Tsunamis, i.e. huge waves generated at sea by earthquakes, underwater volcanic eruptions, landslides etc. can cause destruction to the littoral, tidal and near shore marine environments, undermining their ability to sustain marine life and coastal livelihoods.
Damage done to littoral and sub-littoral areas varies depending on the distance from the epicentre, the exposure to the direction of the tsunami and the nature of the coastal area (rock, coral, beach, etc.) and may have a number of consequences for fisheries and fish habitats. Main impacts result from mechanical damage and physical transport by the wave resulting in habitat loss (e.g. destruction of reef areas and other inshore vulnerable habitats) and eggs, larvae and juveniles contained in the inshore areas been washed ashore. Transportation of material due to receding waters from the flooded habitats results in large amounts of rubble, organic material, sand, mud and any contaminants present in the inundated area moving to the inshore areas further modifying the original habitats. Decreases in bio-diversity could happen simultaneously with a shift in species dominance with possible increase in detrital feeders. The large quantity of sediments and nutrients washed out may boost phytoplankton productivity and possibly favour small-pelagic species.
Massive fish kills affecting large biomasses across relatively large aquatic areas are often triggered by unpredictable environmental conditions resulting from accidental (or sometimes illegal) events such as the release of toxic substances from industrial waste storage facilities (e.g. following a fire or mine tailing impoundment after heavy rains or from malfunctioning waste treatment facilities). As well, heavy winds, resulting in up-welling of oxygen-depleted bottom water, can be the cause of fish kills in stratified coastal waters and great lakes. The fish kills themselves may aggregate oxygen depletion as large amounts of dead organisms accumulate on the bottom. Fish kills can also result from cold spikes (e.g. in the Northern Adriatic) when exceptionally cold atmospheric conditions reduce water temperature in coastal areas, provoking massive kills of sessile or benthic populations (e.g. Norwegian lobsters).
Harmful Algal Blooms
About 300 hundred species of micro-algae can form mass occurrences at times, nearly one fourth of which produce toxins. Scientifically referred to as 'Harmful Algal Bloom' (HAB), they are commonly called "red tides" as the large patches of planktonic concentration may appear red on the surface. Such proliferation of micro-algae - thought to be activated by environmental changes - in marine or brackish waters can cause massive fish kills, contaminate seafood with toxins and alter ecosystems in harmful ways. A broad classification of HABs distinguishes two groups of organisms: the toxin producers, which can contaminate seafood or kill fish, and the high-biomass producers, which can cause anoxia and indiscriminate kills of marine life after reaching dense concentrations.
The impact of harmful micro-algae is particularly evident when marine food resources, such as mussels grown in aquaculture facilities, are affected. Shellfish, and in some cases finfish, are often not visibly affected by the algae but nonetheless accumulate the toxins in their organs. The toxins may subsequently be transmitted to humans through consumption of contaminated seafood and become a serious health threat.
Tankers carry about 1 000 million tonnes of oil per year, about 50% of which is extracted on the continental shelf. On some offshore oil fields shuttle tankers are the main way of delivering hydrocarbons to the onshore terminals. The principal causes of tanker accidents that lead to large spills include running aground and into shore reefs, collisions with other vessels, and cargo fires and explosions. More recently, a number of accidents occurred when vessel structures collapsed during severe weather conditions.
Tanker accidents often result in vast oil spills or massive release of harmful chemicals transported in bulk. According to the International Maritime Organization (IMO), the amounts of oil spilled during tanker accidents in 1989 and in 1990 were 114 000 and 45 000 tonnes, respectively. Some of the most substantial oil spills have been: the Torrey Canyon in 1967 with 95 000 tonnes of oil spilled on the French and British shores; the Amoco Cadiz in 1978 with 220 000 tonnes of oil spilled, the Exxon Valdez in 1989 with 40 000 tonnes and the Braer in 1993 with 85 000 tonnes of oil spilled. The spill caused in 2002 by the Prestige transporting 77 000 tonnes of oil resulted in significant impacts on the fisheries and aquaculture sectors in Galicia, Spain.
The most dangerous are accidents involving underwater storage tanks that contain dangerous substances such as methane. Such incidents are possible and the resulting spills could have disastrous effects on coastal ecosystems, fisheries, aquaculture installations and, in some instances, on human life. Major impacts of oil spills on fisheries and aquaculture are the smearing of nets and fish cages and the tainting of fish and shellfish, rendering them unfit for marketing. Longer-term impacts on the ecosystem depend on the nature of the pollutant and the ecological characteristics of the area.
Complex and extensive systems of underwater pipelines, amounting to thousands of kilometres, are among the greatest environmental risk factors during offshore oil developments, along with tankers and drilling operations. Causes of accidents range from defective material, pipe corrosion, ground erosion, tectonic movements, ship anchors and bottom trawls. Depending on the damage, the pipeline may become a source of leakage or of an abrupt (even explosive) blowout of hydrocarbons near the bottom. The intensity and scale of toxic impacts on the marine biota in the accident zone varies depending on a combination of many factors. It is important to consider that, in a number of cases, the accidental oil and gas spills and blowouts of mainland pipelines threatens coastal marine ecosystems. This can occur when such accidents take place, for example, near big rivers where river pollution eventually affects the sea zone near the river mouth.
Drilling accidents are usually associated with unexpected blowouts of liquid and gaseous hydrocarbons from the well. Only tanker oil spills compete with drilling accidents in regularity and severity. Some rare occurrences could lead to catastrophic situations involving intense and prolonged hydrocarbon gushing. Often they involve hydrocarbon spills and blowouts during drilling operations. Usually, these accidents do not attract any special attention. Nonetheless, the ecological hazards and associated environmental risks can be considerable - especially given their frequency - and could ultimately lead to chronic impacts on the marine environment.