decomposition of juncus maritimus in two shallow lakes of doñana national park

SERGIO ALVAREZ, EUGENIO RICO, Ma CARMEN GUERRERO and CARLOS MONTES

Department of Ecology, Universidad Autonoma de Madrid, E-28049 Madrid, Spainemail: [email protected]

Decomposition of Juncus maritimus in Two Shallow Lakes ofDoñana National Park

key words: decomposition, shallow lakes, macroinvertebrates, chironomids, Juncus,Doñana National Park

Abstract

Santa Olalla and Dulce are two shallow natural eutrophic lakes (”lagunas”) located in Doñana Nation-al Park (south of Spain). Both lagunas have high algal biomass and pH and are surrounded by a macro-phyte fringe of Juncus spp. and Scirpus spp. The effect of the macroinvertebrate community on decom-position rates (k) of Juncus maritimus was measured in both lagunas using the litter bag method overa one year period after which less than 10% of the original litter bag material remained. Two differentmesh sizes, coarse (5 mm) and fine (0.25 mm), were used to evaluate macroinvertebrate communityeffects on decomposition. Decomposition rates were rapid, with k values of 0.0162 day–1 in coarse bagsand 0.0094 day–1 for fine bags at Santa Olalla and 0.0095, 0.0088 day–1 for coarse and fine bags, re-spectively, at Dulce. Both lagunas had similar densities of macroinvertebrates, with a dominance ofshredder-gatherer chironomids. However, densities of scrapers (Ancylidae) increased in Santa Olalla inspring but were almost absent in Dulce. The influence of different physical and chemical parametersand of the macroinvertebrate community on the decomposition process is studied. In spite of somepotential confounding effects in the use of fine mesh bags in standing water systems, it is concludedthat morphometrical characteristics of the lagunas are important in controlling decomposition rates.

1. Introduction

The importance of detritus -defined as non-predatory losses of organic carbon from anytrophic level or inputs from sources external to the ecosystem (WETZEL, 1972)- in aquaticecosystem metabolism has been highlighted by a number of authors (ODUM and DE LA

CRUZ, 1963; MANN, 1988; WETZEL, 1991). In many wetlands – including marshes, swamps,floodplains and shallow lakes (MALTBY, 1988) – primary production is high and only a smallfraction is directly consumed by animals (TEAL, 1962; MANN, 1972). Thus, most organicmatter decomposes and, at different stages of decay, material is consumed by differentassemblages of detritivores. Decomposition emerges, then, as a fundamental ecosystem pro-cess comparable in importance to primary production (MOORHEAD et al., 1996) and as a use-ful management tool, since adequate characterisation of an ecosystem requires informationon both structure (patterns) and function (processes) but current assessment schemes formanagement almost exclusively rest on structural attributes (GESSNER, 1999). Although thedecomposition process is well documented for wetlands in general (see the reviews of PAT-TEN, 1990 and VYMAZAL, 1995) the decay process at an ecosystem level (WETZEL, 1992) isstill unclear owing to the high heterogeneity among and even within standing water systems.Standing water systems have high diversity of detritus (standing dead biomass, abovegroundand belowground material, emergent and submerged vegetation) as well as large spatial andseasonal variability (PIECZYNSKA, 1993). In spite of this diversity, much of the information

Internat. Rev. Hydrobiol. 86 2001 4–5 541–554

VII. Decomposition of Organic Matter in Standing Waters

© WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 1434-2944/01/4-507-0541 $ 17.50+.50/0

underlying the current models of decomposition processes and detrital food webs has beenderived from a fairly narrow range of ecosystems (i.e. aquatic models are strongly influencedby the study of leaf litter decomposition in headwater streams or Spartina marshes) (HEDIN,1991).

