Temperature

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Water Parameter
Temperature
Unit(s): ºC = (ºF -32)/1.8
Target: 24 - 29 ºC
Danger: < 20 ºC

> 32 ºC

Tendancy: Decrease
Increased: Lighting

Pump

Decreased: Evaporation

Chiller

Testing: Daily
Maintenence: Heater

Chiller

Evaporation


Contents

Introduction

The temperature of a tropical marine aquarium should be in the range of 24ºC to 29ºC. Anything over 29ºC and things can start to get stressed, corals, anemones etc. can bleach and some organisms may possibly die. Higher temperatures also increase the rate of metabolism and calcification.

See Heating and Cooling for information on how to maintain tank temperatures.

On a Reef

Lets look at the temperature found around a natural reef, as this is where the organisms have evolve and are collect from before they are placed within a reef aquarium. The water temperature within a natural reef environment is not as 'stable' as many marine aquaria authors lead you to believe. Variations of up to around 10oC have been recorded without corals being killed. The reason that the corals are not killed by such an alteration is the time frame over which the temperature change occurs and previous acclimatisation to such a variation. It will typically be in the order of weeks to months for the time it takes the temperature to change over this entire range.

Seasonal Variation

The dominant temperature changes that occur around a reef are directly related to, and caused by, various effects of the changing of the seasons: winter, spring, summer and autumn. The temperature changes are caused by the following variations:

Solar radiation received by the ocean surface, which is particularly important for the immediate surface temperatures and long term effect of long distance ocean currents. Ocean current variations, with these effecting the entire depth cross-section of the ocean and the type, or origin, of water entering a region. Trade winds, effecting the mixing of the water body and influencing ocean currents. Rain fall, which is important for surface waters and water bodies located close to river mouths. The seasonal temperature variations can be linked to three location parameters to give general trends:

The further the reef is located from the equator the greater the temperature variation. This effect is cause by the variation in the amount of solar radiation received over the year, with a more consistent supply received in the equatorial region. The further inshore the reef is located on the continental shelf, i.e. the close to land, the greater the temperature variation. Water is more easily effected by run off from the land via rivers when close to land, and is located further and more isolated from the temperature stablising effect of deep ocean currents. Seasonal variations in inner shelf regions can be up to 11.5oC. And the shallower the water around the reef the greater the temperature variation. Shallow water receives the same amount of solar radiation, but has a smaller volume through which to distribute it than deep water. At the extreme, shallow water heated by the sun can reach a sufficiently high temperature to inhibit coral growth. This phenomenon has been observed close to shore of Mer Island (Torres Strait, Great Barrier Reef), in the shallow water around Arno Atoll (Marshall Islands), and the beach rock at Heron Island (Great Barrier Reef). The largest temperature series that has been published (Wolanski, 1994) spans 2.5 years from 1980 to 1982. This was obtained from data collect at Green Island, Great Barrier Reef. The data is presented below in Figure 1, along with data from further measurements taken during the same time interval from several other locations, Linnet Reef (mid-shelf), Lizard Island (mid-shelf), Euston Reef (outer-shelf), and Cape Upstart (inner-shelf). These sampling site are spread over a 600km stretch of the Great Barrier Reef, so it can be seen that conditions are relatively consistent along this region, with similar trends occurring at all locations.

Figure 1: Temperature variation of seawater for 5 locations on the Great Barrier Reef (Wolanski, 1994)

The dominant feature that can be seen from Figure 1 is the seasonal variation. The temperature is following a nice sinusoidal cycle. The variation is in the region of 9oC, from a maximum in late February-early March of 29oC (late summer-early autumn) to a minimum in July of 21oC (mid winter). This study also found that temperature variations due to the changes of the tides are very small. The large short-term fluctuations, of up to 1.2oC, that occur over the period of a week are due upwelling of continental shelf slope water onto the shelf. This cooler water rapidly lowers the shelf water temperature, and then when the upwelling stops the shelf water returns back to a similar temperature within a similar time frame. (Wolanski, 1994)

Along the Great Barrier Reef, the average minimum water temperature is well with the limits recognised for coral growth. Coral flourish best in a temperature range of 25oC to 29oC, but they can withstand limited exposures to lows of 16-17oC. At the tip of Cape York, the north most reaches of Australia and the Great Barrier Reef, Booby Island has an average surface temperature from 28.6oC to 25.3oC. The average minimum temperature is 24.2oC in August, and then in December the average maximum is 29.5oC. At the other extreme is Lady Elliot Island, located below Rockhampton, and it is not considered part of the Great Barrier Reef system. Here the minimum average winter temperature falls below 20oC, and moving further south the temperature begins to limit coral growth. At Lady Elliot Island, the average surface temperature varies from 26.5oC in summer to 19.0oC in winter. As can be seen from this data, the seasonal variation in the average surface temperature is from 3.3oC at the northern reaches, up to 7.5oC in the south. (Brandon, 1973) The temperature and temperature variation over the entire length of the Great Barrier Reef actually works out to be approximately linearly related to distance.

It has been reported that corals can in fact adapt to a larger temperature variation. An example is Kingsman (1964) reported flourishing coral reefs on the Oman coast (near Saudi Arabia) in waters having a seasonal variation of 16oC to 40oC, a seasonal change of 24oC! Now these are some remarkable corals. This is a good example of organisms evolving or adjusting to environmental conditions that are outside the range others typically live in.

