Long-term stability in the production of a NW Mediterranean Posidonia oceanica (L.) Delile meadow

Long-term changes in leaf net production of the seagrass Posidonia oceanica (L.) Delile, and in the irradiance arriving at the meadow canopy, were investigated in a NW Mediterranean meadow using a palaeoecological approach. We conducted in situ shade experiments to find the relationships between the carbon stable isotope ratio (δ13C) of plant tissues, leaf net production, and ambient irradiance. The relationships were highly significant and fitted the Michaelis–Menten equation and power functions. These functions were used to reconstruct light and net production using the δ13C of P. oceanica detritus as a palaeoecological proxy along a 150 cm core of P. oceanica matte (a peat-like deposit formed by this endemic seagrass species). The δ13C values of sheath detritus along the core (i.e., the net leaf production) showed a weak but significant decrease towards the present time (R = 0.308; P = 0.02) probably as a result of (1) an increase in DIC availability, and/or (2) a progressive change in the carbon isotopic signature of DIC, both of which are consistent with a rise in anthropogenic atmospheric CO2. The canopy irradiance, reconstructed for the last 1200 years, showed a mean value of 128 µE m− 2 s− 1 with a range of 12.5–280 µE m− 2 s− 1. The reconstruction of the net leaf production for the same period yielded a mean value of 2.5 mg dw day− 1 shoot− 1 with a range of 2.0–2.7 mg dw day− 1 shoot− 1. Both ranges are within the values reported in the literature for present day studies. The absence of significant fluctuations or sudden changes through time suggests remarkable ecosystem stability during the last millennium.