On the other hand, although in Spain there have been several studies of particulate organicmatter (POM) decomposition and cycling in streams in temperate regions (MOLINERO

et al., 1996; LOPEZ et al., 1997) very few have focused on small standing water systems inMediterranean regions even though these lake systems are one of the most widespread aquatic ecosystems in Spain (CASADO et al., 1992). The main features of these systems aretheir temporary nature and great variation in water level among years and, besides their highdiversity, they constitute an important habitat for waterfowl in Spain (FLORIN and MONTES,1998)

Considering that decomposition is a key process to define ecosystem functioning, and thelack of information about decomposition in temporary systems in Spain, the objectives ofthis work are to compare the decomposition rates of the predominant macrophyte vegetationof two shallow lakes of southern Spain, and to assess the effects of the macroinvertebratecommunity and physical and chemical factors on decomposition, integrating informationabout structure and processes. The rate of decomposition may then be used as an indicatorof the decay pathway in these systems. The two studied lakes (Santa Olalla and Dulce) areonly few meters apart but they present differences in water chemical characteristics, and intheir phyto- and zooplanktonic communities (LOPEZ et al., 1991). We hypothesise that therate of decomposition would be high due to the eutrophic condition of both systems butdistinct between them reflecting differences in water chemistry and morphometry and thus,a useful indicator to describe, define and manage each one of them.

This study was part of a broader project that has the objective of defining, describing andclassifying the wetlands of the Doñana area in terms of their primary and secondary pro-duction, nutrient cycling, groundwater interactions and decomposition processes.

2. Material and Methods

2.1. Study Sites

The study was conducted from February 1998 to February 1999 in two shallow lakes (lagunas) located in Doñana National Park: Laguna de Santa Olalla and Laguna Dulce. Doñana National Park islocated on the Atlantic coast of southwestern Spain and is an important stopover for migrating birdsand also sustains some important endangered fauna (Fig. 1). The area is dominated by a Mediterraneantype climate, with an average yearly rainfall of 580 mm, concentrated in autumn and spring. Summersare dry with temperatures that can reach 30˘C in the months of July and August.

Santa Olalla and Dulce belong to the group of peridune lagunas of the “eolian littoral mantle” (MON-TES et al., 1998), which comprises an area of mobile dunes and other of phytostabilized sands (Fig. 1).Both lagunas are located on the northern margin of the mobile dune system at approximately 2500 mfrom the seashore. Both systems are shallow (maximum depth 2.5 m in Santa Olalla and 1.86 m inDulce) and are considered permanent, although in dry years they can become dry. Both lagunas alsoexhibit large variations in water level (70–250 cm in Santa Olalla; 5–186 cm Dulce, during the periodstudied). Santa Olalla has an area of ~45 ha and Dulce has an area of ~8 ha. However, both lagunasare shallow with gentle slopes and these areas can increase considerably in rainy years. Santa Olallaand Dulce are hypereutrophic systems and chlorophyll-a in water samples can reach values of 700 mgm–3 in Santa Olalla and 350 mg m–3 in Dulce. They are alkaline systems with pH values of 7–11. Bothlagunas, especially Santa Olalla, are exposed to high winds with average velocities of 6.1 m s–1 in SantaOlalla and 1.9 m s–1 in Dulce during the studied period.

Although phytoplankton represents the major organic matter input to detrital pathways within thelakes, both systems are surrounded by a macrophyte fringe of Juncus spp. and Scirpus. Sediments are

542 S. ALVAREZ et al.

fine-textured and sandy, changing to sandy-silty in deeper areas. Organic matter content varies betweendifferent zones of the systems, ranging from 30% in deeper zones to 2–5% near the shores. More than85% of organic matter is as fine particulate matter (0.063–0.5 mm).

There have been several studies of phytoplanktonic composition and abundance and their relation-ship with the nutrient dynamics of these lake systems (LOPEZ et al., 1991; TOJA et al., 1991). However,there have been no previous studies on decomposition of the organic matter (of both phytoplanktonicand macrophytic origin) or the macroinvertebrate community associated to the process in the lagunasdespite its importance for the understanding of the ecosystem functioning (MOORHEAD et al., 1996). Themain organic input in these two systems is phytoplanktonic, with average sediment inputs of 1000 gOM m–2 and 2.5 g Chl-a m–2 in Santa Olalla and 2000 g OM m–2 and 2 g Chl-a m–2 in Dulce duringJuly and August (ALVAREZ, unpublished data). However, organic matter input from macrophyte vegeta-tion and from terrestrial origin may also be substantial throughout the year. During the studied periodPOM input from Juncus maritimus alone to the lake shores was about 116 g m–2 (ALVAREZ, unpublisheddata), peaking during December and January months. This amount is lower than the organic matter

