Green, brown and red seaweeds scattered on the beach is a normal occurrence in many coastal regions. The volume of beached seaweed biomass started to increase along the shores of industrialised nations in the 1970s and had become a major nuisance by the 1990s, when mass beaching of macroalgae started to be known as green tides (Fletcher 1996; Valiela 1997). The frequency and scale of green tides increased further in the 2000s (Ye 2011). Though non toxic to humans, seaweed tides overwhelm by their sheer size. Tonnes of seaweed choking the shoreline deter tourists and the dense, drifting mass can block swimmers and small boats from accessing the sea; failure to clean the shore can result in the mass turning into stinking bogs of decaying organic matter, which can produce toxic hydrogen sulphide (H2S) from its anoxic interior and have major negative effects on the already weakened coastal ecosystems (Ye 2011; Arroyo 2012)
Surprisingly, only a few genera of macroalgae are responsible for the massive seaweed tides. Two genera are especially prominent. Species of the genus Ulva, which now includes the former genus Enteromorpha, are primarily responsible for the green tides (Hayden 2003). The thallus (vegetative body) is only one or two cells thick but the shapes can vary even within species and can be sheet-like, tubular or fern-shaped (Blomster 2002). Sargassum—from which
the Sargasso Sea takes its name—is the other genus. The term ‘golden tide’ will be befitting to describe the massive shoaling events it is responsible for—after the apt description of floating Sargassum as “the golden floating rainforest of the Atlantic Ocean” (Laffoley 2011). The Sargassum thallus is leathery, tough and differentiated into features that resemble leaves and a stem and has well-developed gas bladders for flotation. Both Ulva and Sargassum are cosmopolitan, exceptionally species rich genera and can exponentially increase their growth rate in response to nutrients (Lapointe 1995; Teichberg 2010). Whereas most species will only grow when attached to a hard substrate, a few can substantially increase their biomass in a free-floating state, either by increasing the size of the thalli and their fragments, or by making new floating thalli. This is crucial because it is the unattached forms that, by invading new water columns, are able to increase their nutrient supplies, free themselves from competition for limited hard substrates and avoid their many benthic grazers. As a result, unattached forms can build up large biomasses, forming massive seaweed tides. Green tides have occurred all over the world, whereas the golden tides have been restricted to beaches between the Gulf of Mexico and Bermuda. However, they significantly increased their range during a spectacular 2011 event (Fig. 1).
Ulva green tides
The problem of burgeoning Ulva biomass on European and US beaches that began in the 1970s was linked to coastal eutrophication (Fletcher 1996) . As the multitude of negative effects became evident the affected countries took measures to reduce nutrient inputs to the sea from agro and waste sources. A decline in nutrient concentrations resulted in abatement of the problem in the southern North Sea (Van Beusekom 2009). In other regions, particularly along the popular touristy beaches of Brittany, the magnitude of green tides has been steadily increasing since the 1970s (Charlier 2008). Beached seaweed has traditionally been collected and used as fertilizer by local farmers, but by the 1990s it had to be taken away by the truckload.
Sargassum golden tides
Golden tides due to the beaching of floating Sargassum are a regular occurrence in summer along the coasts of the Gulf of Mexico and plague the tourist beaches. Higher nutrient loads of the Mississippi river are largely responsible for the surge of golden tides in the 1980s and 1990s. However, less is known about sargassum in the Gulf of Mexico when compared to Sargasso Sea. Each year since, except 2013, the algae have formed a similar, vast belt. In 2018, that belt was the largest and densest containing at least 20 million metric tonnes of algae (Gramling 2018).
Mitigation or amelioration?
A thorough understanding of the growth trajectories of the massive seaweed tide causing species is not only a prerequisite for developing viable mitigation strategies, but it could potentially help turn this menace into a valuable resource for the fodder industry. Ulva biomass contains a number of compounds of interest to the food-additive industry and a biorefinery plant to process the Ulva biomass collected from the beach or shallow water has recently been established in the region of Brittany plagued by green tides. In the case of Sargassum, also a valuable raw material, harvesting by ship is already regulated in the western Atlantic. Needless to say, harvesting floating macroalgae is the logical and ultimate step in the process known as ‘fishing down marine food webs’. It should also be pointed out that the carbon-to-nitrogen ratio of oceanic Sargassum is around 50:1. (Lapointe 1995) The future potential of green and golden tides could be very different if they become regarded as potential high value crops rather than harmful weeds.
Note: The article is adapted from an article by Smetacek V, et al. 2013. Green and golden seaweed tides on the rise, Available at : doi:10.1038/nature12860
Gramling C, The largest seaweed bloom ever detected spanned the Atlantic in 2018, Available at: https://www.sciencenews.org/article/largest-seaweed-bloom-ever-detected-atlantic-ocean-2018?tgt=nr (2019)
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