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Word Wednesday: Benthic Macrofauna

Wednesday, June 15th, 2016

It’s a small world in the benthos…or is it? Today’s Word Wednesday will explore the benthic community, home of the macrofauna and other bottom dwellers.

Welcome to the benthos! There’s a whole community living on the bottom of bays, continental shelves and the ocean referred to as the benthos (noun) that is home to plant, animal and microbe alike. The organisms found in benthic (adjective) communities vary with depth (coastal to deep sea), bottom type (soft or hard bottom), latitude (arctic to tropical) and many other environmental factors such as salinity and dissolved oxygen concentrations.

Benthic fauna are organisms that live on, in, or at the sediment-water interface and are often classified according to size: microfauna (<0.1 mm), meiofauna (<1 mm), macrofauna (>0.5 mm), and megafauna (>10 mm). One needs a microscope to see microfaunal organisms, while macrofauna are visible with the naked eye. These classifications focus on invertebrate organisms and each size class may be referred to in multiple, interchangeable ways. For example, microfauna may also be called microbenthos or benthic microfauna.

marsh epifauna
An example of the macrofauna found living in the intertidal coastal wetland habitats of the Chandeleur Islands . Photo credit: W. Scheffel

The size classifications originate from the way scientists sample benthic organisms rather than taxonomic groupings. Traditionally scientists collected a sample of the benthic environment and then sieved or strained the organisms from the sediment, classifying the organisms based on the size of the mesh the organisms were retained on. Benthic fauna can be further divided into epifauna, animals that live above the sediment surface (e.g., many gastropods, shrimp and crabs), and infauna, animals living below or at the sediment surface (e.g., polychaetes and bivalves). Epifauna also include organisms that attach to a hard surface, like a barnacle, or organisms that live on the surface of submerged vegetation (i.e. seagrass, salt marsh and mangroves) like amphipods.

Benthic communities are the focus of a great deal of research. Benthic fauna are important members in aquatic food webs. Many of these tiny organisms consume microalgae and in turn are consumed by small fish, crabs and shrimp, which are then consumed by other, larger organisms. A loss of these benthic fauna from natural or a human-induced impact, like the Deepwater Horizon oil spill, could disrupt the entire food web. Because of their importance, benthic communities have been used as indicators of environmental conditions due to the presence or absence of indicator species- that is a species sensitive to or indicative of a specific set of environmental conditions. Benthic fauna are also very important in modifying their habitat.

benthic infauna
A selection of common polychaetes found in the nearshore waters of the Northern Gulf of Mexico. Photo credit: D.Byron

Muddy and sandy sediments get mixed by the burrowing and feeding activities of benthic organisms, a process known as bioturbation. Sediment characteristics such as stability, porosity, and even grain size can be affected by the actions of benthic organisms. Even characteristics of the overlying water column can be affected by the actions of filter feeding benthic fauna. These organisms may be small, but they have a large impact on their surroundings!

ACER scientists in several research groups are studying macrofauna to find out if the oil spill impacted the diversity (species diversity and functional diversity) of these organisms. ACER’s Benthic Infauna, Benthic Microbes, and Nitrogen Cycling groups are examining the impact of macrofauna and microfauna on nitrogen cycling, bioturbation, and energy flow in oiled and non-oiled areas. And ACER’s Wetlands group is collecting epifauna associated with salt marshes and mangroves to detect habitat quality changes in oiled and non-oiled areas. Sampling in emergent vegetation can be tricky, but they accomplish this with a drop and suction sampler, a sampling apparatus we will explore in our next Tool Talk. Stay Tuned!