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The Spatial Distribution of Benthic Ostracods in the Profundal Zone of Loch Ness

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




HUW I. GRIFFITHS
Department of Genetics,
University of Leeds

DAVID S. MARTIN
Loch Ness and Morar Project

Introduction
In the early years of this century the late Sir John Murray commenced pioneer investigations into many aspects  of Loch Ness (Murray, 1904 and 1908; Murray and Pullar, 1910), and some seventy-five years later this was followed by the extended survey carried out by Dr. Peter Maitland and his colleagues (Maitland, 1981), but much of the most recent work, particularly in the study of the benthic fauna, has not yet been published.

A preliminary analysis by Shine and Martin (1988) indicated that ostracod Crustacea ('seed shrimps') appeared to be an important component of the zoobenthos, numerically comprising almost 60% of the invertebrates in the deepest parts of the loch.  Subsequently, both quantitative and qualitative studies of the ostracod fauna of Loch Ness were undertaken (Griffiths, Martin, Shine and Evans, 1993), and since then, additional data on the ostracods of the loch have become available, as well as a small dataset based on samples taken from the profundal benthos of Loch Morar.  This current paper uses all these data to further investigate the patterns of distribution and community structure in benthic lacustrine ostracods.

Methods
Ostracods were collected both qualitatively, by dredging, and quantitatively, with a 15 x 15 cm Ekman grab, and a custom-built 10.3 cm diameter gravity corer.  Ostracods  were extracted by passing sediment samples through a bank of sieves down to 125 m , and then by individually hand-picking specimens from sediment residues under a low-power dissecting microscope.  Ostracods were killed in 30% ethanol, fixed in 70% ethanol, and identified using Henderson (1990), and Meisch

 

(1985) for Potamocypris sp.  Taxonomic nomenclature follows Griffiths and Evans (1993).

Quantitative samples were used to assess ostracod densities in different areas of the loch.  Qualitative data allow investigation of the proportional composition of the ostracod community, both as a whole (Figure 1, 24K), and in Urquhart Bay in the North Basin (Figure 2, 10K).  On two occasions it was possible to extrude cores in such a way as to allow examination of the depth distributions of ostracods within sediments (Figure 3).  Differences in ostracod densities in various parts of the loch were examined by Student's t-test (Parker, 1979).

Both the samples from Loch Morar result from qualitative sampling, but are treated in the same way as those from Loch Ness.  Ostracod diversity from these samples was assessed by Brillouin's Index (HB) (see Magurran, 1988), allowing comparison with diversity values obtained from Loch Ness (Griffiths et al., 1993).

Results

LOCH NESS
During the course of these studies, only the undernoted seven ostracod species were recovered from Loch Ness:

Candona angulata G.W. Müller, 1990

Candona candida (O.F. Müller, 1776)

Cryptocandona reducta (Alm, 1914)

Cypria ophthalmica (Jurine, 1820)

Cyclocypris ovum (Jurine, 1820)

Potamocypris smaragdina (Vavra, 1891)

Psychrodromus robertsoni (Brady and Norman, 1889) 

In addition, Candona neglecta Sars, 1887 was reported from the loch in samples taken during the early 1980s, but has not been identified since.  Of these taxa, specimens of the lotic water species P. robertsoni have only been recovered from the guts of Three-spined Sticklebacks Gasterosteus aculeatus, and are thought to have been ingested in one of the many streams which feed into the loch.  The benthopelagic species Cypria ophthalmica is the commonest ostracod in Loch Ness, making up over 61% of the entire fauna recovered.  In contrast, P. smaragdina and C. ovum have only been encountered as one and two specimens, respectively.

The overall density of benthic ostracods in Loch Ness was reported as 262 340 individuals/m2 by Griffiths et al. (1993), who noted a clumped distribution within the ostracod taxocene as a whole.  It has now been possible to examine densities in three areas within the loch:  the South Basin, the North Basin, and Urquhart Bay. 

The highest ostracod density yet identified is that of the South Basin (mean = 1,835 ostracods/m2; extrapolated range between 0 and 7,284 individuals/m2).  The overall mean density in the North Basin is 125 ostracods/m2 (extrapolated range between 0 and 657 individuals/m2), but rather lower densities occur in Urquhart Bay (mean = 88 ostracods/m2; extrapolated range between 0 and 205 individuals/m2).  Density comparisons within the North Basin are not statistically significant (P >0.05), whereas a significant difference exists between the densities observed in the North and South Basins (t = 2.76, P <0.05).  Ostracod distributions are clumped in all these sub-divisions of the loch.