Decomposition of Juncus maritimus 543

Sevilla

Guadalquivir

Sanlúcar de Barrameda

River

El RocíoRocina Stream

N0 5

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Sta. Olalla

Dulce

Nacional Park Boundary

Disturbed Marshes

Natural Park Boundary

Natural Marshes

Rice PaddlesEcotone

Phytostabilized Dunes

Semistabilized DunesMobile Dunes

Position of litterbags

Figure 1. Location of Santa Olalla and Dulce lagunas in Doñana National Park (Southwest Spain).(–––––) National Park boundary; (------) Natural Park boundary.

inputs in low order streams (CHAUVET et al., 1993; POZO et al, 1997) but lies in the range of organicmatter input of non forested wetlands (PATTEN, 1990). During the study period input of organic matterfrom decomposition of J. maritimus was important because of the high water level that covered impor-tant areas where POM was accumulated. Mechanical damage from animal action (cattle, deer) is a veryimportant supply process of this POM (GOPAL, 1990)

2.2. Methods

Temperature (mercury thermometer with maximum and minimum values recording), pH, redoxpotential (323-A WTW pHmeter) and oxygen concentration (WTW 340-B oxymeter) were measured insitu. Litter bag depth was measured using a metal rod fixed to the bottom of the lagunas.

Brown stems of rush (Juncus maritimus), the most abundant macrophyte associated to the lakes, werecollected on December 1997, which coincided with the maximum senescence and decay of emergentmacrophyte vegetation. Plant material was air-dried on return to the laboratory. Litter bags (20 cm ×10 cm in size) of coarse (5 mm) and fine (0.25 mm) mesh sizes were constructed each one containing5 g (DW) of the collected plant material. On 17th February 1998, 45 bags of each mesh size were placed along two similar shores of the two lagunas at the same initial depth, in zones where rush wasnaturally entering the system (Fig. 1). Bags were distributed along three independent lines and weighedto the bottom using stones. Three bags of each mesh size were retained to correct for changes in masscaused by handling. Three independent replicates of each size class were collected after 5 days to assessrapid leaching of soluble material. Subsequent collections were made monthly intervals until the end ofthe study except for November 1998 and January 1999.

Water level is a critical factor in decomposition control in these highly fluctuating systems. To com-pare between permanent submerged and temporal emergent conditions, some of the bags were alwayskept submerged at both sites, whereas others became dry around the month of August as the naturalwater level receded in the lakes in the summer. Collected litter bags were sealed in plastic bags andtransported on ice to the laboratory. Samples were washed carefully with tap water to remove sedimentand fauna. A 0.075 mm mesh sieve was use to separe fauna from plant litter. Fauna were preserved in4% formalin for identification and counting. Plant litter was oven dried at 75 °C for 48 h to obtain dryweight (DW).

After adjustment for handling losses, mass remaining versus time (days) was fitted to an exponenti-al model:

mt = mo e–kt

Where mt is the mass remaining at time t (in g), mo is the initial mass and k is the decay coefficient.(OLSON, 1963; PETERSEN and CUMMINS, 1974). Analysis of covariance (ANCOVA) on log-transformeddata was used to compare regressions (P < 0.05) (ZAR, 1996). Multiple pairwise comparisons, after ana-lysis of covariance, were obtained by Tukeys Post Hoc test. Linear equations with time in days as inde-pendent variable and exponential equation with time in degree-days as independent variable were alsoused to test another proposed decomposition models in the literature (RYBCZYK et al., 1996). Data onmacroinvertebrate densities were log-transformed and analysis of variance (ANOVA) was used to detectdifferences between treatments. Relationships between different variables were tested using Pearson cor-relation coefficients on transformed data. All statistical analysis were performed using the softwareSTATISTICA.

