: seasonal and spatial variationUPTAKE: seasonal and spatial variation

Total oxygen uptake

03/2009 05/2009 07/2009 10/2009

SOD

, mm

ol O

2 m-2

h-1

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Tem

pera

ture

(°C

)

0

2

4

6

8

10

12

14

16

18

20

AR_3N_4V_2K_1VE_5

Water temperature

Increase of sediment oxygen demand from March to July is likely due to a combination of higher water temperatures and increased organic matter input to surface sediments after the spring phytoplankton blooms. However from July to October the sediment oxygen uptake remains elevated primary due organic input.

ZoneArea,km2

Total O2 uptake, mol h‐1

March May July October

Littoral sand zone  96.4 ‐31.7 ‐67.5 ‐173.5 ‐183.2

Open lagoonmuddy sand

594.7 ‐336.6 ‐832.6 ‐1189.4 ‐475.8

Organic muddyarea

879.0 ‐497.5 ‐879.0 ‐1318.5 ‐1318.5

Total 1570.1 ‐865.8 ‐1779.1 ‐2681.4 ‐1977.5

The risk of hypoxia in the Curonian Lagoon: sediment respiration projection

Phytoplankton  ( when chl a values exceed  to 100 µg  l‐1 )  respiration  in  the water column during night hours can have a major role, with rates over 50 mmol m‐2h‐1. 

Oxygen can be consumed more than 25 times faster!

On average sediments have a low potential to deplete oxygen in the water column: it would take  from 10 to 20 days, assuming irrelevant reareation and limited production.

Near bottom water

Sediments

N2

NO3-

Daily oxygen and nutrient exchange at sediment-water interface

Mud bottom

0.70 0.9

0.1

4.2

16.4

12.3

O2

NH4+

5.3mM m-2

NO3-

0.4mM m-2

SRSi5.9

mM m-2

Cited by: Zilius et al. (submited)Bartoli et al. (submited)

SRP0.5

mM m-2

Phytoplankton

Near bottom water

Sediments

N2

NO3-

Daily oxygen and nutrient exchange at sediment-water interface

Littoral sand

0.4 0.060.1 1.1

0.011.9

O2

NH4+

0.5mM m-2

NO3-

0.6mM m-2

SRSi0.7

mM m-2

Cited by: Zilius et al. (submited)Bartoli et al. (submited)

SRP0.07

mM m-2

Microphytobenthos

Phytoplankton

Open lagoon mud zone (Water column 3 m)

4% 2%

NH4+

NO3- SRP

2%

Dw

>1%

Spring

Sediment

Water column

SiO2

2%

Water column

Sediment

Summer

14%

NH4+

24%

SRP

13%

SiO2

Dw

1%15%

NO3-

Water column

Sediment

Autumn

34%

NH4+

2% 13%

SiO2

Dw

?%2%

SRP

NO3-

Open lagoon muddy sand zone (Water column 1.7 m)

71 mg m-2 126 mg m-2 155 mg m-2

Water column

Sediment

Autumn

26%

NH4+

6% 18%

SiO2

Dw

?%8%

SRP

NO3-

Water column

Sediment

Summer

26%

NH4+

9% 11%

Dw

?%8%

SRP

NO3-

SiO2

10% 1%

NH4+

NO3- SRP

2%

Dw

>1%

Spring

Sediment

Water column

SiO2

6%

71 mg m-2 131 mg m-2 79 mg m-2

1% 0%

NO3- SRP

15%

Dw

<1%

Spring

Sediment

Water column

SiO2

10%

NH4+

0% 1%

NO3- SRP

19%

Dw

6%

Summer

Sediment

Water column

SiO2

36%

NH4+

Water columnAutumn

1%

NH4+

8% 36%

SiO2

Dw

?%22%

SRPNO3-

Littoral sand zone (Water column 1 m)

106 mg m-2 161 mg m-2 162 mg m-2

Driving factors in nutrient exchange at the sediment-water interface and their cycling

K1Mar

K1JulK1Oct

VE5MarVE5July

VE5Oct

V2MarV2July

V2OctAR3Mar

AR3July

AR3Oct

N4Mar

N4July

N4Oct

fluxNH4

fluxNO3

fluxSRP

fluxSRSiDw

Dn

Sed_chl_a

TOM

Temp

Water_chl_a

O2_conc

NH4

NOx

OPD

TOU

pNH4

pNO3_NO2

pSRSi

axis 1-1 0 1

axis

2

-1

0

1

A complex regulating the nutrient cycling and exchange at sediment–water interface:

1) water NO3-

2) water chl a and O2 , sediment chl a3) water NH4

+ and TOM

Explained variance 69%

General conclusions– Organic carbon sources differ between the principal sedimentary

environments and thus could have implication for studying oxygen and nutrient exchange at sediment–water interface their cycling there.

– Total oxygen uptake rates in poor organic sandy sediments are comparable or even higher than those at organic-rich sites. This demonstrates faster benthic metabolism response and possibly higher turnover rates of fresh organic matter in shallow littoral sand rather than in deeper areas after sedimentation pulses of pelagic organic matter.

– In increasing benthic metabolism and organic material flux to surface sediments considerably reduce oxygen penetration depth into sediments overall lagoon.

– In shallower sediments settled “microphybenthos” has an nimportant role in oxygen and nutrient exchange at sediment water interface as well as denitrification capacity.

General conclusions– In spring high amount of bio-available nitrogen delivered into Curonian

lagoon by Nemunas River is initial trigger for further biogeochemical processes in the Curonian lagoon.

– Large seasonal variations occurring in the denitrification, primarily is caused by the dramatic variations in the NO3

- load in the Curonian lagoon.

– Sediments have an effect on nutrient pool in overlaying water column in summer, however it has less importance spring and autumn.

PublicationsZilius et al. BENTHIC OXYGEN UPTAKE IN THE SHALLOW EUTROPHIC LAGOON (CURONIAN LAGOON, THE BALTIC SEA, LITHUANIA) accepted in HydrobiologiaBartoli et al. The role of the bottom sediments in N cycling in the shallow eutrophic lagoon (prepared for Biogeochemistry)Zilius et al. Seasonal nentho-pelagic nutrient fluxes in the in the shallow eutrophic lagoon (prepared for Oceanological and Hydrobiological Studies)

Messages to the stakeholders

• Bottom sediment processes could contribute to the local anoxia events in the Curonian lagoon. However, pelagic cyanobacteria blooms is the most important factor.

• Denitrification rates are in the range typical for estuarine systems and are higher than nitrogen fixation rates. Curonian lagoon is the sink for nitrogen