Improving estimates of ocean primary productivity:Coupling bio-optics into a semi-Lagrangian model of phytoplankton physiology and ocean mixing

由于化石燃料的燃烧和土地利用的变化，大气中的二氧化碳（CO2）越来越多。由于“温室效应”，我们的气候正在变暖。为了预测未来地球会变暖多少，我们需要了解地球上影响气候的所有过程——这些过程不仅发生在大气中，也发生在陆地和海洋中。一旦我们做到了这一点，我们就可以建立计算机模型（即编写计算机程序）来模拟所有这些过程之间的相互作用，然后告诉我们未来的气候可能是什么样子。然而，有许多复杂的生物、化学和物理过程我们还没有完全理解，所以我们仍然需要收集大量的数据，花时间检查单个系统，然后才能正确地模拟整个气候系统。我提议研究的系统涉及海洋中的生物过程。由于热量和气体在空气和海洋之间的流动，海洋影响着气候。目前大气中的二氧化碳含量与海洋中的二氧化碳含量不平衡，所以空气中的二氧化碳目前正在被海洋吸收。这对我们来说是个好消息，因为如果海洋中的二氧化碳更多，大气中的二氧化碳更少，那么全球变暖的速度就会减慢。一旦二氧化碳在海洋中溶解，当表层海水冷却下沉时，它可能下沉到深海，或者它可能通过海洋生物向下运输。一旦它到达那里，它可能在数千年内不会回到地面。如果二氧化碳停留在海面上，就会阻碍更多的二氧化碳进入海洋，所以重要的是要有一种机制将其输送到深海，否则大气中的二氧化碳就会比现在更多。我建议建立海洋生物如何将溶解在地表水中的二氧化碳转化为植物物质的模型。这是大气中二氧化碳向深海生物运输的第一个阶段，因此我们必须充分了解它。海洋中有许多微小的浮游植物（“浮游植物”），只要它们有营养和阳光，它们就能从海水中吸收溶解的二氧化碳并利用它生长。这个过程被称为光合作用或碳固定。这些植物可能会被小型漂流动物、鱼类甚至鲸鱼吃掉，从而将碳转移到食物链的上游。如果这些植物、动物或鱼类死亡，它们可能会从表面沉入深海，将大气中的碳困在海洋内部深处。我们对这些小植物的了解大多来自环绕地球运行的卫星的测量。他们可以测量从海洋表面反射的阳光的数量——这被称为“海洋颜色”——它告诉我们有多少植物生活在海洋表面附近。我提出的研究包括开发一个计算机模型来研究光合作用。这不是一个简单的建模过程，因为随着海水混合或分层（由风和阳光的变化引起），这些小植物在水中上下移动。这一点很重要，因为小植物越深，它们得到的光照就越少，但营养就越多。光照和养分越多，它们的光合作用就越快。为了了解这个过程，我将把植物的数量和固定碳的数量与海洋颜色联系起来。这有两个好处。首先，我们的模型结果将帮助我们准确地了解卫星海洋颜色数据告诉我们的海洋生态系统状态，其次，如果我们想开始将海洋颜色数据添加到我们的模型中，那么我们将知道如何做到这一点。所有这些都很重要，因为它将帮助我们了解目前海洋中正在发生的事情，这将帮助我们预测未来会发生什么。

There are increasing amounts of carbon dioxide (CO2) in our atmosphere from the burning of fossil fuels and changes in land use. Because of the 'greenhouse effect' our climate is getting warmer. To predict how much warmer our planet will become in the future we need to understand all the processes on Earth that affect climate - these occur not only in the atmosphere but also on the land and in the sea. Once we have done this we can build computer models (i.e. write computer programs) which simulate the interactions between all these processes and then tell us what our climate might be like in the future. However, there are many complicated biological, chemical and physical processes that we don't yet fully understand so we still need to collect lots of data and spend time examining individual systems before we can properly model the whole climate system. The system I propose to look at concerns biological processes in the oceans. The oceans influence climate due to the movement of heat and gas between the air and sea. The amount of CO2 in the atmosphere at the moment is out of balance with that in the sea so the CO2 in the air is currently being absorbed by the oceans. This is good news for us because if there is more CO2 in the ocean, and less in the atmosphere, then the rate of global warming will be slower. Once the CO2 is dissolved in the oceans it may sink down to the deep ocean when the surface water cools and sinks, or it may get transported downwards through marine biology. And once it gets there it may not return to the surface for thousands of years. If the CO2 stayed at the sea surface it would hinder the progress of more CO2 entering the sea so it is important that mechanisms exist for transporting it to the deep ocean, otherwise we would have more CO2 in the atmosphere than we do now. I propose to model how the marine biology converts the CO2 dissolved in the surface waters to plant matter. This is the first stage in the biological transport of atmospheric CO2 to the deep ocean and so it is crucial that we understand it fully. There are lots of tiny floating plants ('phytoplankton') in the ocean, and as long as they have nutrient and sunlight they can absorb the dissolved CO2 from the sea water and use it to grow. This process is known as photosynthesis or carbon fixation. These plants may then be eaten by small drifting animals, fish or even whales, thus moving the carbon up the food chain. If these plants, animals or fish die, they may sink from the surface to the deep ocean, trapping the carbon from the atmosphere deep within the ocean interior. Much of what we know about these little plants comes from measurements taken by satellites orbiting the Earth. They can measure the amount of sunlight reflected from the ocean surface - this is known as 'ocean colour' - and it tells us how much plant life is near the sea surface. The research I propose, involves developing a computer model to investigate photosynthesis. It is not a simple process to model because the little plants move up and down through the water as the sea water mixes or stratifies (caused by changes in wind and sunlight). This is important because the deeper the little plants go, the less light they have but the more nutrient. The more light and nutrient they have - the faster they can photosynthesize. To get a handle on this process I will link the amount of plant life and the amount of carbon fixed, to ocean colour. This is good for 2 reasons. First, our model results will help us to understand exactly what the satellite ocean colour data is telling us about the state of the marine ecosystem, and second if we want to start adding ocean colour data into our models then we will know how to do it. All this is important because it will help us to understand what is actually happening in the oceans at the moment and that will help us to predict what will happen in the future.