A comparison of Eastern Boundary Upwelling Ecosystems

Abstract Like all the major upwelling regions, the Canary Current is characterised by intense mesoscale structure in the transition zone between the cool, nutrient-rich waters of the coastal upwelling regime and the warmer, oligotrophic waters of the open ocean. The Canary Island archipelago, which straddles the transition, introduces a second source of variability by perturbing the general southwestward flow of both ocean currents and Trade winds. The combined effects of the flow disturbance and the eddying and meandering of the boundary between upwelled and oceanic waters produce a complex pattern of regional variability. On the basis of historical data and a series of interdisciplinary field studies, the principal features of the region are described. These include a prominent upwelling filament originating near 28°N off the African coast, cyclonic and anti-cyclonic eddies downstream of the archipelago, and warm wake regions protected from the Trade winds by the high volcanic peaks of the islands. The filament is shown to be a recurrent feature, apparently arising from the interaction of a topographically trapped cyclonic eddy with the outer edge of the coastal upwelling zone. Its role in the transport and exchange of biogenic material, including fish larvae, is considered. Strong cyclonic eddies, observed throughout the year, drift slowly southwestward from Gran Canaria. One sampled in late summer was characterised by large vertical isopycnal displacements, apparent surface divergence and strong upwelling, producing a fourfold increase in chlorophyll concentrations over background values. Such intense eddies can be responsible for a major contribution to the vertical flux of nitrogen. The lee region of Gran Canaria is shown to be a location of strong pycnocline deformation resulting from Ekman pumping on the wind shear boundaries, which may contribute to the eddy formation process.

The transition zone of the Canary Current upwelling region

Foreword Preface 1. History of international co-operation in research Jurgen Alheit and Andrew Bakun 2. A short scientific history of the fisheries Alec D. Maccall 3. Habitats Dave Checkley, Patricia Ayon, Tim R. Baumgartner, Miguel Bernal, J. C. Coetzee, Robert Emmett, Renato Guevara, Larry Hutching, Leire Ibaibarriaga, Hideaki Nakata, Yoshioki Oozeki, Benjamin Planque, Jake Schweigert, Yorgos Stratoudakis and Carl D. Van der Lingen 4. Variability from scales in marine sediments and other historical records David B. Field, Tim R. Baumgartner, Vicente Ferreira, Dimitri Gutierrez, Hector Lozano-Montes, Renato Salvatteci and Andy Soutar 5. Decadal-scale variability in populations Jurgen Alheit, Claude Roy and Souad Kifani 6. Biophysical models Christophe Lett, Kenneth A. Rose and Bernard A. Megrey 7. Trophic dynamics Carl D. Van der Lingen, Arnaud Bertrand, Antonio Bode, Rick Brodeur, Luis Cubillos, Pepe Espinoza, Kevin Friedland, Susana Garrido, Xabier Irigoien, Todd Miller, Christian Mollman, Ruben Rodriguez Sanchez, Hiroshige Tanaka and Axel Temming 8. Impacts of fishing and climate change explored using trophic models Lynne Shannon, Marta Coll, Sergio Neira, Philippe Cury and Jean-Paul Roux 9. Current trends in the assessment and management of stocks Manuel Barange, Miguel Bernal, Maria Cristina Cergole, Luis A. Cubillos, Carryn L. Cunningham, Georgi M. Daskalov, Jose A. A. de Oliveira, Mark Dickey-Collas, Dave J. Gaughan, Kevin Hill, Larry D. Jacobson, Fritz W. Koster, Jacques Masse, Hiroshi Nishida, Miguel Niquen, Yoshioki Ooozeki, Isabella Palomera, Suzana A. Saccardo, Alberto Santojanni, Rodolfo Serra, Stylios Somarakis, Yorgos Stratoudakis, Andres Uriarte, Carl D. Van der Lingen and Akihiko Yatsu 10. Global production and economics Samuel F. Herrick, Jr., Jerrold G. Norton, Rognvaldur Hannesson, U. Rashid Sumaila, Mahfuzuddin Ahmed and Julio Pena-Torres 11. Human dimensions of the fisheries under global change Rosemary E. Ommer, Astrid C. Jarre, R. Ian Perry, Manuel Barange, Kevern Cochrane and Coleen Moloney 12.Mechanisms of low-frequency fluctuations in anchovy and sardine populations Alec D. MacCall 13. Research challenges in the twenty-first century Andrew Bakun 14. Conjectures on future climate effects on marine ecosystems dominated by small pelagic fish Pierre Freon, Francisco Werner and Francisco Chavez 15. Synthesis and perspective Dave Checkley, Andrew Bakun, Manuel A. Barange, Leonardo R. Castro, Pierre Freon, Renato Guevara-Carrasco, Samuel F. Herrick, Jr., Alec D. MacCall, Rosemary Ommer, Yoshioki Oozeki, Claude Roy, Lynne Shannon and Carl D. van der Lingen Index.

