Eastern boundary upwelling systems are traditionally explained through the mechanism of along-shore winds. In many coastal regions, prevailing winds blow parallel to the coastline and toward the equator, effectively pushing the warm surface waters offshore. This displacement facilitates the upward movement of cold, nutrient-dense waters from the deep ocean, fueling prolific phytoplankton blooms that underpin the entire marine food web. In this classic picture, the pulse of coastal productivity corresponds closely with wind patterns and intensity.

However, recent observations highlight that this framework falls short of explaining the entire picture in the Benguela upwelling system, located off the coast of Angola and Namibia. Strikingly, seasonal upwelling events are recorded even when winds are conspicuously weak or absent. It is this anomaly that sparked the latest scientific expedition aboard the research vessel METEOR, which departed from Las Palmas with the mission to investigate the unique oceanographic forces at play and their broader climatological implications.

The crux of this enigma lies in the dynamics of coastal Kelvin waves, which are an essential yet often underappreciated component of ocean circulation along eastern boundaries. These waves originate from wind fluctuations near the equator and propagate poleward, traveling thousands of kilometers along the continental margins. Unlike the surface phenomena evident in wind-driven upwelling, Kelvin waves modulate deeper ocean currents and can cause vertical displacements in water masses without significantly altering the sea surface height. This subtle modulation affects the vertical transport of cold, nutrient-rich waters to the surface, contributing to upwelling even in near windless conditions.

Complementing these wave-induced effects is the role of vertical mixing, primarily driven by tidal forces. Turbulence within the upper ocean layers enhances the exchange between colder deep waters and the surface, providing an alternate pathway for sustaining upwelling processes independently from surface wind stress. The interplay of these forces—wave propagation and tidal mixing—is a major focus for Dr. Marcus Dengler and his team aboard METEOR during the M217 “BOCABENO” expedition, which aims to dissect these complex physical oceanographic interactions.

Further compounding the scientific intrigue is the phenomenon known locally as Benguela Niños—periodic marine heatwaves characterized by abrupt sea surface temperature anomalies up to 3°C above average. These thermal events have profound regional impacts, triggering flooding across Angola and Namibia, increasing precipitation in the typically arid Namib Desert, and disrupting fragile marine ecosystems built on the upwelling productivity. Understanding the triggers and progression of Benguela Niños is crucial for predicting their recurrence and mitigating their detrimental ecological and socio-economic consequences.

One of the most significant recent Benguela Niño events occurred in 2021, exhibiting unusual timing by rising late in the upwelling season and notably suppressing phytoplankton proliferation. This disruption cascaded through the food web, resulting in a marked reduction in fish stocks dependent on primary productivity. The genesis of these heatwaves remains contested, with theories proposing influences ranging from the triggering coastal Kelvin wave impulses arriving from equatorial zones, alterations in regional wind regimes, to variations in freshwater discharge, especially from the Congo River basin. The incomplete understanding of these mechanisms underscores the urgency and relevance of ongoing investigations.

Methodologically, the BOCABENO expedition employs a multi-pronged approach to collect high-resolution, multidisciplinary data. Instruments moored to the ocean floor provide continuous time series of physical parameters such as currents, temperature, salinity, pressure, and oxygen content extending down to depths of around 1,200 meters. These data are invaluable for capturing temporal dynamics spanning months and years, facilitating the disentanglement of regular seasonal patterns versus anomalous events.

In concert with fixed moorings, vertical profiling using CTD rosette casts complements the dataset with high-resolution snapshots of the water column at strategically chosen stations. This technique enables meticulous measurement of temperature and salinity gradients alongside oxygen levels, while allowing direct collection of water samples for detailed biochemical analyses—nutrients, dissolved gases, and biological components—which collectively illuminate the ecosystem’s health and functionality.

