depth-time distributions of detritus depositions in lake nemi (volcanic lake of central italy)

Hydrobiologia 202: 185-195, 1990. 0 1990 Kluwer Academic Publishers. Printed in Belgium. 185

Depth-time distributions of detritus depositions in Lake Nemi (volcanic lake of central Italy)

Loreto Rossi, Laura Picciafuoco & Albert0 Basset Ecologia, Dipartimento di Genetica e Biologia Molecolare, Universita’ ‘La Sapienza’ Roma

Received 18 January 1989; in revised form 1 August 1989; accepted 10 September 1989

Key words: lake, detritus depositions, sediment, mathematical model

Abstract

The depth-time distributions of detritus depositions onto lake bottom and the amount of resident organic matter on the upper layer of the bottom have been assessed in a volcanic lake. Depositions were sampled monthly across the lake at four depths (0.2 m; 2 m; 6 m; 30 m) during two years (1983-1984). Organic and ash fractions of sediment cores collected along a depth gradient were assessed in the summer of 1984. The result show: 1. The mean amount of organic matter deposition (size > 10 pm) is 1.24 g me2 d- ’ (dry weight) (i.e.

452.6 g rnp2 year ‘): 2. The metabolism of Large Particulate Organic Matter (L.P.O.M.)) occurs primarily in the littoral zones

not farther than 30 m offshore and the dispersibility of the L.P.O.M. may be predicted by a negative exponential model;

3. The deposition on the bottom comprised under the perimeter of about 80% of the lake surface, is principally autochthonous (planktonic in origin);

4. The detritus settled out of the water column is not completely processed and about half of the total material enters the slow cycle of the sediment on the deepest zone of the lake.

Introduction

Organic detritus is an important component of the lake ecosystems. The input of detritus depends on both lake productivity and features of the lake basin as a whole (Odum, 1988 ; Odum & Heald, 1975; Mann, 1972). The output largely depends on the trophic utilization by macrodetrivorous species that are able to exploit vegetable detritus with sizes >0.07 mm (Wetzel & Manny, 1972; Cummins & Klug, 1979; Gasith, 1976; Rich & Devol, 1978; Rossi, 1985). Hence, the quantity, origin and depth-time distribution of organic detritus are crucial parameters for studies of com-

munity ecology and for the management of aquatic systems.

Allochthonous and autochthonous detritus (dead organic material with any attached micro- organisms) is removed from the water column by sedimentation. The proportions of these two sources of detritus vary case by case, influencing the distribution of benthic faunas and the strategy of lake management. In the lakes having high sinousity and watersheds covered by dense deciduous woodlands, it seems that a significant fraction of the available carbon on lake bottoms is allochthonous in origin and that the role of autochthonous materials increases according to

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the lake surface and depth of the lake (Sorokin, 1972; Wetzel & Otsuki, 1974). Moreover, it has been observed that the phytoplankton production is positively correlated with the processing of settled detritus (Fee, 1979). On the contrary, it is known that pelagic sediments in some lakes are almost completely of phytoplanktonic origin (Gasith, 1976).

Many studies have determined the input and the distribution of P.O.M. (particulated organic matter) in shallow and deep lakes but very little research has been done in volcanic lakes, sampling material by large traps for a long period of time to collect even the larger particles of detritus (White & Wetzel, 1973; Jones, 1976; Molon- goski & Klug, 1980; Lewis, 1987).

In order to determine the availability of organic matter, exploitable by animals as food, on the bottom of a small volcanic lake in central Italy, the main aim of this study was to establish the quantity, the organic content and the depth-time distribution of settling material on the bottom of Lake Nemi. The approach involved: 1) The monthly estimation of the material settling out of the water column at different depth, as both total material (size > 10 pm) and as large particulate organic matter (L.P.O.M. - size > 1 mm). 2) The comparison between the mean organic content of material settling out of the water column and that of the bottom sediments.

