I am sure, all of you have heard of El Niño. Most of you experienced messed up local weather between at least Nov 2015 to April 2016, if not already before and still after that period. Almost all of that was caused by the sever and unusual El Niño event which dominated huge parts of the Pacific starting early in 2015 and lasted until June 2016.
Very often an El Niño event, which tends to increase cyclone activity in the Pacific ocean, is followed by it's counterpart: La Niña. This phenomenon is much less known, although it has huge global influences as well. For example, an increase of hurricane activity in the Atlantic and Carribean. When a La Niña situation occurs after an El Niño, nature doesn't just rectify the lack of trade winds, but amplifies them.
The stronger than normal trade winds push the warm surface water further westward (where they belong), making room for cold, upwelling water along the Ecuadorian and Peruvian coast, which in turn feeds the South Equatorial Current (SEC – also called Humboldt current). While the wind pushes, the SEC sucks the warm surface water away from the Galápagos archipelago, thus enforcing the SEC upwelling on the western flank of the Galápagos platform, the undersea rock massif carrying the Galapagos islands.
In 2015/16 we experience an El Niño event, which was moderate in the Galápagos but extremely strong and long in the central and western Pacific. Is it, as so often, followed by a La Nina? From what several naturalist guides in the Galápagos told me, and from what I have seen myself, it is a clear “probably”. But the NOAA data tell a different tale – at least for here. Though both tales, the observed one and the NOAA one, fit together quite nicely.
In early summer, when the El Niño still persisted in the central and western Pacific (it officially ended in June 2016), unusual growth of an unusual green macroalgae was observed in the western part of the Galápagos archipelago. In the warm, mineral poor surface water of the El Niño, green algae can't grow, at least not abundantly. In those times brown algae dominate the seascape in the tidal and sub-tidal area. After the first growth of this strange bright green algae, and it's dislocation and deposition on the shores of the western islands, it persisted in smaller amounts throughout the months of the cold garúa season. Finally, middle of October, it's growth increased dramatically and on Fernandina Island we walked on a thick layer of drying and decaying green algae, providing breeding ground for millions of annoying flies. This algae was was not the usual species of sea lettuce Ulva lactuca but another species of the same genus, probably Ulva fasciata. The high tide of the October full moon carried the decayed mass out to sea again, so that the coastal walk at Punta Espinoza became less troubled by flies by end of October. At this time we witnessed huge amounts of the same algae, some still alive though torn off the rocky intertidal flats, some dead, dried, and re-immersed, in the waters in front of Punta Espinoza in Fernandina. All that floating stuff hugely impairing visibility while we were searching for feeding marine iguanas under water. At the same time, the normally black beach of Urbina Bay was bright green with dislocated algae – just the same picture as Punta Espinoza ten days before.
Intense growth of macroalgae is a sign of nutrient rich water, which in the Galápagos can have two sources: upwelling of mineral rich water from the Equatorial Undercurrent (EUC – also called Cromwell current), or rain water run-off from the islands, carrying mineral-rich volcanic soil. As it didn't rain in the past half year or so, the only reasonable explanation for the obvious increase in nutrients is upwelling. Looking at the NOAA SST anomaly charts, we can see colder than normal surface water temperatures this year for the first time in May, after the warmer than usual temperatures caused by the El Niño. The only possible source for the cold water is the mineral-rich water from the EUC. Over the following months, upwelling fluctuated, leading to temperatures ± 2 °C from average. The temperature fluctuations can easily be explained by unstable upwelling, sometimes intense, sometimes suppressed. Which would lead to fluctuating nutrient content of th surface water and thus to fluctuating algae growth. When we see the macroalgae lying on the beaches and rocks, it doesn't only mean that a lot grew, but also that it was dislocated, which can happen either by strong wave action, or by moderate wave action when the algae are very dense, long, and thus heavy. Moderate to strong wave action did definitely happen from time to time in the past months, especially in the western archipelago, as several seasick guests of expedition boats can confirm.
So our algae growth observations fit nicely with the NOOA SST data. And from the current cold water (data from November 28) I would expect another boost of Ulva spec. growth right now. Unfortunatly I am not touring the islands anymore, thus relying on input from my naturalist guide friends.
But La Nina conditions of cold, nutrient-rich water do not only influence the marine plant life. This is only the first step, as always: primary production at the base of the food web. So what happened lately in the animal kingdom? Lets have a look at the marine iguanas, which depend directly on green algae. While marine iguanas on Fernandina Islands died in the first months of this year due to the El Niño induced lack of food, they are well fed right now. Not to say fat! While they usually feed on Ulva lactuca, which is also abundantly present, they also seem to like the untypical Ulva species (U. fasciata?), especially when it is freshly washed up on shore. This free delivery of food allows them to feed on land, instead of in the cold water as usual, leading to energy conservation and quicker weight gain. In accordance with their physical status, the male iguanas started early to display breeding colors and territorial behavior, associated with breeding season as well. While strutting their stuff, they are still walking over the carcasses of their mummified conspecifics, the victims of the El Niño.
While the official data (NOAA CPC) display constant, lower than usual SSTs in the central and western Pacific, the average SST in the eastern Pacific (El Niño 1+2 region) is neutral to slightly warmer than usual. With the exception of the western Galápagos waters, thanks to the Galápagos cold pool caused be currently fluctuating upwelling intensity. This upwelling causes marine plant and animal life to flourish, which is typical for La Niña conditions. It remains to be seen if the fluctuating temperatures persist, or if the south-easterly trade winds will soon calm down and allowing for a normal warm and wet season to happen in the Galápagos Islands. Which would mean: back to normal.
NOAA CPC ENSO 1+2 chart
La Nina status from NOAA CPC ENSO Diagnostic Discussion