The Flax Pond ecosystem study: Exchanges of carbon in water between a salt marsh and Long Island Sound1

Transcription

1 The Flax Pond ecosystem study: Exchanges of carbon in water between a salt marsh and Long Island Sound1 G M Woodwell The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts D E Whitney University of Georgia Marine Laboratory, Sapelo Island C A S Hall and R A Houghton The Ecosystems Center, Marinc Biological Laboratory Abstract Flax Pond, a tidal marsh on the north short of Long Island, New York, was used to examine the exchanges of carbon in its various forms between a salt marsh and the coastal waters The marsh removed fine particulate carbon from the tidal water throughout the year; it tended to bo a small source of C as total CO:! and dissolved organic carbon during summer, and a sink for both forms in winter The net flux of total carbon over a year, measured as total COa and as particulate and dissolved organic C, was a small input into the marsh, estimated as about 51 g C mda The data were consistent with others showing the marsh to be a net consumer of chlorophyll throughout the year and a strongly hcterotrophic system in summer and do not support the conventional view of marshes as net sources of fixed carbon to coastal waters Salt marshes exchange carbon in several forms with the rest of the biosphere Most of the carbon is exchanged as gaseous COZ, but another large fraction of the total exchange occurs either as part of the carbonate-bicarbonate system in water or in other dissolved or small particulate form Such exchanges in water may be large if there is a large tidal flux Completely mixed water with a tidal range of a meter or more and a constant difference in concentration of 1 mg C liter-l ( 1 ppm) would result in the net transport of more than 700 g C m-2 over a year, equivalent to the net primary production of many vegetations (Woodwell et al 1973; Whittaker and Likens 1973) This mechanism apparently results in export of substantial quantities of particulate carbon from certain salt marshes into coastal waters (Teal 1962; Odum and de la Cruz 1967) and it has seemed reasonable to assume that all marshes are a source l Research carried out at Brookhaven National Laboratory with support from the US Energy Research and Development Agency, the National Science Foundation ( grant AG-375 ), and The Ecosystems Center, Marine Biological Laboratory, Woods Hole of fixed carbon that is exported in apprcciable quantities and used by other coastal systems (Darnell 1967; Odum 1971; Day ct al 1973; Gosselink et al 1974) This conclusion is attractive and consistent with evidence that marshes are highly productive of fixed carbon and rich in animal life, but it has been based on limited experience Are salt marshes always sources of fixed carbon or only occasionally? When they are sources, are they sources of particulate or of dissolved carbon? When are they sinks? How are other aspects of their function, for example their nitrogen and phosphorus budgets, related to their role in the carbon budget of the coastal waters? To examine these questions we designed a series of studies of a small estuarine marsh on the north shore of Long Island Flax Pond is a 57-ha Spartina alterniflora marsh about 50 km east of New York City It was selected as the marsh on Long Island that offered the best opportunity to resolve the series of questions included in the superficially simple one: What does an estuarine marsh do as a unit of the surface of the earth? The approach required an especially favorable set of circumstances to simplify field measurements LIMNOLOGY AND OCEANOGRAPIIY 833 SEPTEMBER 1977, V 22( 5)

