Phytoplankton in the tropical Pacific
Phytoplankton, the tiny algae that fix carbon dioxide via photosynthesis, represent the base of the oceanic food chain and support essentially all life in the oceans. As such, their production and species composition largely drive the structure and productivity of ocean ecosystems. Phytoplankton biomass is often represented by chlorophyll concentration (Chl), which can be measured by satellite. In the tropical Pacific region, Chl is low except near continents. Being remote and relatively unproductive, these ecosystems remain understudied relative to more productive and accessible regions. However, they contribute significantly to worldwide plankton biomass because of their immense size. They also support a wide variety of pelagic species including most of the world’s tuna harvest, highlighting their economic importance. Understanding the biology of the tropical Pacific is thus very important, particularly in a context of global change where very low-productivity regions are expanding.
What is the “island mass effect”?
Even though they are often considered to be homogeneous and static, the low-Chl “ocean deserts” of the tropical Pacific sometimes exhibit spectacular blooms. Local enrichments resembling oases on land are visible from space, often associated with islands. In this satellite image of the southwest tropical Pacific, chlorophyll concentrations clearly increase near several islands. In turn, this enhanced phytoplankton production supports a rich and varied ecosystem, with increased biodiversity and biomass relative to surrounding areas. This “island mass effect” (after the original paper) is almost ubiquitous across the tropical Pacific. As a consequence, island effects are an important contributor to fisheries potential and to the welfare of islands human populations. They are also responsible for large-scale enhancements of primary production and possibly a significant fraction of the tropical Pacific phytoplankton biomass.
The processes responsible for Chl enhancements near islands are well understood. They include upwelling and mixing in lee eddies formed by flow disturbance or by wind-driven Ekman pumping, nutrient input from island runoff possibly enhanced by human activities, atoll lagoon flushing, coral reef benthic processes, and iron enrichment from the island platform. These processes release micro- (e.g., iron) and/or macro- (e.g., nitrate) nutrients in an overall nutrient-limited environment, supporting enhanced phytoplankton growth. At the regional scale, geomorphic type (atoll vs. island), bathymetric slope, reef area, and population status were found to be the primary drivers of the island effect strength in the tropical Pacific.
The island mass effect has been known for sixty years, and its mechanisms are well understood. However, there is a lack of systematic, large-scale studies. There has been no attempt to characterize the impact of islands on phytoplankton community structure at the basin-scale, despite implications for higher trophic levels and ecosystems functioning. Most previous studies focused on one island or archipelago, and those including reports of phytoplankton community composition are rare. Only two studies systematically investigated the island mass effect for a large number of islands: a 30-year old study based on Chl measured by ships of opportunity in the south tropical Pacific, and a recent analysis of satellite chlorophyll increases and potential drivers for 35 coral reef islands and atolls in the tropical Pacific. Neither study considered phytoplankton community composition. A systematic study of the island effect for the thousands of islands in the tropical Pacific, particularly characterizing biodiversity, is unprecedented.