Darwiniana

ISSN: 0011-6793

sdenham@darwin.edu.ar

Instituto de Botnica Darwinion

Argentina

Pratolongo, Paula; Kandus, Patricia; Brinson, Mark M.

NET ABOVEGROUND PRIMARY PRODUCTION AND BIOMASS DYNAMICS OF

SCHOENOPLECTUS CALIFORNICUS (CYPERACEAE) MARSHES GROWING UNDER DIFFERENT

HYDROLOGICAL CONDITIONS

Darwiniana, vol. 46, nm. 2, diciembre, 2008, pp. 258-269

Instituto de Botnica Darwinion

Buenos Aires, Argentina

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DARWINIANA 46(2): 258-269. 2008 ISSN 0011-6793

NET ABOVEGROUND PRIMARY PRODUCTION AND BIOMASS DYNAMICS OFSCHOENOPLECTUS CALIFORNICUS (CYPERACEAE) MARSHES GROWING UNDER

DIFFERENT HYDROLOGICAL CONDITIONS

Paula Pratolongo1,2, Patricia Kandus3 & Mark M. Brinson4

1Instituto Argentino de Oceanografa (CONICET), Florida 8000, 8000 Baha Blanca, Argentina;paulapra@criba.edu.ar (autor corresponsal).2Departamento de Biologa, Farmacia y Bioqumica, Universidad Nacional del Sur, Coln 80, 8000 Baha Blanca,Argentina.3Departamento de Ecologa Gentica y Evolucin, Facultad de Ciencias Exactas y Naturales, Universidad de Bue-nos Aires, Ciudad Universitaria, 1428 Ciudad Autnoma de Buenos Aires, Argentina.4Biology Department, Howell Science Complex, N-108 East Carolina University, Greenville, North Carolina 27858-4353, USA.

Se estudiaron diferentes atributos funcionales de 2 pajonales de Schoenoplectus californicus (C.A. Mey) Sjak que son muy similares en su estructura y especie dominante, pero aparecen en los extre-mos opuestos a lo largo de un gradiente de influencia fluvial-mareal en el Bajo Delta del ro Paran.Los resultados mostraron una productividad primaria neta area (PPNA) significativamente ms altaen el pajonal afectado directamente por la marea (1999.41 211.97 g m-2 ao-1). En el sitio aguas arri-ba, menos proclive a la inundacin por mareas, S. californicus tuvo una menor PPNA (1299.17 179.48 g m-2 ao-1) y el sistema mostr una mayor capacidad para retener la biomasa producida dentrodel pajonal, con cantidades significativamente mayores de biomasa muerta en pie (1316.00 336.01vs. 112.40 55.05 g m-2) y mayores contenidos de materia orgnica en el suelo (16.20 0.12 % vs.0.70 0.08 %). Los resultados obtenidos en este trabajo sugieren que los flujos superficiales de altaenerga pueden cambiar el funcionamiento de estos pajonales, de un sistema estable acumulador demateria orgnica a un pajonal de rpido crecimiento, con altas tasas de acumulacin de sedimentosminerales.

Palabras clave. Delta del ro Paran, humedales mareales de agua dulce, productividad primaria neta,Schoenoplectus.

Original recibido el 18 de marzo de 2008; aceptado el 20 de agosto de 2008

Resumen. Pratolongo, P.; P. Kandus & M. M. Brinson. 2008. Productividad primaria neta area y dinmica de la bio-masa de juncales de Schoenoplectus californicus (Cyperaceae) bajo diferentes condiciones hidrolgicas. Dar-winiana 46(2): 258-269.

Abstract. Pratolongo, P.; P. Kandus & M. M. Brinson. 2008. Net aboveground primary production and biomass dyna-mics of Schoenoplectus californicus (Cyperaceae) marshes growing under different hydrological conditions.Darwiniana 46(2): 258-269.

