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ety of approaches have been attempted in the past, each with its own set of limitations (reviewed by Furnas 1990). Recently, an approach based on the concept of the cell cycle has been developed (McDuff & Chisholm 1982, Carpenter & Chang 1988). This method is based on the fact that phytoplankton are Unicellular organisms and that the completion of each round of the cell cycle results in population growth. The growth rate can then be estimated by monitoring the lemporal changes in the percentage of the cells that traverse a phase of the cell cycle leading to mitosis This method does not require incubation and is free from potential bottle effects (Carpenter & Chang 1988). Furthermore, cell loss due to grazing (Chang & Dam 1993). import or export, or death (when it is not cell cycle dependent) does not significantly affect growth rate estimation Therefore, comparison of this growth rate estimate, a gross growth rate, with net growth rate obtained with cell counts will allow one to estimate rates of cell loss (e.g. van B1eilswiJk & Veldhuis 1995) In addition, the accuracy of the cell cycle method is presumed not to be affected by varying growth conditions. The first growth rate model of this type (McDuff & Chisholm 1982) was based on the measurement of frequency of dividing cells in the population (Swift & Durbin 1972, Swift et al. 1976) ABSTRACT: The cell cycle approach has several advantages over other methods for estimating phytoplankton SpeClE'Sspecific growth rates In situ. This method has been successfully used in field studies in which the S-G2-M phase was used as a terminal event In the growth rate equation. Recently, the method has been modified, and c 11 cycle proteins have been substituted as the terminal event markel. Unfortunately, many of these markers represent cell cycle phases other than a terminal e ent. WhetilPr such a non-Ierminal event can be used in the cell cycle method wrll have a great impact on the extent of the application of thIs method. Our numerical analysis in thJS report showed thill, With the original growth equation. use of a non-terminal eVf'nt would overestImate the growth rate in most cases The magnitude of the ovcrcstlInation inneas'd with the time interval between the non-terminal event dnd cell diviSIOn as well as with growth rate. However, by solving an equatIOn derived from existing ones, we were able to eliminate the overestimation. To test the validity of this approach, we simulated 33 populations with diffl'rent growth rates, levels of synchrony, ratios of the duration of the G2-M phase to that of the S phase, and shdpes of the S phase fraction curve Estimates obtained with the S phase using the new equation were very close to those obtained with the terminal event (the S-G2-M phase) and to true growth rates, with an average underestimatIOn of about 5 % for the 24 rudimentary sImulation Celses. We conclude that, with the elld of thIS newly derived equation, a non-terminal event could perform as well as a terminal event in estimating growth rates Using this approach for field investigation will only require accurately estimatrng the duration of the non-terminal event dnd the time interval between the non-termllldl event and cell division.

Can a non-terminal event of the cell cycle be used for phytoplankton species-specific growth rate estimation?