3. Results

In both lagunas the highest water temperature was in July-August, when maximum valueswere above 30 °C (Fig. 2). Surface sediments had high pH values throughout the study peri-od, with an average value of pH 9 in Santa Olalla and pH 8.15 in Dulce (Fig. 2), while thewater pH was as high as 11. In both lagunas, water level decreased from the beginning ofthe study and litter bags remained under water until August 1998 (Figs. 2a and 2b). At thistime, some of the bags were moved to a new location to keep them submerged. Oxygen con-centration and redox potential in the sediment surface exhibited sharp variations throughout


the study period (Figs. 2c and 2d). Oxygen concentrations varied from 14–15 mg L–1 duringthe day (due to the photosynthetic community activity) to 5 mg L–1 at night. At the sedimentsurface, and a few cm beneath it, conditions were commonly anaerobic. However, watercolumn and sediments were frequently reoxygenated due to the strong wind mixing the shal-low waters of the lakes. Although significant fluctuations were observed in both lagunas,oxygen concentrations and redox were generally higher in Santa Olalla (Fig. 2c) comparedto Dulce laguna (Fig. 2d) (average values of 93.19%; 8.28 mg l–1 and –34 mV in Santa Olal-la and 81%; 7.57 mg l–1 and –66 mV in Dulce during the study period).

Mass loss from the litter bags followed a similar pattern in both lagunas and under bothtreatments (permanently submerged-PS, and temporarily emerged-TE) (Fig. 3) with 40% ofremaining material around day 100 (May). At this time, and coinciding with summer sea-son and higher temperatures, the rate of decomposition (i.e. rate of mass loss) was morerapid in Santa Olalla coarse mesh bags (Fig. 3a) than in the other treatments. When the bagswere dry due to decrease in water level (around day 190) most of the litter mass had alreadydecomposed or been consumed by detritivores (especially in Santa Olalla). Patterns of massloss were similar between Dulce coarse and fine mesh sizes (Fig. 3b).

The most rapid rates of litter decomposition (Table 1) were for Santa Olalla coarse meshbags under both PS and TE conditions (k = 0.02 d–1 and 0.0162 d–1 respectively). Decom-


Figure 2. Main physico-chemical parameters of the studied systems and average litter bags depth.a), b): (�) Average bag depth (cm); (�) Submerged bags depth (cm); (�) Temperature; (X) pH.c), d): (�) Oxygen concentration (%); (�) Oxygen concentration (mg l–1); (�) Redox Potential (mV).

All values are for sediments surface.


Figure 3. Remaining dry mass (%) of Juncus maritimus under temporarily emerged-TE and per-manently submerged-PS conditions. (�) Coarse mesh bags–5 mm. (�) fine mesh bags–0.25 mm.

(→) Water level reaches 0 cm. TE bags became dried.

Table 1. Regression summary on mass loss data of Juncus maritimus using the simpleexponential decay model in the studied systems (Olalla, Dulce) with the two mesh sizes(Coarse (5 mm)-C-, Fine (0.25 mm)-F-) and the two treatments (temporarily emerged-TE-,permanently submerged-PS-): k, Breakdown rates (d–1); CI, confidence intervals (95%);

mo, estimation of initial dry mass (%).

k (d–1) C.I mo r2

Santa Olalla. C-TE –0.0162 0.0019 85.5 0.88Santa Olalla. F-TE –0.0094 0.00114 82.3 0.88Santa Olalla. C-PS –0.02 0.00206 111.3 0.8Santa Olalla. F-PS –0.012 0.0011 90.8 0.91Dulce.C-TE –0.0095 0.00166 101.5 0.77Dulce.F-TE –0.0088 0.001125 86.35 0.86Dulce.C-PS –0.017 0.0009 133.6 0.9Dulce.F-PS –0.01 0.001 100.6 0.94

position rates were also greater in Santa Olalla than in Dulce, for both mesh sizes and, with-in each laguna, always more rapid in coarse mesh than in fine mesh. Decomposition rateswere also higher under PS conditions than under TE. The ANCOVA analysis revealed sig-nificant differences between lagunas (F = 13.45, P = 0.00042) and mesh sizes (F = 11.89,P = 0.0009) and a slight laguna-mesh size interaction (F = 4.99, P = 0.028). However, afterperforming a posteriori Tukey test, these differences were only significant for Santa Olallacoarse bags, with significantly higher breakdown rates in comparison with other treatments(Table 2).

