The amount of microscopic algae in the water off Sydney has declined, a strong indicator of a downward trend in temperature.

For over a decade, researchers at Macquarie University have kept tabs on the amount of phytoplankton (microalgae) in coastal waters of Australia.

Phytoplankton, though microscopic, have a huge effect on the marine environment.

Their growth produces almost half of the world’s oxygen, and supports the entire marine food chain.

Often appearing in large blooms, called ‘red tides’, they are closely linked to the climate system due to their sensitivity to ocean circulation and nutrient availability.

Shifting climates appear to influence phytoplankton abundance and diversity, and as such they are important indicators of climate-change effects on marine ecosystems.

“We know that the coastal waters of southeast Australia have undergone significant climate-related changes over the past 60 years,” said Dr Penelope Ajani from Macquarie University.

“We wanted to assess the effects of these changes on the phytoplankton.”

“We examined 11 years of samples. Our data confirmed the seasonal pattern of peak diversity in winter, and also that phytoplankton blooms occur most consistently in March, September and December.

“Unexpectedly, we also observed a significant decline in total phytoplankton numbers over this eleven-year period. This decline in abundance was associated with a decline in water temperature,” Dr Ajani said.

Fellow researcher Dr Andrew Allen said: “What these findings tell us is that, although there has been a long-term increase in water temperature in our coastal waters, shorter-term fluctuations can and do occur.

“Such fluctuations significantly affect the phytoplankton, and therefore may have important implications for the entire marine ecosystem”.

The phytotplankton dataset collected and analysed for this study represents one of the longest time series in the Southern Hemisphere. It therefore represents an important baseline for assessing the effects of future climate change on marine ecosystems.

The paper on the decline in relative abundance will be published in the journal Limnology and Oceanography, and is available in PDF form here.