Effects of infauna harvesting on tidal flats of a coastal lagoon (Ria Formosa, Portugal): Implications on phosphorus dynamics

The systematic collection of benthic organisms in tidal flats of coastal lagoons should be taken into account for the management of these systems, once sediment disturbance affects biogeochemical processes by favouring pore water renewal during tidal inundation. The objective of the present work was to evaluate the effects of infauna harvesting on the phosphorus dynamics of muddy and sandy intertidal areas in the Ria Formosa. Sediment cores and overlying water were collected during August 2000 and February 2001 from reworked and undisturbed sediment before and after flooding. Results obtained showed that during the first minutes of flooding there was a marked decrease of phosphate in pore water of disturbed sediments. However, phosphate tidal fluxes from sandy sediment were clearly higher (17 nmol cm−2 d−1 in summer and 3 nmol cm−2 d−1 in winter) than in muddy sediment (0.4 nmol cm−2 d−1 in summer and −0.01 nmol cm−2 d−1 in winter). After muddy sediment disturbance concentrations of iron oxides increased quickly (from 5 to 16 μmol g−1) and phosphate was sorbed onto these iron oxides, resulting in a buffering of phosphate pore water concentrations at low values in the oxidized sediment zone. The estimated P-output from muddy sediment decreased one to two orders of magnitude after sediment disturbance in contrast to sandy sediments in which the impact of infauna harvesting was minimal. Consequently, the P-cycle is influenced by the disruption of muddy habitats in tide-driven systems. Such information could be useful for the management of the lagoon.