The Detritic Compartment in a Posidonia oceanica Meadow: Litter Features, Decomposition Rates, and Mineral Stocks

P.S.Z.N. I: Marine Ecology, 13 (I): 69-83 (1992) 8 1992 Paul Parey Scientific Publishers, Berlin and Hamburg ISSN 0173-9565

Accepted: May 24,1991

The Detritic Compartment in a Posidonia oceanica Meadow: Litter Features, Decomposition Rates, and Mineral Stocks JAVIER ROMERO~ , GBRARD PER GENT^, CHRISTINE PERGENT-MARTINI*, MIGUEL-ANGEL MATEO’ & CBCILE REGNIER~

1 Departamento de Ecologia, Facultad de Biologia, Universidad de Barcelona,

* Laboratoire de Biologie Marine et d’Ecologie du Benthos, Facultt des Sciences Diagonal 645, E-08028 Barcelona, Spain.

de Luminy, UniversitC Aix-Marseille 11, F-13288 Marseille Cedex 9, France.

With 4 figures and 8 tables

Key words: Seagrass, Posidoniu oceunica, detritus, decomposition, mineral stocks, nutrient cycling.

Abstract. The ecosystem associated to the Mediterranean seagrass Posidoniu oceanica shows a clear distinction in two subcompartments regarding turnover time: aboveground and belowground. Aboveground parts (leaves) are highly dynamic, and most of the leaf material is decomposed or exported in less than one year, representing a net loss of nutrients. In contrast, belowground biomass (roots and rhizomes) has a turnover time of the order of centuries, with a consequent accumulation of organic matter in the sediment. The accumulation rates for the single elements rank in the order C > N > P. This ecosystem may be considered as a sink for biogenic elements.

Problem

The functioning of an ecosystem implies the existence of a certain amount of dead organic matter resulting from the budget in space and time between production and decay rates. The standing dead organic matter (necromass) has been recognized as a key compartment in terrestrial ecosystems, as regards both trophic aspects and nutrient cycling, and there is an extensive literature dealing with these topics (OLSON, 1963; HUNT, 1977; SWIFT et al., 1979; MELILLO et al., 1982; SMITH, 1982; MELILLO et al., 1989). In the aquatic environment, research has mostly concentrated on the kinetics of the decomposition processes (BROCK ef al., 1985; GODSHALK & WETZEL, 1978; PELLIKAAN, 1984; VALIELA et al., 1982; for a review, see HARRISON, 1989) and, to some extent, on topics related to the role of detritus as a food source (ODUM&DE LA CRUZ, 1967; for a review, see LEVINTON et al., 1984). Nevertheless, attempts to include these aspects and the necrornass compartment in a general ecosystem model are scarce (PEL-

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70 ROMERO, PERGENT, PERGENT-MARTINI, MATEO & REGNIER

LIKAAN&NIENHUIS, 1988). This may be due to the fact that in macrophyte- dominated systems, only a minor part of the produced material is recycled in situ; the bulk is exported, especially in marine communities (FENCHEL, 1977), resulting in a spatial uncoupling between production and decay.

Consequently, within the framework of a major cooperative project on the Mediterranean ecosystem dominated by the seagrass Posidonia oceanica (L.) DELILE, we have attempted to gain some insight into the necromass compartment. The first stage of our investigation was focused on two basic objectives: (i) To estimate the amount of necromass, especially leaf litter, associated with

the Posidonia bed, and its spatio-temporal variability. (ii) To estimate the decay rates of the plant material.

These questions are of major importance for understanding the functioning of the Posidonia ecosystem, since they relate to basic ecological processes, i. e., nutrient cycling, export of plant material, and detritivore food chains.

