One of the benefits of studying diatoms is that we can produce permanent slides from our samples, which means that we can go back at any time to have another look at samples we collected in the past. As I was writing my previous post, I recalled some other brown films that I had seen in a tidal section of a river and it did not take long to find the slide and put it under my microscope again.
The sample comes from Staithes Beck, a short stream that flows into the North Sea at the old fishing village of Staithes, on the edge of the North York Moors in Yorkshire. I had visited this stream in February 2012 as part of a project and had wondered about the composition of the films. I did not have time to look at them in their fresh state but, instead, viewed them a month or so later when the slides were prepared. Some of the diatoms that I wrote about in The Ecology of Cold Days were here, obviously able to cope with the more saline conditions less than a kilometre from the mouth of the stream. However, the most abundant diatom was a species that I had never encountered in freshwaters: Navicula bottnica. It has the same symmetrical boat-shaped outline as Navicula lanceolata but the arrangement of the striae – the lines on the frustule – were very different. In N. bottnica these were both denser and much more strongly radiate, especially towards the centre. On the other hand, apart from the difference in salinity, it seemed to have a very similar habitat – forming distinctive brown films on the tops of submerged stones in fast-flowing sections of the river in the winter and early Spring.
I did find one reference on the internet to Navicula bottnica living in mucilage-tubes, which is intriguing and makes me wish that I had checked this myself in the Staithes community. I have found that most of the species described as living in tubes are also often found free-living too. The study that recorded N. bottnica in tubes was on the physiology of diatoms in intertidal zones on rocky shores whereas my communities in Staithes Beck were permanently submerged. I suspect that a rocky shore presents a rather more severe challenge to diatoms, with periodic exposure to the air and higher levels of radiation. This would mean that a form of extracellular protection would confer some protection to the diatoms that, perhaps, is not needed in a permanently submerged community.
Navicula bottnica from Staithes Beck, Staithes, February 2012.
Ginnever, N.E. (2014). The photophysiology of rocky intertidal microphytobenthic biofilms. PhD thesis, University of Cardiff, Cardiff.