Abstract
The input of new nutrients by upwelling in shelf waters, and the associated carbon and nitrogen fluxes, can be traced by their stable isotope signatures in organic matter. Here, we analyze the relationships between upwelling intensity and natural abundance of stable carbon and nitrogen isotopes in seston, sedimented particles, and four plankton size fractions (200 to 5000 µm) sampled monthly during 2010 and 2011 in a seasonal upwelling ecosystem. Upwelling modified the seasonal warming stratification by introducing cold and nutrient-rich waters in subsurface layers, enhancing chlorophyll-a and diatom abundance. Seston and sedimented particles were enriched in heavy nitrogen (but not carbon) isotopes linearly with upwelling intensity, indicating a primary effect of upwelling on phytoplankton production. In contrast, all plankton fractions were enriched in heavy carbon isotopes with upwelling, mainly due to the consumption of diatoms. These results confirm the differential effect of upwelling on nitrogen and carbon fluxes in the plankton food web. Direct effects of the new nitrogen inputs on phytoplankton are less evident with the increase of plankton size as nitrogen is repeatedly recycled, while the enriched carbon of plankton suggests the consumption of diatoms during upwelling. We provide linear equations to assess the influence of changes in upwelling intensity on nitrogen and carbon fluxes in seston and plankton in this ecosystem, as well as to estimate reference baseline values for food web studies.
Highlights
Coastal upwelling ecosystems are characterized by higher biological production than ecosystems at similar latitudes but with seasonal variability in nutrient inputs [1,2]
The transfer of nutrients up the food web is assumed to be highly efficient in these systems because of the repetition of upwelling events and the presence of dense plankton populations leading to large populations of fish and other consumers [2]
The similarity in the magnitude and variation of δ15 N in seston and sediment trap particles observed in this study suggests the rapid and complete uptake of the nitrate provided by the upwelling with a minimal process in the water column, as found in other studies of coastal upwelling ecosystems [41]
Summary
Coastal upwelling ecosystems are characterized by higher biological production than ecosystems at similar latitudes but with seasonal variability in nutrient inputs [1,2]. High production events of days to weeks occur through a favorable upwelling season, alternated with nutrient regeneration periods [3,4]. In these ecosystems, upwelling is generally the primary source of new (external) nutrients fueling primary production, but this can be influenced by inputs from continental origins, near large rivers [5]. The transfer of nutrients (mainly C and N) up the food web is assumed to be highly efficient in these systems because of the repetition of upwelling events and the presence of dense plankton populations leading to large populations of fish and other consumers [2]. Upwelling nutrient inputs in the photic layer produce a primary response of phytoplankton growth (blooming of diatoms in a few days) closely followed by increases in bacteria and protozoans (in a few days), and later (in a few weeks) by increases in metazoan zooplankton [6].
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