Seagrass is key for carbon storage but shading from man-made structures is putting seagrass meadows at risk. Associate Editor Nathalie Butt discusses the recent article, Effects of small-scale, shading-induced seagrass loss on blue carbon storage: Implications for management of degraded seagrass ecosystems by Stacey Trevathan-Tackett et al.
Carbon storage in the sea
Given ever-increasing global emissions, natural systems and organisms that can absorb and store carbon are becoming more critical to our attempts to keep global warming below 2°C and mitigate climate change. We are well aware of the importance of trees, forests and soils that act as carbon sinks, but what about blue carbon, the carbon captured by oceans and coastal ecosystems? How important is this blue carbon stock, and how can we ensure that it is maintained or preserved?
Along our coasts and in our oceans, which occupy around 70% of the surface area of the Earth, blue carbon is stored in organisms and in sediment, in four main types of ecosystem: mangroves, salt marshes, algal systems, and seagrass, usually in seagrass meadows. Seagrass can grow at depths of up to 50 metres, is found along the coastlines of all continents except Antarctica, and stores almost 20 Gt of carbon overall (a little more than the total stored in forests in the Democratic Republic of Congo, for example). In addition to this globally important ecosystem service, seagrass also provides habitats and nursery areas, and food, for many marine species, including fish and turtles.
Seagrass at risk
Over the last hundred years or so, about a third of all seagrass beds worldwide have been lost, most rapidly during the last three decades. Weather events such as storms or ocean surges can cause this, but natural processes only account for a small proportion of seagrass loss: human drivers are responsible for most. These drivers can be indirect;through eutrophication from land runoff, or more direct; through dredging or other disturbance. Effects can vary in size, and although small-scale anthropogenic disruptions such as shading from infrastructure (jetties, platforms, etc.), damage from boats and human trampling can themselves be small and local, their impacts can be significant. However, the impacts of these local disturbances are not well known. In their recent paper, Trevathan-Trackett et al. set out to investigate and quantify both the loss of seagrass from shading, and its potential impact on blue carbon stocks.
In their experiment, the authors investigated the effect of shading on two seagrass species, shoal grass and turtle grass, in a human amenity location in the Gulf of Mexico, a highly variable environment. Shoal grass stands are found closest to the shoreline, followed by mixed stands further out, and then by turtle grass only. Using control plots, bare plots, and shaded ‘kill’ treatment plots, rhizosphere (sediment and dead biomass) samples were collected at several points during the study period, and analysed for organic matter content and quality. The largest loss of organic matter was found in the top 8 cm of the shoal grass plots and in the top 1 cm of the turtle grass plots. In terms of carbon content, the study found that even small scale losses of seagrass indicated significant loss of carbon in the top few centimetres of sediment. As this loss is quite shallow, it may be that CO2 emissions could be initially minimised by decomposing seagrass. However, over time, an increase in emissions would be expected from degraded habitats, as with terrestrial natural vegetation habitats.
Reducing shading, protecting seagrass
So how best to address the challenge from increasing shading, due to increased human use of coastal areas? If shade from docks and similar structures leads to seagrass and related loss of carbon storage, how can this potential source of emissions be moderated? Previous attempts to address this issue and raise light levels under docks include using different construction materials, with limited success. As both the timing and the amount of light reduction can influence seagrass survival, using moveable or temporary docks, and adjusting dock orientation, shape and height, could be more effective at reducing shading, and therefore seagrass loss, and thus promoting carbon storage. Managing and reducing small-scale disturbance in this way can help mitigate climate change impacts by protecting blue carbon habitat.
The full article, Effects of small-scale, shading-induced seagrass loss on blue carbon storage: Implications for management of degraded seagrass ecosystems is available in Journal of Applied Ecology.