Newly Described Rhodolith Bed Complex Associated With Shallow Coral Reefs of Palmyra Atoll, Northern Line Islands

Abstract

Rhodolith beds are biogenic marine habitats formed by aggregations of free-living crustose coralline algae. New descriptions of rhodolith beds fill the gaps in our understanding of the global distribution and ecological significance of these understudied habitats. We provide the first characterisation of a network of rhodolith beds associated with coral reefs in the tropical central Pacific. We surveyed the shallow eastern reef flat of Palmyra Atoll to evaluate the spatial extent and biodiversity of rhodolith habitat relative to adjacent coral reefs. We mapped 15 discrete rhodolith beds that collectively covered 1.5 ha. When combined with mixed rhodolith and coral habitat that connected the beds, the total areal coverage of rhodolith habitat was ~15 ha. The benthos of rhodolith beds was markedly different from adjacent reefs. Although coral cover was low in the rhodolith beds, five genera were commonly found as free-living coralliths. Fish abundance did not vary notably between reef and bed habitats, but rhodoliths, particularly Neogoniolithon sp., supported a higher abundance of cryptic invertebrates relative to reef rubble. The dominant rhodolith genera were Neogoniolithon and Porolithon, and a third, less frequent species was Harveylithon munitum. Neogoniolithon sp. was the most abundant and displayed a more structurally complex branching morphology than the encrusting, lobe-forming Porolithon sp. and the encrusting H. munitum. Our description of a previously unknown rhodolith complex in a remote and protected coral reef ecosystem provides novel insights to our understanding of the persistence and function of undisturbed tropical rhodolith bed habitat.