We studied different functional attributes on two Schoenoplectus californicus (C. A. Mey) Sjakmarshes that appear as very similar communities in terms of structure and dominant species, althoughthey are settled on opposite ends along a fluvial-tidal gradient in the Lower Delta of the Paran River.Obtained results showed a significantly higher net aboveground primary production (NAPP) in themarsh directly affected by tides (1999.41 211.97 g m-2 year-1). In the upstream site, less prone to tidalflooding, S. californicus had a lower NAPP (1299.17 179.48 g m-2 year-1) and the system showed ahigher ability to keep the produced biomass within the marsh, with significantly higher amounts ofstanding dead biomass (1316.00 336.01 vs. 112.40 55.05 g m-2), as well as higher organic contentsin soils (16.20 0.12 % vs. 0.70 0.08 %). Results of the present study suggest that high energy over-land flows may change the marsh functioning from a stable system accumulating organic matter to anaggressively growing marsh with higher rates of mineral accumulation.

Keywords.Net primary production, Paran River Delta, Schoenoplectus, tidal freshwater wetlands.

INTRODUCTION

Hydrological characteristics are often conside-red to be critical variables in the structure andfunctioning of wetland ecosystem types (Brinson,1993; Mitsch & Gosselink, 2000). Hydrology con-trols water chemistry, soil characteristics, deposi-tion of sediments, and nutrient availability (Cronk& Fennessy, 2001) and wetland hydrophytes caneven be classified according to their tolerance tosaturation and inundation (Tiner, 1999). Perennialspecies, however, may persist at a site, once esta-blished, in spite of changes in hydrologic condi-tions. For a dominant species to be replaced,physical disturbance (Pickett & White, 1985),increasingly eutrophic conditions, or changes insalinity (Galatowitsch, 1999) may be associatedwith changes in hydrology.

In Argentina, the progradation of the ParanRiver Delta sets up a large sequence of changefrom sediment bars, continuously depositing at theshoreline, to mature islands located upstream, for-med 700-900 years ago (Iriondo & Scota, 1979).As islands develop, deep changes in geomorpholo-gic and hydrological settings of marshes occur.The low elevation of new emergent bars allowsrelatively unimpeded tidal flow to reach the marshsurface. In contrast, mature islands develop an ele-vated levee surrounding the interior marsh, whichget largely isolated from tidal inputs.

In this sequence, Schoenoplectus californicus(C. A. Mey.) Sjak ("California bulrush") formsmonospecific marshes at both ends of the gra-dient. In new islands, S. californicus is the onlycolonizer of bare sediments, and the same speciesforms monospecific marshes on the innerlowlands of mature islands, located as far as 70km upstream from the delta shoreline. In thenewly emergent marshes, tidal inundation followsa regular semidiurnal pattern. In contrast, maturemarshes upstream undergo periods of drawdownof the water table, with less frequent episodes offlooding from tides, but long periods of floodingfrom the Paran River, which may be persistentfor several months under extreme climatic condi-tions, as those observed during "El Nio" periods(Kandus, 1997).

One indicator of a species range of tolerance isits response to different hydrologic conditions.Species that have wide ranges of distribution

across environmental gradients may be among theones best adapted to take advantage of disturban-ces and changes in state, favouring invasionswhere hydrological shifts occur (Newman et al.,1996; Kercher & Zedler, 2004). Schoenoplectuscalifornicus is indigenous to coastal regions of thesouthern North America (Mason, 1957), south toChile and Argentina (Wagner et al., 1990). In Aus-tralia, S. californicus is considered as a potentialenvironmental weed species, which was firstfound in 1995 (Csurhes & Edwards, 1998), and inthe early 1990's specimens of this species werealso collected in New Zealand (Lange et al.,1998). Given the very different hydrological set-tings under which S. californicus marshes deve-lop, we followed net aboveground primary pro-duction (NAPP) and net biomass accumulation tocompare the species response and the marsh func-tions in a newly emergent and a mature island inthe Paran Delta.

METHODS

Study area

The Paran River basin is a 2.6 x 106 km2 fluvialsystem with a mean annual discharge of 17.0 x 103m3 s-1 and an annual sediment load of more than150 x 106 Mg (Orfeo & Stevaux, 2002). Located atthe mouth of this basin, the Paran Delta is a 2.700km2 freshwater tidal system, which is aggradinginto Rio de la Plata Estuary at an estimated rate of70 m year-1 through development of bars that leadto the establishment of new islands (Iriondo &Scotta, 1979; Parker & Marcolini, 1992).

