Student: Adam R. Rivers
Mentor: Ellinor Michel
Introduction
The anthropogenic deposition of sediment on previously undisturbed areas has a profound effect on species diversity and density. This is particularly true in the remarkably clear and productive Lake Tanganyika.
The effects of sediment in Lake Tanganyika have been seen for fish, molluscs and ostracods (Alin et al.
1999; Cohen 1993). The Lake Tanganyika Biodiversity Project identified excess sedimentation of the littoral habitat as “a severe threat “to the diversity of Tanganyikan mollusks (Patterson & Mankin 1998). In addition to their ecological importance, the Tanganyikan gastropods have extrinsic value as models for coevolution and sympatritic speciation.
Excess sediment input is deleterious to gastropods for several reasons. Sediment reduces light penetration, decreasing algal productivity and reducing food for gastropod grazing. Sediments fill in cracks between rocks reducing substrate heterogeneity. This loss of habitat diversity reduces the availability of ecological niches and adversely effects microallopatric speciation. The sediment affects the grazing efficiency of gastropods because of the increase in the inorganic content of detritus (Cohen et al., 1993).
Cohen et al. (1993) observed and increase in inorganic content. Hakazimana (2000) saw increased inorganic content but also noted the presence of organic content. We sought to test sediment composition, algal density and the effect of these sediments on gastropods. We also sought to test the if there was selective foraging occurring at any site. To do so, two sediment-affected and two unaffected sites were selected. Attached and nonattached chlorophyll a and was measured. Snails were collected from sites, held, and their fecal pellets were analyzed for total organic matter.
Methods
Sampling was done at four sites within 5km of Kigoma, Tanzania on the shore of Lake Tanganyika. All sites are rocky shorelines. Two sites, Kalalangabo bay (S 04° 50.907’, E 029° 36.502’) and “Hilltop” are sediment disturbed from agriculture and construction respectively (Hakizimana 2000). “Jacobsen’s” (S 04°
54.64’ E 029° 35.92’) and “Candelabra” (S 04° 50.463’ E 029° 36.279’) sites are not significantly sediment disturbed. The substrates at the Jacobsen’s, Candelabra, and Kalalangabo consisted of quartzite boulders and cobbles and stromatolites. The substrate at Hilltop consisted of Manouvo Red Bed cobbles, aggregates, boulders, and stromatolites.
Two experiments were run to determine the effect of sedimentation. The first experiment measured the amount of chlorophyll a and total organic matter (TOM) attached to the rocks and on the surface of rocks samples from 2 m depth. In a second experiment gastropods were selected from sites and held for the collection of fecal pellets. These fecal pellets were analyzed for total organic matter.
Eight rocks were collected by divers at each of the four sites. Small, flat, surface oriented rocks were selected from a depth of 2 meters. Divers tightly covered a flat potion of the rock with a circular cap of
28.26 cm 2 or 7.065 cm 2 and brought the rock to shore for processing.
An area around the cap was scrubbed with a wire brush and debris was rinsed away from the cap. The cap was removed and non-attached material was gently washed into a collection tray and poured into a sample vial. The vial was then filled to a total volume of 70 ml. The attached material was removed with a wire brush and the slurry from the rock and the brush rinse was collected in a tray then poured into a sample vial and brought up to a total volume of 70 ml.
Once attached and non-attached material was collected from the rock, a subsample was taken for chlorophyll a analysis. The sample vial was vigorously shaken and then 30 ml of the suspension were

29 .
6 ( A b
− A a
)
EVl
A b
= Absorbance
665
A a
= Absorbance
665
– Absorbance
– Absorbance
750
750 before acidification after acidification
E = volume of extraction (ml) v = amount filtered for chlorophyll a (ml) v
V = total volume of algal slurry (ml)
( = area of rock sampled (cm 2 ) l = cuvette path length (cm)
Formula 1 . Chlorophyll a concentration in µ g/ cm 2
Hilltop Kalalangabo Jakobsen's Candelabra species mean st dev mean st dev mean st dev mean st dev
Lavigeria. nassa 24.46 1.75 19.53 1.58 16.15 3.01 21.22 1.37
Lavigeria. grandis - - - -
Reymundia horei - - 1.60
Spekia zonota - -
Lavigeria new sp.M 21.26 1.38
Table 1 . Height in mm of gastropods sampled at each site
Table 2 : Comparative primary production at pelagic stations in Lake Tanganyika
(mg/m3)
Algae Biomass
(mg/m3)
1975 0.1-4.5 50-100
1994 -
1995 0.2-4.5;
2.2
-
426-662
Net. Phot.
