Factors Impacting Primary Production - Online Tutor, Practice Problems & Exam Prep

Net primary production (NPP) in aquatic ecosystems is influenced by light and nutrients, with higher productivity in surface waters due to better light penetration. Key limiting nutrients are nitrogen and phosphorus, which can enhance NPP when added. Eutrophication, the nutrient enrichment process, can lead to oxygen depletion as decaying organisms consume oxygen. In terrestrial ecosystems, NPP increases with moisture, temperature, and nutrients, particularly nitrogen. Tropical rainforests are the most productive, while deserts and tundras are the least due to low precipitation and temperature.

Primary Production in Aquatic Ecosystems

This video, we're going to talk about net primary production or NPP, specifically in aquatic ecosystems. And so really there are two factors that primarily dictate aquatic net primary production, and those two factors are light and nutrients. Now when it comes to light, really it's the depth of light penetration that matters most. So what you should know is that aquatic net primary productivity is actually higher in surface waters where the light actually reaches. And this is because light drives photosynthesis.

And of course, the more photosynthesis there is, the more net primary productivity there will be. And so if we take a look at our image down below on the left-hand side, notice we can see this decreasing gradient of light with depth, where again, the higher surface waters have higher availability of light. But as you go deeper and deeper into the water, the light availability gets lower and lower to the point where if you go deep enough, eventually there won't be any light available at all. And again, light drives photosynthesis, and the more photosynthesis, the more net primary productivity. So notice NPP has a similar decreasing gradient with depth as light does.

Now in terms of nutrients, what you should know is that usually nitrogen and phosphorus are going to serve as limiting nutrients. And so in many cases, it's going to be the addition of these specific nutrients, nitrogen and phosphorus, that will actually lead to an increase in net primary productivity. Now it's also important to recall from previous lesson videos that nutrients have a tendency to sync with detritus or dead decaying matter, which creates an increasing gradient of nutrients with depth, which we can see over here. This gradient of nutrients increases with depth, so there's lower nutrients in surface waters. And as the organisms in the surface water die, the detritus will sink and bring along with it all of the nutrients, which creates this draw of nutrients, this constant draw of nutrients from surface waters down to deeper waters, and that's what creates this increasing gradient.

Now over here on the right-hand side, we have an image to remind you of the process of eutrophication, which we've already covered in some of our previous lesson videos. So recall that eutrophication is the process of an ecosystem becoming nutrient-rich over time. And of course, the addition of all these nutrients is going to lead to an increase in net primary production. However, consequently, this is going to deplete the oxygen content in that eutrophic environment, and this is because all of the life that's supported by that increase in net primary production will eventually die. And the decaying process consumes a lot of oxygen, which ends up depleting the oxygen in that environment.

And so if we take a look at this image down below, notice on the left-hand side we're starting with an environment that has relatively low amounts of nutrients. And with low nutrients, that will only allow for low net primary productivity. And this allows for a high oxygen environment. And so aerobic organisms like fish are able to thrive in these environments. Now again, eutrophication, indicated by this orangish arrow here, is the process of the environment becoming nutrient rich.

And so notice now over here we have a relatively high amount of nutrients. And with high nutrients, that is going to allow for high net primary productivity, which will allow for, which will support a lot of life. But when that life dies, once again, the decaying process consumes a lot of oxygen, depleting the oxygen in the environment creating a low oxygen environment. And so aerobic organisms that require higher amounts of oxygen will end up dying, which is why we're not showing you those same fish here. Now the very last note that I'll leave you all off with is this note that we have here, which is really just a reminder that coral reefs are amongst the most productive aquatic ecosystems on earth.

While open oceans, on the other hand, are actually the least productive per meter squared. But oceans are vast and ginormous. So despite being the least productive per meter squared, that productivity adds up to be a significant amount when you consider how big the oceans are. So this here concludes this video. We'll be able to apply these concepts and problems.

So see you all in the next video.

