lnp logoLNP LOGO
Home Archive
Loch Ness
Loch Ness
Loch Ness Explorers
& Links
Loch Ness

Allocthanous Organic Inputs as an Explanation of
Spatial Biomass Gradients Observed in the Pelagic
Aand Profundal Zones of Loch Ness

Reproduced with the permission of the Scottish Naturalist
Copyright: May be used for private research. All other rights reserved


Loch Ness and Morar Project
Loch Ness and Morar Project
Department of Applied Science,
University of Staffordshire
Loch Ness and Morar Project

A theme recurring throughout the collected papers on Loch Ness appearing in this Volume 105 (1993) of the Scottish Naturalist is spatial variation along the 39 km basin of the loch.

George and Jones (1987) first explored gradients in conductivity and chlorophyll-a in the five largest Scottish lochs, during the multi-disciplinary study by the Institute of Terrestrial Ecology from 1977 to 1980 (Maitland, 1981); in general, phytoplankton gradients reflected catchment richness.  At Loch Ness, all samplings showed increased conductivity and phytoplankton in the North Basin.  Figures 1a and 1b (37K) clearly show the much larger proportion of base rich rock and arable land in the two northern sub-catchments.  Nevertheless, in terms of phosphorus at least, the differences appear small in Jenkins' (1993) Tables.
A paradox arose, therefore, in the 1980s, since acoustic observations showed a persistent increase in fish numbers towards the south (Shine and Martin, 1988).  Since then, Kubecka, Duncan and Butterworth (1993) and Shine, Martin and Marjoram (1993) have confirmed this, and in the latter case have also observed

The Scottish Naturalist: Explanation of Spatial Biomass Gradients in Loch Ness. p259

zooplankton maxima in the south, although this appears to be more wind dependent.  A clue to the paradox may lie in the benthos studies.
Griffiths and Martin (1993) note greater ostracod densities in the South Basin.  The nematodes, oligochaetes, Pisidia, copepods, Cladocera and chironomids described by Martin, Shine and Duncan (1993) also show greater densities in the South Basin.

Core Survey
Bennett (1993), in a 27 core survey, has analysed particle size and organic content along the axis of the loch.  The two Basins were found to be clearly divided by a 'sill' of organically poor and higher particle-size sands off the River Foyers (Figures 2a and 2b, 16K).  On either side of this lie the two deep Basins, of which the South Basin has a 2.0% higher proportion of organic rich sand/silt muds (28.72% - 29.29%) off the river mouths.  The organic inputs to the South Basin, however, are actually even higher, since the total sedimentation rate, judged from a marker layer thought to result from the great flood in 1868 (Anon., 1868), is almost twice that of the North Basin.  The map (Figure 3, 4K) shows the much larger influence of rivers in the narrower South Basin. 

Catchment Differences
None of this is surprising when the much larger area of the southern catchments is considered (Figure 1c), or the greater western rainfall in the Caledonian and Moriston sub-catchments.  Figure 1d demonstrates how this increases their importance in terms of annual flow.
Mansfield (1992) and Bracewell (1993) have shown that lipids from organic matter in Loch Ness sediments are indicative of higher terrestrial plant detritus from the catchment rather than of autochthonous material.  It is therefore proposed that the density gradients observed in fish, zooplankton and benthos may be related to microbial utilisation of the allochthonous inputs dominating the South Basin, as opposed to the low primary productivity which is also limited by light attenuation due to the humic elements of these inputs.

Bacteria Levels
Based upon light extinction and chlorophyll-a data, it is estimated that the dissolved organic carbon exudations for phytoplankton cannot sustain the observed bacterial production levels (Dr. Johanna Layborn-Parry and Mr. M. Walton, pers.

The Scottish Naturalist: Explanation of Spatial Biomass Gradients in Loch Ness.p263

comm.) (Note 1).  Also, bacterial numbers are relatively high in the winter when primary productivity is at its lowest but the river flows are at their highest.  During one sampling along the loch's axis, bacterial numbers were found to double at the southern end (Dr. R.I. Jones and Dr. Johanna Layborn-Parry, pers. comm.) (Note 2).
Humic inputs, previously thought to be recalcitrant, have recently been shown to play important roles in sustaining bacterial production (Moran and Hodson, 1990).  It seems possible that the higher zooplankton, particularly the filter-feeding Cladocera such as Daphnia and Bosmina, may utilise a microbial food source, possibly through heterotrophic nanoflagellates and ciliated protozoa (Porter, Feig and Vetter, 1983).  In this way, allochthonous inputs would find their way to the fish.