Linear regression and exponential models using degree-days were also tested to describethe loss of litter mass over time. However, these models did not improve fit of the data andwere discarded.

After five days, leaching accounted for 5–9% of mass loss depending on the treatment(Table 3). Leaching of the litter was greatest in Santa Olalla and in fine mesh bags.

Temperature showed a positive relation with mass loss in all treatments. (R = 0.93 forSanta Olalla coarse and fine bags and R = 0.89 for Dulce coarse and mesh bags, respectively.P < 0.05). However, no significant correlations were found between macroinvertebrate den-sities and mass loss or temperature. In Dulce laguna, a negative relationship (R = –0.72,P < 0.05) between oxygen concentration and temperature was found.

High densities of benthic invertebrates colonised the litterbags, with over 1200 individu-als bag–1 at Santa Olalla and 600–1000 individuals bag–1 at Dulce. Maximum abundanceswere in June and July (Fig. 4) and greater in coarse mesh litterbags during these months within each laguna. However, there were no statistical differences in the total abundance of


Table 2. Significant differences between breakdown rates (Tukey’s HDS test; P < 0.05).O: Olalla; D: Dulce; C: Coarse; F: Fine; TE: temporarily emerged; PS: permanently sub-

merged. S: significant difference; NS: no significant.

O-C-TE

O-C-TE O-F-TE

O-F-TE S O-C-PS

O-C-PS NS S O-F-PS

O-F-PS NS NS NS D-C-TE

D-C-TE S NS S NS D-F-TE

D-F-TE S NS S NS NS D-C-PS

D-C-PS NS NS NS NS NS NS D-F-PS

D-F-PS S NS S NS NS NS NS

Table 3. Percentage of leaching after 5 days.

% of Leaching (after 5 days)

OLALLA COARSE 7.95OLALLA FINE 8.31DULCE COARSE 4.81DULCE FINE 7.94

macroinvertebrates between the lagunas or between mesh sizes. A slight increase in the totalnumber of animals was observed towards the end of the study period which coincided withan increase in the water level.

Taxa colonising litterbags were similar in both lakes. The most numerous taxon wasChironomidae, mainly Chironominae (Fig. 5), with a dominance of Glyptotendipes pallensand Chironomus sp. Other chironomins found were Parachironomus sp., Tanytarsus sp. andParatanytarsus sp. Chironominae represented more than 80% of macroinvertebrates duringJune and July months (around 87% and 89% of total macroinvertebrate abundance in coarseand fine bags in Dulce and 87% and 77% in Santa Olalla, respectively). Orthocladiinae (Cricotopus sylvestris and Corynoneura sp.) and Tanypodinae (Procladius sp.) were presentin small numbers.

There were higher densities of Ostracoda in Santa Olalla than in Dulce, with the highestdensities values in the Santa Olalla fine mesh bags in July (average 500 individual per bag).In contrast, Oligochaeta (mainly Naididae) were more abundant in Dulce laguna in the sameperiod (Fig. 5b), although they were more abundant in Santa Olalla compared to Dulce inthe first phase of the study in coarse mesh bags (March).

Molluscs were represented by Ancylidae and Physidae. Both taxa were excluded by finemesh. Ancylidae (Ferrissia) increased in abundance in April-May in Santa Ollala coarsebags (Fig. 5a), but were almost absent in Dulce.

Other taxa which were noted but present only in small numbers included Ceratopogo-nidae, Dryopidae (Dryops), Gyrinidae (Gyrinus larvae), Hydrophilidae (Berosus), Baetidae(Cloeon), Pleidae (Plea), Coenagrioniidae, Glossiphoniidae (Helobdella) and Nematoda.

