FAU Institutional Repository http://purl.fcla.edu/fau/fauir This paper was submitted by the faculty of FAU’s Harbor Branch Oceanographic Institute. Notice: ©1985 Elsevier. The final published version of this manuscript is available at www.elsevier.com/locate/ecss and may be cited as: Short, F. T., Davis, M. W., Gibson, R. A., & Zimmermann, C. F. (1985). Evidence for phosphorus limitation in carbonate sediments of the seagrass Syringodium filiforme. Estuarine, Coastal and Shelf Science, 20(4), 419‐430. doi:10.1016/0272‐7714(85)90086‐1 Evidence for Phosphorus Limitation in Carbonate Sediments ofthe Seagrass Syringodiurnfiliforrne F. T. Short", M. W. Davis", R. A. Gibson" and C. F. Zimmermann" "Jackson Estuarine Laboratory , UNH, RFD no. 2, Adams Pi, Durham, N H 03824, bMarine S cience Center, OSU, Newport, OR 97365 and<Harbor Branch Foundation, In c., Box 196, Fort Pierce, FL 33450, U.S.A. Received 11 J anuary 1984 and in revised for m 24 May 1984 Keywords: ph osphorus; seagrasses; nutrients; limitation; carbonate sediments; geochemistry; Syringodium fi lifo rme; Bahamas The seagrass Sy ringodium filiforme was examined in an ecological analysis of plant nutrient requirements and nutrient resource availability. Assessment of the sediment geochemistry in a San Sal vador Island seagrass bed indicated that phosphorus was not readily accessible. to the plants. Ammonium regeneration in the fine-grained carbonate sediments was high, and interstitial con centra­ tions averaged ca. 100 11M while phosphate replenishment to interstitial water was low, and concentrations were generally less than 2 11M . Analysis of the seagra ss leaf tissue content (C: N : P = 1390 : 47 : 1, atomic wt) suggested that nitrogen and ph osphorus were both depleted relative to carbon . However, this high N : P for S. fi lifo rme and th e low concentration of phosphate available in th e interstitial water established the likelihood that plant acquisition of phos­ ph orus was limited. The finding ofhigh root biomass relative to leaf biomass in th ese seagrass beds corroborates this evidence by depicting a method of plant adaptation that increases nutrient absorptive root surface area. Introduction Seagrasses comprise a group of submerged marine angiosperms found worldwide in shallow tropical and temperate oceans (D en Hartog, 1970). Three seagrass species, Th aIassir: testudinum Banks ex K on ig, Syringodium fil iforme Kiitzing, and Halodule wrightii Ascherson, dominate the tropical western North Atlantic and the Caribbean coastal envi ronments. The ecology of S. fi liforme, manatee grass, has received little attention although it is the dominant seagrass in some regions (Phillips, 1960; Thompson, 1978) and may be the preferred food source for herbivorous fish (T ribble, 1981) and turtles (Fenchel et al ., 1979). Early studies of seagrass distribution outlined the descriptive ecology of S. filiforme along the Florida coast (Phillips, 1960; Strawn, 1961). The metabolic acti vity and light-controlled primary production of S. fiIiforme have been examined (Buesa, 1975; Will iams & McRoy, 1982), as have salinity and temperature tolerances (M cM ahan, 1968; McMillan, 1979). However, no att ention has 419 0272-7714/85/0404 19+ 12 $03.00/0 © 1985 Acad emi c Press Inc. (London) Limited 420 F. T . Short et al. been given to the nutrients important for plant growth or to nutrient cycling in S. filiforme beds, particularly in the carbonate sediment environment. Research on nutrient resource availability in both tropical and temperate seagrasses has established a notion that considerable reserves of phosphorus are available in seagrass sediments (Patriquin, 1972; McRoy & McMillan, 1977; Entsch et al., 1983). Recently, studies of seagrass nutrient cycling have emphasized the importance of nitro­ gen (Capone & Taylor, 1977; Iizumi et al., 1980, 1982; Kenworthy et al., 1982; Short, 1983a, b). Phosphorus has been given only secondary consideration and until now has not been examined as a limited resource. The geochemistry of phosphate in carbonate sediments indicates that mineral regeneration is rapid although phosphate does not accumulate in the interstitial water (Rosenfeld, 1979). Low concentrations of phosphate, relative to ammonium, have been observed in a number of studies of carbonate sedimentary interstitial water (Rosenfeld, 1979; Gaudette & Lyons, 1980; Hines & Lyons, 1982). These low dissolved phosphate concentrations may be a result of adsorption of phosphorus on carbonate sediment grains, diagenetic formation of apatite, or replacement of calcium carbonate with a phosphate-rich phase (Bern er, 1974; DeKanel & Morse, 1978; Gaudette & Lyons, 1980). Interstitial water samples from carbonate sediments of seagrass beds also con­ tained low phosphate concentrations, although additional phosphate was extractable from the sediment particles (Patriquin, 1972). The present study investigated aspects ofsediment geochemistry and the ecology ofS. filiforme in the carbonate sediments of an oligotrophic