(mg O
2
/m3 /day)
0.13-3.18
3
0.8-0.86
1996 0.6-1.6;
2.8
-
1998 0.50-1.06 -
1999 0-3.5 -
-
-
-
2000 0.88-1.62 - -
2001 0.41-2.10 27.32-140.38 0.1-4.6
Source
Hecky & Kling, 1981;
Hecky & Fee., 1981
Sarvala & al., 1999
Langenberg & al., 1996;
Salonen & al., 1999;
Sarvala & al., 1999
Langenberg & al., 1996;
Sarvala & al., 1999

drawn out with a 60 mL syringe. This subsample was then filtered through a Gelman Type A/E Glass fiber filter (25 mm in diameter, 1 micron particle retention) attached to the end of the syringe. The total volume of the suspension forced through the filter was recorded and the rest of the 30 mL that would not easily pass through the filter was discarded. A small volume of air was forced through each filter to firmly affix particles to the filter surface. The filter was then removed and placed in 8ml of chilled 90% (v/v) ethanol.
The filters containing the algal slurry were incubated at 4C for 24 hours. Each tube was then shaken vigorously and the filter was removed from the tube with forceps. Each tube was spun down at 1000rpm for 2 minutes. Five ml of supernatant were pipeted off and transferred into cylindrical cuvettes with a 1 cm path length. The absorbency of each sample was measured against a blank of 90% v/v ethanol.
Absorbance was measured on a Spectronic 21D spectrophotometer at wavelengths of 665 nm and 750 nm.
Each sample was then acidified with the addition of 20 ul of 1.5 N HCl. Acid was mixed in the cuvette and allowed to stand for 5 minutes. The absorbance after acidification was recorded at wavelengths of 665 and
750 nm. Chlorophyll a content expressed in µ g / cm 2 was calculated using Formula 1 (Nucsh, 1980).
The attached and non-attached total organic matter were measured separately by filtering the collected slurry not sampled for chlorophyll through a pre-burned, pre-weighed 47 mm diameter 1 micron particle retention Gelman type A/E glass fiber filter. For non-attached samples the entire suspension not used for chlorophyll was filtered. For attached samples the contents of the collection vial were vigorously shaken and then a subsample was filtered and the volume filtered was recorded. Both the attached and nonattached filters were desiccated in a drying oven for 1d at 60C. The dry mass of these samples was measured to 0.0001 g on an Ohaus Explorer 2 analytical balance. The samples were then burned at 550C for 3h in a muffle furnace. The samples were allowed to cool in a drying oven and then weighed.
Snail sampling
A second experiment was conducted to determine the content of gastropod fecal pellets. For each species at each site 60 individuals were collected. Individuals were collected from each site at about 15:00 hr.
Immediately after collection the individuals were scoured with a wire brush to remove algae and sediment.
10 individuals of each species were placed into 0.6 liter open topped containers with 2 mm polyester cloth mesh and placed in a large container of water. Snails were held for 16 hours and then the contents of the water were collected onto a Gelman AE/E 47mm diameter glass fiber filter. After filtration each filter was examined for sand grains, juveniles, and mucus. Non-fecal material was removed from the filter with forceps.
The filters were dried and weighed and then ashed and weighed in the same manner as the sediment samples described previously. The species collected from each site, the mean size and the standard deviation are reported in Table 1.
As a supplement to the loss on ignition study of gastropod fecal content, light microscopy and phase contrast microscopy employed and fecal pellets were imaged. One adult one of each of the following species was isolated and fecal strands collected: L. nassa, L. grandis, L. coronata, L. new sp M, Spekia zonota, Reymundia horei, Nov gen guillemi photographs of coarse and digestive strand fecal pellets were taken using a Kodak DS120 digital camera and microscope mount on an Olympus confocal binocular microscope (Figure 3).
