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The muddy debate: Is the Severn Estuary biologically productive?

Severn Bridge by Philippa Long

Traditionally, the Severn Estuary has been mistaken for an expansive, featureless landscape, dominated by fast-flowing muddy waters that prevent any pelagic biological activity. Although the latter could be true in terms of phytoplankton development, new research has shed light on the vital role that the benthic algal system has on controlling nutrient dynamics in the estuary.

Estuaries form at the margins between the land and the sea. The complex movement and mixing of freshwater and seawater governed by the tide, along with the trapping and recycling of continentally supplied nutrients and sediment, makes estuaries some of the most ecologically viable ecosystems in the world, in line with the biological productivity of coral reefs and tropical rainforests.

The Severn, the largest of 133 estuaries in the UK, has a mosaic distribution of intertidal mudflats, saltmarshes and wetlands, making it a unique habitat for a wide range of species. Alongside nationally scarce plant species, important wildfowl, wader populations and migratory European birds inhabit and refuel in the biologically-rich banks of the estuary. The estuarine waters are also home to over 100 fish species that use the estuary as a nursery, supporting many of the UK’s commercial fish stocks. With such a wide socio-ecological and economic importance, it is clear why the Severn was designated a Special Area of Conservation in 2009.

However, it’s less obvious as to why it has been over two decades since there have been systematic sampling studies in the Severn. Reviews have come and gone during this time, widely associated with renewable energy projects such as the Severn Barrage, but have often repeated findings from the 1990s. Furthermore, any commercially driven studies and their findings are often not disclosed to researchers or the public. This has left, in many aspects, knowledge of the Severn and its current ecosystem condition in a state of limbo. One aspect that’s often overlooked in many hydrological systems and is often overshadowed by carbon, nitrogen and phosphorus, is the element silicon, which may be one of the most important nutrients in the Severn’s environment.
Sand Bay by Holly Welsby

Why is silicon important? 

Dissolved silicon is an important nutrient in aquatic environments, and is essential to siliceous organisms, for example, photosynthetic diatoms, which use dissolved silicon to form their shells (or frustules) made from biogenic silica. Diatoms are broadly categorised as ‘centric’ (round), usually occupying the surface oceans, and ‘pennate’ (long and thin), inhabiting coastal and seafloor environments, including sea ice, and intertidal mudflats such as those in the Severn Estuary.

Despite their small size, diatoms are an important group in supporting most food webs, and due to their abundance, contribute close to half of all surface ocean productivity! Diatoms are a key factor in affecting climate change due to this high productivity, as they remove the greenhouse gas carbon dioxide out of the atmosphere and export the organic carbon from the surface ocean to the seafloor when they die. Dissolved silicon and biogenic silica have been widely used to study marine silicon cycles but the impact that diatoms may have on estuarine cycles, and the potential influence on river silicon inputs to the ocean, has only recently come to light.

Silicon cycling in the Severn Estuary: new research  

After the receding of the tide, large intertidal mudflats form along the shores of the Severn Estuary, which has the second largest tidal range in the world! These nutrient-rich intertidal mudflats are inhabited by pennate diatoms that live in microbial mats, called biofilms, on the mudflat surface. These biofilms, which are visible to the naked eye (the golden-brown shimmer that can be observed on the mudflats at low tide), are low in biodiversity but high in diatom abundance. Biofilms are an important food source to many mud-dwelling creatures, such as estuarine ragworm and laver spire snails, and migratory visitors such as the whimbrel and ringed plover. These ‘sticky’ mats also contribute to sediment stabilization, through the production of an organic rich network around sediment grains, and control nutrient fluxes to the overlying water.
Biofilm on the intertidal mudflats of the Severn by Holly Welsby
Compared to the well-studied carbon, nitrogen and phosphorus cycles, the importance of silicon in the Severn Estuary is less well understood. New research that has been carried out at the University of Bristol has aimed to tackle this gap, with an in-depth, seasonal study of silicon cycling along the Severn river-estuary-marine continuum. Each season in 2016, the surface and bottom waters of the Severn were sampled aboard Cardiff University’s research vessel.

It was found that the strong tidal forces and seasonal river flow fluctuations controlled dissolved silicon and other associated nutrients. In line with previous studies, the high mud water content - referred to as turbidity - limited water column primary productivity by blocking out light. This meant that there was minimal biogenic silica production in the water column itself. Instead, biogenic silica depended on the suspended particulate matter, and displayed seasonal cycles associated with benthic biogenic silica production by the diatom biofilms on the mudflats. In other words, the suspended sediment in the Severn not only originated from the rivers discharging into the estuary, but also from the erosion of the intertidal mudflats. This erosion of the mudflats in this high energy system, led to the suspension of the diatom biofilms, and so increased the biogenic silica concentrations in the water column.

This research has shown that since the 1990s reports, diatom biofilm biomass (i.e. their presence) has increased on the mudflats. These diatoms were also efficient at photosynthesis, resulting in a high potential to cycle silicon. These biofilms break up and reform rapidly between tides meaning that a large amount of silica is shuttled from the mudflats to the water column every day. This benthic biogenic silica export, which is transported further compared to dissolved silicon, could dissolve and replenish the Celtic Sea, with the dissolved silicon ready to be used by plankton that supports our commercial fish stocks.

Severn River in winter by Tim Gregory

Looking ahead 

The Severn Estuary - in all its natural wonders - is a valuable resource in terms of renewable energy, tourism and business. Many of us also call it home. But what does the future hold for these diatom biofilms on the mudflats of the Severn Estuary? In many ways, their prospects are low. With extreme weather events, erosion and coastal squeezing causing a loss to our mudflat and saltmarsh habitats, influx of microplastics and associated toxins, alongside proposals for large construction projects that may alter sediment/nutrient loadings and deposition patterns, the future of these biofilms hangs is in the balance. But based on recent findings, these diatoms are tolerant to the mudflats harsh environmental conditions, which suggests they have the capability to adapt to these adverse conditions. Diatoms are a miraculous species, and their benefits to the estuary is not fully recognised.

We are beginning to understand that there is a limit to the degree that we can modify our environment, but if we could only assign an economic value to this biologically productive system, perhaps the benthic diatoms future on the Severn Estuary mudflats could be aided.

This blog has been written by Cabot Institute member Holly Welsby, from the School of Earth Sciences at the University of Bristol.
Holly Welsby


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