Material and Methods

1. Estimation of litter stocks

Samples were taken using a suction device, as described previously in PERCENT-MARTINI et al. (1991), inside a 35 x 35cm quadrat. A bathymetric transect in Lacco Ameno (see COLANTONI et af., 1982, and MAZZELLA et al., 1989 for a general description of the site) was sampled every three months (July 88, October 88, January 89, April 89, and July 89) at 1 ,2 , 5 , 10,20, and 30m depth. At each depth and on each date, three samples (replicates) - randomly selected over an area of ca. 100-200 m2 - were taken. The samples included both the dead material lying on the sediment and the sediment itself to a depth of 2 4 c m . The number of Posidoniu shoots inside the quadrat was recorded.

The samples were washed inmediately with running seawater, and both the sediment and all the coarse material coming from the belowground parts of the plant (mainly old leaf bases and rhizomes) were discarded. The remainder was sorted into two fractions: (i) Coarse leaf litter (>0.8cm) and (ii) Fine litter (0.1-0.8cm).

The coarse fraction is made up of blade material in a more or less decomposed stage, while the fine litter is more heterogeneous, with variable amounts of tiny leaf debris and fibers resulting from leaf base decay.

Both fractions were weighed separately after drying at 90°C until constant weight.

2. Decay experiments

Classic litter bag experiments were performed at 5 and 20m depth (see the early works of FALCONER el al., 1933; BOCOCK ei af., 1960; for the aquatic environment see, for example, GODSHALK & WETZEL, 1978; JOSSELYN et af., 1986; BROCK et af., 1985). Material for the incubation was collected from living plants; the oldest, senescent leaf of each shoot was choosen in order to simulate a natural leaf abscission. A known amount (30g of fresh weight) of this material was enclosed in a number of net bags (mesh size: 0.lcm). The bags were then placed in the meadow under the foliar canopy, attached close to the sediment, and collected at increasing time intervals (1, 2, 3, 4 weeks; 2, 3, 6 months) Two series of bags were placed at both 5m and 20m in July 1988. Unfortunately, the bags at 5 m were lost 1 month later. Two more series were then initiated in October 1989. At each sampling time, three bags were collected at random and taken to the laboratory. There, the samples were washed, the fauna was eliminated, and the foliar debris sorted into two fractions (coarse and fine), then dessicated, and weighed in the same way as for the litter.

The detritic compartment in a Posidonia oceanica meadow 71

3. Belowground parts

Samples of rhizomes + roots were taken using a diver-held corer (as described in PIRC, 1983) at 5 and 20m depth, penetrating around 20cm in the sediment. Three replicates were taken per depth. The samples were washed carefully and sorted into the following fractions: (i) living rhizomes, (ii) dead rhizomes, (iii) living roots, (iv) dead roots, (v) fine undifferentiated fraction (0.8-0.1cm) - and then weighed as described above. Fractions (i) and (ii) included the attached leaf bases.

The criteria used in fractioning the belowground plant parts are described in FRANCOUR (1990). An approach to the decay dynamics of some belowground parts was carried out using lepido-

chronology (see PERCENT et al., 1989). When Posidonia oceunica leaves die, only the blade falls away; the leaf sheathing base remains attached to the rhizome after leaf abscission, and is then often incorrectly termed a scale. As BOUDOURESQUE ef al. (1983) and PERCENT er at. (1983) demonstrated, the occurrence of annual fluctuations in leaf base thickness makes it possible to date long sections of orthotropic rhizomes. This constitutes a valuable tool for establishing the exact year of formation of a given section of the rhizome or a given leaf base. Consequently, the decay rate of the associated tissues can easily be inferred.

4. Chemical elementary composition

In order to assess the turnover of the various biogenic elements in the different necromass subcompartments, the elementary composition (carbon, nitrogen, and phosphorus) was determined for subsamples - dried at a maximum temperature of 70°C - of the following items: (i) litter from 5 and 20m depth (both fine and coarse), (ii) material from the decay experiments (5 and 20 m), (iii) belowground parts, (iv) lepidochronological samples.