Adding to the expedition’s technical arsenal are specialized turbulence sensors deployed to quantify small-scale mixing processes in situ. These devices capture the intensity and distribution of turbulent energy dissipation, which governs the upward flux of cold deep water that is vital for sustaining the upwelling phenomenon, especially under low-wind conditions. By integrating these datasets, researchers aim to unravel the complex synergy between physical ocean dynamics and biological productivity.

The expedition’s trajectory traces a vital corridor in the tropical Atlantic, departing from the port of Las Palmas and concluding in Walvis Bay, Namibia. Over nearly a month of data collection, the researchers focus on the continental slope areas off Angola and Namibia, regions particularly sensitive to the confluence of atmospheric, oceanic, and terrestrial forces shaping the Benguela system. This comprehensive survey offers new opportunities to validate numerical models and enhance predictive capabilities for climate-driven changes in upwelling intensity and marine heatwave occurrences.

As climate change accelerates, with anticipated alterations in wind patterns, ocean stratification, and freshwater inputs, understanding the nuanced mechanisms governing the Benguela system is more critical than ever. Insights gained from this expedition could not only shed light on regional climate impacts but also inform broader discussions about resilience and adaptation of marine ecosystems highly dependent on upwelling processes worldwide.

Ultimately, the METEOR’s journey represents a significant stride toward deciphering one of the ocean’s most compelling puzzles—how a system seemingly defying classical principles maintains its productivity and how its future trajectory might unfold in the face of a rapidly changing global climate.

Subject of Research: Tropical Atlantic upwelling systems and Benguela Niños marine heatwaves

Article Title: Unraveling the Enigma of Wind-Independent Upwelling and Benguela Niños Off Southwest Africa

News Publication Date: Not provided

Web References: Not provided

References: Not provided

Image Credits: Photo by Philipp Henning, GEOMAR

Keywords

Ocean circulation, Ocean currents, Gyres, Ocean temperature, Ocean waves, Tides, Oceans, Ocean physics, Climate variability, El Nino, Seasonal changes

Understanding the quality of juvenile habitats has long been a challenge for marine scientists and resource managers alike. While many studies have assessed habitat use by quantifying juvenile abundance, this metric alone falls short of capturing the true ecological value of these environments. Abundance measures may indicate presence, but they do not necessarily reflect growth conditions, survival probabilities, or contributions to future adult stocks, critical components in evaluating habitat quality with direct implications for fishery sustainability.

In a groundbreaking comprehensive review published in Biological Reviews, researchers from the International Council for the Exploration of the Sea (ICES) Working Group on the Value of Coastal Habitats for Exploited Species (WGVHES) thoroughly evaluated nearly 900 peer-reviewed studies spanning five decades. Their aim was to critically examine the methodologies employed to assess juvenile habitat quality across marine and estuarine systems worldwide. By synthesizing this vast body of literature, the study exposes significant gaps and biases in current research, paving the way for a more robust and multidimensional approach to habitat evaluation.

The authors revealed an overwhelming reliance on juvenile abundance as the primary indicator of habitat value, with approximately 85% of studies utilizing this singular metric. Common techniques include netting surveys and diver-based visual counts, effective for estimating presence but less informative in quantifying ecological success or habitat functionality. This narrow focus neglects essential biological parameters such as growth rates and survival, which provide a deeper understanding of habitat suitability and the resilience of juvenile populations.

Compounding the issue, while about half the studies incorporate growth-related measurements, a mere 16% have evaluated juvenile survival rates. Survival is paramount because it directly influences recruitment—the transfer of juveniles into the adult breeding population—yet it remains inadequately measured due to logistical and methodological challenges inherent in tracking marine organisms during early life stages.

Even more strikingly, fewer than 10% of all reviewed studies attempted to quantify how juvenile habitats contribute to adult population sizes, widely regarded as the gold standard metric for assessing juvenile habitat quality. Measuring this contribution is technically complex but invaluable for elucidating the linkage between habitat conditions and fishery productivity. The scarcity of such integrative approaches results in a fragmented understanding, hindering effective policy-making and habitat management tailored to sustain viable fish stocks.

