52. Effect of elevated CO2 and nutrients on phytoplankton community structure during spring in the Western English Channel

Matthew Keys (1,2)*, Gavin Tilstone (1), Helen Findlay (1), Karen Tait (1), Tracy Lawson (2).

1 Plymouth Marine Laboratory, Plymouth, Devon, PL1 3DH, United Kingdom.
2 University of Essex, Colchester, Essex, CO4 3SQ, United Kingdom.

Background
Coastal zones support 10-15% of the global ocean net annual primary production and >40% of its carbon sequestration. Elevated CO2 affects the growth rate and biomass of many marine phytoplankton species, though the majority of studies have been conducted on species in culture and there have been comparatively fewer studies conducted on natural populations.

Phytoplankton taxonomy and community structure in the Western English Channel (WEC) has been analysed since 1992 at the WEC time-series (50° 15’N, 4° 13’W) and has shown significant decline in diatoms and Phaeocystis, whereas coccolithophorids and dinoflagellates have increased.

Methods
A fifteen day experiment was conducted on the natural phytoplankton community from the WEC time-series in April 2015, maintained at ambient pCO2 (~380 µatm) and elevated pCO2 (1000 µatm) plus nutrients amended to 8µM nitrate+nitrite and 0.5 µM phosphate in a closed semi-continuous incubation system.

Findings
Total phytoplankton community biomass increased significantly by 50% to ~225 µg C L-1 in the high CO2 treatment. Phaeocystis globosa exhibited a significant response to elevated CO2, increasing from ~60 µg C L-1 to ~185 µg C L-1 and constituted 80% of the total biomass. In the short term, the biomass of chain forming and pennate diatoms increased significantly in the high CO2 treatment, but then declined after 7 days.

Conclusions
P. globosa is an indicator species of water disturbance resulting from eutrophication. OSPAR recommend that waters of good ecological status have low abundance of P. globosa. This study suggests that future CO2 concentrations in the WEC could favour P. globosa, possibly tipping the balance towards poor ecological status in these waters.