Marine Food Webs
In the last section we said that primary production in the ocean comes primarily from microscopic one-celled algae. Less than 2% of the ocean is shallow enough with a firm enough bottom to support larger attached plants. So it is the energy produced by these algae that forms the basis of the food pyramid in the ocean.
(from Duxbury and Duxbury, 1994, An Introduction to the World's Oceans)
The pyramid represents energy in the ocean. The widest part is the microscopic algae. These are at the bottom because they are what converts the sun's energy into carbohydrates usable by everything else. Organisms that can make their own food like this are called autotrophs or producers. Since they have access to the most energy---all the energy from the sun---they have more biomass (living mass) than the layers above them.
Just above the producers are the organisms that depend on them for food. These organisms, from microscopic plankton to whales, cannot make their own food and they are called heterotrophs or consumers. There are several levels of consumers in an ecosystem. Since there are various "-trophs" involved here, this is often called a trophic pyramid and the different levels, trophic levels.
In the pyramid above, typical of the ocean, there is the phytoplankton on the bottom. (Perhaps I should add here that plankton are the very small "drifters" in the ocean. Phytoplankton are the plants or algae, and zooplankton are the animals.)
Note also that at the bottom there are 1000 "energy units" available. The next level up is the herbivorous (plant eating) zooplankton. At this level there are 100 "energy units" available. Carnivorous (meat eating) zooplankton eat the herbivores, these are eaten by carnivorous fish, and in this case, tuna is the top of the line or the apex predator. Actually, in most ocean systems, and perhaps most terrestrial systems, people are probably the real apex. Anyway, notice as you move up that the pyramid gets narrower. This represents the diminishing energy and is reflected in the diminishing biomass. Certainly there are fewer tuna in the world than zooplankters. If you look at the energy units, you see that each layer has only 10% of what the previous layer had. 90% of the energy at each level is lost. Why is this? Well, don't just produce for the benefit of those zooplankton. They've got living, growing, reproducing to do themselves. In doing that, they use up energy. In fact, they use up 90% of the energy they take in. You do too. That's why there are fewer sharks than anchovies in the world. There's simply not enough food to support a lot of big things.
Here's an interesting thing to consider: what are the biggest animals on land? Elephants, right? What do elephants eat? Plants! What are the biggest animals in the ocean? Whales, bigger even than elephants. And what do you think the very biggest types of whales eat? Plankton! Why? That's where the energy is! The biggest fish in the ocean is the whale shark. What do you think it eats? Plankton! So if you want to maximize energy efficiency, you need to eat low on the food pyramid.
Okay, we covered the consumers and producers, but there's one more key category of organisms, without which this system would not work. They are the decomposers. These are mostly bacteria although they get help in their work from the scavengers or detritivores (things that eat detritus or dead stuff). Why do they matter? Well, if they didn't exist on land we'd be up to our ears in dead things, but more important, nutrients that plants need to make food would be stuck out of reach. The decomposers free up those nutrients so they can cycle around again.
This is all neatly arranged, but in the real world, things are much more complicated. There's a reason it's called a food web. This figure shows a typical food web for a fish called a herring. You may have heard of these and you may know that they only get perhaps 10 centimeters long. Note that little herring have to run the gauntlet of being low in the food chain before they grow up. This is true of most organisms. And only a lucky few ever get to be apex predators. Practically everybody is somebody's lunch.
The Marine Environment
It probably seems to you that life on land is a lot easier than life in the ocean, but not really. Life on earth started in the ocean and only recently moved out onto the land. Land is a nasty place. To live on land you have to deal with gravity, dehydration, and large temperature variations, among other things. To live on land, organisms have had to develop specialized systems and appendages to cope with these things. These include, fur, root systems, strong legs, fruits, skeletons, all sorts of complicated things. In the ocean the temperature is fairly stable (your remember all about specific heat, right?), water is plentiful, and you can float and move by friction. In fact, the overwhelming mass of organisms in the ocean are simple and mostly microscopic things.
