As the final part of our oceans and climate change series, this article explores marine ecosystems, looking at primary productivity, microorganisms, and changes in species composition and biodiversity. Have a look at the previous parts of the oceans series. Part 1 includes the key facts on the global ocean. Part 2 explores physical ocean processes and Part 3 explores ocean chemistry and the impacts climate change may have on the marine environment. Written in collaboration with Sir Alister Hardy Foundation for Ocean Science, their original reports and outreach materials can be accessed here.
Primary productivity, phytoplankton and microorganisms
Primary productivity is the rate at which living organisms convert chemical energy to organic substances. In the ocean, it is dependent on photosynthesis by phytoplankton – microscopic marine plants which are responsible for providing food for almost all marine organisms. Phytoplankton are fuelled by the CO2 and nutrients in the deep ocean waters. They are also rich in chlorophyll (a green molecule that absorbs sunlight) which means that they are ideally adapted to harvest the photons in sunlight for use during photosynthesis (where light energy from the sun is converted to chemical energy).
All of this plant growth requires nutrients such as nitrate, phosphate and trace elements (such as iron) which are derived from the deep ocean waters, as well as from terrestrial runoff. The availability of nutrients in the ocean is largely dependent on the vertical density gradient or ‘stratification’ of the upper 50 to 100 m of the water column. This stratification arises from differences in temperature and salinity of the water; warmer and fresher (less saline) waters are less dense (less heavy). The stronger the stratification, the more difficult it is for the nutrient-rich deep ocean waters to become mixed with the surface.
Using observations of decadal trends (over tens of years) in ocean temperatures, salinity, and stratification, as well as model simulations of future scenarios, climate change is predicted to cause increased stratification of the oceans – particularly in the tropics and mid latitudes – due to changes in ocean temperature and salinity. This will reduce the transfer of nutrients to the surface (euphotic zone), and limit primary productivity. However, while the models provide important insights into potential future conditions, we need to use them with care – natural variability in primary productivity is actually larger than the global warming trend, and so it is difficult to securely tie any changes in productivity to climatic variations. What is more, the spatial resolution of the model is too coarse (not detailed enough) to capture the true change in biological processes. Nonetheless, these large scale models allow us to consider the marine system as a whole, at scales that are otherwise not feasible using empirical observations (or measurements) alone.
As well as the millions of microscopic phytoplankton, vast numbers of microbe species (including ciliates, bacteria and viruses) fill the oceans.
There are 100 million times more bacteria in the ocean than stars in the universe
and 10 to 100 times more viruses than bacteria
These microorganisms perform vital functions by processing waste materials and cycling carbon, oxygen, nitrogen, phosphorus, sulphur and other compounds. They take up CO2 from the atmosphere and play an important role in regulating the climate cycle. Some microorganisms have negative impacts on the oceans, and cause disease, toxicity, and harmful algal blooms (rapid increases in algae which starve the other microorganisms of vital resources).
New advances in molecular and biogeochemical tools have revolutionised our understanding of the importance of microbial communities for biogeochemical cycles and ocean health. It is only now that we are able to realise the complexity of these microbial systems. Understanding how these systems, and associated biogeochemical transformations, will respond to climate change is not yet fully understood. This is due to the strong interlinking of microbial communities and marine processes, and the potential for feedback mechanisms.
Changes in species composition and biodiversity
Over the last 30 years, marine ecosystems have changed substantially. This is largely due to warming sea temperatures, which are linked to changes in Earth’s climate. These changes are reflected in shifts in: species distribution; seasonal timing; regime; and biodiversity.
Marine biodiversity is driven by water temperature. Secondary effects of warming, such as changes in the path of currents, may also contribute to expansions and contractions of species ranges. In Europe for example, higher temperature and changes in ocean currents at the edge of the European shelf have caused certain plankton species, which are considered more characteristic of warmer water, to extend their range to the north of Shetland, a shift of 1000 km between the 1970s and 2000/2005. Over the same period, the shrimp-like Calanus f inmarchicus, which are abundant in colder waters and are an important food for fish – especially the larval stages of cod – has retreated towards the Arctic.
Many changes may be associated with the movement of a critical thermal biogeographic boundary of ~9-10°C. This boundary is significant for the locations of many species due to their preferred temperature tolerance (rather like humans, marine species are also adapted to certain temperature conditions). Poleward range shifts of marine biota in the Northeast Atlantic during periods of warming (e.g. the 1920/30s and recent decades) and retraction in cooler periods are well documented and correlated with climate variability represented by the Atlantic Meridional Oscillation.
As species migrate, due to changes in their habitat, they may move into areas which have not previously supported them; they become a new member of the local ecosystem. In these cases such species are called ‘non-native’ or ‘invasive’ species – a species that has been introduced by humans to an area, and may be damaging to the native species. For example, if we were to let lions (non-native) roam free in Europe, it would have major impacts on the local (native) species. Records of non-native and invasive marine species have markedly increased at both global and European scales. These species may cause substantial damage to the diversity and abundance of native populations as well as to ecosystem function.
As the marine ecosystem is highly interconnected through predator-prey relationships, the direct impacts of ocean climate change have ‘knock-on’ effects up the food-chain. For example, recent warmer conditions and associated changes in plankton abundance and geographical distribution have led to the reduced availability of prey fish for some seabirds, which has in turn been strongly linked to recent poor breeding success and reduced survival rates.
Continued ocean temperature rise is likely to have major impacts on species distribution, biogeochemical cycles, and marine resources. If recent patterns are maintained, it is predicted that both the number and spread of non-native species may increase due to climate change (plus ocean acidification) with consequences that are difficult to predict.
- The Marine Climate Change Impacts partnership (MCCIP) is a partnership between scientists, government, its agencies, nongovernmental organisations (nGos) and industry mccip.org.uk
- Charting progress reports at chartingprogress.defra.gov.uk
- Climate Change and European Marine Ecosystem Research at clamer.eu
- Sir Alister Hardy Foundation for Ocean Science at sahfos.ac.uk