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Why Is Seaweed Biodiversity Important for Our Coastal Ecosystems?

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Published: 10th May 2021 in Environmental Sciences

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Biodiversity is a key factor to the stability and longevity of an ecosystem (Bracken and Stachowicz, 2006). Biodiverse environments are more productive because of the relationships between the organisms within them, such as symbiotic relationships (Bertness et al., 1999). There is agreement between numerous studies that the diversity of different species within an ecosystem has an effect on how well the ecosystem functions in terms of stability, productivity and resilience to disturbances (Bracken and Stachowicz, 2006; Loreau et al., 2001Kinzig et al., 2002). This essay will discuss how seaweed biodiversity improves the health and stability of coastal habitats and how the recent increases in biodiversity loss may have detrimental effects on other survival of numerous dependent species.

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Seaweeds are known as “foundation species” in marine communities. They have influence on the structure of a community through their modification of biotic factors, interactions between species and resource availability (Bruno and Bertness, 2001). Seaweeds facilitate interactions between species by making unsuitable environmental conditions more tolerable, for example, seaweeds provide shade and shelter and reduce the effects of evaporation and extreme temperature fluctuations (Bertness and Callaway, 1994; Bulleri, 2009). Seaweeds are a key species in coastal ecosystems as they provide space for marine microorganisms, fishes may lay their eggs in the seaweed’s shelter and they provide structure to the biodiversity of a habitat. Seaweeds are primary producers and are used as bioindicators to provide insight into the level of pollution in the waters of a habitat. (Satheesh et al., 2017). A decline in biodiversity in ecosystem can have important impacts on ecosystem processes such as nitrogen cycling and carbon sequestration (Bracken and Stachowicz, 2006).

It is known that diversity increases the strength of ecosystems and makes them more resilient to negative impacts such as abiotic and biotic stressors. Studies such as one by Stachowicz et al., (2008) have proven this. They found that over the course of 9 months, diversity of seaweeds in this area of Northern California increased the overall seaweed cover in the intertidal zone and reduced the amount of free space, this reduction in free space benefits the community as it would make it more difficult for successful invasions of non-native seaweeds to occur. Such invasions can have negative impacts on the biodiversity of an ecosystem (Armitage, 2017­). Diversity also increases the rate of recovery after disturbances (Stachowicz et al., 2008). This study found that seaweed coverage in the area imposed a  cascading effect on the structure and species diversity of the rest of the organisms in the ecosystem as it is a prime area for colonization by other algae and small invertebrates, creates more tolerable living conditions and as a source of food (Foster et al., 2003, Bertness et al. 1999).

Seaweeds absorb minerals and gases directly from seawater by osmosis through the surface of their blades. The uptake of nitrogen by seaweeds is an important aspect for the survival of habitat because increased levels of nitrogen forms in water is toxic. Seaweed diversity has been noted to enhance nitrogen uptake (Bracken and Stachowicz, 2006). Research on this topic by Bracken and Stachowicz, (2006), demonstrates that with a greater diversity of seaweeds in an environment the amount of nitrate and ammonium taken up by the seaweeds was 22% more than the amounts used by a single species, this is due to the different needs that some seaweeds have to others. The importance of seaweed biodiversity in this case is that elevated levels of nitrogen in marine environments can create toxic conditions and lead to the growth of harmful algal blooms, which can be dangerous to both marine life and human health (Auro and Cochlan, 2013).

The importance of seaweed biodiversity in the battle against climate change has had growing interest in recent years, particularly into how macroalgae add to the carbon sequestration budget of the marine environment. They are known to capture CO2 from their environment and use it to create new biomass, but recent studies have shown that large seaweed mats are absorbing carbon from the atmosphere and then sinking to large depths, meaning the carbon is highly unlikely to return to the atmosphere and add to greenhouse gases (Mcleod et al., 2011; Ortega et al., 2019).