The trophic structure of the macroinvertebrate community showed, in general, a domi-nance of taxa considered shredders-herbivores and, at the same time, collector-gatherers(Glyptotendipes, Chironomus, Cricotopus). Taxa considered primarily collector-gatherers


Total Abundance of Macroinvertebrates

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Figure 4. Total abundance of macroinvertebrates in litter bags (number bag–1). (�) Olalla Coarse; (X)Olalla Fine; (�) Dulce Coarse; (�) Dulce Fine.


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Figure 5. Main macroinvertebrate groups in litter bags (number bag–1). (�) Chironominae (Shredder-gatherers); (�) Oligochaeta (Gatherers, some grazers); (-×-) Ancylidae-Physidae (Scrapers); (�) Ostra-

coda (Gatherers); (�) Orthocladiinae (Shredder-gatherers).

Table 4. Comparision between the breakdown rates obtained in this study and others fromJuncus spp. found in the literature. Only temporarily emerged samples are considered.

k (day–1) Time Time Source(50%, d) (95%, d)

Olalla, Coarse 0.0162 43 185 This studyOlalla, Fine 0.0094 74 319 This studyDulce, Coarse 0.0095 73 316 This studyDulce, Fine 0.0088 79 341 This studyJuncus effusus 0.0032 216 937 BOYD, 1971Juncus squarrosus 0.0013 533 2308 LATTER & CRAGG, 1967Juncus roemerianus 0.00165 420 1818 WILLIAMS & MURDOCH, 1972Juncus roemerianus 0.0017 407 1765 NEWELL et al., 1984

(Ostracoda, Naididae, Tanytarsus, Coeynoneura) had significant abundances in some cases(Fig. 5). Scraper-grazers were only dominant in Santa Olalla coarse bags in spring, with anabundance of 53% of total fauna. Predators were represented by the tanypodin Procladiusand the chironomin Parachironomus. Procladius was rare while Parachironomus, which isalso considered a collector-gatherer, represented 3–12% of chironomins in both Santa Olal-la and Dulce lakes.

4. Discussion

4.1. Decomposition Rates

Decomposition rates (k) in both lagunas were very high, regardless of mesh sizes, com-pared to other estimates for temperate regions (Table 4). For the temporarily emerged bags(TE), in Olalla laguna only 50% of the material in the coarse mesh bags remained after 43 days, and only 5% after 185 days. These rates of litter decomposition are very high for amacrophyte community in a standing water system in temperate regions (where an averagetime period of 400 days is needed to decompose 50% of the material VYMAZAL, 1995) butare similar to decomposition rates for other wetland species in tropical regions with high tem-peratures. In this respect, the average k value for Typha spp. obtained from different studies in Indian wetlands (SHARMA and GOPAL, 1982; MIDDLETON et al., 1992) was 0,0083 d–1 (although this species has generally higher decay rates in comparison with Juncus).

4.2. Physical and Chemical Factors

Temperature, nutrient concentration and oxygen concentration, generally considered themost important factors controlling organic matter decomposition in lacustrine systems(GODSHALK and WETZEL, 1978; WETZEL, 1991), were favourable in these systems. The highwater temperature of the lakes in this study (Fig. 2), particularly during summer, have con-tributed to rapid decay rates by enhancing microbial and macroinvertebrate activity. Manystudies have demonstrated seasonal variation in breakdown rates with faster breakdownduring warmer periods when temperature primarily affects microbial processes (PETERSEN

and CUMMINS, 1974). In this respect, a positive relationship (P < 0.05) was found betweentemperature and mass loss rates, but not between macroinvertebrate densities and tempera-ture or mass losses. Macroinvertebrate feeding seems to be less influenced by temperatureand may sometimes overshadow temperature effects (WEBSTER and BENFIELD, 1986).

