The Marine Benthos

1. Lisa Ziegler : 3241269 Insauf De Vries: 3050561 Candice M Boltman: 2948122 Introduction Earth is made up of terrestrial and aquatic environments, the ocean being the largest. The ocean covers over 360 million km2 of the earth’s surface, 71% of which is randomly spread amongst aquatic environments such as rivers and oceans. Five main zones are found in the ocean, the intertidal/littoral rock, sub littoral/shelf, bathyal, abyssal and hadal. These main zones are categorized by the benthic and pelagic zones. The pelagic zone is all organisms inhabiting communities closer to the surface which is warmer due to more exposure to light, whereas the benthic zone is all organism inhabiting communities at the bottom of the ocean where it is colder and darker due to less exposure to light. The benthic zone also known as the benthos, originating from the Greek noun bathys meaning “depths of the sea”. The benthic zone plays a vital role as it is responsible for many treasured yields and ecological services, (e.g. food for humans, decomposition of organic matter and the removal of waste products). The benthos is home to an enriched biodiversity of communities living together in space and time. These communities are subjected to change due to interactions between the communities and the physical environment. The factors that contribute to those changes are light, temperature, water movement and chemical factors such as salinity and oxygen. These factors will now be discussed according to the environmental gradients that range from 0 – 10000m. Legend 2: Sublittoral Zone Figure 1: Sponges are multi-cellular primitive animals that filter the oceans. Sponges form symbiotic relationships with collected bacteria which create forms of nitrogen from the nitrogen gas in the water that may be nutritional for the sponge. Figure 2: The Red-lined Bubble Snail (Bullina lineate) from the Phylum Mollusca shows its beautiful colouration. Molluscs exist from the intertidal to the deepest ocean. Figure 3: Parasitic crustaceans serve as both hosts and vectors of viruses as well as of parasites and other microbial pathogenic agents. Legend 1: Littoral zone Figure 1: Algae uses the process of photosynthesis to manufacture food from carbon dioxide and water using the suns radiant light energy. Figure 2: Brittle stars are a popular food source to fish and crabs. Brittle stars form a symbiotic relationship with sponges and coral, feeding off the large debris unclogging and cleaning these sessile organisms. Figure 3: Sea urchins are important because it consumes algae, which in turn control algal growth preventing the coral reef from being smothered. Figure 4: Bivalves accumulate heavy metals, bacteria, viruses and toxins in turn filtering water. Bivalves are economically important to human as food and ornaments. Figure 5: Limpets are important grazers as they control barnacle and algal densities. Littoral zone: (0m) Spans from the shallow submerged to the beach habitats, where high and low tides occur frequently. During the low tides, the community is exposed to high O2 levels; therefore they have to be able to withstand wet and dry conditions. The sedimentation is hard rocks which hold water when tides are low as a result of increasing salinity when the water evaporates. Littoral zone experiences strong currents, making it difficult for under-developed organisms to withstand such wave actions. Therefore small organisms adapted ways to counter this by burrowing, attaching or living under rocks. The littoral zone is exposed to full sunlight which leads to fairly moderate temperatures, however can range from extremely hot to near freezing (cold seas). An increase in the turbidity of water caused by phytoplankton could reduce light in kelp communities. The littoral zone is common to small organisms as brittle stars, urchins, bivalves, limpets’ as well present kelp communities which are mainly dominated by large brown algae. (See Legend 1) Figure 1: Figure 1: The Marine Benthos Figure 2: Figure 2: Figure 3: Sublittoralzone: (0-200m) Expands from the low tide to the shelf break also known as the continental shelf. Unlike the littoral zone, wave action is reduced in the sublittoral zone and only exposed to full sunlight when the tide in the littoral zone is low, therefore salinity and temperature is uniform. Rocky, hard substrates become less visible and are replaced by soft sandy substrates as the depth increases gradually. As sedimentation changes so does the organisms found on the seabed, from kelp in the rocky substrates to sponges, molluscs, crustaceans and echinoderms as well as small fish in the soft substrates. ( See Legend 2) Figure 3: Figure 4: Bathyalzone: (200-2000m) The Bathyal zone extends from the continental shelf to the abyssal zone. This region receives very little light therefore the salinity is low as well as the temperature. The pressure force starts to increase as the depth increases. Bathyal sediments are fine grained. Organisms occupying this space are black, red or transparent in colour, making them ultimately invisible in the weak light. Giant squid and hatchetfish are just some of the few bioluminescent organisms that tend to have a poor ability to swim as there is no need to escape predators or visibility is low or absent in some. Bacteria play an important role in deep-sea benthic food webs. (See Legend 3) Figure 5: Abyssal zone: (2000-6000m) The Abyssal zone is located beneath the bathyal zone and expresses complete darkness, high pressure, low current velocities, and low temperatures. The substratum is mainly composed of fine muddy sediments. Reduction in light causes the community to acquire chemoautotrophic bacteria to convert hydrogen sulphide into energy. Due to benthic production being dependent on the contribution of dead organic material produced in the euphotic zone, the abyssal communities are said to be energy limited. Particulate-organic-carbon (POC) is an external factor that drives the patterns of faunal biomass and richness at the abyssal seafloor. (See Legend 4) Legend 3: The Bathyal Zone Figure 1: The giant squid measuring approximately 18 meters in length is the largest carnivorous invertebrate on earth. Figure 2: Hatchet fish use bioluminescent light called “counterillumination” which makes these individuals almost invisible from predators. Legend 5: Hadal zone Figure: 1: Life around hydrothermal vents depend on chemosynthetic bacteria as they use compounds to produce organic molecules which support ecosystems around these vents. Figure 2: The blobfish: Psychrolutesmarcidus, is a gelatinous mass. Feeding on bacteria and invertebrates (crabs) under high pressurised conditions. Legend 4: Abyssal zone Figure 1: The Anglerfish (Lophius piscatorius) have dark-adapted eyes and long feelers to help them locate prey in the dark of the deep ocean. Figure 1: Hadal zone: (>6000m) The Hadal zone, seemingly “lifeless”, is the deepest part of the ocean; it is completely dark, and cold with an extreme amount of pressure. However there are organisms that have adapted to these conditions, such as amphipods, decapods and some fish (blobfish: Psychrolutesmarcidus). The richness of trench communities, thought to originate from the abyssal plains, also declines with depth. Adaptations to high hydrostatic pressures and low temperatures are common. Organisms make use of intracellular protein-stabilizing osmolytes (trimethylamine N-oxide) to maintain enzyme function; it does so by increasing cell volume to prevent effects of pressure. (See legend 5) Figure 1: Figure 2: Figure 1: Figure 2: Conclusion The benthic zone expands from 0 – 10000m. It is evident that changes such as light, temperature, water currents, salinity, oxygen availability, the type of sediment, depth and pressure influence the way communities are structured and operate. Organisms throughout the benthic zone are highly adapted to its specifications and cannot exist in another zone.