Green tides are vast accumulations of unattached green macroalgae usually associated with eutrophied marine environments. The term ‘‘green tides’’ or macroalgal blooms covers a heterogeneous set of events that share two characteristics: they are caused predominantly by green macroalgae, and they have a negative impact on the environment. The term ‘green tide’ has been applied to the phenomenon observed since time immemorial. Green seaweeds lying on the beach are part and parcel life in many coastal reasons. They are vast accumulations of green macroalgal biomass that occur under suitable hydrographic conditions in eutrophicated areas. The eutrophication, or nutrient enrichment, of coastal waters as a result of man's activities is now widely recognized as a major, world-wide pollution threat. An increasing number of worldwide reports are also highlighting the response of benthic algal communities to the impact of nutrients. Usually, these refer to the occurrence of excessive growths of macroalgae and the resultant detrimental ecological and environmental consequences. Notably, these excessive growths are largely comprised of green algae and, as deposits on the shoreline, are commonly termed "green tides". It is only when the nutrients become entrapped in coastal systems and relatively quite high levels are reached that the recipient waters can be termed eutrophic and ecological imbalances ensue.
The great majority of blooms are reported to consist of members of just two genera of macroalgae: Ulva and Enteromorpha are mainly responsible for green tides. These are among the world’s most common fouling algae, which are used as model organisms in studies of spore adhesion. The global problem of green tides has increased both in extent and in its public perception over the last three decades.
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| Green Tides |
In fact, blooms have occurred in every marine water body in Europe, Australia, New Zealand, Hong Kong, the Philippines, Indonesia, India, Egypt, China, South Africa, South Korea and Central America. The proliferation of green macroalgae in the littoral zone is well known all over the world. In Europe, typical cases are found in the northern, the coast of Brittany and the Baltic Sea. In Breton, between 1997 and 2001, sites affected by green tides increased from 34 to 63, and the number of Breton cities affected increased from 60 to 103. In 1986, some 25,000 m3 of Ulva spp. accumulated in Lannion Bay in a single season. In 2004, the total volume was 69,225 m3 and 72 municipalities were affected. During the past 7 years, China has been seriously affected by the excessive growth and drift of macroalgae; to such a degree that algal biomass has become a significant problem. In 2007, for the first time, Ulva prolifera (formerly known as Enteromorpha prolifera) bloomed on a small scale in the center of the Yellow Sea. The green tide was reported in Qingdao in 2007. From that time, green tide outbreaks occurred every year for 7 years, with the largest scale even happening in 2008. Masses of Ulva floated in from the open water of the Yellow Sea and beached a few weeks before the competition was due to start, ensuring prominent coverage by the international media. A 30-km-long bloom was deployed to keep the masses of floating algae out of the bay, and the removal of more than a million tons of algae from the beaches involved 10,000 people at an estimated cost to the province of $30 million. The Ulva strain responsible for the green tide could be overwintering on the sediment surface south of the Yellow Sea as fragments that stem from the summer surface bloom.The same phenomenon occurred in the southwest sea of Korea in 2009, and the green mats were senescent with reduced physiological activity. The green tide reoccurred on the southwest sea of Korea in July 2011, and off the coasts of China and this has become an annual event in the YS (Yellow Sea).
Although the loss of these communities of large macroalgae and seagrasses in eutrophicated waters can frequently be attributed to secondary pollutants, e.g. suspended particulate material, toxic ingredients etc., a number of reports have suggested that they are ousted by algae more readily adaptable to the high nutrient loadings. In general, the great majority of reports refer to an increase in the number of green algae associated with eutrophicated waters. Many of these algae, especially the green algae, are often described by authors as annual, ephemeral, fast growing and opportunistic and are well known as pioneering organisms on recently exposed or denuded surfaces. Essentially, the 'polluted' environment maintains an immature community characteristic of an early seral stage.
However, it is only a relatively small number of algal genera which are universally and commonly reported in eutrophicated waters. With the exception of the brown alga Ectocarpus and to a lesser extent Pilayella, they are almost exclusively representatives of the Chlorophyta, and notably species of the genera Chaetomorpha, Cladophora, Enteromorpha and Ulva. As the only algae present at the polluted sites, they frequently occur in large quantities. When these excessive, mat-like growths of algae are washed up periodically on the tide line they are descriptively termed "green tides".
Particular interest has been directed towards obtaining a better understanding of the biology of the relatively small number of genera (and species) which contribute to the formation of these "green tides". In general, they can be termed opportunistic and are frequently reported as early colonizers or pioneering organisms on newly immersed, denuded or disturbed substrata.
A number of workers have drawn attention to the morphology of many of the "green tide" components and the role this might play in nutrient absorption. They suggest that the frequently recorded filamentous, sheet-like, tubular and ball-like nature of their thalli produces a high ratio of surface to volume which ensures maximum nutrient uptake.
