Decrease of phytoplankton during passage of submerged macrophyte beds in streams and rivers

in deutsch

Decrease of phytoplankton during passage of submerged macrophyte beds in streams and rivers

Author:
Date: Michael Böhme, Boehme@gmx.de
23.03.1998 ©

My question is:

Who knows examples of phytoplankton rich streams or rivers, where during the passage of macrophyte beds water quality improves because of a reduction of seston- or phytoplankton-concentration ?
Who knows papers, where this phenomenon is descibed ?

Details

Eutrophication increased the concentration of phytoplankton and seston of planktonic origin in running waters especially in lowlands. Water quality diminished, the utilization of the water was limited. So there is an interest to reduce the concentration of phytoplankton and seston of planktonic origin. One way is to reduce the nutrient input to the waters. Another way is to alter the morphology of degraded running waters. In some circumstances, macrophytes can help to reduce phytoplankton concentration.

Phytoplankton is part of Seston. Beside phytoplankton, seston embraces other living planktonic organisms (zooplankton, bacteria and fungi) as well as nonliving, mostly organic suspended matter. In slowly running, nutrient rich streams and rivers phytoplankton and its remains often constitute the main part of the seston volume.

In contrast to dissolved substances, seston could be actual eliminated by macrophytes and other compartments of the running water coenosis. Measurements in several running waters within the Spree catchment area have shown a reduction of, for instance, particulate phosphorus up to 97% within some kilometers flowing distance.

Overview on processes which promote retention of seston and losses of phytoplankton, respectively:

- Sedimentation followed by accumulation or decomposition by fungi, bacteria or feeding by zoobenthos
- direct filtration and feeding by zoobenthos, e.g. by Protozoa, Spongi, Bryozoa, mussels and insect larvae.
- elimination, which is caused or promoted by aquatic macrophytes, like:

sieving of sestonic particles,
increased sedimentation around the macrophytes,
provision of large additional areas for Aufwuchs and refugia for direct consumers of phytoplankton from predation by fish,
exudation of substances, which promote the flocculation of phytoplankton or seston, respectively; or which have an allelophatic effect on algae.
competition for nutrients and light, accumulation of nutrients like P and N within macrophytic biomass.

The example of the straightened, open outlet of the Lake Maxsee (east of Berlin, germany) shows, what dramatic decrease of seston concentration is possible in macrophyte rich running waters. From 1994/5/19 to 1994/6/24 the diurnal changes of Oxygen-concentration increased from 2 to 16 mg/l with growing macrophytic biomass. Ceratophyllum demersum (90 %), Potamogeton pectinatus and Hydrocharis morsus-ranae were the dominant macrophytes. Macrophytic biomass increased from 0.5 up to 5 kg FW/m². Five kg fresh wight of Ceratophyllum demersum is approximately equal to 350 g DW.

Phytoplankton, which was imported from the Lake Maxsee, was drastically eliminated during the passage of dense stands of macrophytes. Lake Maxsee water had a seston-concentration around 15 mg/l DW. Main constituents of the seston were Cyanobacteria (Limnothrix redekei and Planktothrix aghardii). When there was a dense and vital stock of macrophytes, seston-concentration was reduced by 89% on average on the first 1.6 km section. The month before and after, without such dense stands of macrophytes, the reduction was only about 50%.

10 km downstream, passing further dense stands of submerged macrophytes, the water was extraordinarily clear with a snorkeling visibility of 6 to 8 m. Only in October, the turbidity rose to a secchi depth of 1 m.

So the question again:

Who knows further examples of phytoplankton rich streams or rivers, where during the passage of macrophyte beds water quality improves because of a reduction of seston- or phytoplankton-concentration ?
Who knows papers, where this phenomenon is descibed ?

Please contact

Michael Böhme, Boehme@gmx.de

	Decrease of phytoplankton during passage of submerged macrophyte beds in streams and rivers
Author: Date:	Michael Böhme, Boehme@gmx.de 23.03.1998 ©