1. Phytoplankton nutrient limitation in the Ria Formosa:
implications for eutrophication assessment
Rita B. Domingues1,2, Cátia C. Guerra1, Ana B. Barbosa1, Helena M. Galvão1
1 Centro de Investigação Marinha e Ambiental, Universidade do Algarve
2 Centro de Oceanografia, Faculdade de Ciências, Universidade de Lisboa

2. What is phytoplankton?
 unicellular, photosynthetic prokaryotic and eukaryotic organisms whose powers
of locomotion are such that they drift freely in the water column
 diatoms, dinoflagellates, coccolithophores, cyanobacteria, …
Why is phytoplankton so important?
 phytoplankton is the main primary producer in most aquatic ecosystems
 phytoplankton is a fundamental player in many ecosystem processes
 phytoplankton is a gauge of ecological condition and change
 phytoplankton can be used to predict the functioning of aquatic ecosystems and
ecosystem responses to natural and anthropogenic perturbations

3. Studying phytoplankton is not straightforward…
 the ecological role played by different phytoplankton groups and species varies
significantly
 phytoplankton in coastal ecosystems is undergoing marked changes
(biomass, composition, phenology) – linked to local anthropogenic influences and
climate change
 links between environmental and phytoplankton changes are difficult to establish
Importance of nutrients
 Nutrient availability is a key driver of phytoplankton dynamics
 Nutrient limitation is a central question for eutrophication control
How do nutrients affect phytoplankton growth, diversity and production?
Assessment of the impact of nutrient enrichment (eutrophication) and
subsequent nutrient reduction strategies

4. Why the Ria Formosa coastal lagoon?
 multiple anthropogenic influences
 very vulnerable to climate change
phytoplankton regulation is a critical aspect
in a fast-changing ecosystem
 lack of information on phytoplankton regulation in the Ria Formosa
 studies are mostly descriptive
Project PHYTORIA – Environmental Regulation of Phytoplankton in the
Ria Formosa coastal lagoon (PTDC/MAR/114380/2009)
Goals
• to determine the limiting nutrients for phytoplankton growth and their
seasonal variation in the Ria Formosa coastal lagoon
• to understand the effects of potential anthropogenic nutrient enrichment on
phytoplankton structure in the Ria Formosa

5. Approaches
 nutrient addition experiments in 2 L microcosms containing natural
phytoplankton assemblages from the Ria Formosa coastal lagoon
 autumn, winter, spring, summer
 addition of dissolved inorganic macronutrients, alone and in combinations, in a
single, saturating pulse
control
+NO3-
+NH4+
+P
+Si
+NP
+SiN
+SiP
+NPSi
 in situ incubation (under in situ T and light)
 phytoplankton growth (epifluorescence and inverted microscopy)
 nutrient consumption

6. Results – N consumption
25 13.5
Autumn 11.5 Spring
N uptake (µM d-1)
20 9.5
N uptake (µM d-1)
15 7.5
5.5
10 3.5
5 1.5
-0.5
0 -2.5
-5
35
30 Summer
30
25 Winter
N uptake (µM d-1)
25
N uptake (µM d-1)
20 20
15 15
10 10
5 5
0 0
-5 -5
 higher N uptake in N-enriched treatments in relation to the control indicate N
limitation phytoplankton in the Ria Formosa is N-limited throughout the
year
 NO3- concentrations up to 12 µM were limiting to phytoplankton growth

7. Results – Establishing nutrient criteria
Application of nutrient limitation criteria based on Ks – problems…
Ks obtained for unialgal cultures under equilibrium conditions
Ks vary temporally, spatially, inter- and intraspecifically
Establishment of nutrient criteria
- should consider the seasonal variation of phytoplankton community (different
spp. are limited in different ways by different nutrients)
- should consider that limiting concentrations don’t have to be necessarily low
concentrations (e.g., in the Guadiana estuary N is limiting up to 20 µM:
Domingues et al., 2011)
B
0,5 **
Community net growth rate (d -1)
0,45 spring **
Initial N = 22 µM
0,4
0,35
0,3 **
**
0,25
0,2
0,15
0,1
0,05
0
control N P Si NP SiN SiP NPSi

8. Results – Phytoplankton net growth
1.1
Summer
Community net growth rate (d-1)
0.9
Increased nutrient uptake 0.7
0.5
0.3
phytoplankton growth
0.1
-0.1
-0.3
-0.5
control Nit Amm P Si NP SiN SiP NPSi
0.45
0.4
Winter
luxury consumption 0.35
Net growth rate (d-1)
0.3
0.25
0.2
0.15
0.1
0.05
0
control Nit Amm P Si NP SiN SiP NPSi

9. Bottle effects on microcosm Effects of nutrient enrichment on Phytoplankton mortality in the
experiments bacterioplankton in the RF Ria Formosa
Effects of ultraviolet radiation and CO2 increase on winter phytoplankton
assemblages in a temperate coastal lagoon (Ria Formosa)
Rita B. Domingues, Ana B. Barbosa, Vanda Brotas
18h15

10. Thank You for Your Attention!
Acknowledgements
FCT project Phytoria – Environmental regulation of phytoplankton in the Ria Formosa
coastal lagoon (PTDC/MAR/114380/2009)
Postdoctoral fellowship awarded to RBD (SFRH/BPD/68688/2010)