Growth Inhibition of Phytoplankton Populations Cultured in Disphotic Zone Water by Insufficient Amounts of Dissolved Organic Carbon

Phytoplankton cultures occurring in disphotic zone water were conducted to examine dissolved organic carbon (DOC) for possible controlling agent of the initial lag period and growth rate. Culture media of various concentrations of DOC were prepared by mixing low DOC disphotic zone water with high DOC surface water. Natural phytoplankton populations showed strong correlations in their lag period with DOC concentrations in the range from 0.75 mgC·l–1 to 1.2 mgC·l–1 in the water (r = –0.833, n = 8) and in their population growth rate (µ) (r = 0.899, n = 8). Similar tendencies were confirmed with a marine diatom (Skeletonema costatum) dominating in the present disphotic zone water by culture experiments. By reducing DOC concentrations in seawater samples by pretreat-ments of ultraviolet radiation, charcoal adsorption and Amberlite XAD-2 resin adsorption, lag periods of S. costatum increased in every case, but their population growth rates were almost identical. These results obviously show that prolonged lag period at least occurs in low DOC water, which can explain the observations by Barber and Ryther (1969) that low photosynthetic carbon uptake rate occurs in newly upwelled low DOC water. It is found that the essential substance to shorten lag periods of phytoplankton cultured in disphotic zone water is a portion of dissolved organic matter, which is poor in disphotic zone water and rich in surface water, and the effect of the substance analogous to Na2EDTA strongly suggests that the substances are organic ligands. 1