Benthic Marine Ecosystems
The Earth is a delicate web of ecosystems which are interconnected and intertwined and have an impact on the rest of the system. Changes or disturbances to any component of this complex web can have wide carrying serious effects often even on components that are seemingly unconnected. One of the major and important components of this complex and fragile system is the benthic marine ecosystem which plays an important role in maintaining the ecological balance. The Benthic marine ecosystem exists at the lowest level of a body of water of an ocean or a lake called as the benthic zone.
It comprises of ‘benthic organisms’ or ‘Benthos’ that live in close association with the soil at the bottom of the water body which is very essential for all their biological activity. Benthic marine ecosystems are often disturbed because of human activity resulting in disturbance to the entire marine ecosystem and ultimately to the overall Earth’s system. Since Benthic ecosystems form such an integral and important part of the entire ecosystem and have such far reaching impacts it is essential to understand the potential threats to the benthic ecosystems and its impacts.
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Threats to the Benthic ecosystems and their impacts and comparison to the pelagic ecosystem Human actvities of trawling and by-catch Trawling has always posed a threat to marine ecosystems as there is a lack of selectivity due to the nature of the trawl net and there is capture of a huge and diverse number of untargetted aquatic life called by-catch. This has a serious impact on the chemical , physical and biological nature of the Benthic marine ecosystem. Huge biomass is removed through trawling.
This results in reduced marine fish landings and productivity as Benthic habitats house fauna which serves as food to many marine life and provides shelter and protection to smaller fishes. Eg A single passage of the beam trawl is known to kill 5-65% of the resident fauna and mix the top few centimeters of the sediment. It has an effect that is similar to forest clear-cutting. (Trawling and by-catch: Implications on marine ecosystem , Biju, A. K. &, Deepti, G. R. (2006)) Trawling causes the most serious physical disturbance on the sea-floor.
They can burrow deep furrows on the ocean floor, remove corals rocks , stir up sediments, smother benthic organisms creating havoc and destroying a lot of marine life and biodiversity. Dragging of trawl nets and formation of sediment clouds may affect the natural balance between physio-chemical parameters and decreases the dissolved oxygen and the benthic fauna composition . Trawling is known to flush out nutrients and contaminants and release lethal gases such as methane , ammonia hyydrogen sulphide thus affecting the life and mortality of organisms.
Benthic communities also play an important role in remineralisation and release of nutrients in marine ecosystems. All this ultimately affects the entire marine ecosystem and aquatic biodiversity impacting the entire ecoystem. However it is seen that the impact of trawling on the pegalic ecosystem is minimal as they are buffered by their physical environment and phytoplanktons and some other organisms which are a major component of this ecosystem are able to vary their cell nutrient content irrespective of environmental changes due to trawling.
However it is somewhat difficult to analyse the imact sometimes as some pelagic species depend on benthic communities for food and shelter during the early stages. Also the effect might just be to increase the population of some species when compared to other species that may not have a very significant impact as these species are relatives at the top of the food chain. Eg The Gulf of Thailand showed an increase in the cephalopod species when trawling was done frequently.
Stable states are often returned to over a duration of time when trawling is stopped which may not happen in case of benthic communities. Trophic cascades in Benthic marine ecosystems : Fishing Fishing is one of the most exploitative acts of human beings . There are many indirect impacts of fishing other than just removal of target and non target organisms. There are many fishing tools which disturb the benthic natural environment by taking out benthic sessile organisms that provide a critical structural habitat important for the organisms activities.
Fishing can cause mechanical effects to the bottom of the water bed by dragging the fishing gear across it resulting in habitat changes and reduction in refuge for animals. Stellwagen Bank, a heavily fished area, was observed in 1993 and 1994 before and after a certain type of fishing gear was introduced. Benthic communities were adversely affected. (Sustaining Marine Fisheries (1999)). Fishing or harvesting in an unplanned manner leads to changes in the population of aquatic life which can disturb the benthic ecosytem . There is a coupling between benthic and pelagic systems both of which are affected due to this activity.
Since benthic fauna and organisms are the target organisms of fish , reduction in their population can result in increase in density and diversity of infaunal species which is a potential threat to the ecosystem. However once the fishing activity is stopped populations do stabilize and return to normal atleast in case of pelagic systems which easily recover. Climatic changes and global warming environment pollution Global warming and climatic changes have a major impact on Benthic communities . Photosynthesis is inhibited due to global warming and ozone depletion in the benthic zone.
Solar UV-B radiations are found to impair the reproductive processes of organisms in the benthic zone. It reduces the size of benthic populatons and reduces their mortality. Coastal areas in many countries which are developing rapidly with major ports, tourist spots and pollution in coastal areas result in deterioration of Benthic populations Eg Lebanon. Persistent Organic Pollutants(POP’s), Radioactive discharges, PCB’s, Oil and Gas discharges and dumping from ships are potential threats to benthic ecosystems. The ill effects of pollution can be summarised into three main ill effects .
It can increase the concentration of Hydrogen ions thus increasing the acidity in those ecosytems, it can reach toxic levels that impair the ability of benthic organisms to grow and reproduce, and it can ultimately disturb the entire food web . ( The Structure of Marine Ecosystems, Steele, J. H. (1974. )) It is also seen that in case of pelagic ecosystems it is not possible to analyse exactly the direct effect of global warming and climatic changes on the pelagic species and they can be misleading and inaccurate and effect would be in terms of predator requirements.
However it is seen that in case of pelagic phytoplanktons there is decoupling between photosynthesis and nutrient intake Shift in nutrient and bio-chemical ratios will result in changes in the ecosystem. Oftentimes newly introduced species ,such as from ballast water from ships, also pose a threat to benthic ecosystems of an area as they disturb the complex and the delicate food web. Eg the Svalbard area of Norway lists 11 new species( microalgae and benthic organisms) as per the European Environmental Agency. Oil spills from ships and pipeline as well as land-based pollution pose a threat.
In the benthic ecosystem hydrocarbon pollution could cause adverse changes such as destruction of species in the lower trophic level, which could be useful in the breakdown processes of carbon , nitrogen and sulphur cycles and also the disturbance to the food web due to extinction of some species. Studies were conducted during the oil spill from the ocean vessel ‘MV River Princess’ regarding the effect of the oil spill on the benthic organisms. ( Ecotoxicological effect of grounded MV River Princess On Intertidal Benthic Organisms off Goa, Ingole, B &, Sivadas, S &, Goltekar, R (2006).
After research and lab tests it was seen that there were reduced levels of oxygen due to reduced benthic biomass and increased respiratory activity by some aerobic bacteria which can mineralize the Hydrocarbons from the oil spill. Population of various fauna were found to be completely altered thus the biodiversity was adversely affected furthur reducing the concentration and mortality of aquatic life. There was a 60% reduction on the number of species available. All these changes had adverse effect on the marine ecosystem. References 1. Biju, A. K. &, Deepti, G. R. (2006). Trawling and by-catch: Implications on marine ecosystem
2. Ingole, B &, Sivadas, S &, Goltekar, R (2006). Ecotoxicological effect of grounded MV River Princess On Intertidal Benthic Organisms off Goa 3. Orheim, O. (2006). Protecting the environment of the Artic Ecoystem 4. Grassle, J. F. 1997. Report to the Alfred P. Sloan Foundation, Workshop to Consider the Scientific and Technical Aspects of a Census of Marine Benthic Species. 5. Steele, J. H. (1974). The Structure of Marine Ecosystems. Harvard University Press, Cambridge, MA. 6. Sustaining Marine Fisheries ,(1999), Commission on Geosciences, Environment and Resources ,Ocean Studies Board