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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 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 by washing with 2M HCI. Sediment was centrifuged ( 12,000 g) and the supernatant discarded. The sample was then digested for 12 h with I 0 ml of 16 M HN0,1. After a second centrifugation ( 12,000 g), I ml of supernatant was buffered with 9 ml of 0. 75 M Iris buffer, then 1 ml of the buffered sample was counted. Microbenthic algal productivity measurements were not made in the Bahamas due to the high carbonate content of the sediment which inhibited the use of the acid hydrolysis portion of the technique. Phytoplankton production was determined by filtering 250-ml volumes of water onto 4 7 mm diameter 0.4 J RESULTS-There was no seasonal variability between the quarterly weight specific photosynthetic carbon uptake rates as measured for the three seagrass species and their associated epiphytic flora (Fig. 2). This is due, in part, to the within-season variability measured for each species. Yearly mean seagrass and epiphyte weight-specific production rates were compared both within and be tween locations using a Kruskal-Wallis non-parametric one-way analysis of var iance. The only statistically significant (P < 0.05) results were: (l) the epiphytes found on Halodule wrightii in Tampa Bay incorporated carbon at a greater rate than the epiphytes on H. wrightii in the Bahamas, (2) the epiphytes on Thalassia testudinum and Syringodium jilijorme in the Bahamas produced at a greater rate than the epiphytes on H. wrightii, and (3) the seagrass S. jili jorme in the Bahamas produced at a lesser rate than its associated epiphytes. In general, seagrasses and their associated epiphytes had statistically similar yearly mean weight-specific production rates-independent of species and lo cation. The yearly weight-specific carbon uptake rate for all epiphytic flora was 1.80 ± 1.21 mg C gm dry wt'h '.This was higher than the mean seagrass car bon uptake rate for all species which was 0.40 ± 0.28. Considering the total seagrass plus epiphyte uptake, seagrasses accounted for 11 to 61 % (X= 22%) of the total. Photosynthetic carbon uptake rates for the seagrass communities are pre 1 1 sented (Fig. 3) in terms of areal production (mg C m 'h or m'h ). The conver sion of seagrass and epiphyte weight specific carbon uptake rates to areal carbon uptake rates is a function of biomass per unit area (Table 1). This conversion was made for comparison with phytoplankton and microbenthic algal production, allowing one number to represent the contribution by the three seagrass species and one number for their associated epiphytic flora at 134 FLORIDA SCIENTIST [Vol. 49 TABLE I. Yearly average standing stock biomass (g dry wt m~ 2 for seagrass species and associ ated epiphytic flora (IR =Indian River, TB =Tampa Bay). Standing stock 2 (gdrywt m ) Species Location sea grass epiphyte Reference Thalassia testudinum Florida 325 Phillips, 1960 Florida (TB) 81 Pomeroy, 1960 • Florida (IR) 48.4 21.9 Jensen and Gibson, This work Florida (TB) 48.8 18.7 , Thiswork Bahamas 5.3 Patriquin, 1972 Bahamas 200 Capone et al., 1979 • • Bahamas 75.4 11.3 Jensen and Gibson, This work Syringodium filiforme Texas 45 McMahan, 1968 Florida (IR) 6.0 2.1 Jensen and Gibson, This work Florida (TB) 20.9 6.4 , Thiswork Bahamas 7.6 0.8 , Thiswork Halodule beaudettei N. Carolina 200 Brylinski, 1971 H. wrightii Florida (IR) 21.7 8.8 Jensen and Gibson, This work Florida (TB) 18.7 6.3 , Thiswork Bahamas 5.9 3.3 , Thiswork Zostera marina Alaska 180 McRoy, 1966 N. Carolina 80 2.5 Penhale, 1977 Denmark 2.3 Borum, 1980 '40% of this standing stock is leaf material, actual standing stock= 32.4. ''Standing stock includes epiphytes. each location (see Materials and Methods). The conversion results in a range of quarterly rates from 7.01 to 30.2 (X= 18.37) mg C m'h' for seagrasses and 10.5 to 27.1 (X= 17.82) for epiphytes. There were no statistically significant differences between the four seasons, with seagrasses accounting for 51 % of the total