Plastic litter has been observed to smother at least 22 species of flora and fauna on the seafloor. Especially sessile organisms are vulnerable to smothering since they are immobile and cannot move to a more suitable location when their habitat changes. Vegetation on the seafloor can suffer from smothering as seen in a study made in Florida: two species of seagrasses (Thalassia testudinum and Syringodium filiforme) exhibited decreased shoot densities after deployment and removal of lobster traps which had been on place for 6 weeks. Smothering was observed to inhibit photosynthesis, broke and abrade blades and crush the blades in the underlying anoxic sediments causing senescence of above-ground biomass. The recovery of seagrass beds took 4 months for T. testudinum, but the effects of smothering were still seen 8 months after removal of the traps in S. filiforme.
Similar results were also obtained in seagrass (Posidonia australis) meadows shaded by plastic films in New South Wales, Australia. As a consequence of shading the growth rate of leaves, as well as shoot number and weight decreased. In addition shading also changed the structure of the epiphyte community growing on the leaves of P. australis: shading reduced the amount of fleshy macroalgae and coralline algae, whereas bryozoans and tube-building worms persisted on the leaves until the end of 3 months long experiment period. Because macroalgal diversity was observed to decline due to prolonged shading, it was proposed that losses in diversity might also occur in fauna dependent on the algae. No recovery of seagrass beds were observed in following 17 months indicating long-lasting effects in these habitats.
When sunken to the bottom, plastic litter can inhibit the gas exchange between the sediment and overlying water, which may lead to local hypoxia or anoxia in the benthos. In fact anoxic conditions have been observed in the sediments under a sunken plastic bag. Reduced oxygen levels in the sediments are suggested to alter the infaunal communities. Field experiments on an intertidal shore have revealed that anoxic conditions prevailed after 9 weeks in the sediment below the plastic bag and resulted in reduced primary productivity and organic matter as well as lower abundances of infaunal invertebrates. Similar results were obtained also with plastic bags made of biodegradable materials.
Interactions between sunken litter and fauna are commonly described in various locations around the world. Deleterious effects of marine litter are regularly detected in coral reefs in several locations. Smothering causes shading of corals which can impair the photosynthetic activity of symbiotic algae. It is also suggested to restrict the water flow and therefore limit nutrition by reducing the particles entering the feeding apparatus of corals and other filter feeding animals. In addition, smothering can also cause tissue abrasion and mortality. In Marshall Islands the amount of hard corals decreased as the coverage of macro-sized litter increased.
Litter on the seafloor can also impede feeding of other than filter-feeding organisms. The foraging behavior of gastropod Nassarius pullus in sandy intertidal beaches of Philippines has been observed to be affected by the plastic litter cover in the foraging area. Increased plastic cover on the beach resulted in decreased foraging efficiency, which was seen in longer travelling times towards the food source or increased burial to reduce the energy costs related to feeding.
The effects of sunken litter are not only restricted to tropical areas: for example in the deep Arctic seafloors 67 % of litter items were associated with sessile megafauna, such as sponges, sea anemones and sea lilies. Anemones, sea stars, crinoids, hydroids and barnacles have also observed to live in contact with marine litter in the submarine canyons near Portugal. In the coast of California litter on the seafloor have been observed to serve as artificial habitats for some fishes and structure-forming invertebrates, who may use plastic as cover or growing platform. Litter embedded in soft sediments may enable the occurrence of species that are usually found only on hard bottoms, changing the composition of the local community. In addition plastic is sometimes worn by hermit crabs instead of natural shells. Therefore, it has occurred challenging to differentiate if the organisms are suffering or benefiting of litter; some sessile organisms may colonize the litter and gain some advantages such as elevated position on the seafloor. On the other hand, sessile animals using plastic as substratum are also suggested to possibly experience some chronic exposure to hazardous substances that can be leaching from plastic matrix.