2 834 Woodwell et al Methods Flax Pond was an attractive site for several reasons First, it was connected through a single channel to Long Island Sound Second, the channel was shallow enough that the flow of water was cxtremely turbulent throughout virtually all of the tidal cycle The turbulence was sufficient for us to assume that the waters were well mixed across the channel and in depth Replicate subsamples supported this assumption Third, topographic maps and field experience showed that the drainage basin of the :marsh was small and supported no continuous freshwater s trcams Monitoring chlorinity of the tidal waters exchanged with Long Island Sound over 24 months showed that subsurface water flow into the marsh was small and there was no statistically significant dilution of the tidal water Over the e-year period from 4 October 1971 to 29 October 1973 the mean chlorinity of the incoming tides was 0018%0 ( *002&) higher than that of the ebbtide (Table 1) The relationship suggested a slight predominance of dilution over evaporation, but the relationship could not be distinguished from zero at the 95% confidence level, and the difference was much too small to affect our analyses The amount of water exchanged with Long Island Sound was measured by integrating the data obtained by planimetering a topographic map that had been especially prepared from stercoaerial photographs (Woodwell and Pecan 1973) Volume was also appraised by measuring depth and flow rates of water through the narrow channel connecting the marsh to the sound These appraisals agreed to within 10% The amount of water exchanged between high and low tides was commonly between 10 and 60 x lo5 m3,, sometimes more, and was about 80% of the high tide volume A continuous record of water depth in the marsh was provided by a recording tide gauge in its center (Tempel 1973) The central location meant that the amount of water indicated by the gauge was a fair appraisal of the total in the marsh at that moment The maximum slope of the water surface within the marsh during strong tidal flows was no more than about 2 cm km-l, insufficient to cause errors that would be important in this work The hypsographic curve used to express volume in units of depth was expressed as a regression that accounted for more than 99% of the variation Water samples were taken in the Flax Pond channel four times per lunar month over 16 months between January 1973 and mid-april 1974 The sampling schedule was fixed a year in advance and, in retrospect, included all normal conditions including several storms The basic sampling unit was one complete tidal cycle Sampling dates were chosen to obtain data for tides that were high at or near 0000, 0400, 0800, 1200, 1600, and 2000 hours We took four subsamples during the flood and four during the ebb The time of each subsample was recorded precisely All water samples were taken with a 6-liter Van Dorn sampler and carried within 30 min to the laboratory, where ph and total alkalinity were determined (Strickland and Parsons 1972) and initial processing for organic carbon was completed Particulate organic carbon ( POC), defined as material retained by a Reeve Angel 984H ultraglass fiber filter, was measured by infrared absorption gas analysis after combustion in oxygen ( Strickland and Par- sons 1972) The dissolved organic carbon (DOC) in the filtrate was measured by pcrsulfate oxidation ( Menzel and Vaccaro 1964; Sharp 1973) Total inorganic carbon was estimated from the pii and total alkalinity data by the Anderson-Robinson technique ( Strickland and Parsons 1972) E luxes of the various forms of carbon were calculated for each tidal cycle by multiplying the quantity of water (based on tide height and volume regressions) ex-, changed during a subsampling interval by the concentrations of carbon measured dur- ing that interval This was done for the four incoming and four outgoing subsamplcs

3 Carbon exchange 835 Table 1 Mean chlorinity and tide data in Flax Pond channel, 4 October October 1973 Means for chlorinity are averages of four subsamples taken during flood and four subsamples during ebb, Basic sampling unit was one tidal cycle One cycle was sampled approximately weekly Chlorinity Flood tides 1'4325O/oo Ebb tides DiEfercnce 0018 Total difference, low to hiqh (mean) 245 m Volume of water exchanged (mean) 3 41 x lo5 m3 Freshwater chlorinity from wells and precipitation at Brookhaven National Laboratory 00040/00 A correction based on chlorinity change was entered into calculations of concentrations in the ebbtide subsamples to normalize for dilution by precipitation or concentration by evaporation, In addition, tidal exchanges were normalized to the same base level to avoid a bias due to small differences in the height of successive low tides These corrections were small in proportion to the net fluxes per tide and did not affect the annual budget The sampling program was designed to explore whether winter-summer, or daynight, or month-to-month differences dominated patterns of carbon flow Analysis of the total variance within the basic sampling unit ( one tidal cycle) was provided by repetition of the sampling unit at weekly intervals A check on analytical variance was maintained by running duplicates from one sample bottle per sampling day, and duplicate sample bottles were taken periodically to provide an estimate of sampling and analytical variance combined Of the two components of variance, that due to intrinsic variance in the water and that due to laboratory analyses or handling, the latter tended to be very small and the former to be larger On the basis of preliminary evaluations of variances of subsamples, we decided to extend the sampling to four samples per lunar month at the expense of replication of subsamples Major reliance for replication was thus on the weekly replication of the entire sampling scheme These procedures enabled two independent analyses of the data First, because the basic sampling unit, replicated four times monthly, was one tidal cycle, a factorial analysis of variance could be used to test the significance of various factors Second, regression curves could be used to define the patterns for the annual or seasonal trends Results and discussion The most important differences in carbon content of tidal waters in Flax Pond occurred in seasonal patterns as shown in Fig 1 The curves are the best leastsquares fit of polynomials to points that are the average of the concentrations observed in the four floodtide subsamples and the four ebbtidc subsamples The peak DOC concentrations were in late summer; the peak POC concentrations were during spring and fall blooms; there was no easily recognized seasonal trend in the total CO2 (total inorganic carbon) The differences between the concentrations of DOC and POC, during flood- and ebbtides suggested that there was a net loss of DOC in summer and a net input of POC throughout the year Calculations of net fluxes (Fig 2) confirmed this pattern and are discussed below The tidal waters that flush Flax Pond were obviously highly variable in C content; the variation through the course of a year ranged up to tenfold This extreme variation emphasizes the complexity of the interactions between different scgmcnts of the coastal zone over time With further measurement the probability is high that additional variation could have been found; extreme variances might be sought between spring and neap tides and with extreme storm tides that occasionally cover the marsh with sediments and may change its basic contours Nonetheless, over time the greatest exchanges of water between the marsh and the rest of the coastal zone occur under the conditions represented in

