Biological oceanography, biogeochemical cycles, and pelagic ecosystem functioning of the east central South Pacific Gyre: focus on Easter Island and Salas y Gomez Island

Abstract

ABSTRACT. The Exclusive Economic Zone of Chile defined by Easter Island and Salas y GómezIsland is in the South Pacific Sub-tropical Gyre (SPSG), putting it at the center of the most oligotrophic and biomass poor waters in the world. Only 10 biological oceanographic expeditions have entered this zone in 105 years . We review key aspects of the plankton ecosystem and biogeochemical function relevant for the understanding of and conservation planning for marine environments. Plankton production is limited by lack of dissolved inorganic fixed nitrogen, not phosphorous. Higher organic nitrogen levels might be biologically unavailable. Short-term experiments suggested iron is not limiting, yet iron still likely limits nitrogen fixation, and thus production, at longer time scales, as the presence of nitrogen-fixers is exceptionally low compared to other ocean gyres. Plankton function is dominated by the smallest unicellular organisms, picoplankton (<3 µm in diameter). The SPSG represents a center of high biodiversity for picoplankton, as well as heterotrophic organisms such as tinntinids, siphonophores, and possibly amphipods, although data for key zooplankton, such as copepods, are lacking. Many groups exhibit negative relationships between diversity and total plankton biomass. High diversity might result from dispersal from a very large metacommunity and minimal competition within functional groups. Whether an island-mass effect causes a real or apparent increase in plankton biomass around Easter Island must be confirmed by high-resolution sampling in situ. Long-term threats to the planktonic ecosystem may include climate change-enhanced ocean stratification and plastic marine debris accumulation. Finally, priorities for future research are highlighted.

Overview of recent advances in oceanographic, ecological and fisheries research on oceanic islands in the southeastern Pacific Ocean

“…In the waters near Easter and Salas y Gómez islands picophytoplankton represents only 3% of the organisms present on average, while the green cyanobacteria Prochlorococcus represents up to 27%. In this area, the abundance of diatoms and dinoflagellates (>15 µm) is lower than that of bacterioplankton (10 millon times less), that of Prochlorococcus (4 million times less), and that of picophytoeukaryotes (50-250,000 times less; Von Dassow & Collado-Fabbri, 2014). Furthermore the biomass of mesozooplankton (>200 µm) is extremely low, 10% lower than that recorded in areas of coastal upwelling and in the Humboldt Current System (Von Dassow & ColladoFabbri, 2014).…”

“…Bacterioplankton and small detritus particulates account for up to 83% of the organic carbon and small unicellular organisms, picoplankton (<3 µm), dominate the plankton in this region (Von Dassow & Collado-Fabbri, 2014). In the waters near Easter and Salas y Gómez islands picophytoplankton represents only 3% of the organisms present on average, while the green cyanobacteria Prochlorococcus represents up to 27%.…”

“…Short-term experiments have suggested iron is not limiting, yet iron still likely limits nitrogen fixation, and thus production, at longer time scales, as the presence of nitrogen-fixers is exceptionally low compared with that of other ocean gyres. Although diatoms known to contain nitrogen-fixing cyanobacteria (Rhizosolenia and Hemiaulus) appear to be well represented in the waters near Easter and Salas y Gómez islands, a diverse group of heterotrophic, nitrogen-fixing bacteria or photoheterotrophic cyanobacteria may dominate nitrogen fixation in the south Pacific subtropical gyre (Von Dassow & Collado-Fabbri, 2014). The prevalence of these nitrogen-fixing organisms could explain the region's lack of response to iron injection, given their functional differences from autotrophic fixers.…”

“…The prevalence of these nitrogen-fixing organisms could explain the region's lack of response to iron injection, given their functional differences from autotrophic fixers. Despite the low rate of nitrogen fixation, plankton communities are maintained in the euphotic zone (Von Dassow & Collado-Fabbri, 2014). …”

Synthesis of the state of knowledge about species richness of macroalgae, macroinvertebrates and fishes in coastal and oceanic waters of Easter and Salas y Gomez islands

“…This information is critical to develop science-based conservation plans. Nonetheless, the patterns of distribution of vulnerable habitats (hydrothermal vents, diversity of seamounts size) reveals gaps in conservation towards the western limit of the EEZ of Chile and in the areas surrounding Easter Island (seamounts of different sizes and depths) where important seasonal concentrations of chlorophyll are observed in comparison with Salas y Gómez Island and the oligotrophic environment characteristic of the Eastern Pacific Gyre (Andrade et al, 2014;Von Dassow & Collado-Fabbri, 2014).…”