seagrass-epiphyte production. Seagrasses had a higher biomass per unit area than epiphytes, accounting for their higher percent contribution to total seagrass-epiphyte areal production as opposed to weight-specific produc tion. The significant (P < 0.05) results of a nonparametric analysis of variance comparing mean yearly areal production rates (Fig. 3) are: (l) phytoplankton in Tampa Bay incorporate carbon at a greater rate than the other three auto trophic components, (2) phytoplankton in the Indian River Lagoon incorpo rate carbon at a greater rate than either microbenthic algae or seagrasses, and (3) phytoplankton in Tampa Bay incorporate carbon at a greater rate than phytoplankton on the Little Bahama Bank. The total primary production in a cubic meter of seagrass bed, i.e., com munity production, and the percent of total production contributed by each of the four autotrophic components is described in Table 2. Total production was greatest in Tampa Bay and lowest on the Little Bahama Bank. Photosynthetic carbon uptake by phytoplankton was greater than all other components in Tampa Bay, and was greater than seagrasses and microbenthic algae in the Indian River Lagoon. TABLE 2. Percent of total primary production contributed by community phototrophic components. z ? w Total Phytoplankton Microbenthic algae Sea grass Epiphyte 3 1 (!) Location (mgCm- h ) (%) (%) (%) (%) 00 2; Tampa Bay 177.9 57.7 10.1 17.0 15.2 Indian River 100.7 72.2 5.1 7.0 15.8 Bahamas 30.7 7.6 58.2 34.2 t;:j z CJJ zt'1 z> t:l C"l @ TABLE 3. Salinity, temperature, and nutrient concentrations for study locations. CJJ 0z 3 I Temperature Salinity Si0 ro; NH N0 +N03 "0 3 3 2 2: Location Date (oC) (ppt) (JLM) JLM JLM JLM ::: > :0 Tampa Bay 10/81 21.0 26.0 1.9 13.8 1.8 0.6 -< "0 02/82 19.0 28.0 8.1 32.4 3.1 0.4 :0 0 05/82 28.0 24.0 t:l 07/82 25.0 25.0 31.9 14.4 4.4 0.2 c:: ...,(") 0 Indian River 10/81 19.5 30.0 19.2 0.6 1.6 0.3 z 02/82 24.0 33.0 29.8 1.4 4.3 1.7 05/82 27.0 30.0 07/82 28.0 26.0 79.4 1.6 3.7 0.2 Bahamas 10/81 29.6 36.1 0.9 0.2 1.2 0.1 05/82 29.0 37.0 07/82 26.0 42.0 5.2 0.5 3.3 0.2 w Ul 136 FLORIDA SCIENTIST [Vol. 49 THAL EPI SYR EPI HAL EPI 8.0 6.0 0 4.0 /""oo ~ 2.0 ..s:: 0 0.0 ...... 3: TAMPA BAY .....>- 10.0 "0 8.0 (J'l u 6.0 (J'l 4.0 ...__....E 2.0 z 0.0 all i 0 1-u INDIAN RIVER LAGOON ::::) 3.0 0 0 0 0::: 2.0 0.. 0 X 1.0 0 0 ~ 0.0 BAHAMAS 1 Fie. 2. Weight specific photosynthetic carbon uptake rates (mg C g drv wt'h- ) for seagrasses (THAL = Thalassia testudinum, SYR = Syringodium filiforme, HAL= Halodule wrightii) and asso ciated epiphytic flora (EPI). Each box represents a quarterly carbon uptake rate; the symbol X represents the vearlv mean. A description of the three locations in terms of temperature, salinity, and nutrient concentrations (Table 3) indicates that the Little Bahama Bank is well flushed with oceanic water and that Tampa Bay and the Indian River Lagoon are brackish water environments, with Tampa Bay receiving the greatest amount of freshwater input. The Indian River Lagoon is highest in Si03 and NO,+ N03 , and Tampa Bay is highest in P04 , probably due to the occurrence of natural phosphate deposits and industrial runoff from local phosphate min ing operations. The Little Bahama Bank location had the lowest concentration of all nutrients. No.3, 1986] JENSEN AND GIBSON-PRIMARY PRODUCTION 137 PHYTO MBA SG EPI 0 120.0 0 ~ 80.0 0 0 0 0 ,...... _, 40.0 I 0.0 ..c: N I TAMPA BAY E 160.0 0 u 0'> 120.0 E ...._., 80.0 1M! 0 z 40.0 0 0 0 1- 0.0 u ::::) 0 INDIAN RIVER LAGOON 0 60.0 0::: 0 Cl. 40.0 20.0 X 0 gg 0.0 BAHAMAS 2 1 1 1 FIG. 3. Areal photosynthetic carbon uptake rates (mg C m h or m h ) for community photosynthetic components (PHYTO =phytoplankton. MRA = microbenthic algae, SC =sea grasses, EPI =associated epiphytic flora). Each box n·pn·sents a quartr-rlv carbon uptake rail'; the symbol X represents the yearly mean. Primary production measurements were made using the carbon-14 tech nique of Steeman Nielsen (1952). This technique has the advantage of high sensitivity, requires only short incubation periods, and is applicable to the