Daily Variation

Daily temperature variations are cause by a large variety of complex factors, but basically they fall into either complex currents involving the mixing of ocean currents and water bodies, or localised heating or cooling. The temperature variations that occur on a reef, both the magnitude and type of variation, are highly dependent on the location of the reef, whether inner shelf, mid shelf, outer shelf, or deep ocean. The temperature variation is larger the further the reef is from the open ocean and closer to shore. The variations are also more pronounced when the reef is located in a region where the mixing of two different water bodies is occurring, such as ocean currents, or in the eddies of a large land mass.

The daily variation in temperature depends on:

  • Location on the continental shelf, deep ocean, outer shelf, mid shelf, or inner shelf.
  • Distance from the equator, which determines the total amount of solar radiation that the water surface and the amount transferred into the ocean currents.
  • Cloud cover, particularly important in shallow water as is has an effect on the amount of solar radiation reaching the surface.
  • Season, (summer, autumn, winter, spring) with this determining the available solar energy, prevailing winds and ocean currents.
  • Depth, with increasing reef depth the temperature becomes more stable.
  • Proximity to a river mouth, and the degree and efficiency of mixing of the water.
  • Ocean currents, which cause a stablising or destabilising effect on the reef temperature depending on its temperature and the degree and efficiency of mixing of the water bodies.
  • Continental shelf upwelling, these can temporarily cause a large temperature change which then returns to normal after the upwelling ceases.
Figure 2: Daily temperature variation in Winter, for 3 locations on the Great Barrier Reef.
Figure 3: Daily temperature variation in Summer, for 3 locations on the Great Barrier Reef.

The following data is a selection from data obtained by the Great Barrier Reef Marine Park Authority, which has a large number of temperature loggers located along the entire length of the Great Barrier Reef. The Great Barrier Reef Marine Park Authority has part of this data available online. The particular data sets used are to help illustrate some of the general trends that can been seen in the water temperatures on a reef, both with season and location. The first data set, Figure 2, is for the period of the 1st to the 8th of July 1995, winter, and the second data set, Figure 3, 20th to the 25th of February 1995, summer. The three locations featured are Magnetic Island (inner shelf, green), Kelso Reef (mid shelf, blue), and Myrmidon Reef (outer shelf, red), which are located off Townsville, Queensland, Australia. This map shows their location within the Townsville area. The solid coloured lines are for the temperature logger located on the reef flat, and the shaded for those on the reef slope. The reef flat loggers are typically at a depth of 0 to 3m and the reef slope ones are 5 to 8m, with the variation due to the tides. The major gridlines are in 1oC graduations for the vertical axis, and 6 hour graduations for the horizontal axis.

The most obvious feature of these figures is the sinusoidal cyclic form of the temperature data, particularly with the inner and outer shelf reefs, Magnetic Island and Myrmidon Reef respectively. The day begins with the temperature slowly on the way down, with a minimum being reached around 7am in the morning. After this the temperature rises steeply to a maximum around 4pm in mid-afternoon, followed by an equally rapid fall ending around 12am. The temperature then continues to decrease into the early morning at a slower rate. The rapid increase from early morning to late afternoon illustrates the effect of solar energy input into the water having a large influence on the temperature. This effect is most dramatic with shallow water locations, the reef flat and when the reef is located on the mid to inner shelf.

The difference between the reef flat and reef slope data is also quite distinct, with the exception of the Kelso Reef data. The largest temperature variation is found on the reef flat, with the same period of variation occurring on the reef slope, but the magnitude has just been reduced. This is mainly due to the closer proximity to the surface, and the solar energy received from the sun causing more heating of the water body because the energy is concentrated in a smaller volume. In the case of Kelso Reef, located on the mid continental shelf, there is very little difference between the reef flat and reef slope temperatures and the cycle each follows. The localised heating of the water in the shallow water at Kelso Reef is off set by the input , displacement by, and mixing of deep ocean water currents, so that very little difference appears.

Seasonal changes in the daily temperature variation is also evident. During summer the highest temperature difference is higher than winter, 2.5oC to 2.2oC for Magnetic Island, 1.4oC to 1.2oC for Myrmidon Reef, and 0.6oC to 0.5oC for Kelso Reef. The seasonal variation in average temperature, as previously discussed, can also be see for all of the reefs.

Variation With Depth

The temperature of the seawater around a reef also depends on the depth. The temperature profile with depth depends on the region and takes three basic shapes. The most common profile on the Great Barrier Reef is essentially straight, with an almost constant temperature. This results from the good mixing of the water at all levels, which eliminates any temperature differences. This is typically due to mixing by strong tidal currents and trade winds. When the trade winds are operating there can be typically only a temperature increase in the upper meters of the water column of a few tenths of a degree. During the summer season the trade winds shut and more pronounce temperature profiles can develop. This creates the second type of profile, which also exists in deeper waters with less mixing. A maximum of around 30oC exists at the surface, which then decreases with an asymptotic curve to around 25oC at 60m of depth. Beyond this there is only very minor temperature changes until well beyond the range that corals typically live.

Rainfall can also have a pronounced effect during the period when the trade winds are not operating. The rainwater tends to stay at the surface as it is less dense than seawater, and has its temperature increased by solar radiation. This then again decreases the density, generating a feedback cycle creating a stratifying effect. This stratifying effect is quite easily broken down again by a short period of strong winds. The final profile type is very similar to that found close to the shore and river mouths. Here the temperature at the surface is greatly decreased due to the cold water input from the river, lowered to around the 20oC mark. As 5-10m depth is reached then the temperature increases rapidly up to around 30oC. Beyond that depends on whether is shallow, well mixed waters or deeper, stagnant as discussed previously.

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