Primary production of Cymodocea nodosa in the Gulf of Trieste (Northern Adriatic Sea): a comparison of methods

Abstract

Net primary production of Cymodocea nodosa, a common seagrass in the Mediterranean Sea, was monitored in Gulf of Trieste using 3 techniques. Production yields obtained by a method based on changes in biomass and leaf export, as indicated by the presence of leaf scars, were compared with 2 other aproaches, a marking technique and the oxygen evolution in situ. In August-September, maximum total standing biomass of 3 stations ranged between 323 and 1020 g dry wt m-2. At the shallowest station, ca 88% of total biomass was below ground, compared to 61% at deeper stations. Leaf area indices varied throughout the growing season between 0.12 and 7.24, indicating a distinct seasonal pattern in leaf production. On average up to 10 leaves were produced shoot-1 yr-1; export rates of leaf material were highest in summer. Leaf production, as estimated by the leaf scar and marking techniques, ranged from 356 to 692 g dry wt m-2 yr-1. Combined annual rhizome-root production was highest at the shallowest station (234 g dry wt m-2) compared to 72 g dry wt m-2 at deeper stations. Annual total net primary production based on the O2-method yielded the lowest results, but the seasonal pattern correlated well with results from the marking technique. Each of the 3 methods, when applied conscientiously according to the desired information, can provide reasonable estimates for the production of C. nodosa, although the O2-technique possibly underestimates production peaks. In order to estimate production yields only, the leaf scar method is recommended as an easily applicable technique. If production rate is one of the parameters of interest, the marking technique appears the most reliable. Average total net primary production was calculated to be ca 620 g dry wt m-2 yr-1 for C. nodosa for this part of the Adriatic Sea. Turnover rates of maximum total plant material and of only leaf material were 1.1 and 3 yr-1 respectively.