Coral reefs are among the most productive ecosystems on the planet, with the primary producers at the base of the food chain (including corals) supporting their complex food webs. Through primary production, reef organisms covert carbon dioxide via photosynthesis into energy (organic carbon) that they and other reef organisms use for food. Although light provides the energy that fuels reef productivity, key nutrients such as nitrogen and phosphorus are also required. Paradoxically, the warm tropical oceans where coral reefs are found are generally very low in nutrients, raising questions about how these rich and diverse ecosystems can be so productive in the oceans where they live.
Charles Darwin in his 1842 Structure and Distribution of Coral Reefs first raised the question of how reefs are able to thrive in the marine equivalent of an “oasis in the desert” within nutrient barren tropical oceans. Now referred to as “Darwin’s paradox”, nearly two centuries later there still remain many questions about how reef ecosystems utilize and recycle nutrients, and more specifically how water motion (waves and tides) provide the critical nutrients that reef communities require.
Scott Reef presents an ideal case study to investigate the mechanisms by which ocean processes support reef productivity. As a largely pristine open-ocean atoll reef completely removed from the influence of land, the health and function of this important reef ecosystem is highly dependent on the region’s oceanography. Throughout our time on Falkor we hope to provide new insight into how ocean dynamics fundamentally influence the growth and health of atoll reef ecosystems, with broad relevance not only to Scott Reef but for the numerous analogous atoll reef systems that are abundant throughout the Indo-Pacific.
Over the past three days we have successfully deployed a number of instruments to characterise the circulation of the whole Scott Reef lagoon system. After initially deploying three deep ocean moorings in the lagoon and channel regions, the focus over the past two days has turned towards deploying a number instruments measuring currents and waves along the shallow reef rim using Falkor’s workboats at over 20 sites. Today we completed the deployment of a number of instruments within north Scott to complement instruments deployed in the south. This full array of instrumentation will help us understand the overall circulation of Scott Reef, and most importantly how both the shallow and deep regions of the reef are connected to the surrounding ocean.
We are now about to head into a period of intensive ship-based sampling where the science team will focus on quantifying the currents and transport of nutrients at several stations throughout Scott Reef (later we will also extend these measurements to a series of shallow reef shoals several hundred kilometres to the north). At each station we will sample for between 12 and 24 hours to resolve the tidal variability, where we will measure currents from the ship with an acoustic Doppler current profiler (ADCP) and will collect water samples at a number of depths using the CTD rosette to measure the concentration of various forms of nutrients that will later be analysed in our labs.
This data will be used to quantify the rate at which nutrients are delivered to various parts of the reef ecosystem, and ultimately the mechanisms that are responsible for the transport. Importantly, the data will be also be integrated with comprehensive reef habitat data being obtained with the ROV and acoustic mapping. This will help scientists to better understand how these rates of nutrient supply correlate with the ecological patterns that are being simultaneously observed on the seafloor.