As part of our oceans and climate change series, this article explores ocean chemistry and the impacts that climate change may have on the marine environment. We focus on ocean acidification and eutrophication. Take a look at Part 1 of our oceans series to get the key facts on the global ocean. Part 2 explores physical ocean processes. Written in collaboration with Sir Alister Hardy Foundation for Ocean Science, their original reports and outreach materials can be accessed here.

Ocean acidification

Almost one third (c.30%) of human-induced CO2 emissions – from burning fossil fuels, for example – since the beginning of industrialisation has been absorbed by the oceans. Without the oceans, this COwould have otherwise been released into the atmosphere. The oceans have therefore played an important role in buffering the effects of climate change.


But this doesn’t mean that the additional CO2 can be forgotten… The problem is that atmospheric CO2 reacts with seawater to produce more hydrogen ions, therefore forming a weak acid (lower pH). Such large inputs of CO2 into the oceans are causing acidification, which can have major impacts on ocean chemistry and ecosystems. Increased acidity leads to a reduction in the availability of calcium carbonate within ocean waters. This means that marine plants and animals that rely on calcium carbonate to produce hard shells are unable to do so. Corals, which are rich in calcium carbonate, are also expected to be particularly vulnerable to changes in ocean acidity. If ocean acidification continues, marine biota may not be able to adapt to the rate of change, meaning that they can no longer survive. The images below, from National Geographic, show the impacts of ocean acidification on a pteropod’s shell. The shell has been placed into sea water with pH levels similar to those predicted for the year 2100. The images show that the shell has corroded significantly after 45 days.

Ocean acidification Nat Geo

Computer model prediction showing that continued acidification will deplete carbonate ions in vast areas of the ocean, making the water corrosive for shell-forming biota (such as pteropods – see below) by the year 2100. Source: National Geographic


Images indicating the corrosion of pteropod shells over a period of 45 days when placed into sea water with a pH level similar to the levels predicted for the year 2100. Source: National Geographic

 At the end of the 21st century, the rate of acidification might reach levels that marine organisms have not experienced for 55 million years

Computer models are able to predict the levels of ocean acidification for difference CO2 emissions scenarios. Based on what we know about ocean acidification processes, we have good confidence in these models. In contrast, understanding of the response of marine biota to this acidification is still at an early stage and will be one of the key components of future research.

NOAA have a series of wonderfully detailed ocean acidification monitoring programs in the Pacific and coastal regions of the USA. These programs collect high frequency diurnal (daily) data, which can be accessed here.


Marine eutrophication

Eutrophication is the term used to describe an increase in the level of nutrients, such as nitrogen and phosphorous, within a water body. This causes an acceleration in algal growth and a reduction in oxygen within the water. Eutrophication is a natural process, but it is often enhanced due to terrestrial runoff and input from rivers. These can contribute sewage, plant matter, and agricultural fertilisers rich in nitrate and phosphate (which excessively boost the growth of phytoplankton vegetation (such as Phaeocystis) in coastal and estuarine areas. These large blooms disrupt the food web, potentially impact tourism, and can severely damage aquaculture systems.


Climate change can also exacerbate eutrophication processes due to changes in: ocean circulation; river runoff; stratification of the water column; ecosystem restructuring; and functioning of the food web. Many of these processes interact, and will have feedback effects on each other. With current monitoring, it is not possible to fully predict which of these changes will dominate and how these will influence eutrophication processes.

An algal bloom near Gotland in the Baltic Sea. The swirls of phytoplankton can be seen against the darker water. Source: NASA Goddard Space Flight Center;  USGS/NASA/Landsat 7

An example of an algal bloom near Gotland in the Baltic Sea. The swirls of phytoplankton can be seen against the darker water. Source: NASA Goddard Space Flight Center; USGS/NASA/Landsat 7