The highly eutrophic nature of the lagunas is also of importance in explaining rapiddecomposition rates. In July and August total nitrogen (TN) reached 3.5 mg L–1 in SantaOlalla and 2.8 mg L–1 in Dulce while total phosphorous (TP) was as high as 0.6 mg L–1 inboth lagunas coinciding with high Chl-a values. High nutrient concentrations and consequenteutrophic conditions are know to accelerate rates of organic matter decomposition in aquatic conditions (WEBSTER and BENFIELD, 1986; PIECZYNSKA, 1993; RYBCZYK et al., 1996)although is not always the case.

Redox potential and oxygen concentrations in the lagunas fluctuated significantly (Fig.2)as a result of wind action, creating turbulence across the shallow lakes. Consequently, sedi-ments were frequently oxygenated and resuspended, probably enhancing the decompositionprocess since numerous studies have shown that decomposer activity is retarded in anaero-bic conditions (NECKLES and NEILL, 1994).

Higher nutrient and Chl-a concentrations and wind velocity (and, therefore, oxygen con-centration in sediments) in Santa Olalla would also explain the slighter higher decomposi-tion rates compared to Dulce (Table 1). In addition, during summer months, macroinverte-


brate densities were higher in Santa Olalla (Fig. 4). However, statistically significant differ-ences in the k values are only found for Santa Olalla coarse bags with respect to the othertreatments (Table 2). This difference can be explained by the loss of 33% of mass from theselitterbags between May and July (Fig. 3). The k value for this period of time was 0.0788 d–1

for Santa Olalla coarse bags in comparison with values of 0,062 d–1 for Santa Olalla fine bagsand 0.0118 d–1, 0.0115 d–1 for Dulce coarse and mesh bags respectively. In Santa Olalla, theoxygen and nutrient exchange, and the mechanical breakdown of litter particles because ofwater movement were enhanced in coarse mesh bags, while in fine mesh bags these effectswere probably attenuated. Fine mesh bags were often clogged with sand and the litter massenclosed had a blackish colour because of more anaerobic conditions. The same effect wasobserved in both coarse and fine mesh bags at Dulce laguna, where oxygenation was less fre-quent and less effective primarily because of the smaller size of the laguna and greater pro-tection from wind by steeper banks around the lake perimeter. In this laguna, during summer,a period of high macroinvertebrate and microbial activity, a negative relationship was foundbetween temperature and oxygen concentrations, evidencing that the oxygen losses due torespiration were not compensated with new oxygen inputs due to mixing processes.

There is evidence that the degradation and dissolution of shallow layers of plant detritusis influenced by solar radiation (DENWARD and TRANVIK, 1998). In this respect, althoughdecomposition progressed faster in the remaining mass under submerged conditions, therewas a slow mass loss in the emerged bags too in spite of the fact that almost no inverte-brates where found under these conditions. This decay under dry conditions could be caused by direct light action, that is an important abiotic factor enhancing decompositionprocesses in these systems (SERRANO, 1992).

4.3. Macroinvertebrates

Many studies have shown that decomposition is more rapid in systems with high inver-tebrate densities (WEBSTER and BENFIELD, 1986). In the studied lagunas, high densities ofmacroinvertebrates were found in summer months (Fig. 4) coinciding with greater losses oforganic matter, especially in Santa Olalla (Fig. 3). However, no significant relationshipswere found between macroinvertebrate densities and mass losses, suggesting a complexmodel in which there is not a direct macroinvertebrate effect on decomposition but an inter-action with increased temperature and nutrient supply.

While conditions within the fine mesh bags were often anaerobic, the fine mesh was notan effective barrier for almost all the dominant taxa with the exception of molluscs. Macro-invertebrate density did not differ between coarse and fine mesh litter bags in both lakes,although densities were higher in Santa Olalla than in Dulce in the summer. On the otherhand, scrapers (mainly Ancylidae) were only significant in Santa Olalla coarse bags in spring(Fig. 5a). The differences in both types of macroinvertebrates and their abundances may alsohelp to explain the higher decomposition rates in Santa Olalla.

