The warming of the world’s oceans continues at an alarming rate.

Satellite measurements of average sea surface temperatures show that February this year was the month with the highest temperatures in the 45-year data series, and that the warming trend continued in May.

New Zealand’s sea temperatures are also reaching record highs. Between 2022 and 2023, the oceans and coastal waters reached their highest annual temperatures since measurements began in 1982, according to data from Stats NZ.

This warming is already threatening coral reefs – the Great Barrier Reef is currently at its hottest temperature in 400 years – and marine life. But it is also changing the ecosystems that form the basis of ocean food webs.

Microscopic algae, also called phytoplankton, are ubiquitous in the surface layers of the oceans. They form the basis of the marine food web and serve as an important carbon sink.

Through photosynthesis, they absorb over a hundred million tons of carbon dioxide every day. The carbon then either sinks to the sea floor when the microalgae die or is “fixed” in tiny animals that graze on the plants.

Scientists are already observing a downward trend in this marine production, which is leading to the expansion of “marine deserts” and the depletion of beneficial microalgae in favor of harmful algal blooms.

Unless we take action to reduce emissions, these changes in microalgae composition are expected to become more severe as ocean temperatures continue to rise globally and regionally in the waters off the coast of Aotearoa, New Zealand.

Changes in microalgal communities in New Zealand

We are already seeing changes in New Zealand’s microalgal communities.

The abundance and activity of microalgae is usually measured by observing chlorophyll A, the pigment most plants use for photosynthesis. Recent reports from Stats NZ show changes in microalgal biomass, with increases in some regions and decreasing levels in others.

In addition to these changes in biomass, studies have also shown changes in the composition of microalgae and reduced biodiversity. This leads to stunted ecosystems that are less resilient to environmental change, have lower productivity and sequester less carbon.

Abrupt changes in microalgal communities can shift ecosystems into an altered state, affecting food webs and fisheries. Such a “regime shift” occurred in the North Pacific in 1977 and 1989 and had far-reaching consequences for the entire ecosystem as well as salmon and halibut fisheries.

More recently, reduced microalgal biomass and food web collapse have been suggested as causes of ‘milk meat syndrome’ in Hauraki Gulf snapper in New Zealand waters.

Harmful algal blooms

Harmful algal blooms also appear to be on the rise in New Zealand. The toxins produced by these microalgae accumulate in shellfish. Eating them can be toxic to humans and animals and threaten the economic stability of the fishery.

The consequences of harmful algal blooms cost the global blue economy over US$8 billion annually, with more than US$4 billion of this related to impacts on human health.

According to data collected by the Ministry for Primary Industries, the increase in harmful algal blooms in New Zealand in 2023/24 resulted in the highest number of mussel fishery losses due to biotoxins this decade.

Rising ocean temperatures can accelerate the growth of microalgae that cause toxic algal blooms, while reducing the nutrient content and size of microalgae species that other marine organisms depend on as a food source.

As our research shows, microalgal toxins are affecting the reproduction and early life stages of shellfish species native to New Zealand. New Zealand’s fishing and aquaculture sectors, together worth almost NZ$4 billion, are already facing fishing shutdowns, stock losses and a decline in larval offspring.

Many of the connections between climate change and toxic, harmful algal blooms are still not fully understood. However, it was clear that the algal bloom in Wellington Harbour in 1998 could cause serious ecological damage. It led to a mass extinction of all marine life on the seabed.

Toxic microalgal blooms can also kill marine mammals or impair their resistance to other stressors such as higher temperatures.

Tackling the challenge

A crucial first step is to understand how microalgal communities might change under different climate scenarios.

This knowledge will enable us to predict and study impacts on the marine environment and develop effective management strategies to protect marine ecosystems and public health.

Knowing when and where harmful algal blooms are likely to occur will reduce the risk to industry and enable effective remediation measures. Improving our knowledge of the impacts of microalgal toxins on human health will help us to safely use waters for recreational purposes and gain clarity on appropriate responses to algal blooms.

These knowledge gaps need to be addressed urgently. Changes in microalgal communities are already evident and are likely to continue at an accelerated pace, with potentially irreversible knock-on effects on ecosystems and marine industries.