This ability to absorb minerals from their surroundings makes marine plants reliable bioindicators for the health of the environment in which they inhabit. Seaweeds, such as Ascophylum nodosum, are often used as bioindicators as they can hold high concentrations of nutrients or pollutants for a long period of time and are generally more accurate than using the seawater for a temporal look at pollution levels in an environment (Vasquez and Guerra, 1996; Stengel et al., 2004).

Kelps such as Laminaria saccharina, Saccorhiza polyschides (Lightfoot) Batters; Alaria esculenta (L.) Greville; Laminaria hyperborea (Gunnerus) Foslie; Laminaria digitata (Hudson) J.V. Laminariales are very common and are native to Irish coasts. Also known as “keystone species”, kelp forests are amongst the most productive and diverse habitats on Earth (Smale et al., 2013). Kelp forests provide structure, habitat and shading, and are a source of food for a large variety of both macro and microorganisms (Kelly, 2005).  Keystone species are those that are fundamental to the function of a habitat, their removal would have significant negative impacts. Amongst the diversity of seaweeds available to settle on there are some that support more life than others, for example, the colonisation of L. digitata is notably less favourable than that of L. hyperborea, this could be due to the location of the plant as L. digitata is better adapted to tolerating wave impact and is therefore found higher up the intertidal zone (Schultze et al., 1990; Kelly, 2005).

Kelps are involved in many important trophic cascades including the well-studied example of kelp-urchin-otter trophic cascade (Estes and Duggins, 1995; Wilmers et al., 2012). In this relationship, sea urchins act as extremely efficient predators of kelp and, without being predated by otters, kelp forest densities would be decimated by unrivalled urchins. In turn, kelps provide shelter and protection to otters, this is an example of the importance of biodiversity in coastal ecosystems (Estes and Duggins, 1995; Lorentson et al., 2010). In recent years, it has been observed that kelp forests in west Atlantic have been subject to invasions by Codium fragile, a chlorophyte, with the potential to take the place of kelp as the dominant species in these areas (Polunin, 2009). This invasion requires sufficient deterioration of kelp cover for C. fragile to take its place, this could be aided by human activity such as harvesting or overfishing (Polunin, 2009; Lorentson et al., 2010). Another driver is the parasitic bryozoan, Membranipora membranacea, which decimates kelp communities but does not affect C. fragile (Levin et al. 2003; Polunin, 2009). This is an example of how climate change is affecting the biodiversity of coastal ecosystems as a non-native species is more resilient to this biotic threat than Laminaria spp. which has continually been threatened by this species. Described as a “synergistic relationship”, this interaction between C. fragile and M. membranacea is expected to have damaging effects on those vertebrates and invertebrates which rely on kelps as a food source and for shelter, leading to reductions in biodiversity of ecosystems (Burridge, 2012).

Non-native species have been identified as a threat to the biodiversity of ecosystems. Along with this, coastal environments can be influenced by a number of other factors such as climate change, eutrophication, overfishing and coastal development. It is currently not definite as to how climate change may facilitate non-native seaweeds, but human activity has definite influence on their dispersal. This is important as seaweeds have a key role in coastal temperate ecosystems (Armitage, 2017).

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On the contrary to this, the introduction of non-native species may actually have a positive effect on the biodiversity of some ecosystems by replacing a native species which impeded the growth of smaller species. Long-term research conducted by Dijkstra et al., (2017) presented results which confirm that the introduction of non-native species to an area increased species biodiversity within site, this is not just withheld to seaweed species but also the primary consumers of these species, and ,indirectly, larger animals further up the food chain. This conclusion was based on the observation of a sub-tidal site along the Gulf of Maine over a span of 30 years and showed a decline in the dominance of native foundation species e.g. Laminaria saccharina, and an increase in invasive seaweeds, these seaweeds, such as Codium fragile, were observed to support a larger variety of species (Dijkstra et al., 2017).

