An empirical approach to the island mass effect in the south tropical Pacific based on sea surface chlorophyll concentrations

Abstract

The island mass effect is defined here as the relationship between sea surface chlorophyll concentration (SSCC) and distance to the nearest island (DNI), assuming that SSCC is an index of biomass in the entire photic layer and that variable currents in this region result in evenness of effects around islands. The study uses about 8500 SSCC randomly sampled by ships of opportunity in the tropical Pacific from 0° to 25°S between 160°E and 130°W, and determination of DNI for each sample. At each place of the field, the mean SSCC value and the SSCC-DNI correlation and regression slope have been computed using all SSCC-DNI pairs available at a distance of <240 nmi from that place. The highest mean SSCC values are found near the equator and originate from upwelling; south of 20°S, relatively high SSCC mean values are found in the western part corresponding to winter enrichment. These large-scale climatic features give more variance than presence or absence of islands, biasing the SSCC-DNI correlation in transition zones; significantly negative correlations (i.e., SSCC increase when approaching islands) are found mainly in a large patch between 13° and 20°S, including Vanuatu, Fiji, Tonga, and Samoa islands. Since these islands have rivers and no closed lagoon, the island mass effect is supposed to result mostly from land drainage. Positive correlations for the Tuamotu Archipelago (low atolls) suggest that the dominant effect could be predation by the coral reef benthic communities. Involving opposite phenomena, island mass effects are weak in the south tropical Pacific and, on the average, SSCC generally does not increase by more than 10% in 40 mi.