Hello and thank you for your great question! It’s so lovely to hear from a budding marine biologist!
I have to start with an apology as it’s quite difficult to give you a short answer to your question. Marine biologists really vary in what they do…
…Some are ecologists, like me, who study animals, plants and the habitats and groups they live in. Ecologists are usually trying to find patterns in nature, across space and through time, to try to understand how different animals and plants manage to live together in one big ecosystem. I find this fun as I get to make maps showing where lots of species are and see how we humans, or the environment (like rocks, volcanoes, ocean currents, etc.), affect where they live. We can use this information to understand the processes affecting species and to ensure their homes are protected both now and in the future (this part is called ‘marine conservation’).
I used to be a deep-sea ecologist, so I studied how creatures without eyes and with really special characteristics survived in deep, dark, hot, and toxic vent environments.
Other marine biologists might focus on one animal or group of animals - like dolphins or whale sharks. These sorts of marine biologists might take photographs or tag species to learn more about their behaviour, or they might do experiments to see how different animals survive in nature or under the pressure we humans put them under when we pollute and warm the oceans, for example.
Other marine biologists and vets learn all about the bodies of marine animals, to understand how they work. This way they can spot diseases or find out things we need to know about to look after any poorly animals.
Then there are the biological oceanographers. This is a big way of saying ‘people who study the biology of the ocean’. These scientists tend to study the chlorophyll and plants in the ocean, sometimes using photographs taken by satellites in space (as the oceans are so vast).
What you can see above is the Alaskan coastal waters ‘blooming’ with phytoplankton (a form of tiny algae) in 2018, as seen using a satellite.
Image credit: NASA/US Geological Survey/Norman Kuring/Kathryn Hansen.
I studied a bit of a mixture of these things at university by taking a course called ‘oceanography’, but there are also courses in marine biology, marine conservation, and marine ecology, if you think one of the sorts of jobs I described sounds more interesting than other ones.
I hope this helps to answer your question! Please feel free to send in any other questions you might have. Keep learning about the ocean in any ways you can - I recommend museums and aquariums or going to the sea if you can one day. No pressure though - books and the internet can teach you so much these days too! There are some really nice videos and activities available on this website, for example: https://worldoceanday.school/resources-2020/.
Good luck with your learning adventure!