Materials and methods

Study site

Lake Nemi is a small volcanic lake in central Italy (area = 1.79 km2; mean width = 954 m; mean length = 1880 m; max depth = 32.4 m; mean depth; 17.0 m). Its watershed is covered by deciduous woodlands while many submerged macrophytes colonize the minimal littoral habitats. Phragmites australis and Myriophyllum spicatum are the dominant species in emergent and submersed macrophyte communities, respectively. No oxygen is present below 5-8 m during large part of the year. In lake Nemi the means of de-

composition rates (detected monthly through the year for allochthonous and macrophytic detritus) between deep and shallow waters, are 2.95% & 0.86 per day (decay coefficient: k = 0.0299) when detritivores are present, and 0.96% k 0.25 (k = 0.0100) when detritivores are ex- cluded (Rossi et al., 1987). An extensive ecological description of the lake and the temporal patterns of phytoplanktonic production, which will be considered in the Discussion Section of this paper, are published in detail elsewhere (A.A.V.V., 1987).

Detritus trapping

The sampling was carried out from January 1983 to December 1984, along two transects. We used bordered traps (collecting surface = 1 m’) of nylon material (mesh size < 10 pm) at four depths (0.2 m, 2 m, 6 m, 30 m) to collect detritus. The respective distances from the shoreline of sampling sites were 0.2 m, 30 m, 150 m, and 755 m. The traps were fixed at distances not less than 30 cm over the bottom of the lake, using lead weights and floating buoys. At 0.2 m the traps were fixed immediately under the water surface. The border of the traps and the distance between the trap and the sediment reduced both the effects of resuspension of settled material and the coloni- zation of the sample by benthic invertebrates limiting the decomposition rate during the inter- sampling period. The material trapped on the 1st day of collection was processed for largest time, and that trapped on the 30th day for the shortest. The integrated mean of decomposition rates did not excede the 14% per month (Rossi, in pre- paration).

At the beginning of each month the traps were collected and immediately substituted by a diver with SCUBA; traps were collected at each depth and were carried in the lab into polythene bags. Direct observation of the content of each sample allowed the removal of the eventual non biological waste and rare colonising animals. Each sample was gently washed with distilled water on a sieve to separate the L.P.O.M. (size > 1 mm) fraction

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from the total deposition. The two fractions, if obtained, were oven dried (60 “C for 72 h) and weighed, ashed in a mufIle furnace (500 “C for 8 h) and weighed again. The weight of organic matters (A.F.D.W.) were the differences between the dry weight (D.W.) and the weight of the ash residue after combustion. The average of material deposition, expressed as g m - 2 d - I, was based on the monthly values of samples which were the averages of 4 samples (2 replicates x 2 year) per 4 depths. At the deepest sites, the L.P.O.M. fractions were very small and ponderal determina- tions of L.P.O.M. were performed on the cumulated collection every 4 months.

Sediments

In June 1984 the organic content of the superficial layer of bottom sediment was assessed. Bottom sediment cores (5 cm long) were collected by a diver with SCUBA using plexiglass tube (diame- ter = 4 cm) at intervals of about 5 m from each other, along 3 transects from the shoreline to the centre of the lake. The cores were immediately frozen after collection to obtain a solid stick and to avoid decomposition of the organic content. In laboratory the cores were divided into 1 .O cm seg- ments to obtain a number of replicates. Core seg- ments were dried (60 ‘C for 72 h), weighed, ashed (500 ‘C for 8 h) and weighed again to assess their organic and ash content.

Model f;tting

The data obtained from the traps were used to calibrate a negative exponential decay model which describes the decrease in the deposition of detritus on lake surface as a function of the distance from shore (Rau, 1976). We applied the Rau’s equation to both groups of our data (i.e. L.P.O.M. and total material), to establish the effectiveness of the model in predicting deposition also on the bottom of the lake and to attempt an indirect estimation of the autochthonous input in the depositions at different bathimetries. The

implied assumptions of the model to be used here, were: 1) that all detritus was originated at the marginal areas of the lake and 2) that all detritus settled on lake surface directly sank to the bottom. The assumptions were due to Rau’s equation describes depositions of allochthonous material on lake surface.

