14/08/2026
These photos from this week show some of the lowest tides this year. It is easy to take the tides for granted but they are fascinating….
Our tidal range is up to about 3m. We almost always have two high tides a day and two low tides. The exception is during very neap tides (half-moon), when sometimes only one high and one low happens in a day.
For each day one of the low tides is lower than the other. During spring tides (full and new moon) the difference can be around 50cm between the two. At this time of year our lowest low tides are in the early afternoon. In January/February they are at night. This is has big implications….
Very low tides are super-stressful for the reef flat corals. They produce lots of mucous to try to avoid dehydration but are vulnerable to UV and dry conditions at this time of year (with daytime exposure), and in the wet-season the night-time extreme lows can coincide with high rainfall. These stresses can cause emersion mortality which has a huge influence on the way shallow reefs grow.
El Nino conditions can also lower the low tides in the Western Pacific by over 20cm and increase them by a similar amount in the Eastern Pacific.
We don’t know of any real-time publicly available tide gauge data on the GBR so can’t say if this week’s low tides were normal lows fitting with tide predictions or influenced by El Nino, but we would love to find out.
For more detail read on ...
El Nino strength is measured by the Southern Ocean Oscillation Index and the July numbers are the lowest since records began in 1876 (the index normally peaks around December to January so is expected to become more negative in the coming months).
Another fascinating thing about the low tides is related to clouds … Marine microalgae, including the symbiotic algae in corals (as well as other parts of the coral holobiont), produce a chemical called DMSP (Dimethylsulfoniopropionate) as an antioxidant to control reactive oxygen molecules created during photosynthesis. DMSP breaks down to DMS which enters the air and becomes the major source of sulphur nuclei which help clouds form (and also make the ocean-air smell). Heaps more atmospheric DMS is produced by exposed corals, so in theory it should help make more clouds over the reef.
Another fascinating thing about these reef flat corals is adaptation to the reef flat conditions. The necessary adaptation to such low tides that happen a few times a year is to short term stress and DMS-rich mucous is a feature. For us, with our coral propagation and planting research interest, we try to select thermally tolerant corals from the local population. Other than blindly trying to pick survivor-corals after bleaching events (which can be unreliable), the only current way to test for these is short term heat-stress assays (MTP and CBASS) but these tests only last a short time whereas real-world marine-heatwave bleaching events last weeks. This illustrates some of the complexity of testing for individual genotype thermal adaption, and the current gaps in knowledge of verifying that short term tests accurately reflect a coral’s real world marine heat-wave fate.
pics and text by John Edmondson
Great Barrier Reef Marine Park Authority Master Reef Guides Visit Queensland, Australia Coral Nurture Program
