I'm using HOBO sensors for monitoring light intensity on shallow coral reefs over a 6-months period. Every 2 weeks the sensors are cleaned from fouling and data is taken with the waterproof shuttle. What logging interval do you recommend using for such long-term period? How to securely deploy the sensors at depth on a sandy/rocky substrate?
Hi,
I have no experience in the ocean studies nor i am aware of the type of the sensor system HOBO is? With a small know-how about the use and deployment of solid state sensors under various conditions, i will try to convey few things . As described by you, the sensor system (assuming Its power unit with data logger) will be deployed on the shallow ocean floor to sense the amount of light received on it. It is assume that the sensing element will be facing the surface of the water from below. As you have to monitor the light received by the coral reef, as a thumb rule, just make sure to keep the height of the sensor element/head at the average height of the entire coral reef floor under consideration. This way you will receive a decent average data of how much light is received by almost all the coral reef in that region.
Regarding the period of logging the system, the ocean floor receives light not only from direct sun but also from the full and partial moon night!!! So i would say, keep the data logging intervals at the smallest interval available in the system while making sure enough data storage is available for that entire 6 months of time. Already the water waves are going to make the light intensity fluctuation a alot on the sensor deployed under it. Getting Average over that amount of data will help to overcome such minor fluctuations easily. No doubt the smallest time interval will get you a tonnes of data over the entire deployed time but having the data of such small interval may become useful in the future analysis of it. Its better to have it rather than latter regret over it for not having it!!! If at all big storage space is not available, keep the logging interval somewhere around 4-5 minutes (i am guesstimating it), that way the changes in the intensity due to relative displacement of the sun/moon position with respect to earth will not be missed to a greater extent.
For deploying the sensor on the sandy/rocky surface, if it has its own mounting system its great all you have to do is make it anchor on the sandy/rocky surface by some means. If it does not come with its own stand/harness, you have to fabricate one. The aim is to keep the sensor at a particular height under water depending upon the average height distribution of the coral reef in that region. An anchor made by using heavy objects/stones should be employed to hold the erected sensor head/element while it faces the water surface from below it. The erected structure should be firmly anchored to the ocean floor so that it won't sway drastically with the waves. Small swaying will not pose that big of a problem while analysing the data if the data logging is done over a smallest interval of time. AS all these things are happening under salty water, you have to be utmost careful about making the entire mounting structure and the sensor system water proof. All the suggested things are almost superficial. There are lot many technical details which cannot be covered in the current explanation.
If all above mention things should sound a bit scary, i would suggest you to take help from the mechanical department students from your university.
Regards
Amit
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