lagoon. This analysis assessed seagrass biomass, plant chemical constituents, plant growth rates, nutrient resource availability, and regeneration ofnutrients in the sediments ofthe seagrass bed. Site description and methods A seagrass bed of Syringodium filiforme was sampled in Graham's Harbor, a shallow semi-enclosed lagoon at the north end of San Salvador Island, Bahamas (F igure 1). This lagoon averages 1-2 m in depth and is protected from oceanic swells and storms by fring­ ing islands and coral reefs , a configuration that allows extensive flushing of the seagrass bed with oligotrophic Atlantic Ocean water while dissipating most ofthe wave energy. A S. filiforme bed located on the west side of Cut Cay was selected for study because it represented a monospecific stand of seagrass in an area of uniform environmental conditions i.e., constant water depth, relatively uniform current flow, and fine-grained carbonate mud. Seagrass plant samples were collected in triplicate with a 16 em diameter plexiglass core sampler to a depth of30 cm in th e sediment and by clipping within a 1/16 m 2 frame. Shoot density was determined from the number of shoots per sample. Shoot height was measured on 10 of the longest shoots representing the top of the leaf canopy in each sample. Rhizome length measurements were made of the total length of horizontal rhizome in each core sample. Root size was measured on selected roots attached to the rhozome and having an intact root tip . Leaf, rhizome, and root portions of the plant samples were separated, washed in fresh water, and dried to constant wt at 80 °C for 24 h to determine plant biomass. The weight ratios for carbon and nitrogen in plant tissue were mea sured on dry plant samples using a Perkin Elmer elemental analyzer. Total phosphorus was determined after persulfate oxidation (M enzel & Corwin, 1975). ).,". N " SOl mp,mg Site .".... '.:." • c. '. ..,Cu t Coy "~ ' . o 2 Mil es I 74° 30" W Figure 1. San Salvador Island, Bahamas, showing the location of the study site west of Cut Cay in Graham's Harbor. Interstitial nutrient depth profiles were determined from duplicate 4·7 ern diameter core sam ples collected by hand, sectioned into 50 ml pol ystyrene centrifuge tubes, capped, and centrifuged for 10 min at 1500 rpm. The interstitial water was removed through the cap with a syringe, injected directly into a Vacutainer, and frozen for NH~, PO~ -, analysis of and Si04 on a Technicon AutoAnalyzer (Zimmerman et al., 1977). Interstitial water samples were also collected using an in situ 'sediment sipper' , These samplers were constructed of hollow PVC stakes having a porous Teflon collar (Zimmermann et al., 1978; Montgomery et al., 1979) located at the required sampling depth (F igur e 2). The stakes were sealed except for a sampling tube extending into the bottom of the stake and a gas port at the top. The sippers were inserted into the sediment to the appropriate depth, argon gas at a very low pressure was used to force all water out through the sampling tube, suction was applied to the gas port with a hand pump to evacuate the sampler, and the interstitial water from around the porous collar slowl y seeped into the hollow stake. The interstitial water sample was removed using argon gas pressure to force the water out the sampling tube where it was collected directly into a Vacutainer. These samples were stored in the dark and transported back to the laboratory where sub-samples were removed with a syringe for immediate analysis of platinum electrode potential (Eh) on an Orion Eh probe, and of pH on a Fi sher combi­ nation pH electrode. The remaining interstitial water sample was frozen for further chemical analysis as described above. The sediment sippers were left in place in the sedime nt for subs equent sampling ofinterstitial water. '. ' · '1 i I !, i I i I I ! . i1 ! t F igure 2. Photo of two 'sediment sippers' used to repeatedly collect sediment inter­ stitial wate r from a specific study site . The sampling method is de scribed in th e text. Not shown is a 1·9 mm ' 0 ' ring that seals the cap to the stake. Nutrient regeneration rates were determined in jar exp eriments by measuring daily formation ofammonium, phosphate and silicate in isolated samples from three sediment depths (Aller & Yingst, 1980). Carbonate mud was obtained using a 16 ern diameter core sampler,S cm depth sections were placed in plastic bags and quickly sealed to avoid excessive contact with air. The mud was homogenized, packed into 50 ml polystyrene centrifuge tubes, and incubated under anoxic conditions in a mud-filled plastic bag at in situ temperatures. The rates of ammonium, phosphate and silicate production were determined by removing three tubes from each depth interval each day, separating interstitial water by centrifugation and extracting samples with a syringe through a hole punched in the plastic lid to avoid air contamination. The amount of adsorbed ammonium and phosphorus in the sediments was determined by extraction.