This post sees me entering the unfamiliar territory of intertidal rock pools for the first time in many years (see “Epiphytes with epiphytes” for my last post on this topic), following a walk along the Northumberland coast starting near Dunstanburgh Castle and making our way, via the Jolly Fisherman pub at Crastor, to Howick Bay. We’ve been visiting the Northumberland coast for many years now but have never seen it so busy: unsurprising on a beautiful sunny day in a year when many people are holidaying in the UK. However, “busy” on the Northumberland coast is hardly the same as “busy” in Bournemouth and Brighton so everyone had plenty of space to enjoy the views and at least a modicum of solitude.
Howick Bay, a couple of kilometres south of Crastor, is a place that is of particular interest to geologists as there is a very clear discontinuity where the Whin Sill – responsible for so much of north-east England’s most dramatic scenery – drops away to expose steeply sloped beds of Carboniferous limestone. The boulders of basalt that the sea erodes away, and the cracks and gaps in the limestone beds, however, then create a rich mosaic of rock pools teeming with intertidal life forms.
My eye was caught by the trails left by some periwinkles on top of cobbles of Whin Sill at the bottom of one of the rock pools. Rather than the slimy trails we associate with terrestrial snails, this one seemed to have rasped a path across the top of the stone as it fed on the thin layer of algae that coated the surface. That, of course, piqued my interest so I decided to take a closer look at whatever it was feeding upon. As I had not anticipated this turn of events, I had not come prepared, meaning that I had to cram a small stone into a plastic bag lurking in our rucksack and carrying this around for the rest of the afternoon. When I was back, I scrubbed the stone with a toothbrush, just as I would if I had found it at the bottom of a stream.
The film that I managed to brush off the top surface of a cobble had the same yellow-brown colour that I find when I look at stones from lakes and streams so I expected to see plenty of diatoms when I peered at a sample through my microscope. What I saw, instead, was a community dominated by narrow branched filaments of the brown alga, Ectocarpus. These have similar pigments to the diatoms and are, in fact, members of the same Phylum (“Ochrophyta”) so my snap judgement may be excused. I did also find several types of diatom were also present in the sample, though the Ectocarpus was by far the most abundant organism. Those diatoms that I saw included some Navicula along with Tabularia and an occasional cell of Licmophora. There were also a few green algae cells and some cyanobacteria, but I’ll write more about those in the next post.
The periwinkle is really “mowing” the biofilm: its mouth is on its underside, surrounded by the strong muscles that enables it to move across the rock surface. The mouth contains a long, ribbon like-structure, the “radula”, which can be thought of as a tongue with minute teeth. These enable the organism to scrape the algae off the rock as it moves albeit relatively unselectively. Brown algae, diatoms, green algae and anything else small enough to be pulled into the gullet will be removed in the process.
Our unanticipated rook pooling session over, we scrambled back up the cliff to retrace our steps to Dunstanburgh. We had delayed our purchase of Crastor’s famous kippers earlier in order to avoid having to carry them in our rucksack for too long on a warm August day but, by the time we got back to Crastor there was a long queue snaking out of the shop and, as our time was limited and our legs tiring, we pushed on.
As we made our way back past the Whin Sill outcrop on which Dunstanburgh Castle stands, and skirted the links golf course just beyond, I reflected that my crab salad at the Jolly Fisherman put me two stages in the marine food chain away from the molluscs of Howick Bay. I eat crabs, they eat molluscs, molluscs eat algae and algae feed on sunshine. Or, to turn this around, algae sustain us all. But if you read this blog regularly, you probably knew that already…
Some other highlights from this week:
Wrote this whilst listening to: Michael Kiwanuka, who we should have seen live at Green Man a week ago.
Cultural highlights: Our first visit to a cinema in six months: to see Babyteeth. Not sure that many films are going to be so unmissable that it is worth wearing a mask for two hours. The low budget ‘subhero’ indie films that we prefer will probably work as well streamed on a laptop anyway.
Currently reading: Afua Hirsch’s Brit(ish). Her series “Africa Renaissance”, currently on the BBC iPlayer, is also worth watching..
Culinary highlight: a five-course tasting menu at Jöro in Sheffield: a very belated Christmas present from our daughter and her partner Sam.