Seagrass: canaries of the sea

By Juliana Imenis, Juliana Nascimento, Larissa de Araujo, Natalia Pirani, Otto Muller and Paula Keshia

Edited by Katyanne M. Shoemaker

In the early 20th century, coal miners frequently carried caged canaries to work. The little birds saved many miners' lives because their sudden death or sickness indicated a possible gas leak. An alarm would sound and the mine would be evacuated.

We could say the canaries were bioindicators, or organisms that indicate a possible environmental problem through their behavior or health status. Today, we no longer have a need to sacrifice the canaries because we have electronic indicators that can tell us about possible mine disasters.

Like the canary, some organisms are extremely sensitive to pollution and habitat alterations; their populations tend to diminish or even vanish quickly after environmental modifications take place. Other organisms may be very tolerant to poor environmental conditions and can sometimes have a population boom in areas where the conditions would be inadequate to the majority of other species. One of these bioindicators is the marine phanerogam, also known as marine seagrass.

Image by Joana Ho

This particular group of plants grow on the sea floor, have elongated straight leaves, and subterraneous stalks, called rhizomes. Seagrass may live completely immersed in water, and they are found in coastal waters of nearly every continent. Despite being known as “seagrass”, this group is closer to the lily and ginger families than grass (Figure 1). They are an important part of the diet of manatees and sea turtles, and they are used as habitat by many other sea animals (Figure 2), including commercially important fish and crustaceans. Although difficult to quantify, seagrasses have a large aggregated commercial value, estimated to be up to 2 million dollars a year. They also play an important role in sequestering carbon into their biomass and sediment, thus decreasing the carbon dioxide (CO2) concentrations in the atmosphere. This helps promote nutrient recycling, coastal protection, and improve overall water quality.

In Brazil, despite controversial information and the necessity of more genetic studies to differentiate the species correctly, there are so far, five known species of seagrass (Figure 3): Halodule wrightii Ascherson; Halodule emarginata Hartog; Halophila baillonii Ascherson; Halophila decipiens Ostenfeld and Rupia maritima Linnaeus. Seagrass are considered to be great environmental quality indicators, because they are very sensitive to light and nutrient availability variations.

Global climate change has many impacts on the marine environment, including the rise of global average sea surface temperatures, variations in pH (ocean acidification), and alterations of ocean currents. These are some of the rapid changes in marine environment that have been seen by researchers, and their consequences are still little known. There are many factors involved in the interactions between environmental variables and biological communities, making overall consequences hard to forecast (Figure 4).

Figure 3 – Identification of seagrass species can be controversial, but nowadays it is defined that there are five species along the Brazilian coast. Marques & Creed 2008.

Seagrass need specific environmental conditions, like low turbidity and high incidence of light. They are suffering local reduction and in some places completely vanishing, indicating that the anthropegenic environmental changes are happening fast, not giving the organisms enough time to respond to the new conditions. The capacity of ecosystems to respond to impact and return/maintain their original conditions is called resilience.

Although the degree and type of impact on seagrass may vary with geography, some hypothesis were generated by the Benthic Habitat Monitoring Network (ReBentos) about how climate change may affect them: (1) the increased concentration of nutrients, given the increased quantity of rain, may cause changes in the community composition, favoring the occurrence of opportunistic species, which can be damaging for the local species; (2) changes in sea surface temperature can affect tropical species, favoring the extension and displacement of their occurrence limits towards higher latitudes; (3) extreme events, like floods and storms, may cause reduction or disappearance of seagrass in a quick and abrupt way; (4) the increased quantity of continental matter in estuaries may affect the abundance and composition of the communities, due to the increased turbidity and salinity changes. On the other hand, the reduction of rain and/or increased penetration of seawater into continental waters could increase or alter the estuarine seagrass' area of occupation; and finally (5) days or week-long heat waves, derived from external events, may reduce or extinguish fields in shallow areas.

As an example of evidences that support these hypothesis, we can mention a study published by the Journal of Experimental Marine Biology and Ecology by Ricardo Coutinho and Ulrich Seeliger, that, in 1984, observed that the species R. maritima, although tolerant with eutrophicated conditions, was shadowed by epiphytes and macroalgae that grew due to an excess of nutrients in the water. Those organisms tangle in this seagrass species, causing reduction on its photosynthetic rates and increasing their drag, facilitating their detachment when subjected to waves and currents. Another example is the study published in the Marine Ecology by Frederick T. Short and collaborators, that in 2006 observed the reduction of H. hrightii through the movement of sediment, caused by stronger and more frequent storms, which buried the fields of seagrass.

Therefore, as mentioned by other authors, we can consider seagrass as the canaries of the sea, important in diagnosing the environment's health in response to global climate change. Certainly, the loss of these ecosystems will bring not only economic loss, but also the loss of biodiversity, a factor that is much more valuable and difficult to measure.

To know more:

COPERTINO, M.S.; CREED, J.C.; MAGALHÃES, K.M.; BARROS, K.V.S.; LANARI, M.O.; ARÉVALO, P.R.; HORTA, P.A. (2015). Monitoramento dos fundos vegetados submersos (pradarias submersas). IN: TURRA, A.; DENADAI, M. R.. Protocolos de campo para o monitoramento de habitats bentônicos costeiros - ReBentos, cap. 2, p. 17-47. São Paulo: Instituto Oceanográfico da Universidade de São Paulo. Disponível em: <>. Acesso em: 04 nov. 2015.

MARQUES, L. V.; CREED, J. C.(2008). Biologia e ecologia das fanerógamas marinhas do Brasil. Oecologia Brasiliensis, v. 12, n. 2, p. 315 - 331.

MCKENZIE, L.(2008). Seagrass Educators Handbook. Cairns: Seagrass Watch-HQ. Disponível em: <>. Acesso em: 30 out. 2015.

MCKENZIE, L (2009). Coastal Canaries. Seagrass Watch, v.39, p. 2-4. Disponível em: <>. Acesso em: 03 nov. 2015.

About the authors:

Juliana Imenis Barradas, CCNH-UFABC, PhD student in the postgraduate program in Evolution and Diversity, biologist, Master in Zoology (UFPB).,

Larissa de Araujo Kawabe, CCNH-UFABC, master graduate student of in the postgraduate program in Evolution and Diversity, biologist.

Juliana Nascimento Silva, CECS-UFABC, undergrad in Environmental and Urban Engineering (UFABC).

Paula Keshia Rosa Silva, CCNH-UFABC, master graduate student of in the postgraduate program in Evolution and Diversity.

Natalia Pirani Ghilardi-Lopes, CCNH-UFABC, Adjunct Professor, Biologist, Assistant Professor, Biologist, holds a doctorate in Botanic (USP).

Otto Müller Patrão de Oliveira, CCNH-UFABC, Adjunct Professor, Biologist, holds a doctorate in Zoology (USP),

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