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 synthes is patterns. Calculated growth rates (mu DNA) were in close agreement with growth rates determined from cell counts in laboratory cultures o f 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 popul ations with the calculated mu(DNA) values provided information on the total specific loss rates of E. huxleyi (caused by grazing, viral infe ctions and autolysis). The initial and mid-exponential phases of E. hu xleyi blooming were characterized by relatively high mu(DNA) values. A t 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) an d no particular trend in time was noticeable. E. huxleyi populations i n enclosures with different phosphate loadings did not show significan t 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 e nclosures 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 olig otrophic enclosure losses were high (0.33 to 0.63 d(-1)) preventing bl ooming of E. huxleyi. We concluded that nutrients were not limiting gr oss E. huxleyi growth but that they affected E. huxleyi losses by chan ging the phytoplankton composition and biomass.