Distance-dependent variation in the tintinnid ciliate assemblage around a tropical oceanic archipelago

Abstract

Tropical oceans are characterized by a strong thermocline that limits nutrient enrichment of surface waters, yet elevated biological productivity near oceanic islands is well documented. Given the ecological importance of tintinnid ciliates in the microbial loop, we investigated how their assemblage structure responds to a potential island‑mass effect around the Saint Peter and Saint Paul Archipelago (SPSPA) under contrasting rainfall regimes. Samples were collected at 24 sites located near and far from the archipelago during periods of maximum and minimum rainfall, using a 10 L Niskin bottle at seven depths. Tintinnid spatial distribution was influenced by distance from the SPSPA and by rainfall regime (ANOVA, p < 0.001). Densities were higher in distant areas and during maximal rainfall, while lower densities near the archipelago may reflect competition with polycystine radiolarians, which increase in abundance closer to the islands. Species richness also differed between zones (permutation test, p < 0.001), increasing with distance from the SPSPA. We recorded 46 tintinnid species overall; all occurred in distant areas, whereas only 19 were found near the archipelago. Assemblage structure varied with distance and rainfall, and shifts in dominant species in distant areas suggest changes in mineral particle availability for lorica construction.