ICES/PICES Zooplankton Production Symposium: Ruben Escribano

​​​​Does climate change matter for zooplankton production in upwelling systems? This is the question Ruben Escribano​ asked participants on the second day of the 6th ICES/PICES Zooplankton Production Symposium.
Published: 10 May 2016

​​​​​​​The 6th ICES/PICES Zooplankton Production Symposium "New Challenges in a Changing Ocean" is taking place this week in Bergen, Norway and ICES news will feature articles from some of the keynote speakers throughout the week. Our second installment from Ruben Escribano integrates main findings, after nearly 20 years of research on population dynamics of pelagic copepods and euphausiids from the coastal upwelling zone, along with climate-induced oceanographic variation, providing insights about mechanisms and factors controlling zooplankton production and how they can be impacted by ongoing climate change.

The manifestation of climate change and global warming in the ocean is non-homogeneous for a variety of regions. Highly productive eastern boundary upwelling ecosystems (EBUE) show increased upwelling and gradual cooling in recent decades. Although this trend is unclear for the California EBUE. Excess of upwelling and extended duration of the upwelling period at mid and high latitude of EBUE regions can have a strong impact on biological processes and organisms.

In EBUEs, the ecological role of zooplankton, as the key link between primary producers and higher trophic levels, may be jeopardized by an extreme increase in upwelling. Although more upwelling may mean more nutrients input into the photic zone with a subsequent increase in primary production, additional physical and chemical consequences for zooplankton habitats may greatly affect community structure, population demography, and vital rates and metabolism of key species. Altogether,​ these effects provide the basis to understand the variability and trends in zooplankton biomass and secondary production in EBUEs.

By pumping nutrients into the photic zone and hence promoting new production, upwellling is the key process controlling biological productions of EBUEs. Upwelling intensity however varies strongly over time and space. Over the latitudinal range, the regime of upwelling follows a pattern of year-round intermittent upwelling at tropical/subtropical areas to a strongly seasonal regime at mid and high latitudes. Interannual variation is also high and mostly related to the ENSO (El Niño Southern Oscillation) cycle. Dominant zooplankton, mainly composed by copepods and euphausiids in EBUEs, have been able to adjust and couple their reproductive cycles and entire life cycles with such variability. However, rapid changes in physical and chemical conditions of the water column in recent decades may override adaptation and acclimation capacity of zooplankton to new altered conditions.

Along with cold water, upwelling rises the oxygen minimum zone and CO2-saturated water into the main habitat of copepods and euphausiids. Low oxygen (hypoxic conditions) combined with low pH reduce survival and development of young stages, and thus limiting recruitment and population biomass and production. Vertical reduction of the oxygenated layer also favors strong aggregation of zooplankton with low tolerance to hypoxia and thus enhancing biological interactions and possibly top-down effects. These combined effects on zooplankton dynamics are illustrated in the figure on the right.

Increased upwelling can also promote offshore advection of plankton, including zooplankton. Studies on mesoscale activity in upwelling regions reveal that large amounts of water along with their properties and plankton can be transported laterally by cyclonic and anticyclonic eddies. These mesoscale eddies can be more than 200 km in diameter, last for months, and transport plankton far away from the coastal zone. Although this process may be viewed as a mechanism to fuel with nutrients and plankton the oceanic regions, for the upwelling zone implies a loss a plankton biomass with a negative impact on zooplankton production.    

Studies on secondary production of zooplankton in EBUEs has not received sufficient attention as to properly address the question on whether global warming and increased upwelling are negatively impacting zooplankton. Few studies deal with biomass and production trends in recent decades and too few works attempt to establish the links between climate variability and production rates. There are certainly methodological limitations to focus on estimates of secondary production of a rather complex assemblage of metazoan organisms like zooplankton, and the C14 ​method to directly measure primary production has no parallel for secondary production. Nevertheless, recent development of automated zooplankton analyses combined with studies on size-dependent vital rates may provide the basis to overcome the taxonomic and ecological complexity of metazoan, and provide a reliable approach to estimate production of the whole community or the bulk of zooplankton biomass.        

EBUEs dynamics clearly depends on upwelling variation which in turn responds to climate variability. Zooplankton dynamics is one of the biological responses to this forcing, and therefore I strongly suggest that climate change does matter for zooplankton production. Altered production at lower trophic levels, such as that of zooplankton, can have a major consequence for productivity and functioning of the whole marine ecosystem in EBUEs. Although EBUEs represents no more than 2% of the global ocean, their contribution to fishery production, ocean-atmosphere exchange of gases, and global biogeochemical cycles, makes them as key regions for sustainability of the global marine ecosystem. 
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​The impact of global warming on zooplankton in a coastal upwelling ecosystems. Increased upwelling linked to global warming rises the oxygen minimum zone, cools down the water column, causes hypoxia, lowers the pH, and generates more offshore advection. A vertically constrained habitat also reduces the extension of diel vertical migration (DVM).  The potential effect is a reduction in zooplankton biomass and production, despite a projected increased in primary production and phytoplankton biomass.

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ICES/PICES Zooplankton Production Symposium: Ruben Escribano

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