Latitudinal environmental gradients and diel variability influence abundance and community structure of Chaetognatha in Red Sea coral reefs

Abstract

The Red Sea has been recognized as a unique region to study the effects of ecohydrographic gradients at a basin-wide scale. Its gradient of temperature and salinity relates to the Indian Ocean monsoon and associated wind-driven transport of fertile and plankton-rich water in winter from the Gulf of Aden into the Red Sea. Subsequent evaporation and thermohaline circulation increase the salinity and decrease water temperatures toward the North. Compared with other ocean systems, however, relatively little is known about the zooplankton biodiversity of the Red Sea and how this relates to Red Sea latitudinal gradients. Among the most abundant zooplankton taxa are Chaetognatha, which play an important role as secondary consumers in most marine food webs. Since Chaetognatha are sensitive to changes in temperature and salinity, we surmised latitudinal changes in their biodiversity, community structure and diel variability along the coast of Saudi Arabia. Samples were collected at nine coral reefs spanning approximately 1500 km, from the Gulf of Aqaba in the northern Red Sea to the Farasan Archipelago in the southern Red Sea. Thirteen Chaetognatha species belonging to two families (Sagittidae and Krohnittidae) were identified. Latitudinal environmental changes and availability of prey (i.e. Copepoda, Crustacea) altered Chaetognatha density and distribution. The cosmopolitan epiplanktonic Flaccisagitta enflata (38.1%) dominated the Chaetognatha community, and its abundance gradually decreased from South to North. Notable were two mesopelagic species (Decipisagitta decipiens and Caecosagitta macrocephala) in the near-reef surface mixed layers at some sites. This was related to wind-induced upwelling of deep water into the coral reefs providing evidence of trophic oceanic subsidies. Most Sagittidae occurred in higher abundances at night, whereas Krohnittidae were more present during the day. Chaetognatha with developing (stage II) or mature ovaries (stage III) were more active at night, demonstrating stage-specific diel vertical migration as a potential predator avoidance strategy.

Keywords:

• Chaetognatha
• coral reefs
• diel vertical migration (DVM)
• latitudinal gradients
• pelagic-benthic coupling
• Red Sea
• upwelling
• zooplankton

Acknowledgements

We acknowledge the support from U. Sommer and his working group (Geomar). We are grateful to S. Kürten (KAUST, GEOMAR) for logistical support during the field surveys, as well as to A. Al-Haj (KAU) for his help with isolation of Chaetognatha. We thank B.H. Jones and N. Zarokanellos (KAUST) for fruitful discussions about the oceanography of the Red Sea. We acknowledge the constructive comments by one anonymous reviewer, whose comments helped us to improve the manuscript.

Disclosure statement

BK and AA conceived the study. BK, ME, RD, and AA carried out fieldwork, analysed the zooplankton samples, and processed the data. KK provided taxonomic identifications of Chaetognatha. BK and KK interpreted the data and wrote the manuscript. AA provided editorial advice. The authors declare that they have no conflict of interest.

Additional information

Funding

This collaboration of the Jeddah Transect Project between King Abdulaziz University and GEOMAR Helmholtz-Centre for Ocean Research was funded by the King Abdulaziz University (KAU) Jeddah, Kingdom of Saudi Arabia, under grant No. T-065/430-DRS. During the writing phase of the project BK has been supported by a postdoctoral fellowship from King Abdullah University of Science and Technology (KAUST, Thuwal, Kingdom of Saudi Arabia).