More from the River Wear

As well as the green patches at the margins of the river, there were also a huge number of small, brown patches floating on the surface, ranging in size from about a two pence coin to a medium-sized leaf.  There seemed to be an almost constant procession of these floating down the river, and they piqued my interest.

I had seen similar mats elsewhere and had a fair idea of what to expect when I put a small part of one under the microscope.  What I saw was a tangle of blue-green filaments, each about 15 micrometres wide, some of which, if I watched closely, glided slowly across the field of view.  These filaments belong to a genus called Oscillatoria, a close relative of Phormidium, which we have met in earlier posts.  Several Oscillatoria species live in the plankton, but this one, probably O. limosa, often forms mats of this type.

Oscillatoria_River_Wear_130

Mats of Oscillatoria limosa and other algae photographed as they floated down the River Wear in July 2013.   The patch on the right is about five centimetres across.  Inset: a single filament of O. limosa.  Scale bar: 10 micrometres (1/100th of a millimetre).

As well as the Oscilllatoria filaments there were also several other algae, including Spirogyra and a number of diatoms.   The tangle of filaments creates a good habitat for motile diatoms.  Indeed, motility confers a competitive advantage as it allows organisms to adjust their position within the mat in order to increase their rate of photosynthesis.  One of the most distinctive diatoms in this sample was the needle-shaped cells of Nitzschia acicularis slowly weaving their way around the filaments and other particles on the slide.

Durham_Spirogyra_stack_#1

A filament of Spiroygra from the mat collected from the Wear in July 2013 with (inset) a cell of the diatom Nitzschia acicularis.  Scale bar: 50 micrometres (1/20th of a millimetre).

My guess is that these mats were formed on the bottom of slower-flowing sections of the river and sheared away from the silty surfaces as the oxygen produced by photosynthesis made them more buoyant during the day.  This, combined with the constant drag of the water over the mats growing on the unstable silty substrates eventually leads to the mats shearing away from the bottom and floating to the surface.

On the evening after I collected these samples, however, there was a heavy downpour and the next morning the river was in spate, with turbid brown water swirling through the arches of the bridges and over the weirs.   All but the more firmly-anchored aquatic plants were dislodged by this, and on my next visit I could see almost none of the floating algae that I have written about here.  Like the grass of the field, they are here today yet tomorrow are gone…

 

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One thought on “More from the River Wear

  1. Pingback: Back to Druridge Bay | microscopesandmonsters

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