Impact of Pestarella tyrrhena on benthic metabolism in sediment microcosms enriched with seagrass and macroalgal detritus

Abstract

The impact of Pestarella (= Callianasa) tyrrhena (Decapoda: Thalassinidea), a common burrowing shrimp in the Mediterranean Sea on sediment-water fluxes (O2, TCO2, NH4+ and NO 3-+ NO2-), sediment characteristics (organic matter, chlorophyll a) and porewater solutes (TCO2, NH 4+ and NO3- + NO2-) was investigated in laboratory microcosms over a period of 42 d. Microcosms containing homogenised fine sandy sediment were amended with either dead Posidonia oceanica leaves or fresh Ulva lactuca thalli. Reworking activity by the animal resulted in a rapid burial of surface deposited organic matter into the sediment. Porewater profiles of both TCO2 and NH 4+ indicated that P. tyrrhena activities result in a significant flushing of porewater solutes. Total sediment metabolism and carbon mineralisation were enhanced in the presence of P. tyrrhena. Metabolism of P. tyrrhena individuals was approximately 3 times higher in Ulva-amended sediment, probably due to a high activity level when nutritious food sources are in excess. Accordingly, animal respiration explained approximately half of the total enhancement in Ulva-amended sediment, while microbial decomposition of refractory seagrass detritus contributed about 4 times more to the stimulated carbon mineralisation than animal respiration. Despite the higher initial addition of carbon in Posidonia-amended sediment compared to Ulva-amended sediment, the amount of excess carbon mineralised was 4.7 times higher in the latter, indicating that mineralisation processes depend on the degradability rather than the quantity of the organic pool. Although P. tyrrhena excretion constituted only a minor part of the total nitrogen mineralised, macrofaunal activities were the major factor affecting total nitrogen mineralisation, and not the enrichment with organic matter.