By Aaliyah Sulaman (BSc Hons Biodiversity & Conservation Biology)
Photo credits: Sean Altern
Marine heatwaves negatively impact marine life. For example, marine heatwave events destroy coral reefs and kelp forests and can even cause mass mortality in fish populations, often leading to detrimental algal blooms.
Climate change is causing more frequent and severe marine heatwaves. Greenhouse gases trap more solar energy as the ocean absorbs more heat and the sea surface temperature rises.
What ARE a Marine heatwaves (MHW’s)
Marine heatwaves (MHWs) are events that occur over some time when the sea surface temperature becomes extremely and unusually warm by transcending a seasonally varying threshold and persist for at least five consecutive days.
RESEARCH ON THE DYNAMICS OF MARINE HEATWAVES AND UPWELLING SYSTEMS
South Africa, in particular, is rich in coral reef and kelp forest ecosystems which can negatively be impacted by marine heatwaves.
Recently, there has been progress in the studies of MHW events. Varela et al. (2018) performed an analysis of temperature trends between coastal areas and their oceanic counterparts around the world. From that study, the authors observed significant differences between regions that experience upwelling and regions that don’t experience upwelling.
On the west coast of South Africa, we find the Benguela Upwelling System (BUS) which brings cold water that is nutrient-rich to the surface. Understanding the consequences of marine heatwaves and the BUS is critical for managing the fisheries and ecosystems of the region and predicting and mitigating future warming events.
The Benguela Upwelling System (BUS)
The Benguela Upwelling System (BUS) runs along Africa’s southwest coast and is an upwelling system that is driven by wind in the ocean. This is caused by the inshore of the Benguela current causing upwelling along the coastline due to southeasterly winds.
The BUS is part of the eastern boundary upwelling systems (EBUS). These upwelling systems are extremely productive areas on the planet. The Benguela current system (BCS), Canary (CanCS), California (CalCS), and the Humboldt (HCS) are the four different EBUSs. The Lüderitz-Orange River is located off the shores of Namibia (Figure 1). It is known to be the strongest upwelling cell, separating the Benguela’s northern and southern regions. In this region (26°C) there are strong winds, excessive offshore advection, and powerful turbulent mixing.
Effects of Climate Change
Climate change is causing more frequent and severe marine heatwaves. Greenhouse gases trap more solar energy as the ocean absorbs more heat and the sea surface temperature rises.
Impacts of MHW events on marine life
It’s believed that marine heatwaves negatively impact marine life. There have been reports on marine heatwave events causing mass mortality in fish populations, often leading to detrimental algal blooms. In 2011, Western Australia suffered an intense marine heatwave (Figure 2) that affected over 2,000 km of coastline for over ten weeks. During this unexpected occurrence, seawater temperatures rose above the physiological threshold (23°C) for net growth for kelp, Ecklonia radiata. As a result, kelp in that location experienced local extinction across 100 km of their northern (warm) range. Because of the extinction of these kelp forests, there were broad changes in species distribution among seaweeds, fish, and invertebrates, leading the way toward the tropicalisation of the ecosystem.
The dynamics of marine heatwaves in the Benguela upwelling system
The BUS response to marine heat waves is uncertain. However, as mentioned above, investigators discovered significant disparities in ocean warming between upwelling and non-upwelling zones. For 92% of the upwelling systems, there were fewer instances of increased temperature at the coast compared to in the ocean. In non-upwelling areas, the rate fell to 58%. Therefore, due to upwelling, oceanic warming seems to be mitigated in the ocean.
As global warming increases, marine heatwave events are anticipated to grow more prevalent and severe. Because of the ecological richness in these areas, it is critical to understand how the BUS responds to marine heatwaves off South Africa and Namibia.
Marine heatwaves have been shown to cause irreversible damage and permanent harm to marine ecosystems. Understanding the consequences of marine heatwaves in the BUS is critical for managing the fisheries and ecosystems in South Africa and predicting and mitigating future warming events. Continuing research in this area seeks to help improve our comprehension of the fundamental causes of marine heat waves and their ecological consequences and to create solutions for adaptive management and conservation considering climate change.
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The above article is an extract from Aaliyah Sulaman Honours’ Project: The response of the Benguela Upwelling System to marine heatwaves off South Africa and Namibia
