freashwater lakes seem (to me) to have very limited ecosystems, as opposed to the oceans. Inn the oceans, you have seaweeds and kelp, corals, sponges, shellfish, and innumerable species of fish; jellyfish, etc. In lakes there are no corals, no sponges, no jellyfish, a few shellfish, and just a few species of fish.
Why are lakes so barren, compared to the oceans?
Lakes don’t necessarily connect to any other bodies of water, and may rely on floods and human intervention to introduce new species. Oceans communicate throughout the world and with rivers.
I’m not sure the premise of your question is accurate. Life is incredibly sparse in the oceans (they’re huge!). All the things you note are just dense areas of life clustered around the shoreline. The open ocean is a lot like space.
There are vast regions of ocean with a very low density of life. Most ocean life is concentrated in areas where currents/runoff brings nutrients close to the surface (where there’s light for photosynthetes to make use of them).
That said, lakes are the watery equivalent of islands. It can take a while for them to be colonized, and they are often dominated by a few species who arrived first.
Believe it or not, there are jellyfish in many lakes. When I was a kid, I once found dozens of them floating near the surface of a lake near my home. My science teacher spent a couple weeks trying to find out what they were and finally settled on Celenterata, which is actually a general term for a large group of different animals. In retrospect, they were probably craspedacusta sowerbyi.
The first thing that comes to mind are that lakes are a lot younger than the oceans. A lot lot lot younger. All lakes.
Consider this; **there have been human beings in North America longer than the Great Lakes have been in North America. ** In fact, most of the lakes in the entire world are much more recent in creation than human habitation of the area they’re in, and all are vastly younger than the oceans, unless you count the Caspian Sea as a lake; it used to be part of the ocean. The oldest real lake in the word is Lake Baikal, which is still relatively young; it did not exist when the dinosaurs walked the earth,
This is kind of difficult to believe if you LOOK at a lake as big as a Great Lake, which are mind-bogglingly huge, but in fact the Great Lakes were not fully formed until the last Ice Age ended. In geologic terms, that’s practically yesterday.
Life has had billions of years to evolve in the oceans. Life has had a miniscule fraction of that time to develop in any lake, and there’s a pretty strong correlation between a lake’s age and its biodiversity.
In addition, freshwater and saltwater are two very different environments physiologically for aquatic creatures. Salt water has a much higher concentration of salt and other chemicals than the internal fluids of most animals, while fresh water has a much lower concentration. An organism that is well adapted to salt water won’t necessarily be able to adapt to live in fresh water.
The oceans are huge, and there have been ocean basins for billions of years. Many different kinds of life forms have evolved in this enormous, ancient environment. On the other hand, the total amount of fresh water on Earth is just a small percentage of the total amount of salt water, and as has been said individual rivers and lakes are not very old (although freshwater environments have been around a long time).
The greater amount of habitat, and longer amount of available time, means that there has been a much greater scope for the evolution of different kinds of organisms in the ocean. Many kinds of the kinds of organisms in the oceans have been not been able to adapt to freshwater environments.