Tides are a very important part of marine life as they influence the community structure on the seashore – from where things live to when they eat. Knowing the timing of tides is essential for all marine users – including boaties, divers, beachcombers and fishers. This newsletter will provide some resources to help you better understand how tides work and their importance in the marine world.
What Are Tides?
Tides are the rising and falling of the ocean. A semi-diurnal cycle (a high tide, dropping to a low tide then rising again) happens approximately every 12.5 hours. Tides are caused by the gravitational pull of the moon and the sun as well as the rotation of the Earth. Have a look at this graphic from NIWA to see how semi-diurnal tides move around our coastline. It can be high tide on one side of the Cook Strait and low tide on the other!
The height of tides is influenced by how the sun and the moon are positioned in relation to each other. This creates spring and neap tides. Spring tides are when the high tides are very high and the low tides are very low so there is a daily difference in the tidal range. Neap tides have a much smaller tidal range – high tides are often lower than normal and low tides are often higher. For a more in-depth explanation of tides, look at this resource Ecology of the Rocky Shore - there is a whole section JUST on tides from page 35.
In and out....in and out....
What happens when the tide comes in? Watch 'MM2: While We're Away' to find out....
Knowing the timing of low/high tides can be very useful when planning your own expeditions to the shore - you don't want to get caught out by an incoming tide! These tidal predications can often be found in your local paper or online such as from Land Information New Zealand (LINZ) or OceanFun. Tide charts are also available in most local newspapers too.
Tides are often displayed in charts – like this one here recorded at the Portobello Marine Laboratory in Dunedin. For those wanting some practice at reading a tide chart, work through this short activity from NZ Maths called 'Tide and Time'.
Intertidal species also have to consider the timing of tides as this determines when individuals will eat, hunt and hide! Have a look at the diversity of species that you might find at low tide and in rock-pools on this poster ‘Life Between Tides’. While exploring this poster, think about how do plants and animals survive both in and out of the water. Do you think this would be difficult to do? What special features might they need to be able to live in these two very different environments?
Thinking about the influence of tides on the intertidal…. Why not star in your own ‘Seashore Drama’! Create the seashore environment with each actor/actress taking on the role of an animal or plant. When underwater, how does it move or catch its food? Have one end of your stage be the sea and the other end be land. Your cast will need to position themselves in the area of the shore where they will survive the best.
Use a long piece of string, extended between two people, to represent the edge of the sea. Starting from the top of the high tide zone, move the string as the tide goes out. The "plants and animals" need to change their behaviour and hold their breath as the string passes over them, and wait until the tide (string) comes back in. How does their behaviour change when the tide goes out? Who has to hold their breathe the longest? How can structures (e.g. shells), behaviours (e.g. movement) and position on the shore help their survival? More information on how to do this activity can be found on 'The Seashore Stage'.
You might have seen these images on our Facebook page. This ‘red tide’ is washed up squat lobsters called Munida gregoria. These ‘red tides’ happen in the summer (usually from November to April) and can occur from the Cook Strait south. Juvenile munida have an ‘instinct’ to swim into the current during this time – and that instinct is so strong that it means that they can get caught out by the tides and get stranded! These images are from Broad Bay, Dunedin but we had reports of other sightings in and around the Otago Harbour. Have you seen them? Let us know!
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We will be covering some of the maths skills needed to estimate the abundance of different species and substrates in your square.