MEPS 320:141-151 (2006)  -  doi:10.3354/meps320141

Biogeochemical implications for phosphorus cycling in sandy and muddy rhizosphere sediments of Zostera marina meadows (Denmark)

Marianne Holmer*, Cristiana Carta, Frede Ø. Andersen

Institute of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark

ABSTRACT: To date, few studies have investigated the implications of sediment biogeochemical conditions on differences in sediment phosphorus (P) pools and P availability in sandy (low-P) and muddy (high-P) siliclastic seagrass sediments. To determine the role of seagrass activity on seasonal changes in sedimentary P, we investigated the solid-phase P pools in seagrass Zostera marina sediments in a Danish fjord. During the Z. marina growth season sediments were fractionated by sequential extractions into 3 (and once into 5) chemically defined groups: loosely adsorbed inorganic P, inorganic P bound to oxidized metals (primarily Fe-bound), P adsorbed to clay minerals, Al and humic acids, Ca-bound P, and refractory organic P. Fe-bound P accounted for about 30% of total P in the muddy sediments, whereas this pool was small in the sandy sediment (<5%). Here the Ca-bound pool was the most important (>80%). The Fe-bound P at the muddy site showed seasonal variations with lower pools during late summer, and it was always lower in the vegetated sediment, except during maximum biomass in July, where Fe-bound P was higher in the deep rhizosphere sediments. The seasonal variation was less at the sandy site, and there was little difference between unvegetated and vegetated sites, suggesting that the vegetation had a limited effect on the sedimentary P pools. However, the presence of vegetation increased the availability of P in the water column at both types of site, as P was released across the sediment–water interface at the vegetated sites compared with an uptake during most of the sampling period at the unvegetated sites, although the fluxes were low and could only account for a minor fraction (<10%) of the seasonal changes in the sedimentary P pools. Estimates of P incorporation in seagrass biomass showed that the seasonal variation in sedimentary P pools were more than sufficient to support the P demand. We hypothesize that the higher P contents in seagrass tissues and the larger biomass at the muddy site are due to high P availability from the redox-sensitive Fe-bound and organic P pools at this site, whereas P may be a limiting factor for growth at the sandy site, where the mobility of P is lower due to binding in the Ca fraction.

KEY WORDS: Phosphorus · Zostera marina · Sediment · Nutrient content · Seasonal variation

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