With the exception of a few ecosystems fuelled by the actions of bacteria on chemical compounds, all other ecosystems are fundamentally powered by the energy from the sun through photosynthesis in plants. Most of you have probably quite early in your school career been taught the concept of food chains or food webs. Typically, at least in Europe, a food web taught to young students would be exemplified with an African savanna food chain or matter pyramid presented as biomass on different levels. In such a food chain or pyramid there will be loads of grass, zebras and other ungulates will be present with ten percent of the biomass of the grass and the lions (it´s always lions, isn´t it!) will be there in a combined biomass of ten percent of the herbivores. The other classic example used a lot where I come from and probably also in northern America would be trees, moose and wolves, with similar proportions between levels. I guess that some tropical countries will use trees of rain forests, monkeys and wild cats, but the relations between the matter in the each level will probably be the same. Thus, the general consensus would be that for every 100 kg (or, if you are so inclined, pounds) of primary producers doing the photosynthesis there would be 10 kg (pounds if you chose pounds above) of herbivores and one kg (well, guess you got the conversion already, but if not, pounds if pounds in a and b) of predator. It is kind of ironic then, that the oceans, which cover approximately 70 % of our planets surface, are not even remotely like that.
Open oceans with visibilities of 50ish meters look blue if you look through them. They can have greenish tinges, which will be the case in nutrient rich waters, where the plants of the ocean, microscopic algae called phytoplankton (just a weird word for plants floating around without major locomotory powers) increase in numbers. In freshwater lakes, the water can be even greener, and in places so green that the visibility is reduced to a few centimeters (a single inch or so). But even in the most green waters around, the total plant biomass will be well below that of the herbivores, which mainly are minute crustaceans.
Furthermore, predators, that is fish, will show an even higher total mass in most lakes than the herbivores. So it seems that very little plant matter sustain some herbivores, which in turn sustain even higher levels of predators. That seems to be weird, counterintuitive and certainly not logical. There is an explanation though and if you check in tomorrow, you will be able to read about it here!