Photoperiodic control of diatom spore growth: A theory to explain the onset of phytoplankton blooms

Photoperiod influences the timing of several growth processes in higher plants and macroalgae. Results from our experiments indicate photoperiodic control of germination and/or growth of diatom resting spores. We suggest that the onset of phytoplankton blooms m northerly areas, and possibly also further south, may be photoperiod dependent and linked to entrainment of phytoplankton resting spores from bottom sediments KEY WORDSPhytoplankton . Spores . Daylength Throughout several years o f investigation w e have observed that the timing of the phytoplankton spring bloom varies little in northern Norwegian coastal waters despite varying environmental conditions between years; the peak of the bloom can always be observed in mid-April (Gaarder 1938, Heimdal 1974, Schei 1974, Throndsen & Heimdal1976, Eilertsen et al. 1981, 1989). T h e seasonal development o f a pycnocline is widely accepted as being a prerequisite for the development o f a phytoplankton bloom (Sverdrup 1953). However, in many inshore northerly areas (fjords) spring blooms occur in thoroughly mixed, vertically homogeneous water columns (Eilertsen et al. 1981, 1989) and cannot be explained b y 'Sverdrup's paradigm' (Sverdrup 1953). One plausible explanation for this may be that although the fjords are deep (200 m or more), blooms may start near the shore where waters are shallower and where the phytoplankters are not transported to below their critical depth. This does not, however, explain how blooms can begin at approximately the same time in coastal waters under highly varying environmental conditions, nor w h y in some years the spring bloom might start at the same time in the open Barents Sea waters as near the coast. It may also be worthy of note that the spring bloom in a shallow (4 m ) saltwater pond near Tromse, Norway, followed essentially the same pattern (timing, species composition) as that in coastal waters despite the difference in environmental conditions (Pedersen et al. 1989). Examination of the literature describing species composition and succession shows that high-diversity diatom blooms m a y occur at approximately the same time at southerly latitudes as at Spitzbergen and in northern Norway (Eilertsen et al. 1989). For example, the spring bloom maximum in the English Channel can be observed around mid-April (Gros & Ryckaert 1983), when the water temperatures and daily insolation values are much higher than those observed further to the north. Thus there seem to be discrepancies between the mechanisms usually invoked to explain the onset o f blooms (stability, light field) and the observed events. It has already been suggested b y Gran (1912) that resuspended resting spores formed seed populations for blooms. W e therefore hypothesised that a large proportion of the phytoplankton contributing to the spring bloom was recruited from dormant cells (resting spores and cells) entrained from the bottom sediments via turbulent mixing during spring. Several preliminary experiments in which bottom sediments were collected during winter and grown in a continuous light regime showed that the source of the cells triggering the spring bloom may be spores contained in the bottom sediments. These sediment cultures comprised all the typical spring bloom species from the region, but the relative numbers of the species present were highly different from those that can be observed in field samples. W e therefore modified the hypothesis to include the idea that the onset o f blooms is regulated b y photoperiod, similar to the regulation of several growth processes in higher plants and macroalgae (Vince-Prue 1975, Evans et. al. 1982, Dring & Luning 1983, Hay 1990). This note presents results from some of the experiments that w e conducted in order to test