The study’s authors emphasize that this research imbalance has profound practical consequences: management decisions are frequently based on incomplete or indirect indicators, misinforming conservation actions and potentially undermining efforts to maintain sustainable fisheries. The disconnect between scientific measurements and policy needs calls for an urgent recalibration of research priorities and methodologies to align ecological insights with resource management.

Dr. Benjamin Ciotti, Associate Professor of Marine Biology at the University of Plymouth and lead author of this influential review, underscores the complexity inherent in measuring juvenile habitat quality. He notes that while existing studies have mostly tackled easier-to-measure parameters such as abundance, there is a clear and growing recognition of the necessity to incorporate integrative, biologically grounded metrics that capture survival, growth, and reproductive contribution. These efforts, although challenging, represent critical steps toward building a scientific framework capable of informing effective conservation and fishery sustainability strategies.

The review also advances a call for spatially and temporally broader studies that better capture the dynamic nature of juvenile habitat use. Shifts in habitat occupancy occur across multiple time scales—from tidal cycles and seasonal variations to longer-term climate-driven changes—requiring monitoring programs that reflect these complexities. By expanding spatial-temporal coverage, scientists can better detect habitat shifts and identify essential fish habitats in a changing marine environment.

Moreover, the researchers advocate for employing a multifaceted assessment strategy that integrates various biological and environmental indicators rather than relying on single-metric approaches. Emerging technologies provide unprecedented opportunities to fill critical data gaps. Tools such as electronic tagging, telemetry, stable isotope analysis, and molecular techniques enable detailed tracking of juvenile fish movements, growth patterns, and habitat connectivity, offering transformative insights into ecological processes that govern juvenile survival and settlement.

The review further highlights the importance of recognizing ontogenetic habitat shifts—the changes in habitat use associated with different developmental stages of juvenile fish and invertebrates. Early life phases often involve transitions across multiple habitats as the organisms grow and adapt to varying conditions. Ignoring these shifts risks mischaracterizing habitat quality and underestimating the complexity of coastal ecosystems that support species at vulnerable life stages.

Collectively, the study lays a critical foundation for a new generation of monitoring and research approaches that move beyond traditional abundance metrics. By emphasizing survival, growth trajectories, and direct contributions to adult populations, it provides a biologically rigorous lens through which habitat quality can be reassessed. This paradigm shift is essential to identify, protect, and restore the habitats that truly underpin marine fisheries and biodiversity.

Looking ahead, Dr. Ciotti stresses the promise held by rapidly advancing technologies. Cameras, electronic tags, molecular markers, and telemetry devices are poised to revolutionize our capacity to decode the intricate ways juvenile fishes interact with their inshore habitats. Harnessing these tools will generate high-resolution data critical for developing robust management frameworks. Ultimately, this research pathway aims to balance conservation goals with the sustainable exploitation of marine resources, ensuring that juvenile habitats receive the scientific attention and protective measures they urgently require.

In conclusion, with fisheries worldwide under mounting pressure, prioritizing juvenile habitat quality represents a vital frontier for marine biology and ecosystem management. The meticulous review by ICES WGVHES sets the stage for enhanced interdisciplinary collaboration and technological innovation, vital for advancing our understanding and stewardship of coastal ecosystems. Protecting juvenile habitats not only preserves biodiversity but directly supports food security and the livelihoods of millions reliant on the ocean’s bounty.

Subject of Research: Animals
Article Title: Measuring juvenile habitat quality for fishes and invertebrates
News Publication Date: 30-Jul-2025
Web References: 10.1111/brv.70050
Image Credits: University of Plymouth
Keywords: Juvenile habitat quality, sustainable fisheries, coastal habitats, fish survival, habitat assessment, marine ecology, ecological metrics, telemetry, habitat connectivity, ontogenetic shifts, fish growth, fisheries management