What are the impacts of marine heat waves?

Diverse marine heat waves, distinct marine life responses

Marine heat waves (MHW) are localised, persistent, and anomalously warm seawater temperature events usually detected at the sea surface level. While being the product of both long-term and short-lived phenomena, MHW are known to affect the natural dynamics of ocean ecosystems. In fact, MHW can pose a serious threat to marine life, impacting marine biodiversity, as various species can respond negatively to their acute and/or prolonged exposure to extreme seawater temperatures. 

This sensitivity is quite species-specific since each marine species has different tolerance thresholds and different reaction mechanisms, depending on its mobility, physiology, or age. On the other hand, even if a given species is not directly impacted by the exposure to high seawater temperatures, it may still be indirectly impacted due to its dependence on the more sensitive species that are part of the lower levels of its food chain. The exposure to the different combinations of MHW characteristics — such as duration, abruptness, magnitude, or the induced heterogeneity of the environment — is also decisive in predicting the impact on the local marine wildlife. For example, during atmospheric-driven superficial MHW, some species may dive deeper toward colder water levels. These aspects are even more pressing, as MHW are becoming more recurrent, longer, and intense.

In the past twenty years, several cases of MHW have been associated with strong impacts on specific species or marine ecosystems. One of the best-studied cases has been the Northeast Pacific Blob, which occurred between 2013 and 2015, with sea surface temperature (SST) anomalies of up to 6 °C relative to 2002–2012 (Gentemann et al., 2017). This event triggered severe consequences for marine life, causing massive die-offs of marine birds, higher mortality of sea lions, reduced stocks of many commercially valuable fish, pronounced shifts in the zooplankton community structure (Cavole et al., 2016), and impacts on the net primary production (Long et al., 2021). Coral reef bleaching, due to warmer seawater conditions during some El Niño events, is another well-known example that also illustrates the effect of MHW on a whole ecosystem.

CAREHeat, a project to help us to better understand MHW and their impacts

As MHW are known to have significant impacts on biological systems, from shifts in species ranges to biodiversity losses, as well as impacts on ecosystem services and human ocean-based economic activities, one of the CAREHeat project objectives is to evaluate the impact of MHW on marine ecosystems, ocean biogeochemistry, and the overall ocean ecosystem’s health. In addition, the project aims to explore how these events impact specific users and sectors, such as aquaculture, fisheries, and marine protected areas.

One of the scientific objectives of CAREHeat is to better understand the effects of MHW on nutrients, oxygen, and pH, and on the carbon dioxide exchanges between the ocean and the atmosphere. Indeed, organisms can also be affected by the indirect effects of MHW, such as nutrient availability, oxygen levels, and energy and carbon flow. For example, the reactions at the first trophic levels (phyto- and zooplankton) triggered by an MHW affect carbon fluxes, which can lead to sequential impacts, up to the apex predators. 

To this effect, several scientific and impact assessment case studies will be carried out by the CAREHeat team, leveraging on the novel algorithms to be developed and the corresponding data products, like the MHW Global Atlas and the MHW4D.

One of the scientific case studies will delve into MHW characterization, with particular attention to the importance of atmospheric fluxes and horizontal advection in triggering specific events. A second case study will focus on the impacts on the ecosystems, namely in key selected levels of the trophic chain (phytoplankton, micronekton — small animals feeding the higher levels of the trophic chain — and apex predators). A third scientific case study will examine the impact of compound events on marine productivity and biomass. 

Furthermore, in the scope of the impact assessment case studies, CAREHeat will work with diverse types of ocean-based sectors and users. An end-user case study will focus on fisheries of tropical wild tunas, in the Pacific Ocean in collaboration with the Pacific Community (SPC). Another case will evaluate the MHW impacts on offshore aquaculture in collaboration with Seaweed Solutions AS (kelp) and Marismar Lda (seabream). The third case study will examine the MHW impacts on coral, sea urchins, and marine birds in the Mediterranean Pelagie Islands Marine Protected Area.

MHW Impact Assessment Case Studies to be developed in the scope of CAREHeat. 

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