primary productivity is a fundamental factor in determining the characteristics of an ecosystem why do arid deserts form in some places while dense tropical forests form in others picture planting the seed of a tree so we have 0 kilg of tree at this point fast forward in time and we've got a big tree that's now 100 kg so where did that matter that 100 kg of tree actually come from well you may know that this is all through photosynthesis plants take in material in the form of carbon dioxide and water and using energy from sunlight they produce glucose there's also oxygen but that's a waste product of photosynthesis and it doesn't contribute to the mass of the plant so I'm ignoring it here the carbon compound that's produced glucose has two possible fates one is to go on to make physical plant biomass forming compounds like cellulose for example the second possible fate of glucose is to be used in respiration which is how plants like all living things release energy for life processes the rate at which energy is converted to biomass after accounting for losses due to respiration is known as the net primary productivity or NPP let's put some numbers to it let's say our tree here has spent 10 years producing glucose and the portion of this glucose contributing to physical plant biomass is 100 kg 100 kg built up over 10 years comes to 10 kg per year and this is effectively the net primary productivity of this tree again the definition for NPP is the rate at which energy is converted to biomass by photosynthetic producers it's important here to point out that this is not generally how we Express productivity we actually tend to think about it in terms of the energy or biomass over a given area rather than for a single tree so it would have units more like kilog per square meter per year or sometimes since we are referring to carbon compounds kilog of carbon per square meter per year using a given area is really useful when comparing different ecosystems and I'll give you some examples in a moment but before that let's think about what the net in net primary productivity means I'll give you an analogy let's say you have a job and for your effort and hard work you get paid $1,000 that's the amount that leaves your employer's bank account for you but sadly you have to pay tax maybe where you live you have to pay 20% tax so $200 of your money goes to the government and you never see it what actually ends up in your bank account the bit you can really observe and make use of is the remaining $800 this $800 that you actually get is called your net income the total $1,000 that your employer paid is called your gross income the maths here is pretty simple your net income is equal to your gross income minus the tax so $1,000 minus the $200 tax this way of thinking works for productivity too plants are able to use an amount of energy to produce glucose the rate at which solar energy is converted to chemical energy in the form of glucose by producers and I'm talking about all the glucose before any of it is used in respiration is known as the gross primary productivity the net gain in biomass of the plant the net primary productivity is all of that material that was produced minus what has been lost to respiration so mathematically the net primary productivity is equal to the gross primary productivity minus the respiratory losses now like I've already said we tend to describe productivity over an area rather than for a single organism allowing us to make comparisons between ecosystems so let's look at some data to get some context according to One Source the average net Prim productivity of a tropical evergreen forest is in the ballpark of 1.1 kg of carbon per m squar per year this is relatively high and that's because tropical forests have year round warm temperatures plenty of rainfall and sunlight all of which contribute to a fast rate of photosynthesis so in the tropics it's possible to convert a lot of carbon from carbon dioxide into plant biomass deserts on the other hand have an aage net primary productivity of more like 0.05 kg of carbon per me squ per year despite plenty of sunlight deserts are distinctly lacking in rainfall so photosynthesis is severely limited and therefore so is the rate of production of biomass we've only considered primary productivity on land but it's also something we can measure in aquatic environments water plants like seaweed contribute a little to to this but the main driver is phytoplankton phytoplankton are microscopic photosynthetic organisms that live in water being photosynthetic they assimilate carbon from carbon dioxide in the same way as plants fueling aquatic food webs the primary productivity of any location aquatic or terrestrial is a significant factor in the overall character of the environment as it provides the source of energy for every consumer higher up the food chain it's possible to determine NPP gpp and R in an aquatic ecosystem with a surprisingly simple experiment click on the next video and I'll walk you through it [Music]