The subsamples were ground to a fine powder and processed through a Carlo-Erba autoanalyzer to determine C and N content. P content was analyzed using ICP (Induction Coupling Plasma) after a wet acid digestion following JACKSON (1970) and DELGADO (1986), modified to a micro-wave digestor by MATEO & SABATB (1989).

Results

1. Litter stocks

Litter stock bathymetric distributions (both coarse and fine) are summarized in Fig. 1. Coarse litter followed a basic pattern, showing an intermediate depth maximum, except in April, and, in terms of seasonal variation, a summer- autumn maximum and a winter minimum.

The fine fraction is heterogeneous and includes small leaf fragments and fibers resulting from leaf base decay. The relative proportion of fibers increased with depth. This heterogeneity results in less clear spatio-temporal patterns. In general, maximum stocks were found at greater depths than for the coarse fraction, and seasonal maxima were shifted towards winter. The relative propor- tions of coarse/fine in the total litter stock clearly decreased with depth.

The few literature data on seagrass litter stocks seem to confirm the quantita- tive importance of this compartment (GALLAGHER et al., 1984; PEL- LIKAAN & NIENHUIS , 1988).

72 ROMERO, PERGENT, PERGENT-MARTINI, MATEO & RECNIER

300 4 Coarse litter I

Ju1'88

0 300

Jan'89

0

300 18 Apr'89

150 0 11.-'---, 300 1 Ju1'89

150 0 L

Fine l i t ter

,+

Fig. 1. Litter stock bathymetric distribution, both coarse (left) and fine (right)

I I 4 fractions. Vertical bars are A

0 10 20 30 0 10 20 30 the standard errors; n = 3 in Depth (rn) all cases.


A weight loss with time was evident in the incubated material. This loss followed a simple negative exponential model, with a high goodness of fit. Although simple exponential models have often been criticised, other models that have been proposed to fit decomposition data (GODSHALK & WETZEL, 1978; WIEDER &LANG, 1982) did not, in our case, increase the goodness of fit or reveal any special biological significance.

Decay rates (computed as k, the exponent of the model) appear in Table 1. Weight loss was more rapid at 5m than at 20m for equivalent periods. A seasonal trend also appeared, with higher decomposition rates in July/January than in OctobedApril. The remaining material inside the bags consisted mainly of coarse fraction; the fine fraction accounted for no more than 10% of total weight in all cases.

The detritic compartment in a Posidonin ocennicn meadow 73

Tablel. Decay rates in the different litter bag experiments. The dates of start and end of the experiment are given, as well as the average temperature during the incubation period and the initial N-concentration in the enclosed plant material. Experiments n and n’ represent decay rates computed over different time intervals using the same data set in order to allow comparisons.

experiment stadend duration depth average t N-content decay rate number: (months) (m) (“C) (% DW) (day-])

1 July/July 1 5 24.1 1.33 0.0100 2 Jul y/July 1 20 16.0 1.32 0.0101 2’ July/January 6 20 17.1 1.32 0.0076 3 October/October 1 5 20.2 0.59 0.0170 4 October/October 1 20 20.0 0.77 0.0078 3’ OctobedApril 6 5 15.7 0.59 0.0062 4’ October/April 6 20 15.5 0.77 0.0031

3. Belowground parts: roots and rhizomes

Data for biomass and necromass of the belowground compartment are shown in Table2. Variability was very high due to the heterogeneity of the meadow, but in all samples large stocks of dead material were registered. No living parts were found below 20 cm under the sediment surface. The fine, undifferentiated fraction showed high values and included the fine litter sampled with the suction device plus the fine fraction buried in the sediment. Values were higher at 5 m than at 20m due to the higher density of the meadow near the surface.

Table2. Standing stocks of both bio- and necromass (g dw . m 2 ) at 5 and 20m depth. Standard errors are given and, in all cases, n = 3.

