deep sea Ecology

Seascapes of the Deep Sea

Citation: Johnson Kellie, Taylor Avery, Socha Annika, Barkyoumb Ellie, Nakamura Koichi, Kaiser Carl L., German Christopher R., Yoerger Dana R., Van Dover Cindy Lee. (2023) Seascape ecology in the vicinity of a Blake Ridge cold seep, Frontiers in Marine Science, 10, ISSN=2296-7745, doi:10.3389/fmars.2023.1198226.

Defining Transition Zones

On land, you might look for changes in the plant life and landscape. For example, the transition from dense, tree-covered forest to grass-covered prairie usually results in a small “boundary area” where grasses, shrubs and trees are all present, battling for dominance where the edges of their habitats run together. In ecology, these edges of habitat are referred to as “ecotones”.

Shrub-like trees and blue sky in the background, low-lying plants, grass, and sand in the foreground.
A transition zone between coastal forest and sand dune ecosystems. WikiCommons

In the ocean, defining ecosystems and their boundaries is a little more challenging because the deep ocean is logistically hard to get to, making ecosystems difficult to survey. NOAA estimates that we’ve only mapped about 25% of the ocean and have physically explored much less than that. But as we explore more of the deep sea, more types of ecosystems continue to be discovered. They are often defined by different characteristics than we typically encounter, like being fueled by methane seeping from cracks in the oceanic crust and bacterial symbiosis rather than by the sun and plants.

Just as the landscape (its topography and plant life) can define a terrestrial ecosystem, the seascape can define a marine ecosystem. A recent study published in Frontiers of Marine Science defined the boundaries and transition zones of a cold seep on Blake Ridge, a deep sea ridge in the Atlantic Ocean, off the coast of southern Georgia. 

Seascapes have a lot of variability. Similar to landscapes, which can contain hills, valleys, and small transitions between forest and meadow, methane seeps define one type of ecosystem and the comparatively barren abyssal plain defines another. Therefore, defining an ecosystem and where it ends is about a lot more than simply depth or latitude. It requires knowledge of all the plants and animals that live there, an understanding of who’s eating who, and the environmental conditions that govern the ecosystem. In order to understand these characteristics of any ecosystem, scientists must conduct surveys. Unfortunately, this is especially challenging in the deep.

Surveying the Seascape

Map of Florida coast with the Bahamas in the south corner. Blake ridge marked with a red location mark off shore between Jacksonville FL and Savannah GA.
Location of Blake Ridge. Screenshot, Google Maps.

In order to define the ecosystem supported by the cold seep from the “non-seep” ecosystems on the rest of Blake Ridge, the study used an autonomous underwater vehicle (AUV) to map the bathymetry, collect photographs, and measure water chemistry to define the boundaries of the cold seep ecosystem.

The photographs were used to conduct surveys of fauna and to study the substrate of the seafloor (like how much of the seep area is covered with rock versus sandy/muddy sediment). The water chemistry measured by the AUV’s sensors is a special kind of chemical reaction that can be measured by voltage, called redox. Redox reactions are the basis of metabolism for all cells, including our own, though instead of using oxygen or doing photosynthesis, cold seep microbes use methane, sulfur, and other compounds. By measuring where redox is taking places around the cold seep, the researchers were able to understand how large the cold seep’s “sphere of influence” might be. 

The seep’s sphere of influence was relatively small, reaching less than 100 meters from the seep itself. The authors reported certain animals as “seep indicators”, distinct from animals outside the seep’s reach. The cold seep ecosystem included mussels, clams, tubeworms, and bacterial mats capable of harnessing energy from the methane and sulfur-rich environment through symbiosis with specialized bacteria. Outside the cold seep, sea stars, urchins, and sea cucumbers dominated. The seep also seemed to be a hotspot for certain predators such as octopus, which prefer to feed on bivalves like mussels and clams, and other animals like shrimp which used the mussel beds as hideouts. The ecosystem around the seep seemed to be more diverse than the non-seep area, suggesting that the seep habitat is also a valuable hunting ground for many kinds of important marine life, including the rare and mysterious beaked whales.

Bright red-orange mussels with a translucent shrimp standing on one of them.
Cold seep fauna; shrimp hiding among mussels harboring chemosynthetic bacteria. NOAA Photo Library, Flickr.

The AUV footage discovered gouged out areas of sediment both within and outside the seep boundary that were possibly caused by feeding beaked whales. Little is known about how these whales forage in the deep sea, though they are known to dive as deep as 2500 m in search of food on the seafloor! Do these deep diving cetaceans have a preference for seeps due to the higher concentration of food? The answer isn’t clear from this study alone, but their discovery of feeding gouges around the seep is an encouraging piece of evidence.

Along with abundant marine life, the AUV footage also found evidence of human pollution surrounding the seep. Glass bottles, coke cans, drink cartons, cardboard, and metal objects from scientific expeditions littered the seafloor at a density of about 2 items per hectare.  

The Importance of Ecological Surveys

Cold seeps are biodiversity hotspots in the deep sea, akin to coral reefs or rainforests in their importance to both biodiversity and “productivity” (the ability to generate energy and food for other organisms). The ecosystems supported by the seep may have benefits that we don’t fully understand, such as providing feeding grounds for large animals like beaked whales.

This study and other deep sea surveys are providing important information to fill in our

Long-bodied whale with a dolphin-like head and small fins.
Beaked whale. WikiCommons

gaps of understanding in how hotspots like cold seeps and vents influence the surrounding seafloor. The surveys found patches of non-seep habitat and transition zones even within the boundaries of the seep, much like meadow clearings in a forest. This challenges a long-held paradigm that seep areas are homogenous and completely separate from other ecosystems. 

Because this seep could become a target for further exploration of methane reserves, foundational studies like this are critical for creating environmental management plans. There is a lot we don’t know about these deep sea ecosystems, including their prevalence and the extent of their influence on deep sea productivity. Exploratory studies like this one are crucial for establishing baseline understanding of deep sea ecosystems so that scientists can study how they are changing in the face of natural variability and human influence.


Cover image caption: Methane bubbles rising from a cold seep covered in bacterial mats. WikiCommons.

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