Kelp surprise

Abstract

A new study reveals a rival to coral in the tropical marine environment. Nigel Williams reports. A new study reveals a rival to coral in the tropical marine environment. Nigel Williams reports. Coral reefs are considered one of the most important tropical marine habitats. They are hotspots of biodiversity and productivity, so their vulnerability to environmental change has prompted efforts around the world to help manage and conserve threatened reefs. But, according to new work, another tropical environment also provides crucial support to organisms and has been somewhat overlooked because of the concern about corals. Kelp forests are mostly seen as key marine environments at higher latitudes, where they are regarded as some of the most productive and dynamic ecosystems with a range of trophic and habitat associations. They are considered to need high nutrient concentrations, low temperatures and bright light. While tropical latitudes exceed light requirements at shallow levels for kelp, nutrient and temperature levels are not suitable for kelp survival, so surface-dwelling kelp forests have never been described. Deeper water kelp populations have been documented but they appear to be rare and have been viewed as relics of shallow-water populations that existed in cooler oceanographic climates. But, in a new study (published online in PNAS) by Michael Graham at Moss Landing Marine Laboratories, and colleagues at the University of California, in Canada and in Ecuador, the team have combined data on all known tropical kelp populations with broader oceanographic data to try to predict conditions that might be suitable for kelp to thrive. The authors considered that tropical kelp habitats could exist at the base of the euphotic zone where there is enough light, nutrients and substrate to sustain growth and survival. They combined data from global remote-sensing satellites and oceanographic repositories containing high-resolution 3D oceanographic data that can be used to determine regions with enough light and substrate material needed for kelp attachment, throughout the tropics. Their analyses suggest that there may be more than 23,500 square kilometres of unexplored habitat at a depth of 30–200 metres that might be suitable for kelp. The model suggests these habitats will exist at minimum depths of 25–107 metres and a maximum depth of 100–236 metres, depending on the ocean basin. The largest and shallowest predicted areas occurred in the eastern regions of the Pacific and Atlantic. More limited areas were predicted in the western Pacific and Atlantic oceans. The eastern aspect of the model covered regions of known distribution of kelp in the Galapagos Islands and the western model predicted the site of two other tropical kelp species off Cabo Frio in Brazil. The researchers also came across a reference to another deep-water kelp species off the Philippines. This species was last known to science following collection from a site predicted by the model to be suitable for tropical kelp. With these predictions to hand, the team decided earlier this year to mount an expedition to the Galapagos to look more carefully for kelp at the sites predicted by their model. Only limited observations of the local species, Eisenia galapagensis, had occurred since its original description in 1945 with specimens largely confined to just one site after the El Niño event of 1982–1983. The researchers' model however, predicted large areas of suitable habitat around the archipelago, with the largest areas predicted along the west coast of Isla Fernandina and the southwest coast of Isla Isabela. And indeed, through scuba dives, the team were able to find kelp populations at predicted sites up to a maximum dive depth of 60 metres. The team were unable to explore any deeper but noted that specimens growing at 50–60 metres were denser, larger and healthier than shallower-growing specimens. The researchers believe that the predictions of their model and their dive findings suggest that these kelp populations may be old and resilient features of the tropical seas. They point out that the Philippines species is the most morphologically and ultrastructurally primitive of current kelp species. “The global significance of these systems likely extends beyond simple ecological curiosity with broader impacts to tropical sublittoral systems worldwide,” they write. Kelp forests may “develop unique communities relative to other tropical systems”, they believe.