In the deep sea, things remain unchanged year in and year out. The temperature is constant. It's constantly dark. The water pressure is constant. At the surface things are a little more variable, but not much. Light intensity changes daily and seasonally and this is especially important in the polar and temperate regions. Surface temperatures change seasonally, but not by much. Likewise, rainfall or lack thereof changes surface salinity, but again, not by much. The organisms that live in coastal water which change the most are much more tolerant of these changes that organisms that frequent the open ocean.
Regulation of Body Fluids
This is a much bigger problem for a land organism than for a marine organism. Consider all the adaptations you have to maintain your fluid balance. You have to drink. You have skin that keeps things both in and out of you. You have kidneys. Since in the ocean an organism is surrounded by fluid, for most species there's no point in going to great lengths to maintain internal fluid different from this. Therefore most marine organisms are permeable and isotonic. In other words, fluids can simply move through their body coating, and their internal fluids have the same salt concentration as their external fluids. They spend no energy in trying to maintain fluid balance, but the down side is that they are intolerant of change. You can't toss an oyster in with your goldfish and expect it to live---an oysters are even among those very tolerant coastal/estuarine organisms.
For the more complex organisms of the ocean, there are enough advantages to being able to maintain an internal environment whatever the outside environment, that they do expend energy doing this. Fish, for example tend to travel, so it's helpful is they have some control over their internal environment. They are semipermeable. This means that water can pass through their skin, but salt cannot. A fish has an internal salinity of about 14 ppt. The average ocean salinity is 35 ppt. Water can pass through the fishes skin by osmosis (the diffusion of water) so what tends to happen? Water leaves the fish for the more salty ocean---it "wants" to balance out those two different salinities. So the fish tends to dehydrate. But unlike you, who would likewise dehydrate drinking salt water, the fish has adaptations to take in the salt water but excrete the salt. It drinks lots of water and secretes the salt out its gills and its highly salty urine. Sea turtles and crocodiles also have special adaptations to excrete salt, and this allows them to live in salty water. They both "cry." Surely you've heard of "crocodile tears." The crocodile isn't sad, he's merely getting rid of salt.
To see if you understand the concepts of osmosis and permeability, see if you can figure out what freshwater fish have to do to maintain their internal 14ppt salinity.
Most marine organisms are more dense than water and so they have a tendency to sink. This isn't good because the surface is where the sun is, so it's where the primary production occurs, so it's where the food is. There are several ways of dealing with this. A lot of marine organisms are mostly water themselves, so they float. Jellyfish are a good example of this. Mammals and reptiles have lungs which help. The mammals also have thick fat layers which both insulate and help them float. Sharks and a lot of the very small zooplankton have high concentrations of oil and you know that oil is lighter than water, so this helps. These very small creatures, as well as the algae, for whom staying in the sun is absolutely vital for photosynthesis, have all sorts of interesting shapes and projections to increase their surface area and make it easier for them to float. Most fish have swim bladders which are gas-filled sacs that inflate or deflate according to where in the water column the fish needs to be. The fish adds air if it wants to go up and releases air if it wants to go down. Some of the faster fish like tuna, some sharks, and also octopus and squid simply have to keep moving or they sink. Then there are lots of creatures that simply live on the bottom.
For a lot of ocean organisms, floating is key so that they stay near food, but by no means is this important for everything. In fact, marine scientists divide the ocean into several distinct environments and each of these presents its own challenges to the organisms there.
The oceanic or pelagic zone comprises the open ocean away from shore and away from the bottom. Sort of the opposite of this is the benthic zone or the bottom. Most organisms are either benthic or pelagic. Within those categories, are several different zones, however. The neritic or coastal zone is the shallow water over the continental shelf. The littoral zone is intertidal. The abyss is the bottom where it's very deep. Most things live where the light reaches. This is called the photic zone. The aphotic zone is where it's dark. And so it goes. All these divisions are artificial, of course. It's one big ocean when you get right down to it. Still, it makes it a little easier to talk about if you can divide things up. These zones are summarized in the figure below.
(from Duxbury and Duxbury, 1994, An Introduction to the World's Oceans)