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Climate Change and Small Pelagic Fish

Seasonal coastal upwelling was analyzed along the NW African coastline (11–35°N) from 1981 to 2012. Upwelling magnitudes are calculated by wind speed indices, sea-surface temperature indices and inferred from meteorological station, sea-surface height and vertical water column transport data. A permanent annual upwelling regime is documented across 21–35°N and a seasonal regime across 12–19°N, in accordance with the climatology of previous studies. Upwelling regions were split into three zones: (1) the Mauritania–Senegalese upwelling zone (12–19°N), (2) the strong permanent annual upwelling zone (21–26°N) and (3) the weak permanent upwelling zone (26–35°N). We find compelling evidence in our various indices for the Bakun upwelling intensification hypothesis due to a significant coastal summer wind speed increase, resulting in an increase in upwelling-favorable wind speeds north of 20°N and an increase in downwelling-favorable winds south of 20°N. The North Atlantic Oscillation plays a leading role in modifying interannual variability during the other seasons (autumn–spring), with its influence dominating in winter. The East Atlantic pattern shows a strong correlation with upwelling during spring, while El Nino Southern Oscillation and Atlantic Multi-decadal Oscillation teleconnections were not found. A disagreement between observationally-based wind speed products and reanalysis-derived data is explored. A modification to the Bakun upwelling intensification hypothesis for NW Africa is presented, which accounts for the latitudinal divide in summer wind regimes

Spatial and temporal seasonal trends in coastal upwelling off Northwest Africa, 1981-2012

The IPCC AR5 provided an overview of the likely effects of climate change on Eastern Boundary Upwelling Systems (EBUS), stimulating increased interest in research examining the issue. We use these recent studies to develop a new synthesis describing climate change impacts on EBUS. We find that model and observational data suggest coastal upwelling-favorable winds in poleward portions of EBUS have intensified and will continue to do so in the future. Although evidence is weak in data that are presently available, future projections show that this pattern might be driven by changes in the positioning of the oceanic high-pressure systems rather than by deepening of the continental low-pressure systems, as previously proposed. There is low confidence regarding the future effects of climate change on coastal temperatures and biogeochemistry due to uncertainty in the countervailing responses to increasing upwelling and coastal warming, the latter of which could increase thermal stratification and render upwelling less effective in lifting nutrient-rich deep waters into the photic zone. Although predictions of ecosystem responses are uncertain, EBUS experience considerable natural variability and may be inherently resilient. However, multi-trophic level, end-to-end (i.e., "winds to whales") studies are needed to resolve the resilience of EBUS to climate change, especially their response to long-term trends or extremes that exceed pre-industrial ranges.

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Under Pressure: Climate Change, Upwelling, and Eastern Boundary Upwelling Ecosystems