In the frontal zone of the delta, semidiurnaltides from the Ro de la Plata Estuary are typicallyabout 1 m amplitude (Vieira & Lanfredi, 1996).However, high tides greater than 4 m (Piccolo &Perillo, 1999) have been reported during sudesta-das (southeasters). Upstream, the Paran Riverseasonally raises its level and occasionally floodslowlands in mature islands, but extraordinary flo-ods occur during "El Nio" at grater intervals. "ElNio" phenomenon has been shown to have a sig-nificant effect on flow patterns and sediment accu-mulation on islands towards the upstream portionof the delta (Karszenbaum et al., 1999). However,flooding patterns downstream seem to be little

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affected by the increase in Paran River discharge(Kandus & Malvarez, 2004).

Schoenoplectus californicus is a perennialreed, with culms developing at nodes located oncreeping rhizomes and with leaves reduced tobladeless sheaths at the base of culms. Schoeno-plectus californicus marshes are the most wides-pread natural community upstream, coveringextended areas in the inner lowlands of matureislands. In the frontal zone, these communitiessettle on narrow strips a few hundred meters. Inthe latter case, S. californicus colonizes emergentbars of bare sediments, and the establishment ofthese monospecific marshes sets the environmen-tal condition for further development of islandsand species replacement (Kandus & Malvrez,2004).

Selected sites were "Bajos del Temor" and"Los Milagros" and located, respectively, in arecently deposited bar in the frontal zone, and inthe inner lowland of a mature island upstream(Fig. 1). In both cases, study sites were establis-hed in nearly monospecific stands of S. californi-cus, with a few associated species present in verylow abundances.

Hydrological settings

In order to asses the hydrological parametersat both study sites, for the specific time frameconsidered for field work, we used water levelsregistered at Paran de las Palmas River station,near Zrate city, and Ro de la Plata station, nearBuenos Aires city (Fig. 1), during the years 2002and 2003 (Direccin Nacional de ConstruccionesPortuarias y Vas Navegables). We used bothmean hourly levels and mean monthly levels todescribe daily and seasonal variations respecti-vely at both sites. As additional information, wealso used mean monthly levels during the years1982 and 1983, coincident with a Paran Riverextraordinary flooding, associated with "ElNio" (Schnack, 2002). We determined overbanklevels at both sites using a time series of LandsatETM+ satellite images, acquired over the sametime period, by relating water level at the corres-ponding station at the acquisition time of thosescenes showing standing water over the studysites.

NAPP and standing biomass

Field measures were made nearly bimonthly atboth sites between July 2002 and August 2003,with higher sampling frequencies (nearly mon-thly) during the growing season. NAPP estimateswere obtained using a technique that combinestagging and harvesting of individual culms (Prato-longo et al., 2005). The tagging technique usedhere has been initially developed for estimatingNAPP of Scirpus giganteus in the same area (Pra-tolongo et al., 2005) but further works de-monstrated that the technique is widely applicableto other wetland plants, including S. californicus(Pratolongo, 2005; Pratolongo & Kandus, 2005).

On the first sampling date, we established 10quadrats on each site in monospecific stands of S.californicus. All live culms in each quadrat (50 x50 cm) were tagged by piercing them at the groundlevel with a steel needle followed by polyesterthread. In this species, nodes remain below thesurface so, as each culm grows the thread risesabove the substrate. On subsequent sampling da-tes, all culms present in previously marked qua-drats were cut at ground level, 10 new quadratswere established, and new plants were tagged. Thecollected material was rinsed and each culm wascut at the level of the thread. The portion below thethread was classified as newly produced tissue, aswell as any new plant that appeared inside the qua-drat. In order to assess the aboveground standingbiomass dynamics, the remaining green and deadbiomass were also separated, and in floweringculms, we classified the spikelets as floweringstructures. Classified material was oven dried for72 h at 60 C and weighed.

Mean biomass production per quadrat and sam-pling period was estimated as the mean sum ofnewly produced tissues of every stem in each qua-drat. Mean annual production per unit area wascalculated by adding the means obtained on everysampling period over the entire year. Since themeans corresponding to different sampling dateswere obtained from different quadrats, they wereassumed to be independent measures, and then theannual variance was calculated as the sum of thevariances for each sampling period. We tested dif-ferences in total NAPP between sites using thetwo-sample Welch's approximate t for the two-tai-led hypothesis (Zar, 1998). We also evaluated dif-

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ferences between sites in the maximum valuesobtained for aboveground live, standing dead, and

flowering structures biomass through Kruskal-Wallis (non-parametric ANOVA) tests.

