Deep-water kelp refugia as potential hotspots of tropical marine diversity and productivity

Abstract

Classic marine ecological paradigms view kelp forests as inherently temperate-boreal phenomena replaced by coral reefs in tropical waters. These paradigms hinge on the notion that tropical surface waters are too warm and nutrient-depleted to support kelp productivity and survival. We present a synthetic oceanographic and ecophysiological model that accurately identifies all known kelp populations and, by using the same criteria, predicts the existence of >23,500 km(2) unexplored submerged (30- to 200-m depth) tropical kelp habitats. Predicted tropical kelp habitats were most probable in regions where bathymetry and upwelling resulted in mixed-layer shoaling above the depth of minimum annual irradiance dose for kelp survival. Using model predictions, we discovered extensive new deep-water Eisenia galapagensis populations in the Galápagos that increased in abundance with increasing depth to >60 m, complete with cold-water flora and fauna of temperate affinities. The predictability of deep-water kelp habitat and the discovery of expansive deep-water Galápagos kelp forests validate the extent of deep-water tropical kelp refugia, with potential implications for regional productivity and biodiversity, tropical food web ecology, and understanding of the resilience of tropical marine systems to climate change.

Fig. 1.

Components of deep-water kelp refuge model for tropical regions worldwide. (a) Distribution of illuminated benthos receiving irradiance above kelp photosynthetic compensation point. (b) Mixed-layer depth. (c) Predicted location of deep-water tropical kelp populations (shading indicates frequency of predicted kelp habitats within 50-km radius). Numbers are locations of known deep-water tropical kelp taxa: 1, E. galapgensis; 2, L. brasiliensis and L. abyssalis; and 3, L. philippinensis. (d) Equatorial cross-section (averaged between 5°N and 5°S latitudes) showing the mixed-layer depth relative to the kelp photosynthetic compensation point.

Fig. 2.

Empirical support of deep-water kelp refugia model in Galápagos Archipelago. (a) Map of illuminated substrate (red) and predicted deep-water kelp distribution (green) with locations of newly discovered (open circles) E. galapagensis populations. (b) Deep-water (60 m) (Left), shallow-water (10 m) (Center), and underwater (Right) E. galapagensis thalli. Note the lengthened stipe in the deep-water thallus that was characteristic of sporophytes found >20 m depth. [Photograph in b Right is reproduced with permission from S. Connell (University of Adelaide, Adelaide, Australia) (Copyright 2007).]