An unexplained bloom in the open ocean
In March 2015, a spectacular bloom was observed from space in the open waters of the western tropical South Pacific. These types of blooms are often driven by local small-scale physical processes such as mixing or uplifting, that supply nutrients from subsurface reservoirs.
Luckily, the bloom occurred during the OUTPACE oceanographic cruise and scientists were able to study it. The ship followed the bloom for five days at a long-duration station, called LD-B. Surprisingly, physical data revealed a stratified water column with no evidence of small-scale physical processes. At the same time, shipboard measurements unveiled very high nitrogen fixation rates, mostly supported by the diazotroph Trichodesmium. These organisms need phosphate and iron to grow. Oceanic currents had advected high-phosphate waters from the east, but the origin of iron remained a mystery. The lack of physical processes ruled out vertical supply from subsurface reservoirs, and atmospheric deposition was low.
A possible island mass effect?
A study by de Verneil et al. (2017) offered the only plausible hypothesis, that iron was provided by an island contact. Indeed, the bloom water masses passed near the Tonga Islands a few weeks before the bloom. However, the LD-B bloom is clearly disconnected from the Tonga Islands so the classical definition of an island mass effect does not apply. Whether or not the Tonga Islands triggered the bloom remained to be demonstrated.
The delayed island mass effect
In a study published in Geophysical Research Letters, we proposed that the LD-B bloom is an undescribed type of island mass effect, which we termed “delayed island mass effect”. These occur when phytoplankton respond so slowly to island fertilization that the bloom becomes separated from the islands as water masses are transported away by oceanic currents. In our study, we used a simple plankton model coupled with surface currents to demonstrate how the Tonga Islands could have indeed remotely triggered the bloom.
This has important implications because island mass effects are classically defined based on a chlorophyll increase near islands. By contrast, delayed island mass effects are open-ocean blooms and could mistakenly be attributed to small-scale local physical processes. As such, island effects on phytoplankton biomass and productivity may have been largely underestimated.
It is difficult to assess the prevalence of delayed island mass effects, because high resolution data are needed. We found that delayed island mass effects may be common near the Tonga Islands, particularly in austral summer. Generally speaking, they may occur when conditions support diazotrophy (warm temperatures and stratified waters) in the presence of islands supplying iron and/or phosphate. Regardless of their frequency, delayed island mass effects can be responsible for unusually strong phytoplankton blooms in a largely oligotrophic environment.
A simple “growth-advection” model
Here are more details on how we used a simple model to represent the LD-B bloom and to demonstrate how islands could have remotely triggered the bloom.
Plankton along current trajectories
We considered the evolution of water masses after they were fertilized by the Tonga Islands. To do so, we used a simple plankton model to represent the evolution of plankton communities over time after an island-driven input of nutrients. We then mapped the result over space using current trajectories. This “growth-advection” model represents a first bloom near the islands (the classical island mass effect) and a second bloom, weeks later, away from the islands (the delayed island mass effect). In the model, and as observed at LD-B, the second bloom is supported by Trichodesmium. It occurs because Trichodesmium grow very slowly and bloom much later than other phytoplankton.
Comparing model and observations
We then ran the model over several months and combined trajectories into maps. We found that the model was able to correctly represent chlorophyll as measured by satellite in the region, including the LD-B bloom. This means that an island nutrient source and oceanic advection are sufficient to explain all blooms for the region and period of study. Our study thus suggests that island mass effects were the primary driver of blooms in the region, and provides a proof-of-concept for the existence of the delayed island mass effect.
Messié, M., A. Petrenko, A.M. Doglioli, C. Aldebert, E. Martinez, G. Koenig, S. Bonnet and T. Moutin, 2020. The delayed island mass effect: How islands can remotely trigger blooms in the oligotrophic ocean. Geophysical Research Letters, 47(2), e2019GL085282, doi:10.1029/2019GL085282 (PDF)