Slides Ph D Xavier De Pedro 2007

Situacions d'anòxia en zones estuàriques sense forçament mareal

una aproximació als balanços de producció/consum d'oxigen

Xavier de Pedro Puente

Tesi doctoral. 2007.

Departament d'Ecologia. Universitat de Barcelona

http://gclub.ub.es/xdp07

Anoxic situations in estuarine zones without tidal forcing

an approach to the oxygen production/consumption budgets

Xavier de Pedro Puente

Ph. D. Thesis. 2007.

Ecology Department. University of Barcelona

http://gclub.ub.es/xdp07

Anoxic events in estuarine zones without tidal forcing

0. Introduction

1. Hydrodynamics

2. Physics-Chemistry & Meteorology

3. Hypoxic episode in 1997

4. Water column

5. Benthos

6. Oxygen budgets

7. Conclusions

(Annexes [4] + CD with software)

Alfacs Bay Ebre River Delta

0. Introduction

ALFACS BAY (Ebro Delta, Spain) Length 13 Km Width 4 Km Surface 40x10 6 m 2 Mean Depth 3.13 m Max. Depth 6.5 m Anoxic events in estu ari ne zones without tidal forcing 0. Introduction 3. Hypoxic episode in 1997 6. Oxygen Budgets 1. Hydrodynamics 4. Water column 7. Conclusions 2. Phys.-Chemistry & Meteorology 5. Benthos Waste Treatment Plant Town Sand Bar (= “ Alfac ”) MEDITERRANEAN SEA Alfacs Peninsula Rice Fields Brackish Lagoons FW Canals Salt Factory “ S1” “ S2” “ S3” Mussel platforms x x x “ Boca” “ Mig” “ Cua”

Oxygen concentration in water may become lower than 3 mg l -1 (“ hypoxia ”)

Anoxic events: rare, but exist

Negative effects on biota (Diaz & Rosenberg 1995)

The problem: hypoxic events (just some summers)

Anoxic events in estuarine zones without tidal forcing 0. Introduction 3. Hypoxic episode in 1997 6. Oxygen Budgets 1. Hydrodynamics 4. Water column 7. Conclusions 2. Phys.-Chemistry & Meteorology 5. Benthos “ S2” - Alfacs Bay

Anoxic events in estuarine zones without tidal forcing 0. Introduction 3. Hypoxic episode in 1997 6. Oxygen Budgets 1. Hydrodynamics 4. Water column 7. Conclusions 2. Phys.-Chemistry & Meteorology 5. Benthos “ OMMEL” O xygen M odel for M icrotidal E stuaries & L agoons + GPP Phytoplankton - Water column respiration + NPP macroalgae +/- Exchange with atm. +/- Transport (advection and exchange) - Sediment & benthic fauna respiration Conceptual Framework - DO > 150% sat. 0 2km { DO = o.m.

Anoxic events in estuarine zones without tidal forcing 0. Introduction 3. Hypoxic episode in 1997 6. Oxygen Budgets 1. Hydrodynamics 4. Water column 7. Conclusions 2. Phy-Chemistry & Meteor. 5. Benthos

1. Hydrodynamics

Potentially: Wind > Freshwater

Usually: Freshwater > Wind

2. Physics-Chemistry & Meteorology in the nineties

Any changes in variables?

Temp., wind speed, solar radiation, net rainfall, nutrients, ...

Results not consistent, in general

Results

Anoxic events in estuarine zones without tidal forcing 0. Introduction 3. Hypoxic episode in 1997 6. Oxygen Budgets 1. Hydrodynamics 4. Water column 7. Conclusions 2. Phy-Chemistry & Meteor. 5. Benthos

3. Hypoxic episode in 1997

16-21 September

Multiparametric sonde (data every 10')

Days 16-19: stratification

Day 20: Wind=>Mixing

Anoxic events in estuarine zones without tidal forcing 0. Introduction 3. Hypoxic episode in 1997 6. Oxygen Budgets 1. Hydrodynamics 4 . Water column 7. Conclusions 2. Phys.-Chemistry & Meteorology 5. Benthos S1 S2 S3 S1 S2 S3 “ S2” Sal. (psu) Temp. (ºC) DO. (mgO 2 l -1 )