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Fig. 1. Location of the study sites in the Lower Delta of the Paran River, Argentina. A, "Los Milagros". B, "Bajosdel Temor"

Soil sampling and biomass decay

Soil sampling in both sites was performedduring December 2002 using a McCauley peatsampler. In the surroundings of the area conside-red for NAPP determination, a soil sampling areaof about 100 m2 was established and 5 soil profi-les were built from 25 cm deep and 4 cm diametersections, centered at increasing depths. The maxi-

mum depth was determined by the McCauley sam-pler characteristics, which preclude taking sam-ples on predominantly mineral soils. At "Los Mila-gros" the high organic content allowed the use ofthe McCauley sampler up to 75 cm depth (3 sec-tions). At "Bajos del Temor", however, just onesection (25 cm depth) could be taken on each pro-file. Samples were stored in hermetic plastic bagsat 4C. Back at the laboratory, samples were oven

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Fig. 2. Mean hourly hydrometric levels registered in Paran de las Palmas River ( ) and Buenos Aires Harbor (----)for a period of A, relatively high and B, relatively low river discharge. Overbank levels at "Los Milagros" ( ) and"Bajos del Temor" (----) are also indicated. Note the damping of tidal amplitudes and the lower occurrence of tidalflooding at "Los Milagros".

dried (80C, to constant weight) and organic mat-ter as percent Lost On Ignition (LOI) was determi-ned as the ratio of weight loss (500C, 3 h) com-pared to the dry weight.

The annual decay rate was assessed only at"Los Milagros". In outlet bars of the frontal zone,the strong daily influence of high energy tidalflows preclude deposition of detritus over the sedi-ment surface. In these high energy environments,dead biomass is promptly exported from the

system, in such a way that the decomposition pro-cess cannot take place over the marsh surface. Todetermine decay rate of S. californicus biomass at"Los Milagros", 30 litter bags were constructedfrom S. californicus dead biomass, previouslyoven dried for 72 h at 60 C and weighed, andwere placed on the marsh surface. The first 5 bagswere withdrawn immediately after placing them inthe field, in order to determine the amount ofmaterial lost by handling. Remaining bags were

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Fig. 3. Mean monthly hydrometric levels registered in Paran de las Palmas River ( ) and Buenos Aires Harbor (----) for A, years 2002-2003 (normal levels) and B, years1982-1983 (during "El Nio" flooding). Overbank levels at"Los Milagros" ( ) and "Bajos del Temor" (----) are also indicated.

withdrawn at 20, 80, 192, 305, and 396 days afterdeployment. The material retrieved from bags wasgently rinsed, and any observable macroinverte-brates and living roots growing inside the bagswere removed. The remaining biomass was thenoven dried and weighed. Finally, mass loss fromeach litter bag was estimated as the change in drymass plus the estimated mean mass lost by han-dling, and we fitted the percent remaining biomassthrough time to an exponential function.

RESULTS

Hydrological settings

Mean hourly hydrometric levels registered inParan de las Palmas River and Buenos Aires Har-bour are shown in Fig. 2. Graphs compare hydro-logical behaviour in both stations for a period ofrelatively high and relatively low discharge of theParan River (Figs. 2A and 2B, respectively) andthey clearly show that semidiurnal tidal oscilla-tions, although delayed and damped still affectwater levels upstream, even during high dischargeperiods. However, if we observe the site specifichydrological traits, the overbank level at "Los

