The use of small, continuous and multispecies cultures to investigate the ecology of phytoplankton in a Scottish sea-loch

Summary

1.	In studying the ecology of phytoplankton in Creran, a sea-loch on the west coast of Scotland, we experienced problems because of patchiness and water movement. It therefore seemed desirable to enclose a part of the phytoplankton and thus fix it in one place for study.
2.	For practical and economic reasons we worked with 20 l laboratory cultures grown in simple apparatus (Fig. 1). Theoretical considerations suggested that culture should be continuous and under a natural cycle of illumination. The cultures were multispecies and should not be called chemostats, although we tried to obtain temporary steady states in which growth balanced dilution (see for example Fig. 6).
3.	Nine experiments were carried out during 1974 and 1975, with durations from 1 to 40 days. In each case the diluent was based on filtered water from Loch Creran.
4.	So long as a large inoculum was used we could grow a phytoplankton in our vessels that for 5 to 30 days was similar to the phytoplankton in the loch from which the inoculum was taken.
5.	Moderate nutrient enrichments coupled with dilution of about 0.2 day−1 allowed the maintenance of a “normal” phytoplankton for the longest time. “Normality” cannot be precisely defined, but includes the occurrence of typical sea-loch species, their growth in parallel with the same species in the loch (Fig. 4), the coherence of growth curves (Fig. 7) and the maintenance of moderate diversity.
6.	The cultures were used in investigations of various aspects of phytoplankton ecology in Loch Creran, including examination of nutrient status, cell division cycles, and microzooplankton grazing. The relationship between ciliates and small flagellates is suggested by Figs. 3 and 7. Investigation of the applicability of the nutrient growth model of Droop (1974, 1975) to the cultured multispecies phytoplankton, is described elsewhere (Jones et al., 1978). Use of this model suggests that N is more likely than P to control phytoplankton growth in Loch Creran.