Data analysis
The mass of attached and non-attached sediment and the percent mass loss on ignition gave an initial estimate of the organic content and sedimentation effects of the four sites. The chlorophyll a data gave an initial estimate of algal density. To understand the effects of sediment/ non-sediment site designation
(sediment status) and the effect of site within a designation a General Linear Model nested MANOVAs were calculated using Systat version 7.0. MANOVAs were run on the dependent variables attached and nonattached chlorophyll a, attached and nonattached dry mass, and for arcsine square root transformed loss on ignition percents.

80
70
60
30
20
50
40
10
0
90
attached
L. nassa fecal content nonattached
U nivariate AN OVA results sedim ented dusturbed vs. undisturbed
--Attached: F = 36.900 P = < 0.001
--N on-attached: F = 29.267 P = < 0 .001
--snail fecal content
(all species): F = 215.42 P = < 0.001
jacobsens undisturbed candelabra
Kalalangabo sedim ent disturbed hilltop
1.0
Least squares means for loss on ignition by sediment status
0.8
Transformed
% loss on ignition
0.6
0.4
0.2
0.0
Non-sediment
Sediment
Sediment Status
Figure 2 . This graph shows the least squares means of arcsine square root transformed loss on ignition percent from the multivariate ANOVA.
The standard error for each site is indicated by the bar.

To analyze snail fecal contents the dry mass, wet mass and the percent loss on ignition was reported. The effects of sediment status, site within the same sediment status and snail species within a site were tested using a General Linear Model MANOVA.
Results
Sediment category effects
Multivariate analysis of loss on ignition ratios (arcsine square root transformed) for attached and nonattached matter indicates that there is a significant effect of sediment status (Wilks-lambda =0.262 F =
38.114, df =2, 27, P <0.001). Subsequent univariate analysis shows that the sediment status effect was highly significant in each case (attached loi: F= 36.900, df= 28, p<0.001; nonattached loi: F = 29.267 df =
28, P < 0.001). This supports the disturbed and non-disturbed sight designations. A multivariate analysis of attached and nonattached chlorophyll a indicates the effect of sediment status is significant (Wilkslambda = 0.594, df = 2, 28, P = 0.001). Univariate analysis supported this observation as well (attached chlorophyll a: F = 7.723, df = 29, P = 0.009, non-attached chlorophyll a: F = 7.859, df = 29, P = 0.009).
This supports the assertion that Jakobsen’s and Candelabra represent undisturbed sites and Kalalangabo and
Hilltop represent disturbed sites.
Site nested ANOVAs
To understand site differences within the sediment category several nested ANOVAs were calculated.
Univariate Analysis of transformed loss on ignition ratios indicates that there is a significant effect of site difference nested within sediment status for non-attached sediment but not for attached material (attached loi: F = 4.646, df = 28, P = 0.018 non-attached loi: F = 3.244, df=28, P=0.054). Multivariate analysis indicated that site nested within sediment status was significant overall (F = 4.026, df = 4, 54, P = 0.006).
A similar nested ANOVA was run to determine the effect of sites nested within sediment status for attached and non-attached chlorophyll a densities. Multivariate analysis indicated a significant overall site effect (
Wilks-lambda = 0.619, df = 4, 56, P = 0.008) Univariate analysis found significant site effect for attached chlorophyll a but not for non attached chlorophyll a (attached: F = 4.646, df = 29, P = 0.018, nonattached: F
= 2.437, df = 29, P = 0.105). There is are site associated differences for chlorophyll a attached matter but not for non attached.
Gastropod differences
Univariate analysis of the transformed loss on ignition ratios for fecal pellets indicates that there is a significant effect of sediment status, sites nested within sediment status and species nested within site. The most striking of these differences is between snails from sites of different sediment status. The F ratio for sediment status differences equals 215.4 (df = 1,54, P < 0.001). The sediment status of fecal contents correlates closely with site disturbance. Univariate analysis of transformed loss on ignition indicates that there is a significant difference of site nested within sediment status (F = 21.874, df= 2, 54, P <0.001).
Site nested within sediment status is significant because snail fecal content closely matched the organic content of the feeding substrate.