5 m 20 m ~ ~

living rhizome 2150 & 1118 1223 f 176 dead rhizome 2743 k 415 2947 t 765 living root 619 f 477 141 f 56 dead root 3308 k 619 945 t 409

698 t 170 tine 491 f 118

total living 2769 1364

total dead 6542 4590

total 9311 5950

4. Selowground parts: lepidochronological data

Sheath decay is shown in Fig. 2. Since there is a certain variability in the number of leaves produced per year (see PERGENT et al., 1989), the data has been normalized for an ideal average sheath. From these data, and assuming again a


Fig. 2. Weight change with time in below- 100 - 5m. ground organs, derived from lepidochronologi-

20rn. cal measurements: sheath data (up) and rhizome data (down).

Sheaths .- : 200 I

m

0 0 4 8 12 16

Years

simple exponential model, a decay rate of k = 0.216.~- ' at 5 m (from 1984 to 1989) and of k = 0.110-y-' at 20m (for an equivalent period) or of k' = 0.086.~-1 (from 1974 to 1989) can be computed. Decay proceeds faster in shallower waters, probably due to both higher hydrodynamism and higher temperature. An additional source of variation - the compactness of the sediment - must be taken into account: at 5 m, there is an unhindered water flow between the rhizomes, which enhances both decay and grazer activity; at 20m, the rhizomes are buried in dense sediment.

Similar data are also shown for the rhizomes in Fig.2. Weight has been normalized for rhizome length, and a value of 100 % has been taken for 1988. The weight increment in the first years is due to the fact that rhizome growth and formation is only completed after 3-4 years (PERGENT et al., 1989). A slight weight loss begins after 8 years, probably related to the loss of functionality of the rhizome.

5. Chemical composition

C, N, and P contents of the analyzed fractions are shown in Table3. Dead (versus living) parts showed a lower mineral content in all cases. The ratio L/D (concentration in the living part / concentration in the dead part) ranked in the order C < N < P, with some exceptions. Nevertheless, the phosphorus content of dead parts was relatively high. These data support the hypothesis that there is a significant phosphorus fraction bound to structural compounds in marine angiosperms, as stated by DELGADO (1986) and VIDAL et al. (1991). In any case, further analyses in successive years (unpublished data) have shown that the N and P content were unusually high in 1988.

C, N, and P contents throughout the decay experiments are shown in Fig. 3, both for the litter bags and for the lepidochronological data.

In the case of leaves, the C-content clearly decreased with time; the N and P


Table3. Carbon, nitrogen, and phosphorus content in differents parts (living and dead) of the plant. Values are expressed in % of the element relative to the dry weight. The SEM is within the range 5-20 % of the mean, with n between 3 and 10. The data are from material collected in July at 20m.

45 -

35

C N P

Corbon - --\+,

0-0 -v

young leaves old leaves litter (coarse) litter (fine) living rhizome dead rhizome living roots dead roots

25 7

15 I , \\-

t

33.8 30.6 29.9 36.6 35.7 34.1 38.4 36.6

I I I

1.49 1.32 0.42 0.45 0.59 0.35 0.45 0.44

Phosphorus -

Fig. 3. C , N, and P content in different parts of the plant during the decomposition process. Left: data from litter bag experiments (note that the experiment at 20 m began in July, that at 5 m began in October). Right: lepidochronological data. Vertical bars are the standard errors.

0.10 3

0.05 ‘.i.-, be*---+/ $ , 0.00 ,

0.127 0.089 0.039 0.031 0.034 0.022 0.017 0.018

-\ A-*4A/-kA ‘-6. I 1 I I

content decreased at 20 m and remained fairly constant at 5 m. For the belowground parts, there was also a slight carbon loss with time. N and P contents decrease sharply in the rhizomes during the first three years, while growth is still proceeding, but remain constant in leaf sheaths.

