Searching for the unseen

New research reveals how deep ocean nutrients keep shallow reef corals nourished in the Indian Ocean. 

Tropical waters are typically low in nutrients. Yet, their shallow coral reefs are highly productive ecosystems that support immense biodiversity.

A study published today by Ph.D. candidate Veronica Radice reveals the importance of deep-water nitrogen to coral nutrition in the remote archipelago of the Maldives in the understudied central Indian Ocean.

Upwelling transports nutrients from the deep ocean to surface waters. These nutrients are a critical source of food for many marine organisms, including those within coral reef ecosystems.

As a wind-driven process, upwelling supports increased biological productivity in the lee of the atolls/islands known as the “Island Mass Effect.” In the central Indian Ocean, the atoll nation of the Maldives experiences upwelling during the dominant monsoon seasons.

At a broad scale, the effects of upwelling are tracked via remote sensing satellites. The technology can detect plumes of primary productivity (chlorophyll) at the ocean surface. This increase in chlorophyll supports local marine megafauna, including manta rays.

However, it is not possible to see the influence of upwelling on reef-building corals simply by looking at a coral reef.

The study

The deeper effects of upwelling cannot be seen by the naked eye. Employing a stable isotope analysis allowed Veronica and her team to investigate the geochemical signature of coral tissues—tracing nitrogen and carbon in coral animal hosts, their algal symbiont partners, and particulate organic matter.

Reef-building corals can depend on both autotrophy (nutrients transferred from their symbionts) and heterotrophy (feeding on resources such as particulate organic matter from the reef water column).

Nitrogen is a particularly important element that helps all living things grow, reproduce, and survive. The researchers found upwelling was the major nitrogen source for all reefs in the area.

They studied three different coral species from shallow and deep reefs of different reef exposures (i.e., oceanic reefs and reefs of the shallower central Inner Sea).

Nitrogen isotopic ratios equivalent to the isotopic signature of deep-water nitrogen were found in three species of corals from both shallow (10 m/ 33 ft depth) and deep (30 m/ 100 ft) coral reefs.

Particulate food sources were also uniform across shallow and deep reefs, providing evidence of a well-mixed water column.

Coral sampling throughout the central atolls showed a similar use of nitrogen among corals collected from reefs directly exposed to oceanic waters, as well as reefs facing the shallower Inner Sea of the central Maldives. However, carbon isotope ratios revealed variability at the species level that indicated different patterns of carbon use across the shallow to deep reef slope.

Where to now?

The “unique geological, physical, biogeochemical, and ecological features of the Indian Ocean” is a scientific theme of the 2^nd International Indian Ocean Expedition (2015-2020).

This focus represents a collaborative opportunity to explore and fill important knowledge gaps in the understanding of the central Indian Ocean, and its response to global climate change.

To further investigate the complex nutrient dynamics of this region, access to an oceanographic research vessel is critical for future sampling.

Although broad scale hydrodynamic processes are a key component of coral reef ecosystems, there is a limited understanding of these influences on shallow tropical coral reefs—especially under global climate change.

In order to understand the responses of coral reefs to shifting climate patterns it is necessary to examine how upwelling affects coral reefs, and investigate how these processes may change in the future.

Read the manuscript:

https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2435.13314

DOI: https://www.doi.org/10.1111/1365-2435.13314

Open source data in Dryad repository: https://www.doi.org/10.5061/dryad.270600g

Follow Veronica on Twitter: @deepcat17