Marine Ecology

Benthic biota of Loch Etive

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Loch Etive in western Scotland. @ Polly Burns

Loch Etive is a sea loch, or officially classified as a fjord, which can be described as “a deep, high-latitude estuary which has been…excavated or modified by land-based ice” (Syvitski et al., 1987). As shown in Figure 1, Loch Etive is situated along the west coast of Scotland (Murray et al. 2003). Loch Etive exhibits a vertically and horizontally well mixed hydrological system, and a biologically productive upper zone.

The ecology of Loch Etive is under pressure from water quality issues as well as trawling methods which disturb the benthic biota, and effectively pose a form of overfishing.

Loch Etive characterisation

caption Figure 1. Location map for Loch Etive on Scotland's west coast Loch Etive meets the sea at Connel, where, due to the tidal conditions, the colliding of the two bodies of water is known as the Falls of Lorna and cuts through the surrounding mountains for 17 miles (Undiscovered Scotland, 2010).

Figure 2 – Map showing position of Connel and Falls of Lorna at mouth of Loch Etive (Get-A-Map, 2010)

There are locations within Loch Etive where its depth is over twice that of the English Channel (Pers. Comm. Griffiths, Dunstaffnage Marine Laboratory) its deepest part being 145 metres (Murray et al., 2003), and therefore is an excellent location to sample deep sea sediments.

Map of Loch Etive showing depths (Murray et al., 2003)Loch Etive is also a relatively stable body of water, and being only three km wide, its also close to the shore (Murray et al., 2003). Due to the rare oceanographic circumstances, the sediment is oxygen enriched and has a temperature of between eight to ten?C (Murray et al., 2003), and therefore within this deep sea sediment there is a incredible array of life considering the habitat, known as infauna (UK Biodiversity Action Plan, 1999). Due to the location of the sediment (deep underwater) investigating what is living in the sediment has to be done using a grab which can efficiently collect the sediment which can then be sieved for anything living. 

Throughout Loch Etive there are large differences between the depths in certain areas, to convey the full range of depths, stations have been assigned to assist in the research (table 1) at certain parts of the Loch (figure 2).


Table 1 – Depths of stations



RE 6


RE 3




RE 5


Although the Loch has a regular change of water coming in (from rivers) and going out (into the sea), there are concerns over the water quality due to a variety of factors including the input of sewage from surrounding communities, particles of materials used for maintenance of surrounding structures and horticultural changes leading to pH disruption (Ardchattan Observer, 2005).

caption Figure 2. Reaches of Loch Etive.

Benthic biota of Loch Etive

Due to the almost continuous changes in water conditions, there is consistent mixing within the water column (on both a horizontal level from interchanging sea water in shallower water, and also vertical mixing when encroaching fresh water is not dense enough to sink below sill level, therefore leaving the dense, deeper water uninvolved in horizontal mixing and only affected by the vertical mixing, often leaving it unchanged for days (Simpson, 2010)). For Loch Etive, circulation in the upper basin is restricted to 60-80m as the water is not dense enough to sink below this level (Brand, 2010). It is only in periods of continuing lack of rainfall therefore leading to reduced fresh water run off, that the tidal water will keep its salinity and deep mixing will occur (Brand, 2010). These unique fjordic conditions create the reintroduction and renewal of nutrients, leading to a large primary production, principally in the upper, lighter zones of the loch.

The Loch had a differing intensity of light hitting it depending on the depth, the shallower zones (euphotic zones) receive the greatest concentration of light, whereas the benthic zone, which is found at the deepest parts of the loch, receives very little or no light. The euphotic zone has the greatest primary production; a study by Wood et al (1973) discovered it to be 70g C m-2 year-1 in Loch Etive. The greater the primary production the greater the mass of debris and waste from organisms in the euphotic zone, which then sinks to the loch floor, and increases production in the benthic zone (Seiburth, 1976). Therefore, theoretically the abundance of life in the sediment should be high. In order the investigate this, it was necessary to study and classify the species found in the sieved mud, as the following demonstrates.