Discussion

1. Litter stocks: models

Bathymetric distribution of litter depends upon the budget of inputs (i. e., from leaf fall) and outputs (i. e. , export and decay). Assuming that decay rates are

76 ROMERO, PERGENT, PERCENT-MARTINI, MATEO & REGNIER

constant with depth (or that its variation is well below the bathymetric variation due to the other two factors), the following model can account for the observed litter distribution:

L = la. (exp [- p - z])(l - exp [- h . z]) [equ- 11 where lo is a scale constant (i. e., the theoretical litter input at 0 meters), p is the rate of decrease of leaf (and subsequently, litter) production with depth, and h is the attenuation rate of the hydrodynamism. The equation was fitted to our data (only coarse litter) using an iterative method (LEATHERBARROW, 1982). The estimation of the parameters of equ. 1 are summarized in Table 4. The fit of the

800 - N

E ': I M

200 -

0

v)

Table 4. Estimated parameters for the model of litter stock bathymetric distribution (for explana- tion, see text).

Fig.4. Variation in meadow density with depth. Vertical bars are the standard errors; n = 12 in all cases. ::::\

I

July October January April July

10 (g ' m-2) 213 414 655 183 212 P (m-9 0.076 0.061 0.067 0.053 0.110 h (m-l) 0.38 0.13 0.025 0.19 0.92

model to our data is good (the predicted values are within 1 SE interval around the mean in 80% of cases, and of 2SE in all cases). Although the model is simple, the parameters seem to be quite realistic. The p rate gives a rough estimate of how Posidoniu production decreases with depth, and its value is close to the d rate from the following equation:

Dz = Do. exp (- d.z) [equ. 21 where Dz is shoot density (shoots-m-2) at depth z. Fitting this exponential model to our data on shoot density (Fig.4) gives an estimation of d near 0.06-m-'. The h rate is of the same order as the proposed hydrodynamic coefficient for Lacco Ameno in various previous works (GAMBI, 1986; GAMBI et uf., 1989). The ability of the model to predict litter stocks led us to conclude that bathymetric distribution of litter is in effect controlled by both production (bed density) and export (hydrodynamism).


In the absence of litter export, bathymetric variation in litter stocks is only due to variation of inputs from leaf fall. Leaf fall data can be obtained from BUIA et al. (1992), assuming that the weight of fallen leaves is the difference between primary production (estimated by the ZIEMAN method) and biomass increase. From this, and taking into account the decay rates estimated, a litter accumulation value can be computed under the hypothesis of zero export, that is

L‘(i) = F(i) exp (- k . tl2) + L(i-t). exp (- k * t) [equ. 31 where L’(i) is the predicted standing litter at time i, Ffi) is the weight of leaf material fallen between times i and i-t, t is the time interval between consecu- tive samplings, k is the decay rate and L(i-t) is the standing litter (observed) at time i-t. Consequently, export per unit surface between i and i-t is

E(i) = L’(i) - L(i) [equ. 41 This model was run using monthly estimates of leaf fall and our data of standing litter; the results are shown in Table 5 .

Table 5. Export rate at different depths, expressed as a percentage of the annual foliar (blades only) primary production.

depth 1 2 5 20 30 (m)

% export 70 73 47 56 52

Although this simple model overlooks some basic aspects of these phenomena (decay rate variations on both a seasonal and a bathymetric basis, different structures of the meadow, effects of the fauna, discontinuity of export events), the values in Table5 are probably a good indication of the order of magnitude of the organic material produced in the meadow and exported, and thus recycled elsewhere. Most of this material (70%) is exported at shallow depths (1-2m), while the export rate is fairly constant from 5 to 30m. For the whole Posidonia oceanica meadow of Lacco Ameno, we can consider that 50 % of the leaf material produced is exported; this is not far from Om’s (1980) estimation. Compared to data for other plants, this value is relatively high: 1-30 % in Zostera marina (BACH et al., 1986), 10-50 % in Spartina (DAY et al., 1973; VALIELA et al., 1978; HOPKINSON & HOFFMAN, 1984); 12 % in Posidonia australis (KIRKMAN & REID, 1979); 1’70 in Thalassia testudinum (ZIEMAN et al., 1979); 90 % in Syringodium filiforme (ZIEMAN et al., 1979); 20 % in mangroves (LUGO et al., 1976). Since Posidonia oceanica decay is not lower than in other species, this high export rate may be attributed to the geomorphology of the zone (open shore).

