Objective

Since the early 1980's exceptional phytoplankton blooms seem to have increased in number and size in German, Danish and Swedish waters. It is now apparent, from long-term trend analysis, that in all regions, annual inputs of phosphorus and nitrogen have increased by a factor of about three during the past three decades, and that overall biomass production has increased by the same order of magnitude. Nevertheless, it is still uncertain whether there is any direct causative relationship between elevated nutrient inputs and the more frequent occurrence of so-called exceptional phytoplankton blooms. Phytoplankton bloom formation depends not only on the supply of nutrients (although this is the main prerequisite for high biomass) but also on a variety of other environmental factors such as light availability and the absence or the ineffectiveness of grazers
There can be no doubt that over the past decade blooms of newly reported, undesirable species have increased in the Baltic transitional area, including Kattegat and Skagerrak. Examples are Gyrodinium aureolum, Prorocentrum minimum, and Chrysochromulina polylepis. In the Kiel Bight, P. minimum has formed blooms annually since 1983 during August/September and thus became a normal component of annual phytoplankton succession.
A similar development has taken place with a new type of phytoplankton bloom which is the main subject of this paper: the naked form of the silicoflagellate Dictyocha speculum (syn. Distephanus speculum). Blooms of this organism first occurred in the Kiel Bight area in May 1983 and have also become common in the species succession of this area.
The first records came from the Danish Alssund where a bloom of naked, previously undescribed flagellates was observed in connection with heavy mortalities of caged fish. It was identified by Moestrup and Thomsen (H. Thomsen, Copenhagen, personal communication) as a naked, (i.e., non-skeleton-forming) modification of Distephanus speculum and was confirmed by observations in Kiel Bight.
Silicoflagellates of the genus Dictyocha are marine chrysophytes bearing a single flagellum and a star-shaped siliceous skeleton composed of tubular elements forming a basal ring and several extruding spines. While D. fibula possesses a quadrangular basal ring and four spines, D. speculum, which is to be found in the Kiel Bight area, has a hexagonal basal ring and six spines.

Results

Naked cells of Dictyocha speculum found in Kiel Bight are more or less spherical and ca. 20 µm in diameter (Fig. 1). They possess one flagellum and ca. 25 chloroplasts. The cell surface often appears to be lobed due to plastids and cells are very sensitive to high temperatures and physical stress (e.g. shaking). In preserved samples, they easily disintegrate after brief storage. The flagellum is mostly shed upon preservation.

Fig.1: Since the early 1980's, besides typical skeleton-bearing forms (S) of Dictyocha speculum, naked forms (N) form massive blooms in May. Cell numbers rise to several millions per liter. Fish death related to theses blooms were reported from Danish waters but could not be confirmed in Kiel Bight

In Kiel Bight, D. speculum exhibited a bloom period of ca. 20 days with maximum cell numbers ~6-7 million cells per liter.
There are good reasons to believe that the persistent occurrence of this new late-spring flagellate bloom is related to changed nutrient conditions due to eutrophication, especially the input of inorganic nitrogen compounds.
The annual cycle of phytoplankton succession in the Kiel Bight area starts with a massive diatom bloom in March/April. At least until the mid-1970's this typical diatom bloom was terminated by the exhaustion of inorganic phosphorus and/or nitrogen compounds while small amounts of silicate were still available. Nowadays, however, lack of silicate seems to terminate the diatom spring bloom, and the remaining nitrate and phosphate are apparently sufficient to support considerable blooms of silicate-independent flagellates in May - among them increasingly unusual and harmful species.
To verify this hypothesis, we followed the change in nitrate and silicate concentrations in inner Kiel Fjord during spring 1984, 1986, and 1988 (the years of naked D. speculum blooms). Whereas silicate was low at the onset of the D. speculum blooms, nitrate concentrations were still consideralbe (Fig. 2). From natural water samples, Redfield gave an atomic Si:NO₃ ratio of 0.94.

Fig 2: Silicate (squares) and Nitrate (crosses) concentrations in spring 1984

Ratios in Kiel Fjord in April and May 1984, 1986, and 1988 were below 1.0 for both surface and subsurface waters and thus indicate silicate rather than nitrate to have been the limiting nutrient.
Comparing nutrient data from 1939 to those of the D. speculum bloom years reveals: Silicate conentrations in March were similar in all years. But compared to 1939, values below 8 µmol l^-1 were reached 20 days earlier in the season in 1986 and 1988 and even 30 days earlier in 1984. Furthermore, data analysis revealed a decreasing trend in winter concentrations of silicate since the mid 1970's (r =-0.63, p < 0.05, n = 17). Consequently, silicate limitation of diatom spring blooms should become more evident as long as anthropogenic nitrogen input into the Baltic Sea commences - thus increasing favorable growth conditions for flagellate blooms in May.
Similar trends of decrasing silicate concentrations and a shif in the nutrient composition have been reported from the adjacent Arkona Sea, the Skagerrak and the German Bight.
In Kiel Bight, D. speculum not only demonstrates that an 'exceptional' bloom may become a recurrent event in the annual special succession but may also be related to the change in environmental conditions due to eutrophication. Although known for years to occur in Kiel Bight as a skeleton- forming species, D. speculum had never built blooms as naked forms before. Thus, changed environmental conditions stimulated this species to grow in a yet unknown form.

Credits

This work was performed at the Institut für Meereskunde Kiel, Germany.

References

Jochem, F. & B. Babenerd: Naked Dictyocha speculum - a new type of phytoplankton bloom in the Western Baltic. Marine Biology 103, 373-379. 1989 
You may also download the whole paper as PDF file