Ocean heat is changing marine food webs – with far-reaching consequences for NZ fisheries and sea life

The Impact of Ocean Warming on Microalgal Communities and Ecosystems

The global ocean continues to warm at a concerning rate. Satellite measurements of average sea surface temperatures show February this year was the highest for any month in the 45-year dataset, and the warming trend continued in May.

Rising Sea Temperatures in New Zealand

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.

The Role of Microscopic Algae

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.

Each day, they take up more than a hundred million tonnes of carbon dioxide through photosynthesis. The carbon then either sinks to the bottom of the ocean as the microalgae dies off or is “fixed” in tiny animals that graze on the plants.

Already, scientists are observing a downward trend in this ocean production, leading to expanding “ocean deserts” and the depletion of beneficial microalgae in favor of harmful algal blooms.

Projected Impacts and Consequences

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.

Apart from these shifts in biomass, studies have also shown changes in the make-up of microalgae and reduced species diversity. This leads to stunted ecosystems which are less resilient to environmental changes, have lower productivity, and capture less carbon.

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 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.

Globally, the impacts of harmful algal blooms cost the blue economy more than US$8 billion per year. More than US$4 billion of this relates to human health impacts.

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.

SDGs, Targets, and Indicators

SDGs Addressed or Connected to the Issues Highlighted in the Article:

1. SDG 13: Climate Action
2. SDG 14: Life Below Water

Specific Targets Under the SDGs:

• SDG 13.1: Strengthen resilience and adaptive capacity to climate-related hazards and natural disasters
• SDG 14.2: Sustainably manage and protect marine and coastal ecosystems to avoid significant adverse impacts, including by strengthening their resilience
• SDG 14.3: Minimize and address the impacts of ocean acidification, including through enhanced scientific cooperation at all levels
• SDG 14.5: By 2020, conserve at least 10 percent of coastal and marine areas, consistent with national and international law and based on the best available scientific information

Indicators Mentioned or Implied in the Article:

• Indicator 13.1.1: Number of deaths, missing persons, and directly affected persons attributed to disasters per 100,000 population
• Indicator 14.2.1: Proportion of national exclusive economic zones managed using ecosystem-based approaches
• Indicator 14.3.1: Average marine acidity (pH) measured at agreed suite of representative sampling stations
• Indicator 14.5.1: Coverage of protected areas in relation to marine areas

Table: SDGs, Targets, and Indicators

The article addresses or is connected to SDG 13 (Climate Action) and SDG 14 (Life Below Water). Under SDG 13, the article highlights the need to strengthen resilience and adaptive capacity to climate-related hazards and natural disasters. This is relevant as the warming of the global ocean and New Zealand’s sea temperatures pose threats to marine ecosystems and fisheries. The article also mentions the projected worsening of shifts in microalgal composition due to rising ocean temperatures, indicating the need for climate action to mitigate these impacts.

Under SDG 14, the article discusses the need to sustainably manage and protect marine and coastal ecosystems to avoid significant adverse impacts. It emphasizes the importance of understanding and addressing the changes in microalgal communities, harmful algal blooms, and their effects on marine life and fisheries. The article also highlights the economic and health impacts of harmful algal blooms, emphasizing the need for effective management strategies and scientific cooperation to mitigate these impacts.

The specific targets identified based on the article’s content are Target 13.1 (strengthen resilience and adaptive capacity to climate-related hazards and natural disasters), Target 14.2 (sustainably manage and protect marine and coastal ecosystems), Target 14.3 (minimize and address the impacts of ocean acidification), and Target 14.5 (conserve coastal and marine areas).

The indicators mentioned or implied in the article that can be used to measure progress towards the identified targets are Indicator 13.1.1 (number of deaths, missing persons, and directly affected persons attributed to disasters per 100,000 population), Indicator 14.2.1 (proportion of national exclusive economic zones managed using ecosystem-based approaches), Indicator 14.3.1 (average marine acidity measured at representative sampling stations), and Indicator 14.5.1 (coverage of protected areas in relation to marine areas).

Source: theconversation.com