FAU Institutional Repository http://purl.fcla.edu/fau/fauir This paper was submitted by the faculty of FAU’s Harbor Branch Oceanographic Institute. Notice: © 1986 Florida Academy of Sciences. This manuscript is an author version with the final publication available and may be cited as: Jensen, P. R., & Gibson, R. A. (1986). Primary production in three subtropical seagrass communities: a comparison of four autotrophic components. Florida Scientist, 49(3), 129-141. '· Florida Scientist QUARTERLY JOURNAL OF THE FLORIDA ACADEMY OF SCIENCES DEAN F. MARTIN, Editor BARBARA B. MARTIN, Co-Editor Volume49 Summer, 1986 Number 3 Biological Sciences PRIMARY PRODUCTION IN THREE SUBTROPICAL SEAGRASS COMMUNITIES: A COMPARISON OF FOUR AUTOTROPHIC COMPONENTS PAUL R. jENSEN AND ROBERT A. GIBSON Harbor Branch Foundation, Inc., RR I, Box 196, Fort Pierce, FL 33450 ABSTRACT: A one-year study was conducted in which seagrass communities in Tampa Bay, the Indian River Lagoon, and the Little Bahama Bank were monitored quarterly for primary pro­ duction, standing stock biomass, and nutrient concentrations. Primary production rates were measured in situ for the following jour community autotrophic components: seagrasses, their associated epiphytic flora, phytoplankton and microbenthic algae. Photosynthetic rates were compared both within and between locations to determine the relative significance of the pri­ mary producers, and the contribution by each to areal, i.e., community production (mg C 3 1 m· h. ). In general, all species of seagrasses, their associated epiphytic flora and microbenthic algae produced at similar rates at all locations. The Indian River Lagoon had phytoplankton primary production rates that were greater than both seagrass and microbenthic algal production rates. Phytoplankton in Tampa Bay had primary production rates greater than all other commu­ nity photosynthetic components, and primary production rates by all photosynthetic components in the nutrient-deplete Bahama Banks were similar. Therefore, the ma;ority of the community primary production, i.e., the production base, in Tampa Bay and the Indian River Lagoon is contributed by phytoplankton. These areas are characterized by anthropogenic perturbations and associated high nutrient concentrations. Historical data indicate that phytoplankton, turtle grass and microbenthic algae were, at one time, equally productive in Tampa Bay. Increased phytoplankton production has reduced the relative importance of seagrasses and microbenthic algae to community production in this area. It is suggested that the relative contributions by the components of the photosynthetic community are affected by nutrient availability. Nutrient en­ richment may induce a shift in the production base from benthic plants to phytoplankton. A shift of this type may be interpreted as an ecological indicator of either eutrophication or environmen­ tal stress in a coastal marine environment. SEAGRASS meadows are a common component of the world's coastal ecosys­ tems. Since Petersen's pioneering work in 1913, a great deal of information has accumulated on various aspects of primary production in the seagrass commu­ nity. These works have established that seagrass meadows are among the most productive marine ecosystems, supporting rich communities of both plant and animallife(McRoy, 1974;Thayeretal., 197S;Jacobs, 1979). 130 FLORIDA SCIENTIST [Vol. 49 It is generally accepted that seagrass meadows generate large quantities of organic material (Thayer et al., 1975) which ultimately enter the detrital food web. This high organic production can lead to the intuitive assumption that seagrasses, being visually the most conspicuous autotrophic component in the seagrass meadow, are the major contributors to primary production in these systems. Seagrasses have been cited as being the major contributor to primary production in certain seagrass meadows (Goering and Parker, 1972; Congdon and McComb, 1979; Jacobs, 1979) but, nonetheless, it should not be con­ cluded a priori that seagrasses are the major contributors to primary produc­ tion in all seagrass meadows. The quantification of all autotrophic compo­ nents must be made to fully estimate the magnitude of each component to the total productivity of the system. The objectives of this research were to quantify seagrass community pri­ mary production rates from three geographically distinct locations, and to examine these rates in terms of: (1) within and between location differences, (2) relative contributions to community primary production, and (3) relationships to local nutrient regimes. MATERIALS AND METHODS-Seagrass beds were chosen as study sites from three locations: (I) Indian Cay, the Little Bahama Bank, Bahamas, (2) Link Port, the Indian River