1. Effects of river damming on
estuarine phytoplankton:
the Guadiana estuary
case study (SW Iberia)
Rita B. Domingues1,2, Ana B. Barbosa1, Helena M. Galvão1
1 Centre for Marine and Environmental Research, University of Algarve, Faro, Portugal
2 Centre of Oceanography, University of Lisbon, Portugal

2. THE IMPORTANCE OF RIVER FLOW
 river flow plays crucial role in the regulation of abiotic and biotic variables
nutrient and light availability
primary and secondary production
coastal fish landings
 dam construction – alteration of the natural flow regime – serious effects on ecosystems
downriver
 e.g., Iron Gates Dam, Danube River
silica inputs to the Black Sea
transition from a diatom-based community to dominance of non-siliceous
phytoplankton
 rupture on diatom-zooplankton-fish food web, based on high diatom biomasses, which
benefits fish and other organisms that rely on planktonic food during their life cycle, can
ultimately be expected from dam construction

3. THE PROBLEM: CONSTRUCTION OF THE ALQUEVA DAM
 the largest dam in Western Europe
 located 140 km from Guadiana River’s mouth
 encouraged extensive scientific research on the
effects of river damming on estuarine ecosystem,
since late 1990´s
 Marine Microbiology Group at the University of Algarve has been studying
phytoplankton dynamics in the Guadiana estuary since 1996
 previous phytoplankton studies focused on
• seasonal variability of phytoplankton and environmental drivers (Domingues et al. 2005, 2007,
2012; Domingues & Galvão,2007)
• cyanobacteria blooms (Rocha et al. 2002; Sobrino et al. 2004; Galvão et al. 2008)
• effects of nutrients and light (Domingues et al. 2011a, b, c)
• tidally-induced variability of environmental drivers (Domingues et al. 2010)
• influence of climate and anthropogenic stressors on phytoplankton variability (Barbosa et al.
2010)
• development of an ecohydrological model (Chícharo et al. 2006)

4.  however… previous studies focused only on specific periods related to the Alqueva dam
construction
 no integration of more than 14 years of data has been done to assess the impacts of the
Alqueva dam on estuarine phytoplankton and to compare the before and after situations
fundamental knowledge for a successful management of estuaries and adjacent coastal
ecosystems
GOAL – understand the impacts of the Alqueva
dam, particularly the effects of different
hydrological regimes; compare phytoplankton and
their environmental drivers before and after the
dam started to operate

5. GUADIANA RIVER AND ESTUARY
 arises in Spain, flows for 810 km and drains between SE Portugal and SW Spain into the
Atlantic Ocean
 last 70 km (tidal limit) – estuary
 area = 22 km2; average depth = 6.5 m; width: max = 800 m, min = a few meters
 mediterranean climate region (hot, dry
summers; moderate winters)
 tides - mesotidal, semidiurnal tides
 vertical stratification - well mixed in middle
and upper estuary; partially stratified in
lower estuary

6. METHODS
 frequent sampling campaigns have been conducted in the Guadiana in the last decade,
focusing on the freshwater tidal estuarine zone (sampling station Alcoutim)
 sampling – ebb flow, neap tides
 abiotic variables
• T, S, O2
•dissolved inorganic N, P and Si
•PAR, Im
• river flow and rainfall (http://snirh.pt)
 biotic variables
• chlorophyll a
• phytoplankton composition and abundance
Portugal
Spain
Atlantic Ocean
 data analysis – data aggregated into 4 distinct periods related to the Alqueva dam
construction and operation:
• 1996-1998, 2001 – pre-filling
• 1999-2000 – intense land excavation period
• 2002-2003 – dam filling period
• 2004-2009 – post-filling period, regular dam operation

7. 0
20
40
60
80
100
Riverflow(m3
s-1
)
B
pre-filling excavation filling post-filling
0
10
20
30
40
50
Summerriverflow(m3
s-1
)
0
100
200
300
400
500
600
Winterriverflow(m3
s-1
)
C
RESULTS & DISCUSSION
 significantly different between the four
periods
 river flow significantly correlated with rainfall
(rS = 0.5330, n= 152, p < 0.0001)
despite intense impoundment, water flowing
into the estuary is still largely controlled by
rainfall
 river flow during summer was significantly
higher in the post-filling period than in previous
phases
elimination of the natural high and low peaks
in river flow, typical of mediterranean climate
regions
221
31 34
26