The effect of species nested within site on transformed loss on ignition was not significant. It appeared that there is not enough degrees of freedom (df = 6) to resolve what differences, if any exist between species at sites.
Discussion
There are striking differences in the organic content and chlorophyll a density of attached and non-attached material from disturbed and non-disturbed sites. These differences manifest themselves clearly in the inorganic content of gastropod feces. Snails at sediment sites are ingesting a higher percentage of inorganic material.

The ingestion of increased inorganic content may have a significant metabolic cost. Increased energy
(digestive enzymes, mucus peristaltic effort) is needed to digest food. To maintain the same caloric intake a greater volume of food must be digested; this necessitates more grazing of greater areas. Increased sediment and elevated physiological stress favors k selected species and threatens apex community species with high degrees of specialization.
Preliminary work done using microscopy indicates that there may be some trophic partitioning although the fecal loss on ignition is inconclusive. One individual from seven different species was collected at
Jakobsen’s and held for fecal collection. Each species produced two distinct types of fecal pellets. One type of fecal pellet, the digestive gland strand (Dillon, 2000), has homogenous well-digested brown particles measuring about 1 micron in diameter. A second fecal pellet that I term “coarse strand” resembles the gizzard strand in some North American snails. This fecal pellet contains large diatoms, algal filaments and other organic matter. Identification of the diatoms found in the course strand indicates that there are differences in diatom content, species and the degree of digestion (Cocquyt pers. comm). This may indicate the presence of selective feeding or microhabitat partitioning. This trophic partitioning may serve to explain the remarkable speciation of Lavigeria.
Recommendations
It would be interesting to test the effects of trophic partitioning and microhabitat preferences by collecting snails grazing particular areas and attached matter they are grazing on. The course strand fecal content could be compared to the ungrazed substrate around the snail and the recently grazed substrate. Diatom and algal counts could be performed under a microscope. If there was selective grazing or a microhabitat preference it could help to explain speciation.
Acknowledgements
I am grateful for the help I received on this project from Team Bio, Mr. Chris (siph) Solomon, Ms. Annie
Socci, and Ms. Karen Hinkley. Thanks to Mr. Pete McIntyre, and Dr. Ellinor Michel for the project guidance, field help and continual support and to Dr. Christine Cocquyt who conducted diatom identifications. I would also like to thank Dr. Andy Cohen and Dr. Kiram Lezzar for their role in administrating the Nyanza Project.
References
Alin , S.R., A.S. Cohen, R. Bills, M.M. Gashagaza, E. Michel, J.J. Tiercelin, K. Martens, P. Coveliers,
S.K. Mboko, K. West, M. Soreghan, S. Kimbadi, and G. Ntakimazi. 1999. Effects of landscape disturbance on animal communities in Lake Tanganyika, East Africa. Conservation Biology 13 :1017-1033.
Cocuyt , C. Personal communication. 10 Aug. 2001.
Cohen , A.S., R. Bills, C.Z. Cocquyt, and A.G. Caljon. 1993. The impact of sediment pollution on biodiversity in Lake Tanganyika. Conservation Biology 7 :667-677.
Dillion . R. T. Jr. 2000. The Ecology of Freshwater Molluscs . Oxford University Press. New York,
Cambridge.
Hakizimana , P. 2000. La proportion de la matiere organique et extraction de la Chlorophylle a dans les sediments des eaux polluees: Sites de Hilltop et Kalalangabo, Lac Tanganyika. In Nyanza Project 2000
Annual Report. Dept. of Geosciences, Univ. of Arizona, Tucson, Arizona, USA.
Menone , C. 2000. Snail diversity at sediment disturbed and undisturbed sites around Kigoma, Tanzania.
In Nyanza Project 2000 Annual Report. Dept. of Geosciences, Univ. of Arizona, Tucson, Arizona, USA.
Nucsh , E.A. 1980. Comparison of different methods for chlorophyll and phaeopigment determination.
Arch Hydrobiol. 14 :14-36.
Patterson , G. J. Mankin (ed.). 1998. The State of Biodiversity in Lake Tanganyika- A Literature Review .
Chstham, UK. Natural Resources Institute.