Response of Posidonia oceanica to burial dynamics

Abstract

Seagrasses are able to colonise and dominate unstable sandy sediments but natural (storms, hurricanes, bioturbation) and anthropogenically induced (harbour building, beach nourish- ment, siltation) sediment arrivals can result in the total or partial burial of meadows. Within this con- text, our main objective was to experimentally establish the ability of long-living Posidonia oceanica, the dominant seagrass in the Mediterranean, to counteract natural or human-induced changes in sedimentation, and to specifically examine plant survival and plastic responses (vertical rhizome growth, branching, number of leaves and sheath length) to different levels of burial intensity, fre- quency, timing, and duration. Shoot population strongly declined with increasing sedimentation, reaching ca. 65% shoot disappearance with a burial level of 4 cm, and undergoing 100% mortality with 9 cm burial. Sediment burial also enhanced shoot population decline regardless of frequency, duration or timing at which the burial event occurred. Nevertheless, at 4 cm burial, we detected some response capacity of plants to burial, consisting of rhizome elongation and rhizome branching. Rhi- zome vertical annual growth and internode length increased by 34% in the 4 cm treatment while it decreased by ca. 34% under high burial, although this response was meadow-specific. Rhizome branching also increased by ca. 36% at medium burial while being reduced by an average of 60% at high burial levels. On the other hand, there was no effect of burial on number of leaves or sheath length. Overall, this work provides experimental evidence that P. oceanica is very sensitive to sedi- ment burial, although it displays a certain capacity to respond, which is site-specific. Our results highlight the importance of sedimentation rates as a limiting factor in determining the distribution of P. oceanica meadows, both under natural conditions and under man-altered regimes.