In general, the export process has been emphasized from the point of view of its role as a nutritional input for adjacent ecosystems; yet it also represents a net loss of nutrients for the seagrass ecosystem (see the last section of this paper).

78 ROMERO, PERCENT, PERGENT-MARTINI, WTEO & REGNIER


Our leaf material decomposition rates are within the range of published data on aquatic angiosperms (HARRISON, 1989), although comparisons are difficult because of different methods, experiment durations, pretreatments, etc. The data in Table 1, although not conclusive, indicate one main source of variation associated with depth; this can be interpreted as hydrodynamism, since decay is greater at 5 than at 20m (for the same time intervals and the same season). The observed seasonal trend can be attributed both to temperature and to the quality of the decaying material (N content of July experiments: 1.33%; N content of October experiments: 0.68 YO), although it should be recalled that N content is not a universal estimator of decay rates. In effect, while the Posidonia N content is low compared to other seagrasses (HARRISON, 1989; DUARTE, 1990), the decay rates are intermediate.

The dynamics of N and P release during decay can be summarized as follows (see Fig. 3 and Table3). A first loss takes place during leaf senescence (11 YO N, 30 % P). How these elements are lost before leaf absicission remains unknown; both an early leaching or a retranslocation process can occur (see FRESI & SAGGIOMO, 1980; PIRC, 1985; PIRC & WOLLENWEBER, 1988). Further losses occur during decomposition (37 YO N, 35 YO P relative to the concentration of the living material), until a plateau is reached (see Fig. 3; material from July at 20 m) that can be interpreted as the refractory fraction (52 YO N, 35 YO P). However, when the initial material has a low nutrient content (see Fig. 3; material from October, 5 m), no further losses occur during decomposition, but the plateau is reached with very similar values.

No nitrogen accumulation during decay was recorded. This seems to be a general feature of seagrasses (HARRISON, 1989), with some exceptions (JOSSELYN & MATHIESON, 1980; RICE & TENORE, 1981). This is a key factor in establishing whether or not leaf litter acts as a sink for nitrogen (as for mangroves: RICE, 1982). Although data on P release during decay are much scarcer, the existence of a refractory fraction seems to be a universal feature of vascular plants. This refractory fraction can be estimated as the P content in the tissues after a certain time of decay expressed as a percentage of the P content of the fresh material. Data in the literature indicate that this refractory fraction can represent between 15 and 90 YO of the initial P concentration, depending on the species and on the decay conditions (Swm el al., 1979; KLUMP & VAN DER VALK, 1984; PELLIKAAN, 1984; VAN DER VALK & ATTIWILL, 1984; FORBS et al., 1988; MENBNDEZ et al., 1989). The existence of such a fraction allows a net export capacity, either to the sediment or to adjacent ecosystems.

3. Mineral stocks

The data presented in this paper allow mineral stocks of different compartments of the system to be estimated; for the sake of brevity, we only present data from 20m (Table6). The ratio deadfliving (in terms of total weight) is around 3.1. This is quite high compared with terrestrial ecosystems, being of the same order of magnitude as for boreal forests (OLSON, 1963; RODIN & BAZILEVICH, 1967). It

The detritic compartment in a Posidoniu oceanicu meadow 79

Table 6. Biomass and mineral stocks in the different parts of the plant. Dry weight in gDW . m-2. C, N, P: g of element. m-2. The data are from the 20m station. For leaves and litter, maximum yearly values are reported.

