Galápagos: we’ve found out why the islands are blessed with such nutrient-rich waters
Every part of the Galápagos’s exceptional and distinctive ecosystem can be traced back to its rich reserves of marine algae. Some animals feed on the microscopic plants directly, others, in turn, feast on them, and so on. Many unique species found only on the Pacific archipelago such as the famous marine iguanas or flightless cormorants, ultimately get their food from this algae.
The abundance of algae – technically microscopic plants known as phytoplankton – is a result of a pool of unusually cold water that is often found to the west of the islands. This cold pool is a result of an upwelling of nutrient-rich deep ocean waters, which is weakest during the hot wet season (December to May) and strongest during the dry Garúa season (May to November).
Scientists have speculated for decades about what drives this Galápagos upwelling and, in the absence of conclusive evidence, some have inferred it is driven by an eastward-flowing current colliding with the islands.
But the key to unlocking the mystery of what causes the upwelling lies in its strong seasonality. First, we found that the coldness of the water to the west of the islands is connected to the strength of local northward winds. This is in marked contrast to the weaker upwelling that occurs throughout the wider equatorial Pacific Ocean, which is sustained by the strength of the prevailing westward winds.
But how exactly do these northward winds drive strong localised upwelling around the Galápagos? We recently explored this question for a study now published in Scientific Reports, in which we used a realistic, high-resolution computer model of ocean circulation in the region. We wanted the model to focus specifically on the effects of local wind strength, excluding as far as possible larger scale variables. This meant we modelled the ocean in its typical annual-mean state for factors like temperature, salinity and water velocity, and then “forced” it with six-hourly changes in atmospheric wind, radiation, precipitation and evaporation based on real-world observations.
To our surprise, this much simplified model was capable of closely reproducing the actual seasonal cycle of the Galápagos cold pool. Close analysis then pinpointed intense turbulent mixing in the ocean as the precise cause of the upwelling. What appears to be happening, to the west of the islands, is northward winds are blowing on so-called upper-ocean fronts – these are bands of abrupt lateral changes in seawater temperature, akin to but much smaller than atmospheric fronts in weather maps. When the wind hits the fronts, this mixes the warm surface water with cooler waters below, and provokes further circulation below the surface which draws still colder water up from the depths of the ocean.
The cold-pool upwelling is highly productive, since more nutrients mean more phytoplankton which means more fish, and so on. The reproductive success of the Galápagos fur seal, Galápagos penguin, flightless cormorant and many other endemic species, is highly dependent upon this upwelling. The seasonal presence of endangered filter-feeding whale sharks in the area is likely also related to these processes. Furthermore, Ecuador’s industrial tuna fleet, one of the largest in the world, concentrates on this region, as does the semi-industrial mainland-based longline fleet.
We then played with the exact location of the islands and their shape within our model. This confirmed that the Galápagos archipelago is almost perfectly positioned to maximise the strength of the wind-generated mixing. Without the upwelling generated by the mixing, phytoplankton growth around the islands would be closer to the more modest levels found much further west in the Pacific. And if this was the case, it would be much harder for the Galápagos to sustain its unique wealth of endemic species.
Our findings demonstrate that Galápagos upwelling is at the very least likely to be strongly influenced by highly localised interactions between the atmosphere and the ocean. This new knowledge will inform plans to expand the archipelago’s marine reserve and help protect against the mounting pressures of climate change and human exploitation.
Alex Forryan received funding from the Royal Society.
Alberto Naveira Garabato receives funding from the Royal Society and Wolfson Foundation.
Alex Hearn receives funding from the Royal Society, PEW Charitable Trust and Galapagos Conservation Trust. He is affiliated with MigraMar, the Galapagos Whale Shark Project, Turtle Island Restoration Network and Fundación Megafauna Marina del Ecuador.
source: The Conversation: Galápagos: we’ve found out why the islands are blessed with such nutrient-rich waters