Carbon assimilation and phytoplankton growth rates across the trophic spectrum: an application of the chlorophyll labelling technique

The chlorophyll labelling technique has been acknowledged to be a useful method for measuring phytoplankton growth rates while avoiding some of the problems involved in calculating growth rates derived from the 14C fixation rates. The results presented here are of experiments comparing phytoplankton growth rates during the summer season in three subalpine Italian lakes: Lago Maggiore, the second largest lake in Italy, and two smaller lakes, Lake Mergozzo and Lake Varese, both included in the Lago Maggiore drainage basin. The three lakes have different morphometric, physico-chemical and biological features. The first goal was to compare two different methods of estimating phytoplankton growth rates starting from 14C assimilation. The second goal of our experiments was to test the hypothesis that growth rates can be quite different across the trophic spectrum, due to the ecophysiological and morphological features of the phytoplankton assemblages. In particular, algal cell size should decrease from eutrophic to oligotrophic systems and growth rates should follow the opposite trend, as they are inversely scaled to the cell size. Two basic conclusions can be drawn. The first is that, in spite of some drawbacks still affecting the use of the chlorophyll labelling technique, this appears to be one of the most promising methods for estimating the growth rates of phytoplankton in situ. The second conclusion is that this method, coupled with information on some algal morphological parameters, can provide useful indications about the functional properties of phytoplankton assemblages living in diverse lacustrine environments.