4 Woodwell et al I 25 v--f----r-r --I-- l I --I 24 t DOC loo- 75- l DOC 0 8 I 4 I - I- L-II--L- I-- 11 I 241v-11 - ;r--i -5o- * l -75- * -100 I I I I I I I_ I L-L L 1 POC 20-0 FLOOD TIDE --0 EBB TIDE I I I I75 - POC 150 I / l25-1 CJ IOO- y 75-, [* T---l - 1-l I---T I I I -T-- r l t TOTAL CO2 8OOf- l 1 i I --; 600/- l l 16 Fig, 1 Concentrations of carbon in water of Flax Pond channel over course of year Each point is unweighted mean of four samples taken on each tide Curves are polynomial regressions OF carbon concentrations on day of year for floodand ebbtides this series of samplings, and the variances encountered here are representative of what might be expected in general With such high variances the more subtle points about C flux are lost; indeed, question arises as to the major issues : What is the magnitude and direction of the net flux? In Table 2 we have summarized the net -600LI_LLz JFMAMJ I Fig 2 Net flux of carbon in water flushing Flax Pond over 12 months Negative values are losses from marsh, Each point is difference be- tween flux on flood and flux on ebb Curves are best lcast-squares fit of net carbon flux data to a polynomial regression flux data for the marsh and show standard errors of the means obtained from the factorial analysis of variance The budget data show that there was not a large net annual exchange of C in all of its forms with the coastal waters The marsh appears to import at present a net of g C m-2 an-

5 Carbon exchange 837 Flux Annual Elux per tide F7hole mar h Per m2 Form (kg) (kg x 10 5 ) (CJI Total CO2-15(345) -1lc241) -19C420) DOC l/4--30/9-248( 76) l/10-31/3 lll(l26) Full year -68( 77) -48( 54) -84(9-d) POC 503tlL4) 352( 79) 613(138) Total org C 435(138) 304( 96) 530(167) nually, but the standard error of this number is large enough that this small influx cannot be distinguished from zero The largest fluxes of C in the water were of course in the carbonate system but the net flux over the year was insignificant (Table 2) There was a simultaneous significant net inward flux of POC of about 61 g C m-2 yr- l DOC exchanges were less clear In winter there was a tendency toward accumulation; in summer there was a loss (Figs 1 and 2) The net for the year was a small loss of about 8 g C m-2, with a standard error of about 9 g m-2 (Table 2) The net flux of chlorophyll through the channel was inward throughout the year ( MO ) ) a pattern that is completely consistent with the particulate carbon flux The marsh is a net consumer of phytoplankton at all times of year ( Moll 1974) Despite the leakage of organic compounds from both living and decaying algae (Hellebust 1965; Fogg 1966; Parsons and Seki 1970) and the high primary productivity of Spartina communities ( Nixon and Oviatt 1973; Woodwell et al 1973; Valiela et al 1976)) there is no systematic loss of organic carbon from the marsh This marsh seems not to be a direct contributor either to the standing crop of fixed carbon in coastal waters or to the l-2 mg DOC liter-l present in the world s oceans The possibility remains that there is a net exchange of fixed carbon in the form of macroscopic algae or particulate matter too large to be included in the water samples Floating organic matter and the or- ganic matter suspended in the upper 05 m of the water column were sampled with a net with a 1 x l-m cross-sectional opening, towed in midchannel so that half of the opening was submerged, The mesh was 25 mm,thirty-five tides between 9 April 1971 and 2 June 1973 were sampled during the period of highest turbulence and maximum transport, 2 h before and 2 h after high tide While extraordinary tides carried as much as 2,700 g of carbon, the average total transport over the 2-h period was well below 60 g C, with no discernible net directional movement over the whole tide The transport of mats of dead Spartina stems outward into Long Island Sound could not be appraised systematically Our observations indicated that transport occurred irregularly during spring tides and unusually high storm tides There is probably substantial variation from year to year in the transport of mats of Spartina stems from the marsh Such fluxes are extremely difficult to appraise, of course The inclusion of the fish populations of the marsh in calculations of the carbon budget suggested that they export a small amount of carbon ( about 20 g m-2 yr-l ) in the fall (Hall and Woodwell in press) These analyses challenge the traditional view of salt marshes as large net exporters of fixed carbon If there is any regular annual net flow of carbon through the channel of Flax Pond apart from fish, it appears to be inward and small This small influx from coastal waters is being added to the net production of the marsh and used in metabolism, in fish production, and in the building of sediments To the extent that the direction of the flux is controlled, the control probably lies in the change in the level of the sea against the land The current rise in sea level assures that the sediments of the marsh are accumulating and provide a sink for fixed carbon (Redfield and Rubin 1962; Armentano and Woodwell 1975) References ARMENTANO, T V, AND G M WOODWELL 1975 Sedimentation rates in a Long Island