Factors Impacting Primary Production Example 1

So here we have an example problem that says interpret the data in the table and notice this table has nutrients added to the experimental culture here in the left-hand column. And then it has the relative uptake of carbon 14 by cultures here in the right-hand column, which is really just a measure of the production of new biomass or a measure of the net primary productivity or NPP where the greater the value is, the more productivity and the more biomass produced. And the problem asks us, which of the following is considered a limiting nutrient in this scenario? And we've got these 4 potential answer options. Now, of course, the limiting nutrient is like the limiting reagent.

So it's those specific nutrients that need to be added for there to be significant increases in the production of new biomass or the NPP. And recall from our previous lesson videos that usually in most cases, it's going to be nitrogen and or phosphorus that are going to serve as the limiting nutrients. However, they're not always going to be the limiting nutrients. It does depend on the scenario. And so we need to really analyze this data here in this table to determine the correct answer.

And so notice that here in the first row, we have a control and the relative uptake is 1.00. So what we need to do is compare all of the other scenarios with this control to figure out what's really happening. And so notice that in the next column, we add nitrogen and phosphorus only, and that leads to a very slight increase in the carbon 14 uptake or a very slight increase in the NPP. Then we add nitrogen, phosphorus, and metals, but we exclude iron specifically, and there's another tiny little jump in the carbon 14 uptake, but not really a significant amount here. But notice that in this last one, when we add nitrogen, phosphorus, and metals, including iron specifically, there is a major drastic significant increase in the carbon 14 uptake, a huge boost in the NPP.

And so really it's going to be iron that is going to serve as the limiting nutrient. What was really limiting the NPP here. And so what we can do is indicate that answer option c, iron, is the correct answer option to this problem. So that concludes this problem, and I'll see you all in our next video.

Which of the following ecosystems would you expect to have the lowest net productivity per cubic meter? 

• A. The open ocean (0m-10m deep).
• B. An estuary.
• C. The open ocean (20m-50m deep).
• D. A coral reef.

Primary Production in Terrestrial Ecosystems

This video, we're going to talk about primary production in terrestrial ecosystems. And so terrestrial net primary production actually has a tendency to increase with increasing amounts of these three factors, which are moisture, temperature, and nutrients. And so in a graph like this one down below on the left-hand side where we have the amount of precipitation, temperature, and nutrients on the x-axis and the net primary production or NPP on the y-axis, we might expect this graph to look something like this here since we know that both of these factors tend to increase with each other. Now when it comes to nutrients, usually it's going to be nitrogen and phosphorus once again that are going to serve as the limiting nutrients. And so usually, it's going to be the addition of these specific nutrients that will actually lead to an increase in the net primary productivity.

Now terrestrial plants have evolved several different adaptations to help them maximize their nutrient uptake. And down below, we're highlighting one very important evolutionary adaptation, which is this mutualistic relationship between the roots of some plants and these nitrogen-fixing bacteria. And so some plant roots have these structures called root nodules that we're highlighting here, and these root nodules house these nitrogen-fixing bacteria that you can see here, which are capable of taking an unusable form of nitrogen gas and converting it into a more usable form of nitrogen that the plants can actually assimilate and use. And this is going to be critical for the nitrogen cycle, which we'll get to talk more about later in our course in a different video. Now, the very last note that I'll leave you all off with, is this reminder that tropical rainforests are actually the most productive terrestrial ecosystem.

And that's no surprise since we know that they receive very high precipitation and temperature throughout the year. While let me get out of the way here so that you can see this better. Deserts and tundra are the least productive terrestrial ecosystems, and that's because they receive very little precipitation and can also have relatively cold temperatures as well. And so this here concludes our lesson and we'll be able to get some practice applying these concepts moving forward. So I'll see you on our next video.

Which of the following ecosystems would you expect to have the lowest net primary productivity?

• A. Temperate broadleaf forest.
• B. Tropical rainforest.
• C. Temperate grassland.
• D. The Sahara Desert.