An Empire Lacking Food
Once viewed as a barren expanse, the deep seafloor is a biologically elaborate ecosystem whose fate is tied to life above, near the sea surface
Diversity of Deep-Sea Life
The rich variety of adaptations to the extremes of food limitation in the deep sea parallels a rich biodiversity of life that would have shocked Edward Forbes. In 1968, Howard Sanders’s comparative study of the seafloor demonstrated that deep-sea diversity exceeded coastal diversity in the temperate zone and approached shallow-water tropical diversity. More recent work suggests macrofaunal diversity in the deep may even rival that of tropical rain forests. In relatively small areas the number of species coexisting in the deep sea is surprisingly high; an area the size of a coffee table on the deep seafloor, it’s now known, could yield more than 300 species.
However, biodiversity in the deep sea is a paradox. High biodiversity in habitats such as coral reefs and tropical rainforests is thought to reflect the variety of environments and resources available there. Such variety supports multiple ecological niches and a richer, more complex community. In contrast, the flat, muddy seafloor, absent of reefs and forest to provide complexity, appears more homogenous. And all organisms rely on and compete for the same basic food source originating from above.
But it turns out that the deep seafloor is not as homogenous as it first seems. Fred Grassle and Howard Sanders of the Woods Hole Oceanographic Institution proposed in 1973 that the seafloor actually comprised a patchwork of microhabitats extending over scales only centimeters long. In their “patch-mosaic hypothesis,” each patch provides a unique set of environmental characteristics—microhabitats—that support a unique set of macrofaunal species. Since then, specifics of these microhabitats have become more clear, as have the reasons why they appear to be more common in the deep sea. The formation of micropatches in part reflects how larger organisms such as urchins, sea stars, sea cucumbers, worms and crabs build burrows, tubes and mounds, and move across the sediment, creating small-scale topography.
The patch mosaic also reflects the nonuniform way in which marine snow collects on the seafloor. Microtopography on the seafloor likely catches marine snow in depressions, much as the irregular surface of a lawn receives an uneven coating of light snow. Carbon sinking from above may also aggregate before its arrival at the seafloor. Larvaceans, a type of plankton, secrete a mucus “house” around themselves that filters particles out of the water. These houses can become clogged easily, and larvaceans shed them about once every four hours. The clogged houses are packed with carbon-rich food and arrive at the seafloor as a clumped food source. Also, the variety of patches persists longer in the deep sea because the consistent, rapid currents that homogenize the seafloor in shallow water are often uncommon in the deep. Thus, a quiltwork of microhabitats can develop, leading ultimately to higher species coexistence.
Patterning in food availability drives biodiversity not only at small scales but at larger scales as well. In 1973, Michael Rex published the first study demonstrating the complex pattern of biodiversity with increasing depth. As depth increases and carbon input decreases, diversity increases on the seafloor. At intermediate depths of around 2 to 3 kilometers, the pattern reverses and diversity begins to decline rapidly. This relationship between productivity and diversity, such that diversity is highest at intermediate levels of productivity, is replicated in a variety of other environments and organisms. Multitudes of papers have been published since then trying to explain this pattern. The most likely explanation for the decreasing productivity and decreasing diversity is the Allee effect. Less food availability supports fewer individuals across species. As a population’s numbers fall, species are more likely to go extinct locally due to random environmental perturbations. This lowers the total diversity of an area.
However, why high food availability would support fewer species is a puzzle. Nearly two dozen hypotheses exist to explain this pattern, and their empirical support varies. Earlier this year, Jim Barry of the Monterey Bay Aquarium Research Institute (MBARI) and I published a study that may reconcile this pattern. In submarine canyons, food can collect at the base of the steep canyon walls. We found that highly mobile megafauna such as urchins, sea cucumbers, crabs and sea stars quickly converge in these areas and monopolize the food source. In such numbers, their activities drastically churn the sediment. This disturbance and lack of food creates harsh conditions for the smaller macrofauna dwelling there. Fewer macrofaunal species, including sediment tube builders whose structures would quickly be destroyed, can survive. Thus, at shallower depths where carbon availability is high, megafauna will be more abundant and may ultimately suppress biodiversity of macrofauna.
Observations in the deep can be of use in clarifying our understanding of other habitats as well. Biodiversity is high in the tropics and declines toward the poles. This latitudinal species diversity gradient (LSDG) is observed in a remarkable variety of organisms in marine, terrestrial and freshwater ecosystems. However, explanations for LSDGs remain as varied as the organisms that exhibit this pattern. Theories point to variability in climatic stability, climatic harshness, temperature, speciation rates, extinction rates, parasitism, predation, competition and food availability. In 1993, Michael Rex for the first time documented LSDGs in the deep sea. Rex and his collaborators found heightened diversity near the equator and depression near the poles among mollusks and crustaceans in the Atlantic Ocean. Later work revealed that forams also followed this pattern. These results indicated that LSDGs were probably not related to temperature. The deep sea varies little in temperature, about 4 degrees Celsius, and can be relatively uniform over large swaths of the ocean. But something else does vary with latitude: the amount and variability of plankton production at the ocean’s surface.
Temporal dynamics in food availability can lead to changes in deep-sea biodiversity over geological as well as annual time scales. Utilizing cores from the Ocean Drilling Program, Moriaki Yasuhara of the Smithsonian Institution and his collaborators have probed the history of minute crustaceans, the ostracods, over the past 500,000 years in the tropical deep sea. The last half-million years saw four glacial-interglacial climatic cycles that radically altered the temperature, currents and plankton production of the oceans. Diversity of ostracods in the deep tropical ocean plummeted during glacial periods and their concomitant lower surface production; consequently, during these periods, the LSDG disappeared. Conversely, during interglacial periods, ostracods in the deep tropical ocean became fantastically rich in diversity.
Over much shorter time scales—at the decades level—the El Niño and La Niña cycles, which shift sea-surface temperatures in the tropical Pacific Ocean, can also change patterns in surface production. A site called Station M—submerged 4,000 meters below the ocean’s surface off Santa Barbara, California—has been monitored consistently for nearly two decades, a rarity in deep-sea research. Work by Henry Ruhl and Ken Smith of the Scripps Institution of Oceanography and MBARI at Station M demonstrates that as El Niño and La Niña oscillations occur in Pacific sea surface temperatures, the amount of phytoplankton varies concordantly. And the diversity and abundance of the megafauna and macrofauna also shift. These exceptional studies uncover remarkable temporal patterning in deep-sea diversity that further links surface production and the deep sea.