Comparison of methods for measuring production by the submersed macrophyte , Potamogeton perfoliatus L

Six conventional methods for measuring primary production of submersed vascular plants were compared to test for previously suggested inherent shortcomings in the O,-exchange techniques. Production was measured for experimental populations of Potamogeton perfoliatus L., from upper Chesapeake Bay in five comparative studies, each including an O,-exchange method. All techniques tested (14C incorporation and 0, evolution for both bottle incubations and undisturbed populations; biomass accumulation and inorganic carbon consumption for intact populations) compared favorably, with mean (molar) ratios of oxygen to carbon production ranging from 0.9 to 1.6. In addition, direct measurements indicated that rhizosphere release of 0, would introduce only small errors (< 5%) in 0, production estimates. Although changes in lacuna1 storage of 0, can result in brief (5-25 min) delays between 0, production and evolution to the external water, simple methodological precautions can overcome such problems. We conclude that all available methods for measuring productivity of this and related species are potentially useful, each having its particular strengths and weaknesses, and the use of more than one method is recommended. Submersed vascular plants are a conspicuous feature of many aquatic ecosystems. Organic production by submersed angiosperms often dominates the annual carbon budgets of lakes and estuaries (Sondergaard and Sand-Jensen 1978; Thayer et al. 1975). Thus, dependable measurement of primary production for these plants is of fundamental importance in the study of shallow-water ecosystems. Various techniques have been applied over the past several decades for measuring production of these macrophytes (see Wetzel 1964, 1974). One of the earliest methods involved direct harvest of plant biomass (Rickett 1922; Westlake 1965), an approach later modified with measurements for elongation of marked leaves (e.g. Zieman and Wetzel 1980) and analysis of population age-structure to correct for mortality (Mathews and Westlake 1969; Carpenter 1980). Observations on the exchange of metabolic gases (CO, and, especially, 0,) between macrophytes and the surrounding medium have also been used I Research supported in part by the U.S. EPA grants R805932010 and X003248010. 2 University of Maryland Center for Environmental and Estuarine Studies Contribution 1678. 3 Present address: Dept. Oceanography, Dalhousie University, Halifax, Nova Scotia, B3H 451. 4 Present address: Biology Dept., Salisbury State College, Salisbury, Maryland 2 180 1. for plants in bottles or under bell jars (e.g. Schemer 1934; Odum 1956). The introduction of 14C isotopes for measuring phytoplankton photosynthesis led to subsequent development of 14C methodologies for submersed macrophytes (Wetzel 1964). In their review and critique of various macrophyte production methods, Zieman and Wetzel (1980) suggested that, with the exception of leaf-marking techniques, all available methods are equivocal and should be used only with caution. In particular, they emphasized the shortcomings of oxygen (0,) techniques, which have been in disfavor since Wetzel’s (1964) earlier review. Zieman and Wetzel (1980, p. 110) concluded that, “[aIdequate evidence exists on both physiological and environmental bases that the oxygen light and dark enclosure method should bc abandoned as applied to submersed angiosperms. ” Results of several recent studies indicate that these caveats may, however, deserve reconsideration (e.g. Westlake 1978; Capone et al. 1979; Kelly et al. 198 1; Sand-Jensen et al. 1982; Lindcboom and DeBree 1982). Several specific problems with O2 methods have been cited, including 0, storage in plant lacuna1 systems, recycling of O2 through coupled photosynthesis and respiration internally within the plant, and O2 transport through the lacuna1 system (from leaves to roots) and release from roots to