Milagros" indicates that, although tidal oscilla-tions are conspicuous in the river, overland flowstrough the marsh surface only occur duringextraordinary high tides. Considering the hydrolo-gical series for years 2002-2003, only on 13 dateswas water level in the Paran de las Palmas Riverhigh enough to exceed overbank threshold, and toallow overland flow at "Los Milagros". At "Bajosdel Temor", in contrast, the lower elevation allo-wed marsh inundation and superficial flows atleast once a day. Mean monthly levels for years2002-2003 show that, even though there is a veryslight trend to the occurrence of higher values infall (March to June) for Paran de las Palmasrecords, there is not a clear seasonal pattern neitherupstream nor at Ro de la Plata station (Fig. 3A).However, monthly levels during "El Nio" eventsshow extremely different responses in both sites(Fig. 3B). During years 1982-1983, mean waterlevels at Ro de la Plata suggested that the frontalzone of the Lower Delta was little influenced bythe extraordinary flooding of the Paran River.Upstream records, on the other hand, showed that"Los Milagros" marsh must have been dramati-cally affected, since mean water levels were per-sistently above overbank threshold for more than 6months.

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Fig.4. Accumulated values of NAPP (g m-2) for S. californicus growing at the upstream site "Los Milagros" () andthe tidal site "Bajos del Temor" (). Values are mean std. error.

NAPP and standing biomass

Net aboveground primary production (NAPP)of S. californicus estimated at "Bajos del Temor"was significantly higher than the value obtained at"Los Milagros" (1999.41 211.97 and 1299.17 179.48 g m-2 year-1, t =7.8, respectively, gl=7,p

production. Besides the higher values of NAPP at"Bajos del Temor", the maximum registered valuefor standing live biomass (used as a proxy for peakstanding biomass) was significantly lower in thissite (661.80 195.62 and 1009.91 265.64 g m-2,respectively, Kruskal-Wallis H=6.64, n=20,p

seasonal pattern (Fig. 5), and dead biomass did notappear at all in samples collected on May (fall).

At "Los Milagros", spikelets begun to appear inSeptember (early spring) and the maximum bio-mass of flowering structures (0.68 0.25 g m-2)was attained in January (early summer), after that,all flowering structures fell off. By blossomingtime, less than 10 % of the living culms developedspikelets. At "Bajos del Temor", in contrast, spike-lets were harvested in every sampling date throughthe year and the maximum biomass of floweringstructures was 68.04 36.89 g m-2 (Fig. 6), anorder of magnitude higher than the peak attained at"Los Milagros" (Kruskal-Wallis H=12.76, n=20,p

miliacea in tidal and impounded marshes, underthe more dynamic conditions at the open tidal sitemarshes a greater net primary production and alower organic matter accumulation were shown. Inthat case, water flow was proposed to be acting asan energy subsidy by performing mechanical worksuch as removing dead matter and toxins and recy-cling or importing oxygen and nutrients.

Observing the species performance, the higherproduction and flowering at "Bajos del Temor"may suggest that S. californicus would have aniche that is more directed to the dynamic condi-tions. Under this assumption, conditions at "LosMilagros" would be far from optimal, and it couldbe expected that S. californicus would be soonovergrown by another species from the successionscheme. However, even after large scale fires andthe massive floods during "El Nio" periods(Karszenbaum et al., 2003) the large areas coveredby monotypic stands of S. californicus were obser-ved to recover the original structure in a few years,suggesting that these marshes constitute a fairlysteady state in terms of permanence and resilience.

Under the more favorable and highly dynamicconditions in the frontal zone, marshes cause deepchanges in topography through suspended sedi-ment trapping and mineral accumulation. Coloni-zation of bare sediments triggers a sequence ofstate changes in which S. californicus is replacedby different plant communities, and leads to theformation of mature islands in the frontal zone,where the species is almost completely displaced(Kandus and Malvrez, 2004). Whether the largemarshes of S. californicus dominating the landsca-pe upstream are relicts of the pioneer marshescolonizing the area 800 years ago, at the begin-nings of the delta progradation, and why theyhaven't been replaced is an open question thatremains still unresolved.

ACKNOWLEDGMENTS

This research was supported by UNESCO-MABunder a MAB Young Scientists Award and by ANPCyTunder project PICT 4503, "Relaciones entre la heteroge-neidad ambiental y las comunidades naturales en la por-cin inferior del Delta del Ro Paran". Partial supportwas provided by a Buenos Aires University Fellowship.We especially thank Dr. James Perry and an anonymousreviewer for their helpful comments that greatly impro-

ved the manuscript. We also thank Julieta Bono, Marie-la Biondini, Mariana Burghi, and Santiago Arias forhelp with fieldwork and sample processing.

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