PeerJ Reviewer Match - Islands impact reef zooplankton isotopes

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Amino acid compound-specific isotope analysis reveals island mass effect subsidies in reef-associated Hawaiian zooplankton

Abstract

The Island Mass Effect (IME) is the nearshore enhancement of primary productivity around islands and atolls relative to offshore waters. Although its physical and biogeochemical drivers are well characterized, the IME’s influence on the diets and distributions of consumers remains poorly resolved. We applied amino acid compound-specific stable isotope analysis (AA-CSIA) to Hawaiian zooplankton sampled across nearshore–offshore and surface–deep gradients to test whether island-derived production alters isotopic composition and trophic structure in reef-associated assemblages relative to offshore counterparts across sites, seasons, and years. Essential amino acid δ ¹³ C values (δ ¹³ C_EAA) normalized to their mean values displayed contrasting nearshore–offshore patterns: lysine and threonine δ ¹³ C values increased with distance from shore, whereas phenylalanine and valine values decreased. These patterns likely reflect shifts in zooplankton diet and the amino acid biosynthetic pathways of their primary producer prey along the coastal–oceanic gradient. Source amino acid δ ¹⁵ N values (δ ¹⁵ N_SAA) declined offshore for lysine and phenylalanine but increased with depth, indicating spatial variation in nitrogen sources and greater reliance on microbially reworked organic matter at depth. Trophic position estimates based on δ ¹⁵ N values of glutamic acid and alanine relative to phenylalanine increased offshore and with depth, consistent with longer food webs and additional microzooplankton trophic steps in offshore waters. Multivariate analysis integrating δ ¹³ C_EAA and δ ¹⁵ N_SAA values clearly distinguished reef, offshore surface, and offshore deep zooplankton assemblages, revealing a conservative isotopic tracer of island-derived production in reef communities. These results demonstrate AA-CSIA’s utility for tracing island-derived productivity to consumers and clarifying biogeochemical connectivity between coastal and open-ocean food webs.

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