IN-SITU GROSS GROWTH-RATES OF EMILIANIA-HUXLEYI IN ENCLOSURES WITH DIFFERENT PHOSPHATE LOADINGS REVEALED BY DIEL CHANGES IN DNA CONTENT

The in situ specific growth rate of the nanoplankton species Emiliania huxleyi can be successfully derived from analysis of diel DNA synthesis patterns. Calculated growth rates (mu DNA) were in close agreement with growth rates determined from cell counts in laboratory cultures of E. huxleyi. For E, huxleyi populations in large outdoor enclosures (temp. = 7.9 to 10.2 degrees C), mu(DNA) ranged from 0.36 to 0.76 d(-1). Combining data on (net) changes in cell number of the enclosed populations with the calculated mu(DNA) values provided information on the total specific loss rates of E. huxleyi (caused by grazing, viral infections and autolysis). The initial and mid-exponential phases of E. huxleyi blooming were characterized by relatively high mu(DNA) values. At a later stage, following the depletion of nutrients in the water, mu(DNA) decreased. Specific loss rates ranged from 0.07 to 0.63 d(-1) and no particular trend in time was noticeable. E. huxleyi populations in enclosures with different phosphate loadings did not show significant differences in mu(DNA); in contrast, differences in loss rates were indisputable. Loss rates were low (0.07 to 0.35 d(-1)) in fertilized enclosures with low and intermediate phosphate concentrations, allowing extensive E, huxleyi blooming. In the fertilized enclosure with high phosphate loadings (PO4 > 2.6 mmol m(-3)) and in the unfertilized oligotrophic enclosure losses were high (0.33 to 0.63 d(-1)) preventing blooming of E. huxleyi. We concluded that nutrients were not limiting gross E. huxleyi growth but that they affected E. huxleyi losses by changing the phytoplankton composition and biomass.