Growth of Blue-Green Algae in the Saidenbach Reservoir and its Relationship to the Silicon Budget

The phytoplankton succession during the summer in the mesotrophic reservoir Saidenbach since 1975 may well be explained by the resource ratio hypothesis. Until 1980, only phosphorus controlled the phytoplankton growth, and diatoms prevailed, because an excesses of silicon existed. From 1981 to 1986, the ratio Si:P often was smaller than 90, a value, critical for the development of the diatom Fragilaria crotonensis. Its reduced growth caused an increased occurrence of blue-greens (mostly Aphanothece clathrata) immediately after the diatom mass development. During these years at first silicon limited phytoplankton growth in summer, later on the growth again was limited by phosphorus. Because of increased Si and P load since 1987 a simultaneous limitation of both nutrients occurs. This leads now to parallel mass developments of diatoms and blue-greens. In order to maintain the positive effect of diatoms (phosphorus transport into the sediment), it is to guarantee a sufficiently high Si:P ratio. If a reduction of P load isn't possible, Si remobilization from the sediment could be increased by artificial changes of the water level.     

The Effects of Sampling Intervals on Phytoplankton Growth and Loss Values Derived from Seasonal Phytoplankton Biomass Variations in an Artificial Lake

Detailed vertical phytoplankton sample series with a good resolution were taken every 3–4 days for almost two years from the artificial Lake Saidenbach. The intervals between samplings were systematically increased by regularly “ignoring” certain samples so that six different annual curves based on different sampling frequencies were obtained to describe phytoplankton variations in the course of the year from a single time series. In this way it was possible to study the effect of the interval between samplings on the results of phytoplankton studies and on the rates of change and turnover calculated from the biomass. The results show that the maximum rates of change and turnover values yielded by half-weekly samplings are some 20–30% higher than those yielded by weekly samplings. Sampling intervals smaller than 3–4 days yield not much increased rates of change and turnovers. This implies that short term changes in the phytoplankton are more accurately detected by half-weekly samplings. It is therefore suggested that, depending on the nature and objectives of the investigations, sampling intervals of 3–4 weeks are sufficient for the winter, but during the growth period (spring to autumn) samples should be taken at least once a week or fortnightly.     

Interactions between Light Situation,Depth of Mixing and Phytoplankton Growth during the Spring Period of Full Circulation

Investigations lasting several years at the artificial Lake Saidenbach have shown that, due to the poor light situation expressed in terms of average daily radiation (Riley 1957), mean daily exposure (Reynolds 1973), extinction depth and the ratio ^zmis/^zeu, no mass development of the phytoplankton should actually occur during the spring complete circulation. In practice, however, the period of complete circulation of the water in spring is not only a prerequisite for, but often also the factor causing, the mass development (usually Asterionella formosa). The poor light situation and the fact that the vertical phytoplankton profile shows significant differences between the plankton concentrations at different depths indicate that the spring complete circulation does not represent a complete recirculation, and thus mixing, of the water down to the bottom but involves only episodic and local partial recirculations interspersed with periods of relatively slight turbulence. The actual mixing depth during this period of complete circulation is therefore obviously less than the mean depth of the water concerned, which is commonly assumed to equal. This permits the algae in the upper layers to grow. Respiratory losses of the phytoplankton at greater depths probably remain slight due to their adaptation to low light intensities in winter.     

Sedimentary Losses in the Reservoir Saidenbach: Flux and Sinking Velocities of Dominant Phytoplankton Species

Sedimentation of phytoplankton was studied in the meso/eutrophic reservoir Saidenbach for two years and measured as biovolume in a sedimentation trap near the bottom. It comes to an annual average of 2.76cm³/m² × d (0.4… 10.9) and is statistically significant dependent on the free water concentration measured 14 days before. This allows flux to be reliably calculated without any direct measurement. The bottom is reached above all by diatoms which form 90% of the deposited algae. The sinking velocity of the diatoms is mainly determined by their physiological state: During growth phases low sinking velocities (0.1 … 2m/d) were found, while in decline phases they increased considerably (>6m/d). The highest average sinking velocities found among large diatoms were calculated for Fragilaria crotonensis (3…4m/d), the lowest for Melosira italica (1.5…2m/d). The values for Asterionella formosa, Synedra acus and Diatoma elongatum were between them. Turbulence has no influence on sinking velocity (usually, spring full circulation sees the highest sedimentary losses), but plays an essential part in the survival of the population in free water. Permanent redistribution prevents diatoms from sinking out from the euphotic layer, this “inoculation” making further development possible. On an annual average, phytoplankton forms approximately only one quarter of the whole trap sediment (max. 62%). The flux of the dry weight of seston (2.33g/m² x d on an average) reflects the changes in phytoplankton flux negligibly only and does not reveal any relation to it. so that seston flux is not suitable for determining phytoplankton sedimentation. But it is possible to calculate this at a probability of 65 to 94%, by using the free water concentration measured 14 days before.