Lagoon, Florida, and (3) Lassing Park, Tampa Bay, Florida (Fig. 1). These areas are on approximately the same latitude (27-28 °N) yet exemplify distinct environments representing a range of nutrient regimes and varying degrees of human interference. Tampa Bay is an industrialized estuarine system, altered by dredge and fill operations, harbor and channel construction, and effluents from domes­ tic and industrial waste waters (Wilkens, 1979). The Indian River is a long narrow coastal lagoon, experiencing variable flushing rates and mixing with oceanic waters. Increased organic load and turbidity can be attributed to canal dredging and drainage from land development projects (John­ son, 1983). The effects of industrialization are less than in Tampa Bay. The Little Bahama Bank is a broad shallow-shelf lagoon. It is a pristine environment, well flushed by oceanic waters, with low nutrient concentrations and few anthropogenic influences. During a one-year period each location was sampled quarterly to measure: primary production rates of four selected autotrophic components, standing stock biomass, and physical and chemical parameters. The community autotrophic components monitored were: ( 1) the three most abundant seagrass species (Thalassia testudinum Banks ex Konig, Syringodium filiforme Klltzing and Halo­ dule wrightii Asherson), (2) associated epiphytic flora, (3) phytoplankton, and (4) microbenthic al?ae. The communities were all in ca. 1 m water depth and therefore were spatially defined as I m of water overlying 1 m 2 of seagrass meadow. The only major autotrophic components not considered were the macroalgae, which are found either attached or free-floating, and the emer­ gent wetland vegetation. Primary Production Measurements-Triplicate measurements were made between 0900 and 1400 hours of each sampling period to determine primary production rates of the selected compo­ nents. All primary production measurements were made at the collection sites using in situ radio­ 14 isotope tracer techniques with 1 ml NaH C03 (Amersham Corp.) at concentrations between 8-14 1<Ci/ml and incubation periods of 0.5 h. Equations 1-3 were used to calculate primary production: 3 1 phytoplankton production (mg C m' h ) = 14 2 ( C assimilated) x (' C available) x (1.05) (I) 14 ( C added) x (time) x (volume filtered/chamber volume) 2 1 seagrass and epiphyte production (mg C m' h ) = No.3, 1986] JENSEN AND GIBSON-PRIMARY PRODUCTION 131 83 ATLANTIC OCEAN FLORIDA 2 7 D' Frc. I. Locations of sea grass meadow sampling sites. 132 FLORIDA SCIENTIST [Vol. 49 14 12 ( C assimilated) x ( C available) x (I .05) x (areal dry weight) (2) 14 ( C added) x (time) x (sample dry weight) x (area) 2 1 microbcnthic algal production (mg C m h ) = 14 12 ( C assimilated) x ( C available) x ( 1.05) (3) 14 ( C added) x (time) x (area of core) Separate Plexiglas chambers were used for the in situ incubation of ( 1) sea grass blades plus epiphytes and (2) microbenthic algae; phytoplankton were incubated in glass boiling flasks. The seagrass-epiphyte chamber allows for the insertion of a seagrass blade, plus associated epiphytic flora, into the chamber without removing the plant from the sPdiment. Chambers were filled with filtered seawater prior to short shoot insertion and, after insertion, inoculated with carbon-14 to allow isotopic uptake by both the seagrass and the associated epiphytes. Microbenthic algal photo­ synthetic rates were determined for the upper 2.5 em of sediment using a 2.0 em inner diameter Plexiglas corer which also acted as the incubation chamber. Carbon-14 was added to the chamber and the intact sediment core; the chamber was then scaled and returned to the sediment for incubation. These chambers were designed to minimize disturbance of the sample population (see Heffernan and Gibson, 1983, for detailed description of apparatus). Incubation procedures and sample processing for scagrasses, epiphytes and microbenthic algae were performed as described by Heffernan and Gibson (1983), with the substitution of 0.5 h for 3.0 h incubation periods. Following incubation periods, seagrass samples were rinsed with filtered seawater to remove unincorporated carbon-14, and the epiphytes were separated from the seagrass blades using the freeze-drying and scraping technique of Penhale ( 1977) under the low magnifica­ tion of a dissecting microscope. Macrofauna! components of the epiphyte population were re­ moved and discarded. Seagrass and epiphyte samples were solubilized for liquid scintillation counting using a modification of the methods of Gange ct al. ( 1979). Microbenthic algal photosyn­ thetic production was determined using a modification of previous methods (Van Raalte ct al., 1974) as follows: unincorporated carbon-14 was removed from the sediment