In general, a dominance of taxa considered shredders-herbivores as well as collectors-gath-erers, is found in the studied systems. While most species of the subfamily Chironominaeare regarded as collector-gatherers, the most abundant species found in the lagunas of thisstudy (Glyptotendipens pallens and Chironomus sp.) are classified as shredder-herbivores,collector-gatherers. Both species are also burrowers (miners and tube builders) (MERRIT andCUMMINS, 1988), and were frequently observed within the litter material perforating the Jun-cus stems. The great abundance of these shredder species may explain the high processingrates. There are abundant references in the literature about the impact of chironomid larvaein increasing organic matter decay rates and nutrient (N,P) exchange (JOHNSON et al., 1989;ANDERSEN and JENSEN, 1991). Chironomids, mainly of the chironomidae family, are oftenpresent in large numbers (up to 100000 m–2) in sediments of shallow eutrophic lakes (HAN-


SEN et al., 1998). The predominant species found in this study are similar to those describedin an alkaline pond in the Netherlands (KOK and VAN DER VELDE, 1994) and in a shalloweutrophic lake in Czech Republic (DVORAK, 1996) evidencing a similar macroinvertebratestructure that may be widespread in these shallow eutrophic lake ecosystems.

5. Conclusions and Perspectives

Decomposition rates for macrophytes were very rapid in these eutrophic systems and prob-ably are also evidencing a rapid processing of phytoplanktonic detritus and nutrient recycling.This is also consistent with the organic matter fractions found in the sediments, where lessthan 10% is comprised by CPOM (>1 mm). Summer (from May to August) was a criticalperiod for decomposition. Higher temperatures interacted with increased macroinvertebratedensities to accelerate decomposition rates. It is not clear yet if higher temperatures are favou-rable to growth of macroinvertebrate populations or, since there exists a positive relationship(P < 0.05) between temperature and mass losses but not between temperature and macro-invertebrate densities, to microorganisms which enhanced decomposition rates therefore providing an increased food source for the macroinvertebrates. Further research is being conducted at this respect studying the evolution of microbial communities associated withdecomposing litter in shallow lakes of Doñana. Nevertheless, the effect of macroinvertebratein the decay process appears to be important due to the predominance and high abundanceof chironomids with a known effect over decay rates (HANSEN et al., 1998). However, finemesh bags did not exclude the majority of taxa, so this effect could not be isolated.

Decomposition was more rapid (P < 0.05) in Santa Olalla coarse bags. Consequently,when the rest of conditions are similar (temperature, macroinvertebrate densities), oxygenconcentration and mechanical breakage are critical in controlling the decay process. Consid-ering that morphometry is underlying these factors (oxygen concentration and mechanicalbreakage) the k value is providing information about the differences between the twosystems in this aspect. The bigger size and surface-volume ratio, flatter banks and more windexposition of Santa Olalla emerge as factors increasing decomposition rates.

Fine mesh bags were not an effective barrier for most of the macroinvertebrate, with verysmall size, but they modified the physical and chemical conditions (oxygen concentrationand mechanical damage) exhibiting reduced k values with respect to coarse ones, althoughinvertebrate densities where similar between both mesh sizes. As it has been previously stated, fine mesh bags for studying litter decomposition in standing water systems must compromise with the exclusion of fauna while minimising the differences between externaland internal environmental conditions (BOULTON and BOON, 1991).

Importantly, taxa with large degree of feeding plasticity were dominant in the lagunas.Both shredders and collectors trophic groups of lotic aquatic systems would correspond toa single shredder-gatherers group in these lentic systems. These considerations regarding theuse of fine mesh bags in systems were anaerobic conditions can be easily provoked artifi-cially, and the greater feeding plasticity of the invertebrate community should be taken intoaccount when studying plant litter processing in standing water systems.

6. Acknowledgements

This project has been financed by the CICYT (HID97–0321-C02–01). SERGIO ALVAREZ was sup-ported by a personal grant from the Ministry of Science and Education. The authors would like to thankIGNACIO GRANADOS for his help in the Chironomidae identification, FERNANDO BARAJAS for his fieldassistance and PAULINE GRIERSON and two anonymous referees for their critical reviews of themanuscript.


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decomposition of juncus maritimus in two shallow lakes of doñana national park

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