four autotrophic components in question. The major criticisms of the technique for seagrasses are that labeled carbon can be released from the leaves (Sonder gaard, 1981 ), translocated to other areas of the plant or internally recycled (Bittaker and Iverson, 1976) or exchanged between leaves and epiphytes (Harlin, 197 5), all leading to the underestimation of photosynthetic carbon incorporation. In support of the technique, Penhale and Smith ( 1977) and 138 FLORIDA SCIENTIST [Vol. 49 Brylinski ( 1977) have independently shown the release of labeled dissolved organic carbon to be low ( 1 to 4%) in comparison to total carbon incorpo rated. Brylinski (1971) found linear carbon-14 uptake by Thalassia testu dinum and Halodule wrightii for 0.5 h to 2.0 h incubation periods. Short incubation periods (i.e., 0.5 h) minimize the loss of labeled carbon via respira tion, translocation, recycling and exchange, and therefore approximate a rate of total carbon uptake between net and gross. DISCUSSION-Yearly means of carbon-14 uptake rates were, in general, sta tistically similar for seagrasses, epiphytes, and microbenthic algae at all three locations. This similarity of means is, in part, attributed to the variability measured both within and between the quarterly production rates. Those means which were not statistically similar reveal that phytoplankton produc tion in the Indian River Lagoon was greater than seagrass and microbenthic algal production, and that phytoplankton production in Tampa Bay was greater than all other components, resulting in this area having the highest total community primary production per cubic meter. The Little Bahama Bank location exhibited the lowest total community primary production of any area. Pomeroy ( 1960), working in the Boca Ciega Bay portion of Tampa Bay, in water less than 2 m deep, found phytoplankton, turtle grass, and microbenthic algae to be equally productive. Two decades later, the relativ~ contribution by phytoplankton to the Tampa Bay system has increased, reducing the relative importance of seagrasses and microphytobenthos in community production. This shift in production base towards phytoplankton is largely attributed to an 81% reduction of seagrass cover in Tampa Bay (Lewis eta!., in press). Seagrass meadows have been physically destroyed (e.g., dredge and fill operations) or reduced via competitive interactions with micro- and macroalgae for the di minished irradiances experienced in an eutrophic environment. Though no historical data are available for the Indian River Lagoon, it is speculated that a similar shift in the production base has occurred in this area-also spurred by reduced seagrass populations and the eutrophication process. Biomass measurements indicate Thalassia testudinum beds have the high est standing stock (g dry wt m'). This is in agreement with other available data that indicate Thalassia and Zostera are the genera that attain the highest bio mass (McRoy and McMillan, 1977). The yearly standing stock estimates are lower than many reported in the literature with possible explanations being: ( 1) lyophilization was used to obtain dry weights as opposed to oven drying, (2) epiphytes were not included in the measurem~nts, (3) only the above-ground photosynthetic portion of the blades were considered, and (4) location depen dent density differentials. The sampling strategy was not designed to discern seasonal trends in standing stocks though others have found them to exist (Sand-Jensen, 1975; Aioi, 1980). Epiphyte biomass, which comprised as much as 36% of the total standing stock, supports the work of Capone and co-workers ( 1979) who found epi phytes to contribute 2 7-44% of the total biomass in the Bahamas. Tampa Bay No.3, 1986] JENSEN AND GIBSON-PRIMARY PRODUCTION 139 and the Indian River Lagoon had the highest epiphytic biomass; this conforms to the correlation found between high epiphyte biomass and areas of high nutrient concentration (Phillips et a!., 1978; Borum and Wium-Andersen, 1980). Epiphyte and microbenthic algal production rates corroborate the im portance