Abstract
Enhancement of phytoplankton biomass near island and atoll reef ecosystems—termed the Island Mass Effect (IME)—is an ecologically important phenomenon driving marine ecosystem trophic structure and fisheries in the midst of oligotrophic tropical oceans. This study investigated the occurrence of IME at Rangiroa Atoll in the French Polynesian Tuamotu archipelago, and the physical mechanisms driving IME, through the analysis of satellite and in situ data. Comparison of chlorophyll-a concentration near Rangiroa Atoll with chlorophyll-a concentration in open ocean water 50 km offshore, over a 16-year period, showed phytoplankton enhancement as high as 130% nearshore, over 75.7% of the study period. Our statistical model examining physical drivers showed the magnitude of IME to be significantly enhanced by higher sea surface temperature (SST) and lower photosynthetically active radiation (PAR). Further, in situ measurements of water flowing through Tiputa Channel revealed outflowing lagoon water to be warmer, lower in salinity, and higher in particulate load compared to ocean water. We suggest that water inside Rangiroa’s lagoon is enriched in nutrients and organic material by biological processes and advected as a result of tidal and wave forcing to coastal ocean waters, where it fuels primary production. We suggest that a combination of oceanographic and biological mechanisms is at play driving frequency and magnitude of IME at Rangiroa Atoll. Understanding the underlying processes driving IME at Rangiroa is essential for understanding future changes caused by a warming climate and changing environmental conditions for the marine ecosystem.
Highlights
Phytoplankton production at the base of the food web provides a fundamental source of energy fueling entire marine ecosystems and production of the world’s fisheries (Duarte and Cebrian, 1996)
We found that during 75.7% of the 16-year study period, Rangiroa Atoll displayed localized nearshore enhancement in chlorophyll-a, which is the Island Mass Effect (IME), chl
We investigated the factors that influence the nearshore to offshore chlorophyll-a difference via a suite of physical predictor variables using a generalized additive model (GAM) with normal error distribution assumption
Summary
Phytoplankton production at the base of the food web provides a fundamental source of energy fueling entire marine ecosystems and production of the world’s fisheries (Duarte and Cebrian, 1996). Besides direct inputs from land, coral reef organisms can modify biogeochemical characteristics of water flowing over reefs through a multitude of biochemical mechanisms (e.g., Wyatt et al, 2010, 2012; Nelson et al, 2011), such as nitrogen fixation, nutrient regeneration, and recycling from other biota (Atkinson, 2011) and increase nutrient availability surrounding oceanic islands. Organic material and other sources of nutrients generated via coral reef ecosystem processes can be flushed out of the atoll via wave forcing (Callaghan et al, 2006) and tides (Chevalier et al, 2017) and provide nutrients to the surrounding oligotrophic waters. Many studies have presented significant spatial and temporal variations in IME, as well as diversity in key forcing mechanisms and their importance in driving IME (Dandonneau and Charpy, 1985; Charpy, 1996; Charpy et al, 1997; Martinez and Maamaatuaiahutapu, 2004; Gove et al, 2016; Sauzède et al, 2018). Gove et al (2016) analyzed chlorophyll-a measurements recorded over 10 years via satellite for 24 islands and 11 atolls in the tropical western Pacific and found the primary drivers of IME magnitude to be reef area, bathymetric slope, geomorphic type, and population status
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