Coral reefs support an abundance of organisms despite being surrounded by oceanic waters characterized by low nutrient levels. Over more than a century of research, scientists have debated whether life on coral reefs is self-sustaining or whether reef organisms extract nutrients from the open ocean that in turn subsidize organic production within the reef system. This dissertation focuses on one guild of coral reef consumers - fish that feed on zooplankton from the water column. Pairing two independent metrics of fish diet - gut content analysis and stable isotope analysis followed by a mass-balance mixing model - I provide direct evidence that zooplankton from the open ocean comprise a significant proportion of the diet of fish inhabiting both offshore and nearshore reefs. In a study of feeding behavior of the planktivorous fish Dascyllus flavicaudus, I document that this species feeds selectively on certain taxa within the zooplankton assemblage. Oceanic copepods (Oncaeidae and Corycaeidae) were over-represented in fish gut contents relative to their abundance in environmental zooplankton samples. Non-random feeding by D. flavicaudus resulted in a 2 to 6-fold increase in the contribution of oceanic prey to fish diet beyond that expected under random feeding. The natural spatial variability in the zooplankton assemblage on coral reefs has the potential to affect not only fish diet but fish growth. I examined the relationship between zooplankton abundance, fish feeding and fish growth using a field experiment where juvenile fish were transplanted to reef habitats spanning a range of ambient zooplankton densities. The resulting spatial patterns in fish growth support the hypothesis that spatial variability in the abundance of zooplankton prey can significantly affect fish growth. Fish transplanted to locations with turbid waters exhibited low growth rates, suggesting that changes in land use practices which alter water quality may have deleterious effects on planktivorous reef fishes and that factors such as turbidity can act to de-couple fish growth from zooplankton abundance. This dissertation provides a landscape-scale perspective of planktivorous fish as links between oceanic and coral reef food webs and highlights the effects of fish behavior and reef habitat on cross-ecosystem exchange.
Thesis or Dissertation
Scripps Institute of Oceanography, UC San Diego