The form of the equation to be used here is: d = do ecvk”) (where d, = deposition at a dis- tince x from the shoreline (g m - 2 d - ‘), do = deposition on the shoreline (g m - 2 d - ‘) x = distance from the shoreline). The parameter k is the coefficient of dispersibility and can also be calculated from the equation itself using two values of depositions respectively on the shoreline (d,) and on a fixed distance (d,) from the shoreline (k = (- l/x) In d, d,)). Since our data regards the deposition on the bottom of the lake, in computing the equation we considered the distances from the shoreline of our submersed sampling sites. The expected values for the depths of 6 m and 30 m were obtained by calculating the k value directly from the general equation above; the experimental deposition on the shoreline was utilized as d, and that observed at 2 m depth as d,. Expected values of deposition for the depths of 2 m-30 m and 2 m-6 m were calculated in analogous way. The average of k’s values was used to determine the expected deposition at the four study depth/distance offshore.

Results

Total deposition

The average of materials that settled out of the water column (deposition) with sizes not lower than 10 pm was 453 g rnp2 year ’ (Dry Weight) corresponding to 1.24 g m - 2 d - ’ with an organic matter fraction of 46.5 %. 693 g m - 2 year - ’ of material were collected by traps at the shoreline, while315gmp2year - ’ were collected at the lake centre. The distributions of the depositions were assessed with respect to both depths and period of time (months). The quantities observed for the same months of the two consecutive years were similar and the samples collected in the different

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transects at the same depth were not significantly cant power curve (Fig. 1, 3). The temporal distri- different. The standard errors of the monthly butions of depositions, at 0.2 m as well as at the averages for each depth rarely excede 10% 2 m sites, showed maximum values coincident (Table. 1). with the fall period and were significantly hetero-

The largest amount of materials was trapped at geneous throughout the year (x2 test P < 0.01, the shoreline sampling sites (0.2 m) and the depth Table. 1). Neither of these last characteristics distribution of total deposition follows a signili- occured at a depth of 6 m and 30 m. In fact, at

Table 1. Detritus depositions (g m- 2 d- r of dry weight f SE.) monthly collected at 4 depths respectively located at 0.2,30,150, 755 m offshore. Percentages of ash free dry weight are in brackets. L.P.O.M. depositions (g m-’ d-r of dry weight f S.E.) are alsoshown.** = p -C 0.05,ns = notsignificant,fortheheterogeneityofcolumn(contingencytableanalysesformonthlydistributions, x2 test). The means joined by bars are not differents (Rank Sum test).

Month Depth

0.2 m 2m 6m 30 m Means + S.E.

January

February

March

April

May

June

July

August

September

October

November

December

0.874 * 0.09 (70.8%)

0.800 + 0.04 (65.1%)

0.763 f 0.02 (67.5%)

0.826 f 0.07 (71.3%)

0.995 & 0.05 (64.4%)

0.818 + 0.03 (60.6%)

0.674 + 0.04 (64.1%)

1.213 + 0.13 (58.7%)

2.682 k 0.57 (71.3%)

6.130 f 1.72 (79.2% 1

5.968 k 0.98 (80.9%)

1.129 f 0.17 6’4.6%)

0.399 + 0.03 (58.1%)

0.394 f 0.08 (53.0%)

0.970 f 0.04 (47.7%)

1.744 f 0.05 (41.7%)

1.140 f 0.06 (41.5%)

1.193 * 0.75 (33.9%)

1.032 + 0.24 (39.5%)

1.322 + 0.43 (40.1%)

2.537 + 0.22 (42.8%)

4.007 + 0.40 (47.7%)

0.440 f 0.03 (47.3%)

0.454 * 0.05 (49.9%)

0.384 & 0.05 (40.1%)

0.311 * 0.05 (42.5%)

0.774 + 0.06 (37.6%)

0.994 k 0.03 (35.3%)

0.960 f 0.07 (35.8%)

0.938 + 0.04 (36.1%)

