Ask the Expert
By Kate Mackey

Question: Are studies underway to find out how climate change has affected plankton numbers and distribution, and therefore the food chain? Are there any discernible trends?

Answer: What a great question! This is a timely topic and yes, there are many scientists investigating the impacts of climate change on plankton communities around the globe, including the Mackey Lab.

The term ‘climate change’ refers to a series of complex phenomena that can affect plankton in many different ways. Phytoplankton are microscopic algae that drift in the ocean. Although tiny, these microbes are a big deal! They produce about half of the oxygen on Earth by recycling carbon dioxide via photosynthesis. They also serve as the initial food source for virtually all ocean creatures, being consumed by zooplankton, which in turn serve as food for larger animals. Thus, as the question implies, anything that disturbs the phytoplankton will very likely affect marine food chains and the ecosystem as a whole.

A central component of the human-caused climate change that our planet is currently experiencing is global warming, which is caused by emission of greenhouse gases (mainly carbon dioxide, but also methane and nitrous oxide) that people have introduced into the atmosphere since the Industrial Revolution. Due to its sheer mass and high ability to store heat, about 90% of the excess heat from global warming is stored in the ocean. This has resulted in a 0.85 °C increase in average sea surface temperature between 1880 and 2012. The heat also causes the water to expand, contributing to sea level rise.

The increase in sea surface temperatures (or SST for short) is expected to affect global phytoplankton distribution in at least two important ways: directly, through alterations to cellular metabolism/growth, and indirectly, via alterations to vertical mixing of ocean waters.

Our planet has distinct SST latitudinal zones with temperatures ranging from ~ 30 °C in tropical waters to ~ -2 °C at the poles. In general, phytoplankton can maintain growth over a surprisingly wide range of temperatures, but every species has a unique optimum growth temperature. Some types of phytoplankton thrive in colder waters, while some like it hot! In this way, global SST is one of several environmental variables controlling the growth and distribution of diverse phytoplankton species. When SST change beyond the ideal range for the phytoplankton inhabiting a given region, the consumption and allocation of resources within their cells are affected. Proteins and other cellular molecules have to function in suboptimal conditions, and that reduces the efficiency of photosynthesis in converting carbon dioxide into organic carbon. In the long run, this reduces the amount of carbon that would otherwise be buried in the deep ocean, reinforcing global warming in what scientists call a “climate feedback”.

At the same time, increased SST prevents vertical mixing of the water column due to the difference in density between the warm surface water and cold, nutrient-rich deep water. Without adequate mixing, nutrients cannot reach the sunlit surface ocean where phytoplankton grow, and hence phytoplankton are starved for essential resources (nitrogen, phosphorus, and iron). Therefore, the integrated response of marine phytoplankton to increased SST will be a blend of both the direct temperature-induced changes to cellular biology and indirect changes of surface nutrient availability.

The importance of changes in SST to phytoplankton biology, as explained above, is unquestionable. However, the way in which these climate change-related factors have actually impacted marine plankton globally is still a topic of much interest within the scientific community. Some researchers think that the warming of SST has caused significant negative impacts to marine phytoplankton around the globe. For example, back in 2010, Boyce and collaborators published a study in the prestigious scientific journal Nature arguing that phytoplankton concentrations have declined in eight out of ten ocean regions at a global rate of ~1% of the global median per year. According to this estimate, the amount of phytoplankton around the globe today would be about half of what existed in the 1950’s! These findings were contested by a number of researchers, some of whom argued that phytoplankton abundance might have increased in certain regions as a response to higher SST, offsetting the losses at other sites.

This debate-a vital element of the peer review process, which maintains the integrity of the scientific method-continues, as new methods and analytical tools evolve. One of the most exciting areas of research within this field is the use of Earth System models to predict the changes in phytoplankton community structure in the next decades and centuries, as SST continues to rise globally. The Earth System approach views Earth’s processes as intimately interconnected, and focuses on the ways that land, atmosphere, and ocean processes interact. Recent studies have shown that global warming and its consequent changes to ocean circulation will likely cause the geographic distribution of phytoplankton species to change. For example, in models of the North Atlantic Ocean, entire phytoplankton communities move in space in response to changing ocean conditions. It is possible that over the coming century, most but not all studied species could shift northeastward in the Atlantic, moving at a rate faster than previously estimated.

Finally, the capacity of phytoplankton to evolve and adapt to new environmental conditions will play a role in determining how they will fare as Earth’s climate changes. Phytoplankton are able to reproduce both sexually, by exchanging gametes, and asexually, by cellular division in which they make identical copies of their cells. Cellular division can double the population size in a matter of hours to days, and it is this quick generation time that can facilitate rapid evolutionary adaptation by the phytoplankton. Many exciting laboratory experiments indicate that phytoplankton species have the capacity to evolve over a few years in response to single environmental factors such as changes in carbon dioxide concentration or temperature. The question now is, how will those results obtained from laboratory experiments of single factors on individual algae species translate to the much more complex real world ocean?

Our team and other scientists at the University of California in Irvine are currently studying the combined effects of higher temperatures and changing resource availability on phytoplankton communities using a combination of laboratory, field, and modelling approaches. Ultimately, we aim to predict how these changes will affect atmospheric carbon dioxide levels and marine food webs around the globe.

Special thanks to my graduate students Johann Lopez and Joana Tavares for their collaboration on this article.

Kate Mackey is an Assistant Professor of Earth System Science at the University of California, Irvine.