http://digital.csic.es/bitstream/10261/49073/1/CanC_review_PiO2009.pdf

Sub-regional ecosystem variability in the Canary Current upwelling

Fisheries constitute an important economic sector for Morocco, where the species Sardina pilchardus represents the main landings. In acoustic evaluations conducted along the Moroccan coast since 1995, the absence of juveniles in 1996 and 1998 and the collapse of the sardine stock between 1996 and 1997 represent the main events until 2002. Sardines are known to be microphageous planktivores and thus are sensitive to environmental variability. A biogeochemical model coupled to a hydrodynamic model (ROMS) was run over the Canary Current System (1991-2002) to investigate the environmental factors that could have played a role in the variability of the sardine spawning. A grid refinement (1/12 degrees) centred on the Saharan Bank (SB) region was built to study the main spawning ground of sardines off northwest Africa. The volume of the potential spawning habitat (PSH) of sardines was defined as a function of depth, temperature and salinity, which are included in the ranges 0-200 m, 15-21 degrees C and 35.8-36.8, respectively. Our modelling frame was able to reproduce the seasonal cycle of temperature, phytoplankton concentration and PSH over the SB. It also captured the warming associated with the negative index of the North Atlantic Oscillation of 1995-1997 and allowed a description of the inter-annual variability of the PSH. Our experiment shows that the volume of PSH was much reduced in 1996 compared to the other years. The results suggest that the delay between high abundance of plankton and favourable spawning conditions is a good proxy for determining the recruitment failure of sardine in this oceanic region.

Environmental control of the recruitment of sardines (Sardina pilchardus) over the western Saharan shelf between 1995 and 2002: a coupled physical ⁄ biogeochemical modelling experiment

The three eastern boundary ecosystems comparable to the Benguela ecosystem (BCE) display differences and commonalities. The California (CalCE) and Humboldt Current (HCE) ecosystems are continuous topographically, whereas the Canary Current ecosystem (CanCE) is interrupted by the Gulf of Cadiz and the Canaries archipelago. All have similar regimes of equatorward flow over shelf and slope associated with upwelling and a subsurface poleward flow over the slope, though in the HCE multiple flows and counter-flows appear offshore. All systems exhibit year round upwelling in their centre and seasonal upwelling at their extremes as the trade wind systems that drive them migrate north and south, though the HCE is strongly skewed toward the equator. All systems vary on scales from the event or synoptic scale of a few days, through seasonal, to inter-decadal and long term. Productivity of each system follows the upwelling cycle, though intra-regional variations in nutrient content and forcing cause significant variability within regions. The CanCE is relatively unproductive compared to the CalCE and HCE as a result of differences in large scale circulation between the Pacific and Atlantic. The latter two systems are dominated by El Nino-Southern Oscillation (ENSO) variability on a scale of 4-7 years. Physical modeling with the Princeton Ocean Model and the Regional Oceanic Modeling System has advanced recently to the stage of reproducing realistic mesoscale features and energy levels with climatic wind forcing. Operational forecasting by these models with assimilation of sea surface temperature and other data is successfully implemented in CalCE. On longer time scales, the Lamont-Doherty Earth Observatory model is able to hindcast El Nino variability over the long term up to 2 years in advance. Empirical ecological models in all three systems have attempted prediction of permissible catch level (fractional Maximum Sustainable Yield), recruitment, catches or onset of migration with lack of continued success, partly because discontinuous or inadequate observations hamper model implementation and assessment. Moreover, empirical models tuned to particular environments fail when fundamental regime shifts occur. One of the most successful approaches is that of intensive monitoring of catch and environmental parameters linked to an informal Operational Management Procedure (OMP) to inform fisheries management off Peru. This OMP contributed to preservation of anchovy stock during the 1997-8 El Nino but remains to be formalized or tested under varying conditions. Prediction on time scales of global warming are uncertain because physical climate models still disagree on whether upwelling will intensify or weaken. Possible scenarios on decadal scale based on warming or cooling of waters in the Eastern Boundary Current systems can be proffered, albeit with little confidence at present. Future approaches for all systems, including the BCE, will in the long run likely combine coupled atmospheric/ocean models with biological process models. Judicious application of purely statistical modeling based on inherent time series properties will assist, though such techniques are unable to cope with regime shifts.

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Souad Kifani

Papers

Sub-regional ecosystem variability in the Canary Current upwelling

Environmental control of the recruitment of sardines (Sardina pilchardus) over the western Saharan shelf between 1995 and 2002: a coupled physical ⁄ biogeochemical modelling experiment

Modelling, forecasting and scenarios in comparable upwelling ecosystems: Californie, Canary and Humboldt.