Meiofaunal production and energy transfer efficiency in a seagrass (Posidonia oceanica) bed of the Western Mediterranean

Measurements of secondary production of meiofaunal assemblages are limited, and for most seagrass systems no information is available yet. In the present study meiofaunal production was estimated in a Posidonia oceanica bed in the NW Mediterranean Sea using the summation of biomass increments and different equations based on the production:biomass ratio (i.e. biomass turnover). Total meiofaunal production ranged from 7.5 to 13.2 gC m-2 yr-1, according to the approach utilised (mean value 10.0 ± 1.9 gC m-2 yr-1, median value 9.6 gC m-2 yr-1). These values are among the highest reported worldwide, indicating that seagrass systems are hot spots of meiofaunal production. Despite the numerical dominance of nematodes, harpacticoid copepods were responsible for about 50% of total meiofaunal production, followed by polychaetes and nematodes (34 and 12%, respectively). Daily meiofaunal production rates were highest in February-March (21.3 mgC m-2 d-1), late May-June (31.4 mgC m-2 d-1) and October-December (44.6 to 52.0 mgC m-2 d-1). Analysis of daily secondary production of the dominant taxa suggested that different meiofaunal components dominated in different periods. During February-March, May-June and September-October harpacticoid copepods displayed highest production rates, but in November and December daily meiofaunal production was largely accounted for by polychaetes and nematodes. Temporal analysis of total meiofaunal production also revealed that, conversely to what is expected for temperate environments, only ca. 15% of the meiofaunal carbon was produced in summer, whereas 23% was observed in spring and 45% in autumn. Temporal changes in total meiofaunal daily production were apparently not dependent upon temperature, but rather reflected changes in the content and bio-availability of sediment organic matter in the seagrass bed. The seagrass P. oceanica was responsible, together with microphytobenthos, for high primary production values (169 to 300 gC m-2 yr-1). The fraction of total benthic primary production potentially consumed by meiofauna (as a measure of seagrass trophic efficiency) ranged from 10 to >50%. These estimates provide evidence of the relevance of meiofauna in benthic energy flows. At the same time, the production efficiency in the Posidonia bed (calculated by the ratio of total meiofaunal production to total primary production) was only 3.0 to 5.3%. These values, comparable to those observed in estuarine/organic-rich systems, are 2 to 10 times lower than those estimated in Cymodocea nodosa or Zostera noltii seagrass meadows, indicating that the Posidonia system is less efficient than other seagrass systems in channelling primary production to higher trophic levels.