The two vertically sub-divided cores provide comparatively little information on depth distributions.  The first 6.0 cms were examined of a core taken from 215 m in the North Basin (Figure 3a, 6K).  Here C. ophthalmica occurred throughout the sediments, presumably burrowing deeply to feed.  Candona angulata occurred to a depth of 5.0 cm, but C. reducta (which belongs to an infaunal genus) was only represented by a single individual.  In the first 2.0 cms of the core taken from 150 m in the South Basin (Figure 3b, 6K), ostracods were present at higher densities.  Only C. ophthalmica and C. angulata were present, the former in comparatively high numbers in the top 1.0 cm layer.  The  depth to which ostracods actually penetrate Loch Ness benthic sediments is currently unknown, although it is surprising to find the benthopelagic form C. ophthalmica at these depths in the sediments.

 

LOCH MORAR

The Loch Morar samples were both taken from a depth of 300 m on 8th June 1992, and the fauna identified in the two samples are listed below:

The fauna of the two Loch Morar samples are fairly similar to the samples from Loch Ness, except that one benthic species (C. angulata) is absent, although the free-swimming form C. ovum is present in relatively high numbers.  Species richness (four taxa) is similar to that found in much of Loch Ness, and in other

Vol 105, The Scottish Naturalist: Benthic Ostracods in the Profundal Zone of Loch Ness p144

 

oligotrophic temperate lakes, but HB values are within the upper range of the values reported from Loch Ness (Griffiths et al., 1993).  As far as we are aware, these samples (from 300 m) represent the deepest records yet known for C. ovum.

Discussion

Griffiths et al. (1993) have shown that no correlations exist between depth and either ostracod diversity or the abundance of individual species within the loch.  Comparison of the ostracod communities in the North and South Basins (Figure 1) shows that species-richness is lowest at the southern end of the South Basin, where the influence of the River Oich is greatest (Bennett, 1993).  Species-richness remains at either 3.0 or 4.0 throughout much of the loch's length, only reaching 5.0 in the upper, shallower reaches of the North Basin.  Within Urquhart Bay (Figure 2), which comes under the influence of the Rivers Enrick and Coiltie, the community appears to be a little more uniform in composition, although ostracod distributions are still clumped (see Elliott, 1977).

It is clear that ostracod densities are considerably higher in the South Basin than in the North, and this is believed to reflect the higher sedimentation rate of the South Basin, coupled with quantitative increases in the levels of organic inputs.

 

Vol 105, The Scottish Naturalist: Benthic Ostracods in the Profundal Zone of Loch Ness p145

However, this increase is not accompanied by increases in either diversity or species-richness.  The latter never rises above 4.0 in the South Basin, and several samples consisted only of C. ophthalmica and C. angulata, both of which are eurytopic, hardy species.  Candona candida was largely absent from the South Basin, except in deep-water samples, possibly suggesting that C. candida is sensitive to some aspect of the conditions associated with higher levels of sediment deposition.

The sediments of the deeper parts of the loch are primarily of laminated muds, with areas of sand or organic-rich sands in the vicinities of river mouths (Bennett, 1993).  However, there appears to be little linkage of fauna and sediment type, even when the sandy, organic substrata of Urquhart Bay are compared with the remainder of the loch.  This provides further evidence that the restricted ostracod fauna of Loch Ness is (to some degree at least) a product of the homogeneity of the benthic sediments (Griffiths et al., 1993).

The use of ostracods in palaeolimnological analysis at Loch Ness has so far proved impossible.  The sediments are predominantly acidic, which causes rapid post-mortem degradation of ostracod valves.  Despite this, it is believed that the loch has represented a comparatively stable environment, at least since 1868, when a catastrophic flooding event (Anon., 1868; Barron, 1985) resulted in the deposition of a layer of clay throughout the benthos (Bennett, 1993).

The benthic fauna of a given water-body must largely be determined by site-based variables (abiotic interactions), the influence of biotic constraints becoming more apparent over time (Forester, 1991).  Griffiths and Evans (1992) have argued that the co-occurrence of several related ostracod species (e.g. congeners) reflects niche-packing, since species which are essentially similar in their modes of life partition the resources available.

At Loch Ness, many of the benthic samples contain three closely-related species: C. angulata, C. candida and C. reducta, plus  C. ophthalmica.  Of these, the first two are semi-infaunal, and C. reducta is markedly so.  Cypria ophthalmica, however, is free-swimming and hardy.  Combinations of these species (or a variant composed of C. neglecta rather than C. angulata) are also present in Loch Morar, at Cosemeston Lakes in South Wales (Griffiths and Evans, 1991) and elsewhere (e.g. Löffler, 1975; Jónasson, 1978; Danielopol et al., 1985).  This may suggest some sort of 'stable' ecological unit, pre-adapted to partitioning simple benthic habitats.  Since this faunal unit is so widespread, it also suggests the question, - to what extent are benthic niches pre-empted? - (see Krebs, 1985).  The presence of this fauna may simply reflect the inability of other groups to survive in

Vol 105, The Scottish Naturalist: Benthic Ostracods in the Profundal Zone of Loch Ness p146

these environments, but its recurrence perhaps does suggest a colonising fauna which, once established, is difficult to displace.  The presence of such a faunal element, exhibiting a high degree of niche pre-emption, would also act to limit benthic colonisation, and restrict incoming colonists to littoral habitats, or to other, possibly less favoured, niches.

