An unexplained bloom in the open ocean
In March 2015, a spectacular bloom was observed from space in the western tropical South Pacific. Chlorophyll concentrations reached close to 1 mg m-3, a ten-fold increase relative to background concentrations. These types of blooms are often driven by small-scale physical processes such as mixing or uplifting that locally supply nutrients from subsurface reservoirs. Luckily, the bloom occurred during the OUTPACE oceanographic cruise and scientists were able to study it. They followed the bloom for 5 days at a long-duration station, called LD-B.
Surprisingly, physical data revealed a stratified water column with no evidence of small-scale physical activity. 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. High-phosphate waters had been advected by currents, but the origin of iron remained a mystery. Local stratification ruled out vertical transport or mixing, and atmospheric deposition was low.
The only plausible hypothesis was offered by de Verneil et al. (2017), who suggested that iron was provided by an island contact. Indeed, the bloom waters 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 (IME) does not apply. As such, whether or not the Tonga Islands triggered the bloom remains to be demonstrated.
A new type of IME: 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 IME, which we termed “delayed IME”. Delayed IMEs 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 IMEs are classically defined based on a chlorophyll increase near islands. By contrast, delayed IMEs 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 IMEs, because high resolution data are needed. We found that delayed IMEs 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 IMEs 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 demonstrate how islands could have remotely triggered the bloom.
We considered the evolution of water masses after they passed 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 IME) and a second bloom, weeks later, away from the islands (the delayed IME). In the model, the second bloom is supported by Trichodesmium. It occurs because Trichodesmium grow very slowly and bloom much later than non-diazotrophic phytolankton.
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 all blooms for the region and period of study can be explained by an island nutrient source and advection by oceanic currents. Our study thus suggests that IMEs were the primary driver of blooms in the region, and provides a proof-of-concept for the existence of the delayed IME.
Reference: 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, in press, doi:10.1029/2019GL085282 (PDF)