Resource Distribution
The way in which inputs distribute their resources is also of interest.  Often the river water will be denser than the loch surface water and will deliver the sediment load as an interflow (Figure 4, 19K colour chart), which may account for the large quantities of non-migrating Cladocera sometimes found in and beneath the thermocline.  Once in the water column, the inputs will be vigorously mixed and widely transported by the turbulence induced by shear due to wind stress (Figure 5, 5K).  However, the deeper return current generated by the prevailing south-west wind will tend to confine inputs to the south.

Physical Considerations
The regularity of the Loch Ness basin, and its orientation south-west to north-east in line with the prevailing winds, renders it particularly physically dynamic.  Furthermore, turbulence has been suggested as a mechanism for aggregation of dissolved organic matter (Wotton, 1984; Petersen, 1986).  The role of aggregates (known in oceanography as marine snow) is important, since in oligotrophic waters aggregates provide centres of enhanced microbial activity (Caron, Davies, Madin and Sieburth, 1982 and 1986).
In May 1991, while the interflow (Figure 4) was being recorded off Urquhart Bay, the water temperature only varied from 7.9oC at 15 m to 7.0oC at 87 m, but with a suggestion of a weak thermocline at about 80 m coinciding with the interflow depth.  A Marine Snow Camera, from the Institute of Oceanographic Sciences, lowered from a fixed station 5.0 km to the north, showed some

The Scottish Naturalist: Explanation of Spatial Biomass Gradients in Loch Ness.p265

interesting changes in the distribution of particles (Mr. W. Hillier, pers comm.) (Note 3).

Figure 6a (8K) shows the greatest number of particles to be above and within the thermocline at between 60 m and 80 m.  There is another smaller increase at 140 m.  Figure 6b (10K), illustrating mean particle size, and Figure 6c (23K), illustrating cumulative frequencies, show that the change at 70 m is confined almost entirely to an increase in the population of the largest size category (2.0 mm), which could be zooplankton, and that the change at 140 m involves a relative decrease in the smallest size class (0.5 mm).  In terms of diameter, these appear to correspond to a relative increase in particles over 1.5 mm long, and a decrease in particles of less than 1.0 mm long, respectively.
Figure 6d (8K) shows the grey level (more particles = lower grey level), and the notable feature is the higher grey level below 100 m, thus indicating much clearer water, probably the hypolimnion.  One speculation is that these results may show aggregation taking place and being confined to two areas of possible turbulence.

In conclusion, it is proposed that the gradients observed in the density of biota along the loch's length can be ascribed to allochthonous inputs of particulate and dissolved organic matter in the South Basin.  These would mostly be delivered to the water column at depth as interflows, where they may remain confined to the belt of turbulence at the thermocline.  They would then be transported along the loch through shear, while they are utilised by microbial plankton and passed up the food chain.  Thus the cause of the biological gradients is to be found in the sediments and its ultimate effect in the southern fish concentrations.


1.  M. Walton and Johanna Laybourn-Parry: Functional Aspects of the Microbial Plankton in Loch Ness.  Paper read at British Ecological Society's winter meeting and A.G.M., University of Lancaster, 15th-17th December 1992.

Johanna Laybourn-Parry and M. Walton: Studies of the Plankton of Loch Ness - The Microbial Loop.  Poster paper: displayed at British Ecological Society's winter meeting and A.G.M., University of Lancaster, 15th-17th December 1992; and at Scottish Freshwater Group's 50th Meeting, University of Stirling, 2nd-3rd February 1993.  Later published as abstract (Laybourn-Parry and Walton, 1993)

The Scottish Naturalist: Explanation of Spatial Biomass Gradients in Loch Ness.p268.

2.  The Loch Ness and Morar Project's 'Length Run' programme in support of the University of Lancaster's study, funded by N.E.R.C. -  Plankton Community Dynamics in a Large Oligotrophic Freshwater System (Loch Ness).

3.  A camera system designed to record 'aggregates' in the marine environment.


The authors would like to express their best thanks to all the collaborators of the Loch Ness and Morar Project whose diverse work has been drawn upon in the construction of this hypothesis, and in particular to Mr. William Hillier for his analysis of the Marine Snow Camera data.


Anon.  (1868).  Great floods in the north.  Inverness Courier, 6th February 1868.

