Ocean heat is changing marine food webs – with far-reaching consequences for NZ fisheries and sea life
As the ocean warms, toxic algal blooms are on the rise in the waters off New Zealand, causing the highest number of shellfish harvest closures in a decade.
Anne Rolton Vignier, Scientist in Shellfish and Algal Biology, Cawthron Institute, Kirsty Smith, Scientist in Algal Ecology, Cawthron Institute
29 August 2024
The global ocean continues to warm at a concerning rate.
New Zealand’s sea temperatures are also hitting record highs. Between 2022 and 2023, oceanic and coastal waters reached their warmest annual temperatures since measurements began in 1982, according to Stats NZ data.
This warming is already threatening coral reefs – the Great Barrier Reef is the hottest it’s been in 400 years – and marine life. But it is also reshaping ecosystems at the very basis of ocean food webs.
Microscopic algae, or phytoplankton, are ubiquitous in the surface layers of the ocean. They represent the foundation of the marine food web and serve as a substantial carbon sink.
Unless we act to cut emissions, these shifts in microalgal composition are projected to get worse as ocean temperatures continue to rise, globally and regionally in the waters off Aotearoa New Zealand.
Shifts 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 tracking Chlorophyll A, the pigment most plants use for photosynthesis. Recent reports by Stats NZ show shifts in microalgal biomass, with increases in some regions and declining levels in others.
Abrupt shifts in microalgal communities can drive ecosystems into altered states, affecting food webs and fisheries. Such a “regime shift” happened in the North Pacific in 1977 and 1989, with far-reaching consequences for the the entire ecosystem and salmon and halibut fisheries.
More recently in New Zealand waters, lower microalgal biomass and a collapsing food web have been implicated in the cause of “milky flesh syndrome” in snapper from the Hauraki Gulf.
Harmful algal blooms
Harmful algal blooms also appear to be on the rise in New Zealand. The toxins these microalgae produce accumulate in shellfish, and their consumption can be poisonous for people and animals and threaten the economic stability of fisheries.
According to data gathered by the Ministry of Primary Industries, the rise in harmful algal blooms in New Zealand during 2023/24 resulted in the highest number of shellfish harvest closures from biotoxins this decade.
Rising ocean temperatures can accelerate the growth of microalgae that cause toxic blooms, while reducing the nutritional quality and size of microalgae species other marine organisms depend on for food.
As our research shows, microalgal toxins affect the reproduction and early life stages of shellfish species indigenous to New Zealand. New Zealand’s fisheries and aquaculture sectors, collectively worth nearly NZ$4 billion, already face harvest closures, stock losses and reduced recruitment of larvae.
Toxic microalgal blooms can also kill marine mammals or make them less resilient to other stress factors, such as higher temperatures.
Addressing the challenge
Understanding how microalgal communities might change under different climate scenarios is a crucial first step.
This knowledge will help us forecast and investigate the downstream effects on the marine environment and develop effective management strategies to safeguard ocean ecosystems and public health.
Knowing when and where harmful algal blooms are likely to occur will lessen the risk for industry and enable effective restoration efforts. Improving our knowledge of the impacts of microalgal toxins on human health will enable safe recreational water use and give clarity on appropriate responses to algal blooms.
Filling these knowledge gaps is urgent. Changes in microalgal communities are already evident and will likely continue at an accelerating pace, with possibly irreversible knock-on effects on ecosystems and ocean-based industries.
Anne Rolton Vignier receives funding from the New Zealand Ministry of Business, Innovation and Employment.
Kirsty Smith receives funding from the New Zealand Ministry of Business, Innovation and Employment.
This article is republished from The Conversation under a Creative Commons license.
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