8.  variability in river flow contradicts previous predictions for the Guadiana estuary:
significant reduction in river flow, especially during summer
alarming!
Toxic cyanobacteria blooms – recurrent problem in the Guadiana
estuary, during summer, associated to low freshwater flows
 major increase on the frequency and magnitude of cyanobacteria blooms was
expected with dam construction
pre-filling excavation filling post-filling
0
10
20
30
40
50
60
Cyanobacteriaabundance(x107
cellL-1
)
C
No data
However…
 river flow during summer increased in the
post-filling period
 cyanobacteria abundance decreased significantly
420 x 106 12 x 106 cells L-1 97%
 strong negative correlation between cyanobacteria
abundance and river flow (rS = -0.48, n = 69, p < 0.0001)
Cyanobacteria abundance

9.  river flow is clearly a relevant regulator of phytoplankton dynamics
 most problems associated with river damming are a direct consequence of changes in
freshwater flow
nutrient inputs biological productivity
0
50
100
150
200
250
Nitrateconcentration(µM)
A
pre-filling excavation filling post-filling
0
50
100
150
200
250
DSiconcentration(µM)
C
 nutrient concentrations were mostly similar (N, P) or
higher (Si) after dam filling in relation to the pre-filling
phase
N
Si
 e.g., construction of the Aswan High dam in the Nile River
caused a reduction of 90% in freshwater flow
decreased nutrient inputs to the Mediterranean
disappearance of phytoplankton blooms
collapse of Eastern Mediterranean coastal fisheries

10. pre-filling excavation filling post-filling
0
5
10
15
20
25
30
35
40
Chlorophylla(mgm-3
)
 however, phytoplankton biomass and abundance decreased significantly in the post-
filling period…
0
2
4
6
8
10
12
14
16
18
20
Diatomabundance(x106
cellL-1
)
A
No data
Diatom abundance
Chlorophyll a
0
20
40
60
80
100
120
Greenalgaeabundance(x105
cellL-1
)
B
No data
Green algae abundance
river flow during spring-summer
light limitation of phytoplankton growth
nutrient inputs & turbidity
light availability in the water column

11. -0,1
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
control N P Si NP SiN SiP NPSi
Communitynetgrowthrate(d-1
)
**
**
**
**
B
light limitation nitrogen limitation
In: Domingues et al (2011) Est Coast Shelf Sci (doi: 10.1016/j.ecss.2010.10.033); Domingues et al (2011) Est Coast Shelf Sci (doi:
10.1016/j.ecss.2010.12.008)
0
0,1
0,2
0,3
0,4
0,5
control I1 I2 I3
Communitynetgrowthrate(d-1
)
B
*
****
pre-filling excavation filling post-filling
0
20
40
60
80
100Im(µmolphotonsm-2
s-1
)
B
 light availability was slightly lower in the
post-filling in relation to the pre-filling period

12.  alterations in river flow alterations in phytoplankton dynamics
 phytoplankton abundance and biomass decreased significantly in the post-filling period
 river flow controls abiotic variables that regulate phytoplankton dynamics (nutrient and
light availability)
 disappearance of cyanobacteria blooms – beneficial to the ecosystem
 decrease in diatom biomass is detrimental to higher trophic levels
diatom fish
 alterations in phytoplankton biomass affect fish abundance and species composition
phytoplankton planktivorous fish
phytoplankton carnivorous fish
Adapted from Chícharo et al (2006) ECSS 70: 39-51
wet
dry
planktivorous fish carnivorous fish
 sardine (planktivorous fish) is the most
economically important fish in the Algarve
 when carnivorous fish
dominate fish catches…
average loss of 3 M €

13. CONCLUSIONS
 river flow is clearly a critical regulator of abiotic and biotic variables in the Guadiana
estuary
 human regulation of river flow and disruption of natural flow regimes
negative impacts on the estuarine ecosystem
 elimination of natural peaks in river discharge and a more constant river flow
throughout the year decline in phytoplankton biomass
 “ecological flows” should be avoided for extended periods
 establishment of river flow that mimics natural river discharges, to prevent negative
impacts on the natural patterns of biological communities

14. © Pedro Morais
Thank You!
Rita B. Domingues is supported by a postdoctoral fellowship awarded by the Portuguese
Foundation for Science and Technology (SFRH/BPD/68688/2010).