Contrasting ENSO Types With Satellite‐Derived Ocean Phytoplankton Biomass in the Tropical Pacific

Observed variations in the tropical phytoplankton community structure and biogeochemical processes have been linked to the El Niño Southern Oscillation, a driver of large‐scale natural climate variability on interannual timescales. Satellite bio‐optical algorithms have allowed us to derive complex biological parameters from the surface ocean via remote sensing, providing a scientific platform to investigate biological relationships with climate indices. Studies have focused in‐depth on contrasting types of the ENSO types with various physical parameters with only a few recent studies focusing on satellite‐observed chlorophyll‐a, with however none focusing on phytoplankton biomass itself. Here we review the types of ENSO and its effect on backscattering‐based biomass using different statistical techniques, over the 1997‐2007 period. We also contrast the responses of phytoplankton biomass with those of chlorophyll‐a and their physical drivers in various types of ENSO. Signatures of various ENSO types are observed in the physical and biological fields

The contrasted phytoplankton dynamics across a frontal system in the southwestern Mediterranean Sea

Numerical simulations have shown that finescale structures such as fronts are often suitable places for the generation of vertical velocities, transporting subsurface nutrients to the euphotic zone and thus modulating phytoplankton abundance and community structure. In these structures, direct in situ estimations of the phytoplankton growth rates are rare; although difficult to obtain, they provide precious information on the ecosystem functioning. Here, we consider the case of a front separating two water masses characterized by several phytoplankton groups with different abundances in the southwestern Mediterranean Sea. In order to estimate possible differences in growth rates, we measured the phytoplankton diurnal cycle in these two water masses as identified by an adaptive and Lagrangian sampling strategy. A size-structured population model was then applied to these data to estimate the growth and loss rates for each phytoplankton group identified by flow cytometry, showing that these two population parameters are significantly different on the two sides of the front and consistent with the relative abundances. Our results introduce a general method for estimating growth rates at frontal systems, paving the way for in situ exploration of finescale biophysical interactions.</p

A very oligotrophic zone observed from space in the equatorial Pacific warm pool

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We also acknowledge Coriolis (http://www.coriolis.eu.org), AVISO (http://www.aviso.oceanobs.com/duacs), the CERSAT (http://cersat.ifremer.fr), OSCAR (http://www.oscar.noaa.gov), and RSS (http://www.ssmi.com), for sharing the freely available data we used. We are grateful to Thierry Delcroix for constructive discussions during this work We thank three anonymous reviewers for their very valuable remarks. This work was supported by CNES (Ocean Surface Topography Science Team program). F. L benefited from CNES funding. 1 ELSEVIER SCIENCE INC NEW YORK REMOTE SENS ENVIRONThe analysis of the SeaWiFS chlorophyll archive shows a quasi-persistent strip of oligotrophic waters (chl < 0.1 mg m(-3)) extending over about 20 degrees longitude in the eastern part of the equatorial Pacific warm pool. Other space-borne data sets (scatterometric wind, microwave sea surface temperature (SST), altimetric sea level, and surface currents) were used together with barrier layer thickness derived from Argo floats to investigate the variability of the oligotrophic zone and of its eastern and western boundaries, and to propose processes that could explain why surface chlorophyll is so low in this region. The eastern limit of the oligotrophic waters matches the eastern edge of the warm pool and moves zonally both at seasonal time scale and with the El Nino/La Nina phases whereas the western limit moves mostly at intraseasonal and interannual time scales. On average, about half of the surface of the zone is occupied by very oligotrophic waters (chl < 0.07 mg m(-3)) located in the eastern part. The degree of oligotrophy of the zone increases when its width is maximum during boreal fall and winter and during El Nino events. Oligotrophy in the eastern part of the warm pool most likely persists because of the lack of vertical or horizontal penetration of nutrient-rich water due to the following processes. 1/ The equatorial oligotrophic warm pool is bounded poleward by the oligotrophic subtropical gyres. 2/The deep nutrient pool prevents strong vertical nutrient inputs into the euphoric layer and the barrier layer above it potentially reduces the efficiency of mixing. 3/ During westerly wind events, mesotrophic waters in the far western basin are too distant from the oligotrophic zone to be efficient nutrient and phytoplankton sources, and become nutrient and phytoplankton depleted during their eastward advection. 4/ Nutrient-rich waters from the central basin and nutrient-poor surface waters of the warm pool do not blend because of subduction at the eastern limit of the oligotrophic zone

Basin-scale biogeochemical and ecological impacts of islands in the tropical Pacific Ocean

International audienceIn the relatively unproductive waters of the tropical ocean, islands can enhance phytoplankton biomass and create hotspots of productivity and biodiversity that sustain upper trophic levels, including fish that are crucial to the survival of islands' inhabitants. This phenomenon, termed the island mass effect 66 years ago, has been widely described. However, most studies focused on individual islands, and very few documented phytoplankton community composition. Consequently, basin-scale impacts on phytoplankton biomass, primary production and biodiversity remain largely unknown. Here we systematically identify enriched waters near islands from satellite chlorophyll concentrations (a proxy for phytoplankton biomass) to analyse the island mass effect for all tropical Pacific islands on a climatological basis. We find enrichments near 99% of islands, impacting 3% of the tropical Pacific Ocean. We quantify local and basin-scale increases in chlorophyll and primary production by contrasting island-enriched waters with nearby waters. We also reveal a significant impact on phytoplankton community structure and biodiversity that is identifiable in anomalies in the ocean colour signal. Our results suggest that, in addition to strong local biogeochemical impacts, islands may have even stronger and farther-reaching ecological impacts

The Delayed Island Mass Effect: how islands can remotely trigger blooms in the oligotrophic ocean

In oligotrophic gyres of the tropical ocean, islands can enhance phytoplankton biomass and create hotspots of productivity and biodiversity. This “Island Mass Effect” (IME) is typically identified by increased chlorophyll concentrations next to an island. Here we use a simple plankton model in a Lagrangian framework to represent an unexplained open ocean bloom, demonstrating how islands could have triggered it remotely. This new type of IME, termed “delayed IME”, occurs when nitrate is limiting, N:P ratios are low, and excess phosphate and iron remain in water masses after an initial bloom associated to a “classical” IME. Nitrogen fixers then slowly utilize leftover phosphate and iron while water masses get advected away, resulting in a bloom decoupled in time (several weeks) and space (hundreds of km) from island‐driven nutrient supply. This study suggests that the fertilizing effect of islands on phytoplankton may have been largely underestimated. Plain language summary In the poor and nutrient‐depleted waters of the tropical Pacific, islands act as sources of nutrients fertilizing nearby waters. These nutrients are consumed by microscopic photosynthesizing algae, the phytoplankton. The resulting phytoplankton enrichments (blooms) in turn support productive ecosystems. This phenomenon, termed the “island mass effect”, has been known for sixty years and is classically defined by increased chlorophyll (representing phytoplankton biomass) next to an island. Blooms also occur in the open ocean and are usually attributed to vertical processes such as mixing or uplifting that locally supply nutrients from subsurface reservoirs. In this paper, we demonstrate that a different type of island mass effect exists, where the phytoplankton response is delayed because they grow very slowly. These blooms are supported by the nitrogen fixer Trichodesmium. Since phytoplankton get carried away from islands by oceanic currents while they grow, this can lead to a bloom located hundreds of km away with no apparent connection to the islands. Nutrient inputs by islands followed by advection can thus trigger remote blooms in the open ocean. Our study suggests that the fertilizing effect of islands may currently be largely underestimated, particularly in the warm waters of the tropical Pacific where Trichodesmimum is common