The Ecology of Tidal Freshwater Marshes of the - USGS National ...

CHAPTER 3. ECOSYSTEM PROCESSES3.1 PRIMARY PRODUCTIVITYIntroductionThe primary productivity of an ecologicalsysteln, community, or any part ofsuch a system, is defined as the rate atwhich various organisms, chiefly greenpl ants, assimilate and synthesize gaseousand dissolved inorganic substances intoorganic inatter. The total amount oforganic matter produced by green plantsduring a particular time interval istermed gross primary production. The rateof storage of organic matter in planttissues in excess of the respi raioryutilization by plants during the period ofmeasurenlent is known as net pri!naryproduction.The organic satter produced by vascularplants, phytoplankton, and benthicalgae in the tidal freshwater habitatserves as an energy source for variousheterotrophic organi s-as. Much 1 ive materialcan be consumed in situ by variousherbivores. Microbial populations decomposeand utilize a large fraction of thedead plant material on the marsh surface.Detritivores further fragment decomposingplant remains. A1 though a significantportion of this organic matter is utilizedand stored within the marsh habitat, alarge fraction may be exported out of thesystem. Tidal currents and wind encouragethe entrainment and transport of organiccarbon to downstream estuarine locations.Migrating consumers may feed within thehabitat and then move on. It is estimatedthat salt marshes export about one-ha1 f ofthe net pritnary production to adjacenttidal waters (Teal 1962, Odum and Skjei1974) ; however, a comparable figure is notavail able for tidal freshwater marshes.Production EstimatesThe biomass and primary production oftidal freshwater wetlands have beenreported to be very high (Odum 1978).Numerous estimates of standing crop andannual aboveground net production existfor the dominant species of vascularplants occurring within this habitat(Table 7); however, there are only a fewestimates of total net communi ty production(Whigham et al. 1976, Dournlele 1981).The existing data on biomass and productionshow a great deal of variability bothwithin and between vegetation types, yetit appears that the total net communityproduction of these marshes generallyranges from 1,000 to over 3,500 g/m2/yr(Odum 1978). Productivity measuresreported for saline wetlands fall withinthe saoie range as those for tidal freshwatermarshes, within a given latitudinalzone (e.g., mid-At1 antic) ; the biomassproduction of fresh tidal wetlands may begreater than higher sal ini ty communities(Whigham et al. 1976).Obtaining accurate estimates of netproduction in tidal marshes, either on aper-species or community basis, is difficultbecause of (1) seasonal patterns ofbiomass allocation, (2) the heterogeneityin plant community composition, (3) seasonalbiomass turnover due to leaf mortality,decomposition, and herbivory, and (4)the inherent problems in measuring theproduction of belowground plant parts(Whigham et a1 . 1978). Traditionally,investigators have compared the productivityof tidal wetland vegetation by measuringpeak aerial standing crops. However,there is no objective way of pinpointingthe exact moment at which the peak cropexists; therefore, the potential for errorwith single harvest methods is high(McCormick and Somes 1982). By restrict-