Restrict water movement and velocity can lead to increased sedimentation rates, it can interfere with oxygen transport, and it can also restrict plankton transport, with obvious implications for the supply of food for suspension feeders. It will also restrict larval movement, preventing the settlement and recruitment of polychaete and bivalve larvae. The algal mats would also interfere with surface feeding deposit feeders and influence epifaunal associations. The large mats of algae would also shade and reduce the photosynthetic activity of underlying benthic seagrass and algal communities as well as physically press down and abrade them.
The sudden appearance of massive amounts of algae will always influence the ecological system to some extent. The seaweed covers the ocean surface in large scale and results in low solar transmission, leading to hypoxia under the seaweed. It takes time for recovery to occur. It can also influence the drainage system in the intertidal zone to some extent. Although green tides can reduce the nutrients in the seawater to some extent and prevent the redevelopment of red tides, such outbreaks also greatly affect marine ecosystem. Green tides can influence other algae’s growth by allelopathy (the inhibition of growth in one species of plants by chemicals produced by another species) nutrition competition; long-term green tides are likely to be a threat to the survival and reproduction of ocean zooplankton; green tide can also threaten marine macroorganisms, such as scallops, abalone, and so on. When green tides die down, they decay gradually and give off a foul odor in the irradiance of sunlight. This, in turn, causes environmental pollution and interrupts sightseeing and marine sports.
The worldwide occurrence and ecological impact of "green tides" have aroused considerable concern and much consideration has been given towards the development and implementation of various control methods. Although there is some evidence that grazing by invertebrates can exert some control on biomass development, in the majority of "green tide" situations and during the peak periods of biomass production, such natural biological control methods are totally inadequate and treatment is required. A better understanding of the origin and persistence of green tide blooms is desirable in order to address the problems they cause. Even simple taxonomic identification is confounded because the unattached algae are often morphologically atypical.
Green tides occur in many coastal regions of the temperate zone of our planet. Ulva spp. is the most prevalent of the algae found in such events. The frequency, duration, and intensity of algal blooms are related to a number of biological, chemical, and physical factors; however, many of these complex relationships have not yet been identified. There are some very clear linkages between land-based activities and green tides. However, many green tides occur in relatively pristine waters and show no influence from pollution or other human activities. Indeed, green tides can occur in a variety of areas and hydrographic conditions, including upwelling regions and pristine coastal waters with moderate nutrient levels. Green tides are currently not managed sufficiently. Management options include prevention, control, and mitigation. Prevention is directed at minimizing anthropogenic impacts that may contribute to bloom formation. Legislated reductions in chemical and nutrient inputs may successfully reduce algal growth in certain cases where such acts can be fully enforced. We currently lack sufficient insight into mechanisms regulating blooms of any green tide species to implement effective species-specific controls.
The case for a direct connection between spreading coastal eutrophication and the worldwide upsurge in the incidence of green tides is compelling. However, curbing eutrophication requires significant investments in infrastructure and agricultural practices in the catchment area and can take years to implement, and even longer to take effect.
Probably the most widely used treatment involves the physical removal of the troublesome algal biomass. This particularly applies to situations where large quantities of weed are present e.g. 85 000 m3 of harvestable Ulva deposits on beaches in Brittany and an estimated 1-1.2 million tons of Ulva growing in the Venice. Usually, the algae are mechanically removed following its accumulation on the tide line, and transported to landfill sites. In some situations, the algal biomass is removed in situ.
Other control methods which have been considered include:
- Improving water circulation and current flow and thereby, preventing the buildup of nutrients;
- Lowering the salinity of the contaminated waters by a controlled freshwater influx in order to kill the algae.
- Removal of nutrients from effluents at the tertiary sewage treatment stage or at least regulating the nutrient composition to produce an effluent less favorable to algal growth.
- Reducing the discharge of nitrogen and phosphorus pollutants.
- Strengthen the control of the source.
- Establish a sound and responsive system of environmental protection.
Nutrient extraction and regulation is, however, an expensive process and would not be appropriate in regions where the nutrient loading was derived principally from agricultural runoff or if the bulk of nutrients are from diffuse sources via river run-off; the latter would require long-term reductions in the use of fertilizers. One interesting prospect is the removal of nutrients by mariculture techniques. The eutrophicated regions could provide suitable conditions for the commercial production of economically important species. There are still unanswered questions regarding the bloom-forming species because of their unusual physiological and ecological characteristics. Given the ecological importance of this bloom, an understanding of the causation and potential management of this nuisance phenomenon must begin with the correct identification of the algae involved.
Finally, harvesting floating macroalgae are the logical and ultimate step on the process known as ‘fishing down marine food webs. The future impact of green tides could be very different if they become regarded as potential crops rather than harmful weeds.
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