1. A conceptual model of annual Ostreopsis cf
A conceptual model of annual Ostreopsis cf. ovata blooms in the northern Adriatic Sea based on the synergic effects of hydrodynamics, temperature, and the N:P ratio of water column nutrients
Stefano Accoroni1,2, Patricia M. Glibert2, Salvatore Pichierri1, Tiziana Romagnoli1, Mauro Marini3, Cecilia Totti1
1CoNISMa, Università Politecnica delle Marche, Ancona Italy
2University of Maryland Center for Environmental Science, Cambridge MD USA
3Consiglio Nazionale delle Ricerche), Ancona, Italy
Summer School, Tunis, 26th-28th May 2015

2. Ostreopsis distribution
From Accoroni, 2013
Only two species:
Ostreopsis cf. ovata, Ostreopsis cf. siamensis 2

3. Study area
Adriatic Sea
Passetto
ANCONA 3

4. Methods Study period: 2007-2012 from April-June to November
Frequency: 15 dd before and 5-7 dd after 1st cell appearance
Sampling of undisturbed substrata:
macroalgae (3 replicates)
pebbles (3 replicates)
water column (1 replicate)
Environmental parameters: T, S, meteomarine conditions, nutrients (3 replicates)
Ulva rigida
Dictyota dichotoma
Hypnea musciformis
Hypnea musciformis 4

5. Methods
Treatment of substrata to obtain the complete removal of Ostreopsis cells.
Substrata measurements:
Thalli → fw, dw, area
Rocks → area
Identification, counting and measurements at inverted light microscope.
Statistical analysis: ANOVA, Tukey test
Abundances expressed as
cells g-1 fw
cells g-1 dw
cells cm-2 5

6. Temporal trend and relationships with T
Appearance: end Jul/early Aug
Peak: abundances reach 104 cells cm-2 (106 cells g-1 fw, 107 cells g-1 dw) in late summer (Sep-Oct)
Decline: end Oct/early Nov
2007: °C
Totti et al., 2010, Harmful Algae
2009: °C
Accoroni et al., 2011, Mar. Pollut. Bull.
2011: °C
2012: °C
Accoroni et al., 2015, Harmful Algae
2010: °C
Accoroni et al., 2012, Cryptogamie, Algologie 6

7. Relationships with temperature
A trigger role of temperature may be hypothesized, as cyst germination occurs only at T=25 °C in experimental conditions.
T=25 °C
Vegetative cells
Double-walled cysts
Accoroni et al., 2014, Harmful Algae

8. Relationships with temperature
Accoroni et al., 2015, Harmful Algae
This would suggest that other environmental factors beside temperature may affect the development of Ostreopsis cf. ovata blooms 8

9. Relationships with nutrients
DIN: µmol l-1
PO4: µmol l-1
2009
Optimal N:P ratio: ~ 16 (Vanucci et al., 2012; Vidyarathna and Granéli, 2013)
PO4 concentrations were significantly higher in bloom onset than in either bloom maintenance phase or the pre-bloom conditions (p < 0.01)
N:P ratio was significantly lower in bloom onset (24.5) than in pre-bloom conditions (139.2, p < 0.05)
2010
2011
2011
Accoroni et al., 2015, Harmful Algae 9

10. The bloom onset: temperature and nutrients
2009
2011
2010
2012
Accoroni et al., 2015, Harmful Algae
A water temperature threshold of 25 °C plays a key role in the germination of O. cf. ovata cysts and therefore in bloom onset, and an N:P ratio around Redfield value is a necessary condition to allow cell proliferation.

11. Growth rate: temperature and nutrients
2009
2011
2010
2012
Accoroni et al., 2015, Harmful Algae
Significantly higher growth rate in N:P <24.5 and T >20 °C than at higher N:P ratios and temperatures <20 °C (p < 0.05).

12. The bloom maintenance: temperature and nutrients
2009
2011
2010
2012
Accoroni et al., 2015, Harmful Algae
Once Ostreopsis cysts germinated, vegetative forms seem to actively proliferate even if temperature values decrease
A slight increase of N:P ratio during the bloom maintenance is likely attributable to Ostreopsis uptake
Once Ostreopsis cysts germinated, its vegetative forms seem to actively proliferate even if temperature values decrease
Alike, while high growth rates (stimulated by an injection of P-rich water) may allow blooms to initiate, adaptive physiology e.g., (metabolic dissipatory strategies, allelopathic and mixotrophic interactions) would enable the bloom maintenance at less than maximal growth rates and at non-optimal N:P ratios.

13. Relationships with hydrodinamism
Passetto S 5 10 15 20
25-Mar
9-Apr
24-Apr
9-May
24-May
8-Jun
23-Jun
8-Jul
23-Jul
7-Aug
22-Aug
6-Sep
21-Sep
6-Oct
21-Oct
avg macroalgae
cells cm -2 3
Passetto E 5 10 15 20
25-Mar
9-Apr
24-Apr
9-May
24-May
8-Jun
23-Jun
8-Jul
23-Jul
7-Aug
22-Aug
6-Sep
21-Sep
6-Oct
21-Oct
avg macroalgae
cells cm -2 3
Abundances were significantly higher in sheltered than in exposed sites (p < 0.01 and in 2007 and 2009, respectively, Totti et al., 2010, Harmful Algae; Accoroni et al., 2012, Harmful Algae). 13

14. Conceptual diagram
Accoroni et al., 2015, Harmful Algae 14

15. Conclusions
The bloom of O. cf. ovata in the N Adriatic typically occurs in late summer in sheltered rocky coasts, with max values of the order of magnitude of 104 cells cm-2 (=106 cells g-1 fw).
Calm conditions are a prerequisite for blooms. Then O. cf. ovata blooms appear to be triggered by a combination of optimal temperature and available nutrients.
A water temperature threshold of 25 °C plays a key role in the germination of cysts and therefore in bloom onset, and an N:P ratio around Redfield value is a necessary condition to allow cell proliferation.
The synergy of higher temperatures and optimal N:P ratios resulted in a higher net growth rate of O. cf. ovata cells.
After the onset, blooms can be maintained at temperature values even below 20 °C and at N:P ratios that are in excess of the Redfield ratio.
The increase of N:P ratio during the bloom maintainance is likely attributable to Ostreopsis uptake