Study to explore why otherwise healthy lakes all over the world turning green

Researchers at Nelson’s Cawthron Institute are co-leading a collaboration of international scientists working to understand an emerging environmental threat to lakes all over the world that were previously considered to be in good health.

From Lake Tahoe in the USA to Lake Wakatipu in Queenstown, pristine lakes around the world are being carpeted by bright-green slimy algae growing on the lake bed, especially during the summer. This relatively recent phenomenon is deeply concerning to scientists because it indicates that many of the world’s most iconic clear lakes could be degrading at an alarming rate.

Cawthron Institute scientist Dr Simon Stewart is co-leading the research financed through MBIE’s Catalyst: Seeding fund.

Dr Stewart said that typically, when a lake experiences degradation, the water becomes enriched in nutrients and choked with algae, but some of these lakes that have algae mats growing on the lake bed have clear water that is free of nutrients, puzzling scientists.

Explaining the challenge, Dr Stewart advised “the ‘green bottom’ lake problem is caused by filamentous algal blooms (FABs). These blooms need lots of light, so they’re occurring at lake edges where people swim and play, which means they’re impacting on people’s enjoyment and use of lakes, but they’re also bound to be having a negative impact on the ecosystem of these lakes - exactly the type and degree of impact is unknown at this point because we just don’t have the data.

"The fact these blooms are occurring in essentially pristine lakes is an indicator that freshwater ecosystems are rapidly changing and the blooms could be early-warning indicators of ecosystem change, but we have a puzzle to solve because traditional monitoring methods can’t tell us much about the problem.

Co-researcher Professor Marianne Moore from Wellesley College in Massachusetts, USA, who has  conducted research in the world’s largest lake, Russia’s Lake Baikal, said she was not surprised to find that human sewage contributes to FABs in Lake Baikal, but was surprised that this wasn’t a common denominator in FAB’s worldwide.

Professor Moore noted "the prevalence of FABs in other clear-water lakes of the world amazed me, especially those where the cause remains a mystery."

Dr Stewart said scientists typically monitor lake health and other types of algal blooms by taking water samples, usually from the middle of the lake, but because the water samples from lakes with FABs are generally clean, other approaches will need to be developed.

He added “we need to respond by developing new approaches and scientific techniques to understanding and monitoring these nearshore habitats to get early warning when a bloom is beginning and find out what’s causing it, because it could be any or all of the environmental stressors we know about, like climate change, nutrient pollution, pesticides or invasive species, or it could be something else - we really need to dedicate scientific resource to understanding the problem."

Dr Stewart said there are several lakes in New Zealand that are already experiencing these blooms, including Upper Maori Lake near Ashburton, Lake Wakatipu and Lake Taupo, concluding “our team have been sampling and monitoring these blooms in a few lakes in New Zealand, including Taupo, to shed light on what is driving these blooms, but unlike the offshore waters in lakes where in some instances monitoring data goes back 100 years and the theory is well established, we’re starting from a fresh slate so we’re having to work smarter.

"This is why the global research partnership is so important - by sharing knowledge and resources with partners worldwide we can all make progress together."

Dr Stewart hopes further funding can be secured to continue research into determining why these FABs are occurring and improving our understanding of lake ecosystems.

Image: Algal bloom in Lake Waihol. Credit: Otago Regional Council/Juliet Milne.