Bennett, S.  (1993).  Patterns and Processes of Sedimentation in Loch Ness. B.Sc. Dissertation, University of Staffordshire.

Bracewell, C.E.  (1993).  A Geochemical Study of Natural and Pollutant Compounds in Loch Ness, Scotland.  M.Sc. Dissertation, University of Newcastle-upon-Tyne.

Caron, D.A., Davies, P.S., Madin, L.P. and Sieburth, J.M.  (1982).  Heterotrophic bacteria and bactivorous protozoa and oceanic microaggregates.  Science, 218: 795-797.

Caron, D.A., Davies, P.S., Madin, L.P. and Sieburth, J.M.  (1986).  Enrichment in microbial populations in macroaggregates (marine snow) from surface waters of the North Atlantic.  Journal of Marine Research, 44: 543-565.

George, D.G. and Jones, D.H.  (1987).  Catchment effects on the horizontal distribution of phytoplankton in five of Scotland's largest freshwater lochs.  Journal of Ecology, 75: 43-59.

Griffiths, H.I. and Martin, D.S.  (1993).  The spatial distribution of benthic ostracods in the profundal zone of Loch Ness.  Scottish Naturalist, 105: 137-147

Jenkins, P.H.  (1993).  Loch Ness sediments: a preliminary report.  Scottish Naturalist, 105: 65-86.

Kubecka, J., Duncan, A. and Butterworth A.J.  (1993).  Large and small organisms detected in the open waters of Loch Ness by dual-beam acoustics.  Scottish Naturalist, 105: 175-193.

Laybourn-Parry, J.E.M. and Walton, M.C.  (1993).  Studies of the plankton of Loch Ness, Scotland.  2. The microbial loop.  Verhandlungen der Internationalen Vereinigung fur Theoretische und Angewandte Limnologie, 25: 459.

The Scottish Naturalist: Explanation of Spatial Biomass Gradients in Loch Ness.

Maitland, P.S.  (Ed.) (1981).  The Ecology of Scotland's Largest Lochs:  Lomond, Awe, Ness, Morar and Shiel.  Monographiae Biologicae, Vol.  44.  The Hague: Junk.

Mansfield, C.A.  (1992).  A Study of Biogenic and Anthropogenic Compounds in Sediment Cores from Loch Ness, Scotland.  M.Sc. Dissertation, University of Newcastle-upon-Tyne.

Martin, D.S., Shine, A.J. and Duncan, A.  (1993).  The profundal fauna of Loch Ness and Loch Morar.  Scottish Naturalist, 105: 113-136.

Moran, M.A. and Hodson, R.E.  (1990).  Bacterial production on humic and non-humic components of dissolved organic carbon.  Limnology and Oceanography, 35: 1744-1756.

Petersen, R.C.  (1986).  In situ particle generation in a southern Swedish stream.  Limnology and Oceanography, 31: 432-437.

Porter, K.G., Feig, Y.S. and Vetter, E.F.  (1983).  Morphology flow regimes and filtering rates of Daphnia, Ceriodaphnia and Bosmina fed on natural bacteria.  Oecologia, 58: 156-163.

Shine, A.J. and Martin, D.S.  (1988).  Loch Ness habitats observed by sonar and underwater television.  Scottish Naturalist, 100: 111-119.

Shine, A.J., Martin, D.S. and Marjoram, R.S.  (1993).  Spatial distribution and diurnal migration of the pelagic fish and zooplankton in Loch Ness.  Scottish Naturalist, 105: 195-235.

Wooton, R.S.  (1984).  The importance of identifying the origins of microfine particles in aquatic systems.  Oikos, 43: 217-221.

Received July 1993

Mr. Adrian J. Shine, Loch Ness and Morar Project,

Loch Ness Centre, DRUMNADROCHIT, Inverness-shire IV3 6TU.


Mr. David S. Martin, Loch Ness and Morar Project,

Loch Ness Centre, DRUMNADROCHIT, Inverness-shire IV3 6TU.


Miss Senga Bennett, Loch Ness and Morar Project,

Loch Ness Centre, DRUMNADROCHIT, Inverness-shire IV3 6TU.


Miss Rosalind S. Marjoram, Loch Ness and Morar Project,

Loch Ness Centre, DRUMNADROCHIT, Inverness-shire IV3 6TU.



Back to the Archive Room  




Loch Ness Allocthanous inputs, spatial biomass