1.690 f 0.32 (34.2%)

1.205 f 0.15 (36.6%)

1.339 + 0.09 (38.4%)

1.180 k 0.09 (37.2%)

0.510 + 0.06 (36.8%)

0.407 * 0.05 (38.5%)

0.748 f 0.07 (35.7%)

0.958 + 0.03 (33.1%)

0.997 + 0.08 (34.2%)

0.814 f 0.04 (35.5%)

1.110 f 0.09 (32.8%)

0.849 + 0.05 (33.7%)

0.812 f 0.07 (34.5%)

0.648 + 0.08 (36.1%)

0.840 f 0.03 (32.4%)

0.780 f. 0.03 (33.0%)

0.879 k 0.04 (34.7%)

0.924 it: 0.04 (35.9%)

0.601 + 0.12 (51.2% f 8.1)

0.616 & 0.16 (48,4% + 6.9)

0.876 + 0.06 (46.8% + 7.5)

1.095 * 0.22 (46.0% + 8.6)

1.051 f 0.28 (43.6% + 7.2)

0.949 f 0.08 (41.1% f 6.5)

1.052 f 0.02 (43.1% f 7.1)

1.097 f 0.15 (42.9% f 5.4)

1.849 f 0.45 (46.2% k 8.6)

3.024 + 1.25 (49.3% + 10.4)

1.949 f 1.34 (49.9% * 10.7)

0.728 f 0.18 (49.7% * 8.8)

/ \ I \

Means & S.E. 1.906 f 0.5g** 1.303 * 0.30** 0.891 + 0.12”” 0.863 + 0.03”” 1.24 + 0.20”” (69.0% +_ 2.O)** (45.3% + 1.9)“” (37.4% + 0.6)“s (34.3% + 0.4)“” (46.5 + 0.94)“5

\ / \ I

L.P.O.M. 1.639 + 0.04 (70.7% + 3.0)

0.501 f 0.10 (65.3% k 2.1)

0.241 + 0.04 (68.1% * 1.9)

< 0.00001 (not deter.)

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0.2 m and 2 m deposition values were much higher in the period September-November than in the period December-August while at the 6 m and 30 m depth sampling sites, the depositions were uniform with small temporal variances (heterogeneity not significant). Below 2 m depth, the depositions were always of about 1 g m- 2 d - ’ with variances of 0.18 and 0.01 at 6 and 30 m respectively (Table. 1, Fig. 1).

As an average for the whole bottom of the lake, the distribution of the total material showed a peak between July and November. Similar trend was observed for the algal density and the two temporal patterns were signiticatively correlated (Fig. 2).

L. P. O.M. and model fitting

The amount of large particulate organic material (L.P.O.M. > 1 mm) in total deposition, is shown as mean per depth in Table. 1. It decreases with the increase of the depth and the fraction (%) of L.P.O.M. in the total deposition diminishes strongly along the depth gradient. At 0.2 m the mean of L.P.O.M. constitutes 86% of the total deposition and at 30 m its proportion is reduced to less than 0.001% (<O.OOOOl gmM2 d-l). The organic content of L.P.O.M. is about 6%70%, independently by the depth (Table. 1). Fitting both the L.P.O.M. and the total

material data against the depth, to a negative

J FMAMJJ ASOND

MONTH (1993 - 94 1

Fig. 1. Monthly deposition on the lake bottom at 4 depths. Upper line (dark + white areas) = total deposition (dry weight). Lower line (dark areas) = depositions as ash free dry weight. (xx) = significative positive correlation between dry weight and

ash free dry weight.

