FORMATION OF RESTING SPORES BY LEPTOCYLINDRUS DANICUS (BACILLARIOPHYCEAE) IN A CONTROLLED EXPERIMENTAL ECOSYSTEM 1

Leptocylindrus danicus Cleve became the dominant phytoplankton species, comprising 70–80% of the total assemblage, in one of the CEPEX Controlled Experimental Ecosystems (CEE) at Saanich Inlet, British Columbia (Canada). In the first week of June, when nitrate levels were reduced below 0.5 μm, the majority of the L. danicus cells present in the CEE formed resting spores. The spores were heavily armored with spines and appeared to sink unmolested to the bottom of the CEE. Four continuous cultures were started with an inoculum from the CEE and, in a 24 h period when N became depleted, 86% of the L. danicus present (∼80% of the total phytoplankton assemblage) formed resting spores. A daily dilution culture with nutrients kept at saturating levels was started at the same time from the same inoculum and continued for 2 wk beyond the spore formation event in the N‐limited cultures. No spores were observed in the nutrient‐saturated culture, indicating that N limitation was necessary to trigger spore formation. Spores were kept in the dark at 3 and 10 C. After 36 and 97 days, a large percentage of the spores germinated. After 214 days, >1% of the spores were still capable of germination. Laboratory studies with L. danicus isolated from the CEE confirmed that N limitation was the primary factor triggering spore formation. Microscope observations of these cultures indicated that the spores were formed by auxospores following sexual reproduction.