Biodiversity loss is the greatest environmental challenge facing humanity. The reduction of biodiversity globally is mainly caused by human activity which degrades and impedes natural habitats. Seaweeds are drivers of habitat formation and support, they provide shelter and nutrients to the many organisms and communities that interact with them. Seaweed species are also economically important so their loss would be felt not just by marine organisms but populations worldwide. Human activity such as urbanisation and urban sprawl, especially in coastal locations, is a cause of dramatic decline in the populations of key seaweed species. Conservation and restoration of these species must be further enforced, and their importance must be more widely known. An example of the effect human activities can have on habitat forming is species is that of Phyllospora comosa, which was once a common species in the coastal habitats around Sydney, Australia but was decimated in the 1980s due to a constant input of sewage waste into the habitats. A study by Marzinelli et al., (2013), observed and quantified the organisms within healthy communities of Phyllospora habitats and compared them to habitats formed by Ecklonia radiata. Although there were similarities between the two habitats, Phyllospora supports a larger number of large invertebrates, which brought the conclusion that the restoration of damaged habitats was most important for the biodiversity and stability of numerous species in these coastal areas (Marzinelli et al., 2013).

A study by William et al (2013) investigated the effects of several stressors on seaweed biodiversity and abundance in northern California. Investigating biodiversity at two different tide heights, researchers use screens as blockades, so temperatures were decreased, predators were removed, and extra nutrients were added to the survey area. The study methods reduced temperatures in the area, increased humidity, reduced predator stress and caused a rise in nitrogen levels in both the seaweed and seawater (Williams, Bracken and Jones, 2013). It was found that seaweed biodiversity and abundance were impacted by the tide height, physical stressors and predators. The effects that physical stress and herbivores had on seaweed biodiversity were not related to one another and impacted the seaweed independently, this is an example of how seaweeds can express resistant responses to multiple different stressors at a time and their capabilities of adapting with environmental changes. Interestingly, it is noted that nutrient availability had no significant influence on seaweed biodiversity (Williams, Bracken and Jones, 2013).

The commercial harvesting of seaweeds has been a driver for the depletion of species abundance and biodiversity. Kelps are harvested globally for many different uses such as; a food source, animal feed, cosmetics and for medicinal purposes (Lorentson et al., 2010). This is due to the pigments found in these species which have been found to have a number of health benefits such as anti-coagulants and anti-oxidants in Laminarans. Apart from kelps, there are many other seaweeds which are traditionally harvested for a variety of purposes such as Ulva spp., Porphyra, Ascophylum nodosum and Chondrus crispus. These species are all harvested both by hand and mechanically, and with both of these there are significant negative impacts which may occur when overharvested. Harvesting can cause long-term deterioration to both the in-demand species but also the dependents on this species, which in turn negatively impacts the biodiversity of the disturbed, harvested location. Recovery of seaweed populations after harvesting is slow, a study by Mathieson and Tveter, (1975), discovered that only an 80% recovery was made by harvested C. crispus populations after 9 months. This highlights the importance of regulations needed to reduce the chance of overharvesting without giving seaweed populations time to recover adequately.

A high biodiversity of seaweeds in coastal areas is indicative of a healthy ecosystem with great species richness (Bertness and Callaway, 1994). As foundation species, seaweeds are often considered the most important part of the coastal ecosystem due to the fact that if they were not present in these areas, many other organisms would struggle to survive in these locations (Bruno and Bertness, 2001). A high diversity of seaweeds also shows that water quality in the area is suitable for a high diversity of life and not just tolerable to some (Matthiessen, 1995; Stachowicz et al., 2008). It is unfortunate that with an increase in human activities such as overfishing and harvesting, habitats such as kelp forests are becoming less resilient to the attack of invasive seaweed species, which can reduce the biodiversity of ecosystems and remove important shelter and food sources for some organisms (Armitage, 2017). For the future, as invasions become unavoidable, there are some actions that may be implemented for the protection of natural biodiverse seaweed accumulations such as increased regulation in aquaculture industries and improvements in the water quality monitoring and management. The importance of seaweed biodiversity can be felt at all taxonomic levels as they are the primary producers in numerous food chains and many seaweed species are economically important for these reasons the maintenance of seaweed biodiversity must also be prioritised, especially with the most biodiverse species population in Britain and Ireland being located at Finavarra, Co. Clare.

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