JFMAMJJASOND

deposition proportionj I = 0.7 p < 0.05). The algal density was estimated by sum of mean densities through 30 m of water column at lake centre (755 m offshore) (Altinito S. et al., 1987) and of mean densities at the shoreline. -@- Algal

density ( lo6 cells/litre) -I-J- Detritus deposition km 1. -*d-l

MONTH

l y=1.701 e -o’3ggx (m depth) r =. gg

p<O.OOl -0.023

0 y=1.639 e ?m offshore)

exponential model, it results a good tit (P < 0.001) when L.P.O.M. data are used and no tit when data of total material are used. This last set of data shows good fit with a power curve (Fig. 3). The expected exponential decrease in detritus deposition with respect to the distance offshore, as obtained by Rau’s equation, is overlapped with the L.P.O.M. observed data. The observed total material and its expected quantity are instead very different. The model underestimates the depositions at 6 m and 30 m depth (Fig. 3). The aver-

Fig. 2. Time variations of proportions of detritus deposition and of algal density. The two variables are significatively correlated (vc~,,l density proportion) = 0.034 + 0.54 x (detritus

A y=1.409 x -O” 7o (m depth) r _o,gg

ii.

p < 0.001

n y=,,go6 e-“‘oo63Tm offshore)

?

Fig. 3. Depth distributions of L.P.O.M. (0) and of total material (A). Data detected by traps (solid lines) and expected by Rau’s equation (dashed lines) (see text). In abscissa, in parenthesis, are shown the meters offshore of the sample sites.

age values of k (coefficient of dispersibility), obtained calibrating Rau’s equation with our data, are 0.023 for the L.P.O.M. dta (kzm = 0.0395; k,, = 0.0128; k30m = 0.0159) and 0.0063 (kzm = 0.0127; kern = 0.0050; k30m = 0.0010) for total depositions.

Organic contents

A. In the total deposition The amount of organic fraction (A.F.D.W.) of the total material (D.W.) and that of the total material itself present in the traps throughout the year, were positively correlated at all 4 depths (Fig. 1). The amount of organic fraction is also significantly correlated with the depth and follows

00

40

20 I_

I T

191

a negative power curve (Fig. 4). At the shoreline sample sites, the organic fractions of materials in the traps showed temporal heterogeneity (x2 test, P < 0.05) and values very high throughout the year (about 70%). At the other three depths, the organic fractions of the materials in the traps were <46% with no significant temporal variations (Table. 1).

B. In the sediment cores The organic fraction of upper sediment showed no significant difference among replicates collected at the same depths nor within each core; the results are reported as average organic fraction observed at each depth (Table. 2). The resulting organic fractions of upper layer of bottom sediment were very low at 0.2 m (2.9%)

DEPTH

m

Fig. 4. Trends of organic fractions in total detritus depositions (m) and in sediments of bottom layers (0) at different depths. The figures fit with power curve (deposition) (y = 52.2 x- .I““; r = - 0.97; p < 0.01) and direct logarithmic model (sediments)

(y = 11.2 + 5.3 lnx; r = 0.88; p < 0.05) with respect to depth.

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Table 2. Percentages of organic matter (ash free dry weight) in bottom layers at different depths.

Depth 0.2 m 3m 5m 10m 15m 20 m 25 m 30 m X + 2 SE.

% Organic k 2 S.E.

2.91% 22.17% 18.03% 12.51% 26.12% 28.27% 28.81% 31.36% 21.35 f 1.89 k 2.03 + 3.04 k 5.32 k2.41 + 2.09 + 0.14 + 2.09 rf: 6.86

and much higher at 30 m (31.4%). A significant correlation was determined between the percent of organic material and the depth, according to a positive logarithmic equation (Fig. 4, Table. 2). Thus, along the depth gradient the percentages of organic material in the traps and those in the cores of sediment follow opposite patterns (i.e. the difference between the percentages decreases as the depth increases). In fact, organic fraction differences range between 67.1% (69.0%-2.9x, near the shoreline) and 2.9% (34.3x-31.4%, at the 30 m depth) (Tables. 1, 2 and Fig. 4).