Anoxic events in estuarine zones without tidal forcing 0. Introduction 3. Hypoxic episode in 1997 6. Oxygen Budgets 1. Hydrodynamics 4. Water column 7. Conclusions 2. Phys.-Chemistry & Meteorology 5. Benthos

4. Water column processes

Seasonal dynamics for:

4a. Plankton production: Gross (PGP), Net (PNP)

Chlorophyll a (Chl.a: mg m -3 )

Productivity (pp_chla: mg O 2 mg -1 Chla h -1 )

PGP (mg O 2 h -1 )

4b. Physical processes

Exchange with atmosphere, Hydrodynamics

4c. Water column respiration

4a. Plankton Production

Productivity of Chl a (pp_chla): estimation processes (A) and (B)

(A) through all net diel changes (B) through net diel changes minus “physics” average pp_chla (net) = 38 mgO 2 mgChla -1 h -1 DO (mg l -1 ) days “ Net diel change”

4b. Exchange with atmosphere

Wind aeration (DO wae ): f(T, S, WS ) (Wanninkhof, 1992)

Ca= [DO] in water at 100% sat. of O 2 , given a T. and S.

Cw=[DO] in water

k=O 2 transfer coefficient (L T -1 )

u= wind speed (m s -1 )

Sc=Schmidt number

T=temperature (C), S=salinity (psu)

4c. Water column respiration (BOD 5 )

BOD 5 through incubation of field water samples in sealed bottles for 5 days in 2 Temp.

BOD rate per hour linearly interpolated, and corrected to field temperature

Same range as nightly change in dissolved oxygen in field water column, in general

Overall results for Water column (4a + 4b + 4c)

Winter: active [Chl. a] (correlated with DO, p<0.05)

Summer: 54% variance in POC explained by Chl.a, but Chl.a not correlated with DO: detritic phytoplankton

upper layer

5. Benthos 5a. Changes in benthic vegetation in 90's

New colonization by 2 macroalgae species!: Changes in Temp.? Nut.?...

Anoxic events in estuarine zones without tidal forcing 0. Introduction 3. Hypoxic episode in 1997 6. Oxygen Budgets 1. Hydrodynamics 4 . Water column 7. Conclusions 2. Phys.-Chemistry & Meteorology 5. Benthos Caulerpa prolifera Alsidium corallinum 1986 1997-Spring

Chemically defended: Caulerpenine , Domoic acid

Anoxic events in estuarine zones without tidal forcing 0. Introduction 3. Hypoxic episode in 1997 6. Oxygen Budgets 1. Hydrodynamics 4 . Water column 7. Conclusions 2. Phys.-Chemistry & Meteorology 5. Benthos

5b. Benthic production & respiration

field measurements (light, DO, Sed. resp.)

laboratory P-I incubations of macroalgae

Macroalgae selfshading

(5b) Sediment respiration

3h Incubations of sediment with benthic PVC chambers: base rigid cilinder + flexible conus on top.

Clear and dark chambers.

(5b) Macroalgae selfshading

As a function of Biomass

Anoxic events in estuarine zones without tidal forcing 0. Introduction 3. Hypoxic episode in 1997 6. Oxygen Budgets 1. Hydrodynamics 4 . Water column 7. Conclusions 2. Phys.-Chemistry & Meteorology 5. Benthos K b =0,00274 m 2 gPS -1 Experiment: 5 Light level and 5 Biomass simulations

(5b) Summary of results

Time needed to reach anoxia in simulated scenarios in Alfacs Bay. End of Summer, Station “S3”, lower layer

Anoxic events in estuarine zones without tidal forcing 0. Introduction 3. Hypoxic episode in 1997 6. Oxygen Budgets 1. Hydrodynamics 4 . Water column 7. Conclusions 2. Phys.-Chemistry & Meteorology 5. Benthos Hurricane Bob (USA) (Peckol & Rivers 1995)

(5b) Summary of results (cont.)

