I had more or less decided to cancel my fieldwork when I heard the weather forecast a couple of weeks ago. The last remnants of Hurricane Gonzalo were blowing themselves out over western and northern Britain and the prediction was for heavy rain over the Pennines. I heard the rain during the night and checked the river levels in the morning. They were not outrageously high, but the trend was upwards and common sense dictated that I wait a day and try again. You can see from the hydrograph below just how quickly the river rose and then fell again over the course of 24 hours. Even by 09:00 the following day the river was not at its lowest, but at least it was safe to wade in and collect some samples.
River Levels in the River Wear at the Environment Agency’s monitoring station at Stanhope, 20th – 22nd October 2014 (http://apps.environment-agency.gov.uk/river-and-sea-levels/120695.aspx?stationId=8196). The arrows indicate the approximate time of the rainfall associated with Hurricane Gonzalo.
A sign that the river was still higher than usual was that some cushions of moss that are usually well clear of the water were at or, in a few cases, still below water level. There are a few species of moss, including members of the genus Cinclidotus and Racomitrium aciculare, that seem to thrive on the upper parts of boulders. They spend most of their life out of the water, then just a few days each year submerged. Those few days will bring down suspended material which will become entrapped around the dense network of stems in the cushions and, in the process, provide a nice little “compost heap” that will keep the moss supplied with nutrients. The stems also trap water, through capillary action, so that the cushions stay moist for some time after the water levels have dropped again. Yet such generalisations do not explain why a few genera thrive in this habitat whereas others are never found here.
Much of the magic of rivers is associated with organisms which are rarely, if ever, seen: the fish, of course, but also otters, kingfishers and other vertebrate organisms. I’m also fascinated by the microscopic world of rivers. Mosses have the opposite problem: we always see them on visits to rivers yet almost never notice them, let alone spend time unravelling the stories that they can tell us. I am reminded of Mungo Park’s words back in 1795 (see “More about mosses…”). Perhaps I should write a little more about the extraordinary diversity of mosses over the next few months?
Cushions of Cinclidotus mucronatus just above water level in the River Wear at Wolsingham, October 2014.