6 838 Woodwell et al marsh determined by = Pb dating Limnol Lauff [ea], Estuaries Publ Am Assoc Adv Oceanogr 20 : Sci 83 DARNELL, R M 1967 Organic detritus in re- PARSONS, T R, AND SFKI 1970 Importance lation to the estuarine ecosystem, p and general implications of organic matter in In G H Lauff [ea], Estuaries Publ Am aquatic environments, p l-27 In D W Assoc Adv Sci 83 Hood [ea], Organic matter in natural waters DAY, J W, W G SMITII, P R WAGNER, AND W C STOWE 1973 Community structure and carbon budget of a salt marsh and shallow bay estuarine system in Louisiana Center for Wetland Resour, Louisiana State Univ Publ LS U-SG Focc, G E 1966 The extracellular products of algae Oceanogr Mar Biol Annu Rev 4: GOSSELINK, J G, E P ODUM, AND R M POPE 1974 The value of the tidal marsh Louisiana State Univ, Publ LSU-SG p IIALL, C A, AND G M WOODWELL In press Spatial and seasonal patterns of standing crop, productivity and diversity of fishes in Flax Pond, a Long Island estuary Fish Res Bd Can IIELLEBUST, J A 1965 Excretion of some organic compounds by marine phytoplankton Limnol Oceanogr 10 : MENZEL, D W, QNI) R F VACCARO 1964 The measurement of dissolved and particulate carbon in seawater Limnol Oceanogr 9: MOLL, R A 1974 The phytoplankton community of a temperate zone salt marsh PhD thesis, SUNY, Stony Brook 122 p NIXON, S W, AND C A OVIAYI-IY 1973 Ecology of a New England salt marsh Ecol Monogr 43 : ODUM, E P 197 I Fundamentals of ecology, 3rd ed Saunders -, AND A A DE LA GRUZ 1967 Particulate organic detritus in a Georgia salt marshestuarine ecosystem, p In G H Inst Mar Sci (Alaska) Occas Publ 1 REIWIELD, A C, AND M RIJBIN 1962 The age of salt marsh peat and its relationship to recent changes in sea level at Barnstable, Massachusetts Proc Natl Acad Sci 48: SI-IARP, J I-I 1973 Total organic carbon in seawater Comparison of measurements using persulfate oxidation and high temperature combustion Mar Chem 1: STl\ICKLAND, J D, AND T R PARSONS 1972 A practical handbook of seawater analysis, 2nd ed Bull Fish Res Bd Can 167 TEAL, J M 1962 Energy flow in the salt marsh ecosystem of Georgia Ecology 43: TEZMPEL, N R 1973 An inexpensive recording tide gauge Limnol Oceanogr 18 : VALIELA, I, J M TEAL, AND M Y PERSSON 1976 Production and dynamics of experimentally enriched salt marsh vegetation: Belowground biomass Limnol Oceanogr 21: WHITTAKER, R II, AND G E LIKENS 1973 Carbon in the biota Brookhaven Symp Biol 24 : WOODWELL, G M, AND E V PECAN 1973 Flax Pond: An estuarine marsh Brookhaven Natl Lab Tech Rep BNL p ~ P H RICH, AND C A HALL 1973 Carbon in estuaries Brookhaven Symp Biol 24: Submitted: 4 March 1975 Accepted: 25 January 1977