Trout in hot water

Brown trout

10 October 2014 by Eoin O'Gorman

Understanding the effects of climate change on ecosystems is a complex business. Eoin O'Gorman describes how trout living in geothermally-heated Icelandic streams are helping.

The patchwork quilt of Reykjavík's red, white and blue rooftops disappears in the rear-view mirror of our jeep as we set out for the Hengill geothermal valley. You might think of Iceland as a cold, dark country hiding beneath a permanent blanket of snow and ice. In the summertime, though, the grass is lush, the ancient lava fields are alive with green mosses, and the countless rivers and waterfalls twinkle in the near 24-hour daylight. The splendid isolation we feel as we leave the main road and weave along a rocky dirt track into the heart of Hengill is a constant reminder of how magical this country is.

For now it remains one of the last untouched natural beauties on our planet, although geothermal energy expansion threatens to change this in the coming decades. Iceland will also feel the force of climate change more rapidly than many other countries, with the fastest rates of global warming predicted in the Arctic region. It is for this very reason that we are about to begin another day of fieldwork in Hengill.

Our field site sits on a volcanically active zone, although thankfully an hour's drive from the infamous Eyjafjallajökull ('island mountain glacier'), whose volcanic ash grounded flights across Europe a few years ago. Hot magma is much closer to the Earth's surface in these zones - only a few kilometres deep - leading to a mixture of cool patches and hot spots in the landscape. Luckily for us, the surface is warmed by heat radiating from beneath the ground, rather than by upwelling sulfurous geothermal fluids as in more extreme environments like Yellowstone Park. This lets us examine the natural communities of living things found in similar environments at a range of different temperatures, which is representative of the planetary warming we might see in the coming decades.

We park next to the Hengladalsá ('hanging dales river'), which cuts through the valley between two small mountains. There are 15 small streams flowing into this river, all within a mile of each other, but with temperatures ranging from 5 to 25°C. The chemistry of the streams is typical of Iceland in general and does not vary with temperature, making this the perfect natural experiment on the ecological effects of warming.

Before and after photos showing the effects of warmer water on algae growth

The effects of warmer water on algae growth

Since the streams all come from the same river stem, we might expect them to contain very similar freshwater communities. Yet our past research has shown that this is not the case. Some species (such as midge fly larvae) become less abundant or disappear completely as stream temperature increases; others (such as the freshwater snail and blackfly larvae) do the opposite. Amazingly, the only fish in the system, brown trout, are mostly found in the warmer streams, even though they have to spend more energy to survive in these conditions. We think the explanation may be that the heat speeds up the breakdown of organic matter and nutrients are recycled more quickly, leading to more resources for everything living in the stream. Algae also grow faster in the warmer streams, which leads to more invertebrates grazing on them. This provides the trout with far more food than they enjoy in the colder streams, helping them to meet their high energy demands.

This is a really interesting example of how our expectations for the impacts of warming are not always matched in nature, and understanding why is critical if we are to predict future change. It also makes it difficult to know how much of any change in community structure is directly due to rising temperatures and how much is down to these large, hungry fish. And so today, we begin an experiment to unravel this puzzle.

Hot streams in a warming world

Over the past few days, our team has built fences out of steel bars and plastic mesh, hammering them in place at three points, each 15m apart, across six streams with different temperatures. This has created two sections, or reaches, in each stream which we will use to study the trout. Our first job today is to fish all the trout out from between the fences and put them back in the water downstream - effectively excluding them from our experiment. We do this by passing a small electric current through the water, which makes the fish swim towards our equipment so we can lift them out with a net. We do the same thing in a stream that is not part of our experiment, but this time we keep the fish in buckets and select five similarly-sized trout to place in the lower reach of our six experimental streams.

Aerial view of Hengill

Aerial view of Hengill

Importantly, we have already collected samples before the experiment. These include rock scrapes to tell us which algal species are present, and samples from the stream bed to count and weigh the invertebrate species. We will take more samples after we end the experiment in five weeks. This design allows us to see what changes are down to seasonal effects, background variation between the reaches, temperature, and of course whether or not trout are present. In effect, we are using the Hengill streams as a natural laboratory to determine the importance of brown trout as top predators, and how that role changes with temperature. And so moderate geothermal areas like Hengill are not only excellent natural experiments for monitoring the effects of temperature on freshwater life - they are also excellent venues for experimental manipulation.

To highlight that point, our American collaborators have carried out a low-cost whole-stream warming experiment in Hengill over the past few years by using two streams that run side by side, but differ in temperature by almost 20°C. They use gravity to pipe water from the cold stream into coiled tubes in the warm stream - without mixing - then redirect it back into the cold stream. Using this design, they have managed to warm the lower reach of the cold stream by about 4°C. Comparing before and after photos really paints the proverbial thousand words: what was a stony sub-Arctic stream has become completely covered in green algae, with a decrease in the abundance of midge larvae and the arrival of freshwater snails - just like we see in the geothermally warmed streams at Hengill.

Our aim over the coming years is to expand the research we have been performing at Hengill to similar geothermal areas, not only in Iceland, but right around the Arctic Circle. By doing the same thing in many places, we can find out if the patterns that we detect apply everywhere. We can also see if ecosystems that are more or less diverse and productive respond in the same way. We will continue to use the data we have already collected to build better models to predict how warming will affect the structure of plant and animal communities and the way energy flows through food webs. Most importantly of all, our research provides a new way to understand the unavoidable problem of global warming.

Dr Eoin O'Gorman is a NERC postdoctoral researcher working with Dr Guy Woodward in the Department of Life Sciences at Imperial College London.