The impact of extreme weather on ponds

The Met Office recently revealed that the UK is experiencing hotter days and more “tropical nights”. This is in addition to the country being exposed to more weather extremes over the last 10 years than previous decades.

With this in mind, it’s safe to say that extreme summers and winters are becoming increasingly challenging for pondkeepers. Dave Hulse, technical consultant at Tetra UK offers advice for fish keepers on how to cope with severe seasonal weather conditions.

The summer of 2018 was a real scorcher. According to the Met Office, it was the joint hottest on record, shared with the infamous summer of 1976. However, we should not confuse this extreme ‘weather’, (short term temperature, pressure, precipitation conditions), with ‘climate’ – the average of these values over a long period of time.

There is no doubt that the climate of the UK is shifting. The Met Office reports that the Central England Temperature has risen by about a degree Celsius since 1980, with 2006 being the warmest year on record.

How does pond water react to extreme seasonal weather changes?

With the prevalence of warmer summers on the rise, pondkeepers should consider the impact of this on their garden pond. Interestingly, a considerable amount of heat is required to raise the temperature of liquid water. In fact, the amount of energy required to increase the temperature of one gram of a substance by one degree Celsius is called the ‘specific heat capacity’, and water has the highest specific heat capacity of any commonly occurring substance, far higher than any metals for example.

This high specific heat capacity of water is vital in biology; it means that water can buffer changes in air temperature. As such, on the hottest day ever recorded in the UK (when the mercury soared to 38.5˚C in Faversham in Kent) the temperature of lakes, ponds and rivers would not have risen equally high. Data on the average temperatures in these freshwater environments across the country would be rather meaningless as they are all so variable, however data from the Met Office for UK sea-surface temperatures reveals a rise of about 0.7˚C over the past three decades, and we can infer average freshwater temperatures are rising slightly too.

Hot summer weather: what challenges do pondkeepers face?

It is the extremes of weather than can be a problem for pondkeepers. In the intense heat of a hot, dry summer, the main challenge for outdoor fish is meeting their oxygen demands. Warmer water can hold less oxygen, yet at higher temperatures the fish require more oxygen because their metabolic rate depends on the water temperature.

As a result, in the summer months pondkeepers must ensure they are aerating their ponds as much as possible, via fountains, waterfalls or with an air pump. Look out for signs of oxygen distress in your fish, such as gasping at the surface or hanging around filter outlets or waterfalls where the dissolved oxygen is marginally higher. With the help of Tetra’s Test O2 kits, pondkeepers can test oxygen levels in their pond easily and precisely, allowing them to react before it gets too serious.

It’s also worth mentioning that shallow ponds are much more susceptible to an increase in temperature in hot weather, with a subsequent drop in dissolved oxygen. A pond for goldfish should be at least three ft deep in places, for Koi carp, a five ft. depth is a minimum. What’s more, do not rely on so-called ‘oxygenating weeds’, as these will add precious oxygen during the daylight hours, but this will cease at night.

It is not only the pond fish that are susceptible to deoxygenation, the precious bacteria that colonise the biofilter, breaking down toxic waste ammonia into nitrate are also utterly dependent on dissolved oxygen.

With this in mind, the vigilant pond keeper should be testing ammonia and nitrate levels in the pond water frequently. The concentration of both should be zero, as any rise above this could be a symptom of these bacteria struggling due to a lack of oxygen. Tetra’s Pond FilterStart Bacteria can help ponds recovering from an oxygen shortage, as it combines two types of natural, highly active bacteria to promote optimal pond care. The solution is able to boost effective biological filtration and eliminate ammonia and nitrate, helping to replenish lost bacteria to restore biological balance in your garden pond.

Combatting colder weather

When considering climate change in the UK, we tend to focus on hot weather, but in fact the Met Office predicts that over the year, and on average, we will see more extreme weather events. Hot summers may be a more common feature, but there may also be extremes of precipitation or sudden extremes of cold weather in the winter months.

For the pond keeper in winter this presents some very unique challenges. Oxygenation is rarely a problem for ponds in cold weather. In winter, the water is around 4˚C, and will hold much more dissolved oxygen than it could in the summer, yet the metabolic rate of fish has decreased so they require less oxygen. Any aerating devices still running are likely to cool the water too much for the fish, as they circulate the pond water in contact with the icy cold air. In winter, the pond water can be circulated near the surface, only minimising the contact between water and air. Fish must still be monitored for signs of oxygen distress, but this is unlikely in winter, compared to high summer.

In summary, the changing UK climate poses huge challenges for pondkeepers. The unique properties of water mean that rapid profound changes in water temperature will not arise, however, sustained seasonal weather extremes should be on pondkeepers’ radar. Looking out for signs of water deoxygenation in long bouts of dry, warm weather, as well as monitoring your pond’s aerating features during the colder months is advised, to ensure the water quality of your garden pond is not compromised, providing an optimum environment for your fish through the seasons.

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