Forms and availability of sediment phosphorus in carbonate sand of Bermuda seagrass beds

Abstract

Primary production by seagrasscs in tropical and subtropical carbonate sediments often appears to be phosphorus (P) limited, and several studies have sought to identify the possible sources of P for long-term growth. Here, we quantify concentrations of particulate and dissolved P and fluoride (F-) in carbonate sediments, sediment-water P exchange, and leaf-tissue P concentrations in three seagrass beds in Bermuda. We also present data on the sequential extraction of P and F-from the sediments at each site. Total sediment P (TP,,,) in the upper 20 cm ranged from 650 to 1,250 mmol P m-2 and was some 500-fold larger than the pool of P dissolved in pore waters. Loosely adsorbed inorganic P comprised 2% of TPsed, while a surface-bound inorganic P pool extracted by dithionite buffer constituted 15-20%. Most of the remaining P and nearly all of the Ca and F-present in the sediment was recovered when the remaing sediment was dissolved in an acetic acid buffer solution. This pool includes calcium-fluoroapatite (CFA), which is considered the primary diagenetic sink for P in carbonate sands. Dissolved inorganic P and Fwere both elevated in the rhizosphere pore waters from the three seagrass beds. In combination with our analysis of sediments by sequential extraction, this result indicates that the carbonate matrix of the sediment is being dissolved in the rhizosphere, with a resulting release of P to the pore water and hence to the seagrass roots. We suggest that this is important in the P economy of these subtropical seagrass (Thalussia testudinum) beds and note that the carbonate-bound P pool is large enough to sustain seagrass P requirements for decades. Nonetheless, primary production in such seagrass systems can remain P limited if the rate of P release from sediment dissolution is too slow to support maximum seagrass growth rates. Analysis of seagrass leaf tissues revealed that P availability was highest at the site with the highest anthropogenic influences: This site was also characterized by the highest sediment P release, indicating that high P availability may also be partly due to higher release of P as sediments are dissolved. Phosphorus was released from the sediment mainly as dissolved organic P causing a net efflux of dissolved P at all sites in April and at two sites in August.Carbonate sediments of tropical and subtropical coastal waters are thought to trap phosphorus (P) efficiently by strong adsorption of dissolved inorganic P (DIP) (i.e. phosphate) onto CaCO, particles. This mechanism of P immobilization is inferred from adsorption experiments where excess DIP disappears rapidly from solution when added to sediment slurries (deKane1 and Morse 1978; Morse et al. 1987; Short et al. 1990; McGlathery et al. 1994), and from stoichiometric nutrient regeneration models that indicate a depletion of DIP relative to ammonium in sediment pore 'Present address:

Nitrogen as the limiting nutrient for eutrophication in coastal marine ecosystems: Evolving views over three decades

“…Note that these tropical carbonate systems may become N limited as anthropogenic nutrient enrichment accelerates and rates of P sorption decrease (McGlathery et al 1994;Howarth et al 1995;Jensen et al 1998). From his studies of N and P dynamics in lownutrient tropical lagoons, Smith (1984) was impressed at how P acted as the master variable, with N being responsive over time, just as was the case in the ocean system described by Redfield (1958) and the lake experiments described by Schindler (1977).…”

The role of organic acid exudates in liberating phosphorus from seagrass‐vegetated carbonate sediments

“…Pore-water pH and F 2 changes are suggested as proxies for sediment (specifically carbonate fluoroapatite) dissolution and have been used in laboratory experiments and in the field to determine P release (Rude and Aller 1991;Jensen et al 1998;Burdige and Zimmerman 2002). Our dissolution experiments showed that F 2 was liberated from sediment cores at high OA concentrations, but that increased F 2 concentrations were not present with equal H + additions in the form of HCl, suggesting that OA are more efficient at dissolving carbonate fluoroapatite minerals than simple changes in acidity (such as the acidity provided by OA-biotic mechanisms).…”

“…A number of studies have investigated total sediment dissolution rates and seagrass uptake rates and found that P i concentrations provided by rates of bulk sediment dissolution are not sufficient to relieve P-limitation (Jensen et al 1998;Gras et al 2003;Nielsen et al 2006). Recent evidence suggests that there are increased rates of sediment dissolution in marine rhizosphere sediments, and that high rates of oxidation caused by seagrass O 2 release may provide the required amount of acid to explain enhanced rates of dissolution (Ku et al 1999;Burdige and Zimmerman 2002;Burdige et al 2008).…”

A high‐resolution pore water sampler for sandy sediments

“…comm. ;Erftemeijer and Middelburg 1993;Jensen et al 1998). The shape of the profile is the net result of sources of PO 4 3Ϫ (mainly decomposition of organic matter and sediment dissolution), sinks of PO 4 3Ϫ (primarily uptake by seagrasses and sediment immobilization), and a loss of PO 4 3Ϫ (presumably dominated by molecular diffusion) to the water column and, to some extent, to the sediment below our measurements.…”

“…In previous studies, we have used sippers extensively for pore-water extractions from both vegetated and unvegetated carbonate sediments (McGlathery et al 1992, pers. comm;Jensen et al 1998). Part of our motivation for developing this new technique was the need to obtain porewater samples at a much finer depth resolution, to accurately estimate fluxes across the sediment-water interface and transformation rates of solutes within the sediments.…”