When

Anoxia?:

from days

to infinite

Anoxic events in estuarine zones without tidal forcing 0. Introduction 3. Hypoxic episode in 1997 6. Oxygen budgets 1. Hydrodynamics 4 . Water column 7. Conclusions 2. Phys.-Chemistry & Meteorology 5 . Benthos

6. Oxygen budgets

Remarks & Results

Physical and biological processes

Dynamics in space and time

General equation:

DO upp. mgO 2 l -1 DO low. mgO 2 l -1

Sensitivity analyses:

(Jørgensen’93, Beck’83)

(time averaged)

Calibration for Chl.a, Rsed (S2, Summer, lower layer):

Chl.a*0.21

Rsed*1.67

Anoxic events in estuarine zones without tidal forcing 0. Introduction 3. Hypoxic episode in 1997 6. Oxygen budgets 1. Hydrodynamics 4 . Water column 7. Conclusions 2. Phys.-Chemistry & Meteorology 5 . Benthos S2 S3 o.m. in sediment (Palacin 1990)

Anoxic events in estuarine zones without tidal forcing 0. Introduction 3. Hypoxic episode in 1997 6. Oxygen budgets 1. Hydrodynamics 4 . Water column 7. Conclusions 2. Phys.-Chemistry & Meteorology 5 . Benthos Annually averaged budget: 26.5 mgO 2 m -2 d -1 Net biological production: 326 gC m -2 y -1 “mesotrophic – eutrophic”; (Nixon 1995)

7. Conclusions

Most saltwater net inflow: by freshwater inflow (+50% speed); and lesser: wind, atmospheric pressure

Nutrient concentrations: not much change in 90's, but an increase in DIN (respect to 1986-87) in drainage channels from the rice paddies. Attributed to changes in the agricultural practices at the Ebro River delta.

Hypoxic-anoxic situations do not occur every year. Reason: not a single cause alone.

Macrophytes: important role controlling anoxic events

from a few days to infinite is the time needed to reach anoxia, depending on light and biomass scenarios.

Whole ecosystem O 2 budget:

Slightly positive in anual basis: 26.5 mg O 2 m -2 d -1

(+++) Winter & Summer, and (-----) Spring & (--) Autumn

Main producers: phytoplankton > Macroalgae

Main sinks: Sediment Resp. > Exchange with Atm., Plankton

O 2 exportation to air and open sea.

Model (OMMEL) fit: good for most 1997.

Possible source of discrepancies:

Underestimation of advective and exchange processes (seiches, internal waves, real freshwater inflows through channels or phreatic water) from November to March.

Dissolved Oxygen most sensitive to water turbidity

Plus: phytoplankton biomass and productivity, sediment respiration (lower layer) | wind speed, saturation irradiance, BOD (upper layer)

80% of Chl.a end summer lower layer: inactive

The presence of macroalgae: shift in sign of oxygen balance in the lower layer

In annual basis, whole ecosystem (including physical processes) has oxygen incomes and outcomes quite balanced, for the whole bay and all water column. Only benthos in station “S3” (“Cua”) is slightly heterotrophic

Net biological production in Alfacs: 326 g C m -2 y -1 ( “mesotrophic – eutrophic”; Nixon 1995 )

Hypoxic events are not caused by an only factor, isolated and identifiable, but for the coincidence in space and time of a set of them:

high temperature, water column turbidity, high macrophyte biomass and sediment respiration and wind absence, among others.

Hypoxic situation seems to be kept while these conditions do not substantially change and there is:

little deep water renewal, lack of advective flow by intrusion of salt water from the exterior, or by lack of non-advective flow with the upper layer.

Hypoxia decreases and can end up disappearing when:

either occasional strong winds or weaker winds but more continued, which break or erode water column stratification, or when there is deep water renewal by external marine water entrance, caused by, for instance, freshwater inflow through the channels.

Range of maneuver to reduce risk of anoxia is low

Management chance: freshwater inflows to the bay

Acknowledgements

Department of Ecology. University of Barcelona.

Dr. Javier Romero (head and Ph.D. supervisor)

Collaborators:

Dr. Marta Pérez, Marta Manzanera, Olga Invers, Pere Renom, Fiona Tomàs.

This Ph. D. work has been possible due to a 4 year grant from the Spanish Ministry of Education and Science, and a 7 month Marie Curie (EU - FP5) Fellowship in Norway