of these components as primary producers in the seagrass community (Jones, 1968; Penhale, 1977; Borum and Wium-Andersen, 1980; Cattaneo and Kalff, 1980). Seagrass weight specific production rates are lower than those recorded by others (Buesa, 1975; Capone et a!., 1979; Williams and McRoy, 1982), and can possibly be attributed to the separate consideration of epiphyte and seagrass production. An overestimation of seagrass standing stock and primary production rates may result from not considering these components 1 separately. Seagrass areal production rates (mg C m'h ) also are lower than those reported in the literature (Phillips, 1960; Pomeroy, 1960; McRoy, 1966; Capone et a!., 1979; Dillon, 1971; Penhale, 1977) and are attributed to the standing stock estimates of which the production calculation is a direct func tion. Macroalgae, at times, are visually the dominant autotrophic component in the Indian River Lagoon. They occur in the form of seasonal aggregates of drift algae which can obtain an average biomass of 164 g dry wt m 2 over a 0.15 km2 seagrass meadow (Virnstein and Carbonara, in press). Quantifying the contribution of this component is a difficult task, but one which needs to be addressed for a comprehensive assessment of primary production in the Indian River Lagoon. A comparison of seagrass community primary production rates was made between three locations in the Indian River Lagoon (Heffernan and Gibson, 1984). This study recorded large variabilities in primary production rates both temporally and spatially within the lagoon. The location referred to as Link Port was in closest proximity to the Indian River Lagoon site sampled by our study. Comparison of the primary production rates measured between the two studies, and within each study, for this location, indicates high variability and necessitates the use of caution in defining relationships between seagrass com munity primary producers. Investigations on coastal marine primary production have indicated sea grasses to be the major contributor to community primary production in some areas (Jacobs, 1979; Cogdon, 1979; Goering and Parker, 1972). The presence of seagrasses in a community should not lead, a priori, to the conclusion that seagrasses are the major contributors to primary production in that commu nity. We emphasize the importance of examining all autotrophic components in ascertaining the importance of any one as a primary producer in the sea grass community. In areas where seagrasses do not contribute significantly to total primary production, they still provide many valuable community func tions, i.e., as substrata for epiphytic organisms which are a food source for grazers (Fry, 1984 ), in reducing water currents and inducing sedimentation, and in increasing sediment stability and promoting nutrient cycling (den Har tog, 1977; Zieman and Wetzel, 1980). It is suggested that monitoring community primary production, both abso- 140 FLORIDA SCIENTIST [Vol. 49 lute and relative, provides important ecological data which may be inter preted as an environmental indicator of community "health." Disturbances within a community may be associated with nutrient enrichments, resulting in increased phytoplankton populations, reduced water column light transmit tance, and the inhibited growth of benthic macrophytes. The loss of benthic plant cover, and its associated function in sediment stability, further increases water column turbidity-hastening the eutrophication process. The response of the photosynthetic community to this type of stress may be observed as a shift in the production base, e.g., from seagrasses to phytoplankton. Documen tation regarding the magnitude 'or type of shift in primary production rates may lend insight into the effects of anthropogenic perturbations on the coastal marine environment. LITERATURE CITED AIOI, K. llJilO. Seasonal change in the standing crop of eelgrass (Zostera marina L.i in Odawa Hav, central Japan. Aquat. Bot. 1>:343-354. BITTAKER, H. F., AND R.L. IvERSON. 1976. 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