Discussion

The quantity and quality of organic detritus settling out of the water column depend on many factors which may be internal or external to the lake. The quantity usually ranges between 0.9 and 2.0 g m-* day-’ (Rau, 1976; Gasith, 1976; Molongoski & Klug, 1980). The value determined in the present study is within this range (1.2 g m - * d - ‘) even if it may be a little biased by both the decomposition occured during a month and the mesh size (< 10 pm) of the trap used. This last one, in principle, resulted in the loss of the ultra- fine material during the lirst days after the trap immersion. However, we were interested in collecting detritus, major than 10 pm in size, exploitable by detritivores and influencing their distribution on the bottom. On the other hand, the bias due to the rate of decomposition during a month with animal exclusion, is very low (about 14%, see method). It did not excede the standard error of the average of the trapped material. On the contrary, the data obtained seemed strongly related to two main characteristics of Lake Nemi: 1) the watershed is small, steep, covered by

deciduous vegetation, and many aquatic plants colonize the shallow water zones. These factors influence the input of large size detritus along the coast, 2) the pelagic primary productivity is restricted to the first 5-8 metre depth and, in the central zones of the lake, the remaining part of water column (20-25 metres) is anaerobic for a long part of the year (A.A.V.V., 1987). Conse- quently, the planktonic detritus, as well as other kinds of detritus present on the surface of the lake center, may reach the bottom with a partially degraded form, increasing the organic standing- crop on the bottom (Cummins & Klug, 1979). The analysis of our data emphasize the following:

- the input of material producted around the perimeter of the lake (allochthonous and macrophytic in origin) strongly affects the total detritus depositions as far as the belt of the 2 m depth (i.e. less than 10-15x of the total lake surface); that is, the standing-crop and the origin of the organic matter on the lake bottom varies according to the depth -.

- about half of the total organic input is decom- posed within a year -.

The first point is supported by both the significant seasonal trend of the total deposition, and by the high proportions of L.P.O.M. in the sampling sites at 0.2 m and 2 m depth (i.e. 30 m offshore). The maximum of the total material is, in fact, synchronous with the leaf abscission period for deciduous woodland and for emergent macrophytes which, in central Italy, occurs between September and November. Therefore, the Autumn period is crucial for the contribution of organic plant material to Lake Nemi metabolism. This aspect agrees with the observations of other authors for temperate lakes (Pieczynska, 1972; Gasith & Hasler, 1976). Moreover, the great pro-

193

portion (86%) of L.P.O.M. in the total trapped material near the shore, sustains the important contribution of the allochthonous and macrophytic detritus to the deposition. The distribution of detritivores determined in Lake Nemi, is limited to the belt of O-2 m depth (Rossi et al., 1987) and this agrees with the observed large L.P.O.M. availability at these depths. The type of material in the traps was easily identifiable as the remains of vegetables living around the lake. Moreover, it is known that the size of detritus is linked to the origins of the detritus itself. The large particulate organic matter (L.P.O.M.) is chiefly a decaying product of the allochthonous detritus as well as of the detritus from aquatic macrophytes. Very small size particles are, instead, phytoplanktonic in origin as well as resuspended material (Gasith & Hasler, 1976; Cummins et al., 1973).

On the contrary, a poor influence of the autumn input on the deposition collected from the major deep bottoms seems to be sustained (i) by the very low proportion of L.P.O.M. in the traps at 6 m and 30 m, (ii) by the not significant temporal heterogeneity of the quantities of the total trapped material at these depths and, (iii) by the convergence of the organic fractions of the sediment in the cores and of the materials in the traps.

The small fractions of L.P.O.M. agree with our expectation and are affected a) by the short lake radius that allows a redistribution along the coast of materials floating on the lake surface (Rau, 1976); and b) by the very abundant benthic detritivores, in shallow water areas, which are able to process dead leaves to F.P.O.M. (size < 1 mm) with a rate of 3-4% a day, strongly reducing the biomass of the autumn input of L.P.O.M. (Rossi et al., 1987). The remaining material of the processing, containing a very small organic fraction (see also Table. 2) should be unlikely able to reach the major deep bottoms with significant quantities. In fact, the organic fractions of the material in the traps at 6 m and 30 m depth are too high with respect to those of the sediment cores in the shallow water.

