As the most important primary producer in the marine ecosystem, phytoplankton forms the foundation of the structure and function of the marine food webs. Most of the primary productivity is turned into phytoplankton biomass, which is a good indicator to reflect the spatio-temporal distribution of marine organic matter production and dynamics of the phytoplankton community. Consequently, the estimation of phytoplankton biomass has ecological significance in marine ecosystem studies.
Several methods have been established to estimate phytoplankton biomass, such as cell counting method, chlorophyll a determination method, cell carbon and nitrogen contents determination method, conversion of cell volume to biomass method etc.. Among them, the conversion of cell volume to biomass method is relatively more accurate as with this method, the cell volume is worked out and organic matter content per cell was measured. The results could be used to address phytoplankton biomass at the cell level and the change of ecosystem function. Therefore, this method gradually becomes the most common and effective method to estimate phytoplankton biomass.
The key of conversion of cell volume to biomass method is to study the relationship between cell volume and cell organic matter (carbon, nitrogen or others) content. In the past several decades, there were a number of studies on biomass of diatom, but fewer studies on dinoflagellates. It is needed to study dinoflagellates cell volume and cell organic matter contents.
Ten common dinoflagellates were investigated to determine the relationship between cell volume and their contents of carbon and nitrogen. The morphological characteristics of ten species were observed using a Nikon ECLIPSE TE2000-U optical microscope. Then cell-geometry analogous models were built accordingly. The model of Alexandrium tamaren, Alexandrium affine and Protoceratium reticulatum is sphere, that of Akashiwo sanguinea, Amphidinium carterae, Prorocentrum donghaiense, Prorocentrum gracile and Prorocentrum minimum is ellipsoid, that of Gonyaulax spinifera circular cone and half sphere and Lingulodinium polyedra circular cone and circular truncated cone. From these models, the cell volumes for each dinoflagellate can be calculated by microscope measurement including cell length, width or diameter and breadth. Furthermore, cell carbon and nitrogen content was determined using a CHN analyzer. Then the relationships between both carbon and nitrogen contents and cell volume could be established. Large differences were observed among the ten dinoflagellates in cell volume, carbon and nitrogen contents. The range of cell volumes are from 2.97×10²μm³ (Amphidinium carterae) to 4.50×10⁴μm³ (Akashiwo sanguinea). Amphidinium carterae had the lowest carbon and nitrogen contents (54.50 and 11.42 pictogram per cell), while Gonyaulax spinifera had the greatest carbon and nitrogen contents (2238.00 and 482.28 pictogram per cell), showing a difference of a factor of 40. It was more appropriated to analyze these relationships by logarithmic models due to the wide range of cell volume, which was mentioned by Verity et al (1992). In the present study, the cell volume thus determined is positively correlated to their cell carbon and nitrogen contents significantly (P<0.0001), which indicated that the cell carbon and nitrogen contents would increase with cell volume increased. On the other hand, the cell volume thus determined is negatively correlated to their cell carbon and nitrogen contents per unit volume significantly (P<0.0001), which indicated that the smaller cells contained more carbon and nitrogen per unit volume. In order to allow conversion among cell volume, carbon and nitrogen, in the marine ecology field, it is important to establish regression equations between these parameters.