References

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

Barron, H.  (1985).  The County of Inverness.  Third Statistical Account of Scotland, Vol. 16.  Edinburgh:  Scottish Academic Press.

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

Danielopol, D.L., Geiger, W., Tölderer-Farmer, M., Orellana, C.P. and Terrat, M.N.  (1985).  The Ostracoda of Mondsee: spatial and temporal changes during the last fifty years. In:   Contributions to the Paleolimnology of the Trumer Lakes (Salzburg) and the Lakes Mondsee, Attersee and Traunsee (Upper Austria).  (Ed. D.L. Danielopol, R. Schmidt and E. Schultze).  Pages 98-121. Mondsee: Eigenverlag Institut für Limnologie. 

Elliott, J.M.  (1977).  Some Methods for the Statistical Analysis of Samples of Benthic Invertebrates.  Freshwater Biological Association, Scientific Publication No. 25, Second edition. Ambleside.

Forester, R.M.  (1991).  Fliocene-climate history of the western United States derived from lacustrine ostracodes.  Quaternary Science Review, 10: 133-146

Griffiths, H.I. and Evans, J.G.  (1991).  Some freshwater ostracods (Crustacea, Ostracoda) from South Wales.  Freshwater Forum, 1: 64-72.

Griffiths, H.I. and Evans, J.G.  (1992).  A simple notation scheme for the description of time-averaged ostracod assemblages (Crustacea, Ostracoda) by their taxonomic composition.  Journal of Micropalaeontology, 11: 31-35.

Griffiths, H.I. and Evans, J.G.  (1993).  An annotated check-list of British Pleistocene, Holocene and modern freshwater Ostracoda.  Journal of Micropalaeontology, 12: in press.

Griffiths, H.I., Martin, D.S., Shine, A.J. and Evans, J.G.  (1993).  The ostracod fauna (Crustacea, Ostracoda) of the profundal benthos of Loch Ness.  Hydrobiologia, 254: 111-117.

Henderson, P.A.  (1990).  Freshwater Ostracods.  Synopses of the British Fauna (New Series), No. 42.  London:  Linnean Society.

Jónasson, P.M. (1978).  Zoobenthos of lakes.  Verhandlungen der Internationale Vereinigung für Theoretische und Angewandte Limnologie, 20: 13-137.

 

Vol 105, The Scottish Naturalist: Benthic Ostracods in the Profundal Zone of Loch Ness p147

Krebs, C.J.  (1985).  Ecology: The Experimental Analysis of Distribution and     Abundance. Third edition.  New York: Harper & Row.

Löffler, H.  (1975).  The evolution of ostracode faunas in Alpine and prealpine lakes and their value as indicators.  In: Biology and Palaeobiology of Ostracoda.  (Ed. F.M. Swain).  Bulletin of American Paleontology, 65: 434-444.

Magurran, A.E.  (1988).  Ecological Diversity and its Measurement.  London: Chapman & Hall.

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.

Meisch, C.  (1985).  Revision of the recent West European species of the genus Potamocypris (Crustacea, Ostracoda).  Part 2: Species with long swimming seta on the second antennae.  Travaux Scientifiques du Musée Nationale d'Historie Naturelle de Luxembourg, 6: 1-96.

Murray, J.  (1904).  Loch Ness.  Part V: Biology of Loch Ness.  Geographical Journal, 24: 442-443.

Murray, J.  (1908).  Bathymetrical survey of the fresh-water lochs of Scotland.  Appendix VII: Notes on the biology of the lochs of the Ness basin.  Geographical Journal, 31: 64-67.

Murray, J. and Pullar, L.  (Eds.) (1910).  Bathymetrical Survey of the Scottish Fresh-Water Lochs.  Vols. 1-6.  Edinburgh:  Challenger Office.

Parker, R.E.  (1979).  Introductory Statistics for Biology.  Second edition.  London: Edward Arnold.

Shine, A.J. and Martin, D.S.  (1988).  Loch Ness habitats observed by sonar and underwater television.  Scottish Naturalist, 100: 111-199.

 

Received May 1993

Mr. Huw I. Griffiths, Department of Genetics,

University of Leeds, Woodhouse Road, LEEDS LS2 9JT.

 

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

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

 

 

 

 

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Spatial Distribution of Ostracods in Loch Ness