SCOPE Nー36 - 03/2000 -

Lower Rhine river
Silicate limitation of algal development


The production of phytoplankton was analysed in the three main branches and sedimentation areas of the lower Rhine river in Holland. A model based on carbon and silicate metabolism derived from sampling programme data which followed the water flow as it moved downstream.

Data were based on samples taken on five dates in 1988, one each month from April to August. Water was collected from the surface of the river at several points across its width. The same body of water was sampled over a 96 hour period, as it moved down the three branches of the river, the speed of movement being calculated on the basis of the average flow rate. The samples were analysed for photosysnthesis (carbon absorption at different light intensities), for dry weight of suspended matter and chlorophyll-a. Sky irradiance was obtained from local weather stations and the vertical light attenuation of the water was measured at the sampling stations.

======== Phytoplankton development model ========

A plug-flow model of phytoplankton development was constructed on the basis of :

The photosynthetic production was modelled on the basis of the sample data (measured photosynthetic rate of samples, sky irradiance data) and by application of reduction factors related to light attenuation in the water column (for all algal species) and to silicate limitation (applied to diatoms only).

======== Silicate limitation ========

It was assumed that phytoplankton growth was not limited by nitrogen or phosphorus, as average concentrations of soluble nitrogen and phosphorus in the lower Rhine were 5 mgN/l and 0.3 mgP/l respectively (1988), whereas silicate in the river water was sometimes depleted in summer.

The dissolved silicate concentration was modelled as affecting diatom growth, algal silicate uptake and algal cellular Si/C ratio (standard Si/C ratio = 0.6 mgSi/mgC, minimum 0.3).

The fraction of diatoms in the algal population was derived from cell counts taken on the five sampling dates, varying from 53?86% diatoms at the most upstream point and changing little as the water moved downstream.

Silicate was assumed to be not released from algae except after settling to the river bed, and a literature derived estimate of 10 mgSi/m2/h was used for release from the sediment.

Dissolved silicate concentrations at the upstream sampling point were observed to be lower during algal blooms (May, July and August) and to decrease as the water moved down the Rhine.

======== Algal grazing and biomass loss ========

Respiration losses, as a function of water temperature, were based on literature values from Janse and Aldenberg 1990 and Westlake 1980.

Settling rates, again based on literature values fro shallow lakes from Janse and Aldenberg 1990, were taken to be 3% per day for 5m deep water, which may be an underestimation for the sedimentation areas of the lower Rhine.

For zooplankton grazing of algae, the Janse and Aldenberg lake-derived functions were also applied, despite the fact that in much of the lower Rhine rotifers are the dominant grazer with crustaceans reaching substantial numbers in the lowest reaches.

======== Modelling results and conclusions ========

In periods without algal blooms (April, June), chlorophyll-a concentrations increased linearly as the water moved down the river. For the other months, an initial increase was followed by a substantial decrease further downstream, thought to be the result of accelerated zooplankton grazing (cf. Ruyter van Steveninck et al 1990).

The model correctly predicted significant phytoplankton losses due to zooplankton grazing during algal bloom conditions. Simulated zooplankton densities were in the range of 0.1?0.2 mgC/l, not far removed from the maximum zooplankton densities observed in the Rhine of 0.3 mgC/l (van Zanten and de Ruyter van Steveninck 1991).

The model predicted changes in dissolved silicate concentrations along the river matching with observations and diurnal variations agreeing with literature data.

The model indicates that the daily photosynthetic production rate can reach 50% of the algal biomass, enabling significant algal production even within the relatively short residence times resulting from river flow.

The model suggests that silicate limitation can decrease this algal production rate by a factor of 0.4?0.5, a reduction factor comparable to that observed in dilute plankton systems such as in Narragansett Bay (Kremer and Nixon, 1978). This is in agreement with certain field observations, such as the halving of phytoplankton photosynthetic capacity after dissolved silicate depletion in the Rhine (de Ruyter van Steveninck et al. 1992), or the observation of dead or poor condition diatoms at stations with low silicate concentrations (Bijkerk, in de Ruyter van Steveninck et al. as above).

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"A model of phytoplankton production in the lower Rhine verified be observed changes in silicate concentration". Journal of Plankton Research, vol. 15, nー 6, pp 659-682, 1993.

W. Admiraal, Aquatic Ecotoxicity Section, University of Amsterdam, Kruislaan 320, 1098 SM, Amsterdam. S. Mylius, Inst. Theoretical Biology, Leiden University, PO Box 9516, 2300 RA, Leiden. E. de Ruyter van Steveninck, IHE ? Aquatic Ecology, PO Box 3015, 2601 DA, Delft. D. Tubbing, National Inst. of Public Health and Environmental Protection, PO Box 1, 3720 BA, Bilthoven, Holland.


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