Throughout the depths there are a gamut of polychaete worms, which shows they have the ability to survive at a variety of depths. Although they are all different genus, the class polychaetea contains a significant number of different organisms, which all vary in their depths of occurrence. The Scalibregma polychaete worm was found at both RE5 (140 m) and RE6 (60 m) which shows that is able to successfully live in both shallow littoral zones and benthic depths.

Benthic ecology

caption Brittle star.Phyllum Echinodermata, class Stelleroidea,
order Ophiurida, family Amphiuridae, genus Amphiura
The organisms found in the deepest parts of Loch Etive, although mostly small, play an important role in the ecosystem of the loch overall. As phytoplankton in the shallower waters die, they cluster together and sink to the depths, where they are eaten by larger organisms in the benthos, such as the brittle star (Ampihura) or decompose into the sediment where they support life on a bacterial level. These bacteria are fed on by protozoa and in turn benthic worms and other larger organisms. These larger benthic organisms are fed on by larger carnivores such as fish, which in turn reinforce the populations of sea birds and mammals, such as seals (Tett, 2003).

Materials and methods

The sediment was collected from four different stations at differing depths. The sediment was collected using a grab (figure 4) which was deployed over the side of the boat on a long wire. Figure 4 – Grab used for collecting sediment (Allison, 2010) The grab was equipped with a mechanism that caused the grab to snap shut when it hit the bottom of the sea, closing around a sample of the sediment. The grab was then pulled up to the surface and the sediment was emptied into a crate (figure 5). This sediment was then put into a large sieve and hosed with water (figure 6) which pushes the majority of the sediment through the sieve and back to the sea. Therefore the marine organisms are left exposed where they can be carefully removed and placed in a bucket of seawater from the surrounding sea.

caption Scpiochaetoptcrus: Phyllum Annelida, class Polychaeta, order
Canalipalpata, family Chaetopteridae, genus Spiochaetopterus

This process was repeated until all sediment was washed away and all visible organisms collected. The grab was deployed at each of the four stations so a variety of organisms from a variety of depths could be studied and identified back at the lab. On return to the lab the four buckets of the benthic organisms were brought out and microscopes were set up. The microscopes had magnification powers of 4x, 10x, and 40x. The organisms were gently removed from the bucket and put into smaller, individual containers for closer examination. Under the microscope, minute details on the organism could be seen and this enables the animal to be drawn in detail and thus identified.

Conservation Issues

Loch Etive, as well as being an area of great beauty, plays an important role in the local economy with a series of businesses that use the loch as a crucial part of this. Although these may be beneficial for the local economy, they could be detrimental to the benthic environment. As in most areas, the over fishing of the Loch Etive is a primary concern. The least ecological method of fishing is trawling, where a net is simply dragged over the sea floor to catch everything that falls in front of it. According to a study by Somerton (2001) there are three main ways in which this particular method of fishing is so damaging to the benthic environment. Firstly, the sediment on the sea floor is damaged by the equipment used, creating unnatural sculpturing. Furthermore, the benthic organisms that are pulled up out of their habitat can be directly killed by the equipment, changes in pressure and surrounding environment, and also due to direct exposure to predators.

Additionally, the trawl may remove other organisms from within the ecosystem, (whether the target species or not), which again will cause a change to the benthic organisms circumstances (Somerton, 2001). It varies on the exposure to this method of fishing and the sea floor itself on how long it takes to recover, varying from one year to twenty (Somerton, 2001). According to Fishing Argyll, the advice given to anglers who wish to fish in Loch Etive, is to fish in places inaccessible to trawlers, which lessens the pressure on the ecosystem. The salmon fish stocks in Loch Etive have been struggling due to the decrease in water quality, the threat of farmed salmon escaping and out competing them for food and other resources, and also the sea lice that are abundant within farmed salmon stocks (Argyll Fisheries Trust, no date). A decrease in fish stocks can cause a shift within the ecosystem which in turn can affect the benthic community negatively.


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Personal Communications

  • ALLISON, K., 2010. Field Course [photo album]. Dunstaffnage Marine Laboratory, Oban, Scotland. GRIFFITHS, C., 2010. Dunstaffnage Marine Laboratory, Oban, Scotland





Burns, P. (2012). Benthic biota of Loch Etive. Retrieved from


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