The not significant temporal heterogeneity of the total material trapped at 6 and 30 m depth may be related to the resuspension of bottom tine

material, Resuspension occurs in December- February (A.A.V.V., 1987) and could balance the peak of June-September detritus input that seems principally planktonic in origin at these depths (Fig. 2). In fact, the convergence between the organic fraction of the total trapped material and of the cores belonging to the deepest bottom suggests a similar origin (i.e. planktonic) of these materials. Moreover, it is known that the remaining quantity of recalcitrant substances of the decomposition process of the detritus, depends on the origin of the detritus itself, rather than on the type of the processing (Petersen & Cummins, 1974; Moran & Legovich, 1988). The autochthonous origin of the deposition in the central part of the lakes has often been demonstrated (Gasith, 1976). In the pelagic water of Lake Nemi the phytoplanktonic production is very high (Altinito et al., 1987) and, as we showed above, the direct influence of no-planktonic matter on the depositions seems restricted to the coastal area (10-15x of the lake surface). Moreover, if the negative exponential model is solved to predict the deposition of L.P.O.M. on lake bottom at a determinate distance offshore, the ‘dispersibility’ (k) of large particulate organic matter results overlapped to the one observed. The coefficient (k) is less precipitous with respect to that one determined by Rau (1976) in Findley Lake because the size of Nemi’s watershed is larger than Findley’s and allochthonous materials are lighter. However, L.P.O.M. data fit with the exponential model, also if they are plotted against the depth instead of the distance offshore. In this case, the coefficient of dispersibility results 0.399 (Fig. 3) similarly to that obtained by Rau. How- ever, it seems confirmed the effectiveness of Rau’s equation in the deposition prediction on the bottom of the lakes.

The data of total deposition do not follow an exponential curve and the expected value obtained by Rau’s equation significantly underestimate the actual deposition at 6 m and 30 m depth. The data fitted with a different decay model (power curve Fig. 3) and they did not vary signiti- cantly at two major depths. The underestimate obtained when total deposition data are used to

194

calibrate the equation may be also an indirect estime of the planktonic input that reaches the deepest traps. In fact, the similarity between the depositions at the two major depths seems related to both the resuspension which is higher at 6 m-30 m than 0.2 m-2 m, and to the common (6 m and 30 m depth) planktonic input - this can reach the deoxygenated bottom partially de- gradated, increasing the organic fraction of the deposition. On the other hand, the autochthonous input seems to play an important role in the accretion of the lake sediment (Gasith, 1976). More- over, also if specific tests are required to establish the ecological relationship between plankton and detritus, our data can support the important role of the plankton detritus in the accretion of the sediment of more than 80% of the Lake Nemi area.

The material settling out on the bottom at the major depth is also recycled in a slower rate than that in shallow water. Based on the difference between the average organic fraction in the traps (46.5%) and that of the upper sediment (21.3%) the overall decomposition efficiency of settling material was, according to Gasith (1976), [(46.5%-21.3x)/46.5% = ] 54%. This figure, therefore, represents the material entering the quick cycling pool; 46% enters the slow cycle of lake sediment principally on the major deep bottom of Lake Nemi. High fractions have been observed also in other eutrophicated lakes (Jewel1 & McCarty, 1971; Lawacz, 1969) and that observed by us is likely because the deepest waters are without oxygen as long as 2/3 of the year in Lake Nemi. The decomposition in anoxic condi- tions shows, in fact, very low rates (Barnes et al., 1978; Reed, 1979).

In conclusion, our data show that in Lake Nemi : (1) the metabolism of L.P.O.M. occurs primarily

in the littoral zones not farther than 30 m offshore and may be predicted by a negative exponential model.

(2) the deposition on the bottom comprised under the perimeter of about 80% of the lake surface, is principally autochthonous and pre- sumably planktonic in origin.

(3) the detritus depositions are not completely processed and about half of the total material enters the slow cycle of the sediment on the deepest zone of the lake.

Acknowledgements

The research work was supported by grants of M.P.I. and C.C.E. contract.

References

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depth-time distributions of detritus depositions in lake nemi (volcanic lake of central italy)

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