~~

dry weight C N P

leaves 384 123.6 5.4 0.41 living rhizomes 1223 393.8 7.2 0.42

roots 140 53.8 0.6 0.02

total living 1747 571.2 13.2 0.85

leaves 136 40.7 0.6 0.05 rhizomes 2947 1004.9 10.3 0.65 roots 945 345.9 4.2 0.17 fine 698 255.5 3.1 0.22

dead

total dead 4726 1646.9 18.2 1.09

TOTAL 6473 2218.1 31.4 1.94

would be even higher with sampling to a greater sediment depth. This ratio drops if we consider single elements C (2.6), N (1.3), and P (around 1.4). These values are obviously related to turnover rates of the different elements.

Using P/B data from O n (1980) and ROMERO (1985), the turnover time of the different necromass subcompartments (that is, the standing necromass of a given subcompartment divided by its yearly production) for each of the three elements can be estimated (Table7). The turnover time is Iow for the aboveground parts, on the order of months. For the belowground part, the turnover time is much longer, from years to centuries. As shown in Table7, both N and P recycle faster than C .

Even our rough estimations at this preliminary stage of work demonstrate that the Posidoniu ecosystem acts as a sink for the various biogenic elements. In effect, amounts of the order of 25 g C m-2. y-* (and associated elements: see Table8) are diverted to a very low turnover subcompartment (rhizomes plus roots). In contrast, the leaf subcompartment is much more dynamic, and decomposition and export lead to low residence time of old leaf material within the meadow. Since both immobilization and export represent net losses of biogenic elements for the system (summarized in Table 8), we conclude that at least 35 % of the plant production is new production (in the sense of DUGDALE & GOERING, 1967), and will need an external nutrient supply to proceed.

Table7. Turnover rates (year-’) of the different parts of the plant for the different elements (C , N, and P). Turnover rate for a given subcompartment is calculated as the annual production of the subcompartment divided by its maximum annual biomass. Data from the 20m station.

C N P

leaves 4.56 rhizomes 0.03 roots 0.01

14.22 0.05 0.01

11.73 0.05 0.01


Table 8. Summary of nutrient losses for the Posidonia ecosystem (g of element. m-*. y-I). The requirements have been estimated using data of primary production and data in Tables3 and 6.

requirements immobilization exported total % (rhizomes + roots)

N 8.12 0.36 2.11 2.47 30.4 P 0.62 0.022 0.11 0.132 21.3

Summary

This paper represents a preliminary attempt to investigate the detritic compartment of a Mediterranean seagrass ecosystem (Posidonia oceanica). The amount of dead organic matter (necromass) within the system has been evaluated, taking into account both leaf litter and dead rhizomes and roots, as well as the mineral stocks (C, N, and P) it represents. Decay rates of leaves and rhizomes have been estimated, and turnover time for each element within each subsystem derived. The leaf compartment is highly dynamic, and within one year all the material produced is either decomposed or exported; yearly export can be estimated as 50% of the produced leaf material for the whole meadow. Turnover time of the belowground biomass is higher, of the order of centuries. The belowground parts act as a sink for biogenic elements, annually diverting from the general nutrient cycling up to 25 g C, 0.4 g N, and 0.02 g P * m-2. Taking into account both export and burial, we conclude that at least 35 % of the yearly plant production must be supported by external nutrient inputs, a value that also represents new production.

Acknowledgements

This work was supported by EEC grant EV4V-0139-B. Field work was carried out at Ischia, where laboratory and diving facilities were kindly provided by Dr. LUCIA MAZZELLA and her staff at the Laboratorio di Ecologia del Benthos de la Stazione Zoologica di Napoli. Thanks are also due to P. BULTEEL and P. COULON for their help. The Spectroscopy Service of the University of Barcelona, from where ELIONOR PELFORT deserves a special mention, gave technical support for the phosphorus analysis.

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Documents

The Detritic Compartment in a Posidonia oceanica Meadow: Litter Features, Decomposition Rates, and Mineral Stocks